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Articles from 1997 In September


Software Development Simplified with Requirements Management Tool

Software Development Simplified with
Requirements Management Tool

Improves the definition, management, and communication of project requirements

For medical device manufacturers, keeping track of FDA's increasingly complex and ever-changing design and manufacturing requirements can be a real challenge. As a result, they are seeking out tools that can help.

A developer of software for medical devices, Software Remodeling Inc. (SRI; Plano, TX), discovered such a tool. SRI is currently developing software, electronic, and mechanical subsystems for an infusion pump.
There are a variety of FDA guidelines and standard operating procedures that SRI must meet. Safety-critical requirements must be reviewed with extra scrutiny to ensure the code is accurate and the software is thoroughly tested. Testing the potential hazards of every device subsystem--software, electronic, or mechanical--is very important. For every potential failure mode, team members must perform a hazards analysis.

To help achieve this, SRI team leaders implemented RequisitePro--a requirements management tool developed by Boulder, CO­based Rational Software--to improve the definition, management, and communication of project requirements throughout the development life cycle.

SRI's senior software engineer, Cecelia Rogers, is responsible for the definition of software requirements and the development of the software requirements specification (SRS). The SRS is a critical document because it describes exactly what the device needs to do to meet customer and safety requirements. The document also becomes part of the 510(k) submission to FDA.

SRI uses RequisitePro to trace all requirements that need to be tested and to specify how each requirement will be tested (i.e., by analysis or demonstration). Developers and testers can simply print out the hazards analyses and testing matrices for selected requirements and their attributes and include it with the 501(k) submission.

"One of the main benefits we have received from implementing RequisitePro," Rogers says, "is the ability to trace requirements from the highest-level specifications through software design documents and individual test plans. This traceability not only ensures that customer needs are met but also keeps us from implementing rogue requirements."

Prior to using RequisitePro, requirements were managed manually or with an in-house, proprietary system. Developers would have to export requirements data from a PC in a text file and then import them into a relational database--a time-consuming process. Additionally, a requirement change could not be traced or communicated effectively.

Because RequisitePro allows users to custom design fields and attributes within its requirements repository, SRI has created its own assignment field. Team leaders use this custom field to communicate responsibility for specific requirements to other members of the team, both inside and outside the SRI organization. "I can assign requirements to external project team members without having to generate a separate memo or E-mail. The assignment field tells people in real time what they should be working on," says Rogers. "When changes are made to the project documents or the requirements database, RequisitePro matrices bring attention to any conflicts. These flags have become a communication tool for us and have made us more productive," she explains.

"RequisitePro has greatly improved our project management process," notes SRI co-owner Jeff Rogers. "The tool helps us manage document releases and reports quickly and easily. What used to be a one- to two-hour job of manual processing is now a five-minute job with the click of a mouse."

For more information on Rational Software, call 303/444-3464.

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MPMN September 1997: Products Featured on the Cover

MPMN September 1997: Products Featured on the Cover

Hydrocolloid adhesive extends wear time of medical devices

Suitable for applications requiring skin protection, cushioning, and long wear time, a hydrocolloid adhesive product resists breakdown from exposure to body fluids and other moist conditions and remains intact and pliable for up to seven days. The nonirritating synthetic formulation does not support growth of microorganisms and allows skin to breathe and function normally. Uses include products for ostomy, incontinence, tracheostomy, prostheses, and cushioning. 3M Medical Specialties, 3M Center, Bldg. 275-4E-01, P.O. Box 33275, St. Paul, MN 55133. Phone: 800/228-3957.

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Precision metal stampings available in various quantities

A contract manufacturer provides precision metal stampings to the medical device industry. The firm has expertise in wire EDM, jig grinding, and CNC machining and can design and build progressive dies, custom rubber molds, and plastic injection molds. Short and long production runs can be accommodated. Straton Industries, 180 Surf Ave., Stratford, CT 06497. Phone: 203/375-4488.

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Multilayered sheath tubing offers walls as thin as 0.002 in.

Offering good kink resistance and coefficient of friction, a line of multilayered sheath tubing includes a variety of inside diameters and walls as thin as 0.002 in. The tubing comes in a range of colors or in a radiopaque version and can be bonded with other thermoplastic materials for multiple uses. The company offers assembly and testing services in a Class 100,000 cleanroom facility. Putnam Plastics Corp., 130 Louisa Viens Dr., P.O. Box 779, Dayville, CT 06241. Phone: 203/774-1559.

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Soft and Elastic TPE Provides an Alternative to Latex

Materials

Soft and Elastic TPE Provides
an Alternative to Latex

Can be easily overmolded onto other components

Teknor Apex Co. (Pawtucket, RI) has introduced a soft and elastic thermoplastic elastomer (TPE) that provides a low-cost, nonallergenic alternative to latex rubber.

Beyond latex replacement, Tekron 96-E0807A-03NT opens new design possibilities because it can be easily overmolded onto other components, imparting valuable functions without costly assembly steps. According to Teknor's Peter Galland, the softness and elasticity of the new compound make it a good candidate for use as a cushioning layer on system components that need shock and vibration control.

The compound is a TPE alloy based only on 21 CFR-listed feedstocks (polyolefin and SEBS rubber). It has a rating of only 3 on the Shore A durometer test and an ultimate elongation test value of 1100%. "The durometer appears to be lower than that of any other TPE on the market, and the elongation is among the highest available," says Galland. The compound's surface is smooth and dry to the touch, and it can be formulated for greater optimal transparency than latex. It can be extruded or injection molded, and applications may include seals, gaskets, plunger tips, linings, and overmolded coatings or cushions.

Tekron 96-E0807A-03NT is part of a series of high-elongation grades developed by Teknor Apex Plastics Div. to provide functional and tactile properties similar to those of latex rubber without the possibility of allergic reactions. Recently the company also introduced a medical-tubing grade of Tekron TPE, designated 95X0832E-55, with a Shore A hardness value of 55.

For more information, contact Teknor Apex Plastics Div. at 401/725-8000.

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Coatings

Process Microfuses Radiopaque Markers to Intravenous Devices

Allows visibility on x-ray film and fluoroscopes

Implant Sciences (Wakefield, MA) has developed a process for microfusing radiopaque markers to intravenous devices for visibility on x-ray film and fluoroscopes. The microfused surface is biocompatible and can achieve 100% attenuation of a fluoroscopic spectrum.

This proprietary process is a form of "coupled" unbalanced magnetron sputtering, which results in an extremely dense and well-adhered microfused coating of an x-ray-absorbing alloy. Radiopaque alloys consist of gold, platinum, iridium, palladium, rhodium, or a combination of these. Microfused radiopaque coatings create a surface up to 15 µm thick on either polymers or metals.

Microfused radiopaque coatings from Implant Sciences eliminate the need for crimping and swaging of precious-metal bands, which can be abrasive and can even loosen, shift, or fall off the device in use. Because of the malleability of metal and the process itself, compressive stress is created in the microfused materials, resulting in surfaces that can bend and flex. This makes them suitable for such applications as stents and coils.

In addition to radiopaque microfused coatings, the company offers ion-implantation services and other microfusion technologies such as microfused ceramic coatings, microfused antimicrobial coatings, and microfused iridium hard coatings.

For more information, contact Implant Sciences at 617/246-0700.

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Packaging

Adhesive Coating Technology Solves Packaging Problems

Ensures seal integrity and resistance to sterilizer creep

An advance in adhesive coating technology developed by Philadelphia-based Perfecseal solves many of the performance problems associated with the standard coated Tyvek lid stocks used for medical device packaging. Designated SBP2000, the adhesive formulation is designed to ensure critical seal integrity and resistance to sterilizer creep.

Both features are important in medical device packaging because of exposure to rigorous EtO sterilization procedures, high temperatures and humidity, and short cycle times. In addition to high bond strength and creep resistance, the adhesive provides wide heat-sealing capability, high porosity (low Gurley rating), and good adhesion to silicon-treated polyester. Thermal adhesion under environmental stress and excellent cold flexibility help ensure package integrity and maintenance of sterility under the extreme environmental conditions encountered in global distribution.

Perfecseal also provides thermoplastic flexible packaging and heat-sealed coated Tyvek and paper, film and foil laminations, multilayer coextruded and oriented films; peel pouches; custom thermoformed trays and die-cut lids; pharmaceutical labels; flexo and rotogravure printing; and vacuum metallizing.

For more information, contact Perfecseal at 800/999-7626.

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Medical Device Exporting Seminar Announced

Medical Device Exporting Seminar Announced

Tips to be offered by current exporters

The Southern Ohio District Export Council, representing the tri-state area of Kentucky, Ohio, and Indiana, will conduct "The Medical Device Conference: New Markets, New Methods." The two-day seminar will take place in Ft. Mitchell, KY, on September 30 and October 1, 1997.

The conference, which will kick off World Trade Week, offers seminars and workshop sessions for companies that currently export or are considering exporting products. Cultural, financial, logistical, marketing, regulatory, and legal issues will be covered on two tracks.

Among the highlights of the conference will be a keynote address from Terri Zavada, the director of global strategy and analysis for the Health Industry Manufacturers Association.

For more information, contact MidMark at 937/526-4604.

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Perfecseal Awarded Shingo Prize

Has manufactured packaging materials for 25 years

One of this year's nine Shingo Prizes for excellence in manufacturing has been awarded to Philadelphia-based Perfecseal Inc., a manufacturer and marketer of packaging materials for the health-care industry. The company, which began manufacturing disposable medical packaging in 1972 and employs more than 200 people, was recognized for its efforts in achieving excellence in all phases of the manufacturing process.

The Shingo Prize is named for Shigeo Shingo, the manufacturing expert best known for developing the revolutionary Toyota Production System. The award is given to companies in the United States, Canada, and Mexico that excel in productivity and process improvement, quality enhancement, and customer satisfaction.

For more information, contact Perfecseal Inc. at 215/673-4500.

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Panasonic Forms New Medical Unit

Will use advanced digital video technologies

Panasonic Broadcast & Television Systems (Secaucus, NJ) has announced the formation of a new company unit dedicated to the medical and industrial imaging markets. The newly formed Panasonic Medical & Industrial Video Co. (PMIVC) will focus its efforts on the sale and support of the company's line of CCD cameras, video recording equipment, monitors, and related system components for the medical, dental, diagnostics, machine vision, test and measurement, quality control, and various industrial imaging industries.

PMIVC's parent company, Matsushita Electric, is one of the world's largest manufacturers of broadcast, professional, and consumer video systems. As a result, the new company benefits from its R&D in emerging technologies such as digital imaging and recording, nonlinear recording, high-definition television, and LED displays.

For more information, contact PMIVC at 914/358-1801.

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NuSil Expands Material Testing Services

Tests include durometer, tensile strength, thermal conductivity, and viscosity

NuSil Technology (Carpinteria, CA) has introduced an expanded material testing service. For over a decade the company has performed silicone testing per ASTM E595, and now all forms of silicone materials may be tested, including fluids, gels, adhesives, elastomers, and dispersions. Specific tests include durometer per ASTM D2240, tensile strength per ASTM D412, thermal conductivity per ASTM C177, and viscosity per ASTM445.

Customers who purchase NuSil's silicone products can use the service to perform nonstandard tests or determine suitability of shelf-life extension.

For more information, contact NuSil Technology at 805/684-8780.

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BioScreen Testing Acquires Biotechnics Labs

Offers microbiology, chemistry, and toxicology services

BioScreen Testing Services (Torrance, CA) has acquired Biotechnics Laboratories (Los Angeles), a specialist in contract medical device testing. According to company president Bradford Rope, BioScreen will now be a full-service provider of microbiology, chemistry, and toxicology testing for medical devices.

Richard Schlesinger, a diplomate of the American Board of Toxicology with 40 years of industry experience, has been appointed vice president of toxicology services, and Richard Sullivan, PhD, will become the director of technical and regulatory affairs.

For more information, contact BioScreen Testing at 310/214-0043.

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For more information on liquid-level and flow sensors from Gems Sensors, call 860/747-3000.

PROFILE: Sensors Provide Accurate Liquid-Level Monitoring in Immunoassay Analyzer

Often, OEMs must work closely with outside suppliers to effectively integrate components into a product's design. This is particularly true in the medical device industry, where designs are scrutinized to ensure safety and effectiveness and where shortening the development cycle is essential to marketing a product successfully.

One company that has discovered the importance of working with suppliers early in the product design process is the diagnostics division of Abbott Laboratories. The manufacturer is developing an immunoassay analyzer that mixes chemical reagents with blood samples and performs multiple tests on more than 200 different blood specimens per hour. Light photon analysis on the blood/reagent solutions within the instrument provides physicians with vital information on a patient's condition.


Critical Design Factors

Carefully measured chemicals are delivered to the analyzer through a system of siphons from reservoirs at the bottom of the apparatus. Reliable delivery of the reagents during the testing process is critical. If the tanks run empty and air is siphoned into the system, not only will an expensive repriming of the chemical reagents have to be performed, but hundreds of unhappy patients will need their blood drawn for another test. On the other hand, because chemical reagents are expensive, virtually all must be emptied from each container before a new one is used. Thus, Abbott required a system that would inform the technician about how many more tests the bottles could go before needing replacement.

This posed a challenge. There needed to be three bottles with three level sensors to comply with various siphoning applications. Two of the bottles are designed to be changed by the lab technicians, but the third bottle is refilled automatically from a reservoir while being monitored by a liquid-level sensor.

Finding Solutions

Once Abbott's engineers had determined the analyzer's requirements, they contacted Gems Sensors (Plainville, CT), a manufacturer of liquid-level and flow sensors. After extensive research and consultation, Abbott accepted Gems Sensors' suggestion to use float-switch technology because it is considered more practical and less expensive than other technologies.

The Gems product that best fit the solution was the LS-350 series of liquid-level switches. The small polysulfone-bodied switches are well suited to be used in shallow tanks and reservoirs. They use a magnet-equipped float that rises and lowers with the liquid level along the unit's stem and can have as many as four actuation levels. The magnetic field generated inside the float actuates a hermetically sealed, magnetic reed switch mounted in the stem. The reservoirs proposed by Abbott measured less than 18 in., just right for the LS-350 models. Although the chemical reagents in Abbott's analyzer remain at room temperature, the sensors can withstand temperatures from ­40° to 225°F and pressures up to 250 psi.

Fine-Tuning the Design

Gems engineers used preliminary sketches supplied by Abbott to design a customized LS-350 for operation within the new instrument. They combined a level sensor and draw tube to siphon the reagents up into the machine. This design is easier for the operator to handle. Gems also designed a threaded bottle cap into the sensor assembly for one complete unit. The analyzer's operator needs only to unscrew the entire one-piece sensor assembly from the bottle, insert the sensor assembly down into a new bottle, and screw the top back on. Having one hole in the bottle instead of two reduces the possibility of outside contamination. The reservoir containers are made of standard clear polypropylene. Their concave bottoms typically allow fluid to accumulate around the edges of the bottle, and so, to accommodate these concave bottoms with centered tops, Gems engineers designed an angled mounting configuration to ensure less waste of the expensive fluids.

Gems engineers customized the sensor to fit the individual requirements of the three reservoirs. The first two, known as Trigger 1 and Trigger 2, are 7-in. bottles outfitted with 6 1/2-in. single-point level sensor switches.

As the chemical decreases in the tank and the polypropylene float reaches a preset actuation level near the bottom, an alarm sounds and a timer alerts the tester that there is a specified amount of tests remaining before the chemical level gets low enough for air to be drawn in. To ensure that an operator cannot inadvertently hook up the wrong sensor, thereby contaminating the solutions, the matching switches were made foolproof by color coding the bottle tops and outfitting one with a two-pin connector and the other with a three-pin connector.

The third reservoir, which is approximately 18 in. tall, holds the buffer, an important chemical solution used in immunoassay testing to neutralize both acids and bases. The liquid-level sensor in the buffer reservoir has three separate switch actuation levels: high, low, and low-low. Unlike the first two reservoirs, this bottle is not changed by the operator. It is replenished through a tube that, along with the sensor and the siphoning device, comes through the top of the bottle from a fourth reservoir holding the buffer solution supply. When the chemical gets to the low setting, an alarm goes off to alert the operator that the refilling sequence should begin. When the sensor's float reaches the high position, an alarm tells the operator to stop. However, if the refill mode does not kick in and the chemical gets to the low-low level, the sensor signals the analyzer to shut down. The integrity of the rest of the blood samples is thereby protected.

Conclusion

As the immunoassay analyzer was honed to meet customer needs over the past year, the design of Gems' LS-350 sensor has been modified. In addition to customizing an existing level sensor, Gems completed the assembly by incorporating a PC board with PVC cable preterminated with a connector, thereby allowing Abbott personnel simply to plug in the sensors.

Gems also modified its FS-150 polypropylene flow switch to control the delivery of saline solution into the new analyzer. The company added lead wires and two-pin connectors, identical to those on the LS-350, for easy hookup during the manufacturing process. Gems will continue this evolutionary process as additional modifications are required. With the help of a supplier, Abbott engineers are succeeding in developing a well-designed, functional product.

For more information on liquid-level and flow sensors from Gems Sensors, call 860/747-3000.

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SPOTLIGHT ON ADHESIVES

SPOTLIGHT ON ADHESIVES

Noncurable PSAs

A company has developed noncurable pressure-sensitive adhesives dispersed in three different solvents: ethyl acetate; 1,1,1, trichloroethane; and hexamethyldisiloxane. The dispersion is applied either to the skin or to the device. When the solvent evaporates, a tacky silicone layer is left behind, enabling its adhesion to another object through application of pressure. A curable, cross-linking PSA that cures to a solvent-resistant tacky coating is also available. Nusil Technology, 1050 Cindy Ln., Carpinteria, CA 93013. Phone: 805/684-8780.



Conductive silver epoxies

A line of conductive silver epoxy adhesives is used for bonding circuits, flat cables, and wave guides, as well as RF and EMI shielding. Tra-Duct 2902 is a two-part paste formulation made of refined pure silver and epoxy resin components free of any copper or carbon additives. The adhesives develop strong electrically conductive coatings and bonds between dissimilar materials, including metals, ceramics, glass, and plastic laminates. Their ability to cure at room temperature makes the adhesives suitable for cold soldering, a practical replacement for hot soldering of heat-sensitive components. The product comes in a kit for fast, easy mixing and dispensing or in premixed, frozen disposable cartridges. Tra-Con Inc., 45 Wiggins Ave., Bedford, MA 01730. Phone: 800/872-2661.



Optical-grade epoxies

A line of optical-grade epoxies is suitable for bonding, coating, and encapsulating fiber-optic components. The adhesives can be used in the assembly or manufacture of medical devices, optical filters, LEDs, photo diodes, and lenses. Designed to maximize the optical and mechanical properties required in fiber-optic or optical use, the materials offer good adhesion, high-temperature properties, and chemical resistance. Epoxy Technology Inc., 14 Fortune Dr., Billerica, MA 01821. Phone: 800/227-2201.



Multigrade cyanoacrylates

To compliment a line of UV-curable medical device adhesives, a company offers multigrade cyanoacrylates suitable for such difficult-to-bond materials as acetal, nylon, rubber, and nontransparent plastics. The Medi-Cure 220-series adhesive is USP Class VI compliant, has a thermal range of ­55° to 80°C, and offers shear strength up to 4200 psi, depending on the substrate. The company also manufactures a line of UV/visible light curing lamps and conveyor systems. Dymax Corp., 51 Greenwoods Rd., Torrington, CT 06790. Phone: 860/482-1010.



Biocompatible epoxy

A two-component low-viscosity epoxy resin system is designed for bonding, sealing, and potting applications. The USP Class VI­compliant EP21LV system has a noncritical one-to-one mix ratio (by weight or volume) and can be cured at ambient or elevated temperatures. Physical strength properties can be adjusted by varying the mix ratio. A mix ratio of two parts resin to one part hardener optimizes strength, rigidity, and hardness, while a mix ratio of one part resin to two parts hardener enhances impact strength, toughness, and flexibility. The cured polymer system demonstrates good adhesion to similar and dissimilar substrates. Master Bond Inc., 154 Hobart St., Hackensack, NJ 07601. Phone: 201/343-8983.



Electrically conductive adhesives

Conductive adhesives meet temperature range requirements from ­55 to 150°C and produce a bond that cures quickly at room temperature. After curing, the bond or seal is flexible, waterproof, and chemically resistant. The product's consistency and high conductivity enable optimum silk screening for engineering design applications. Shelf life is up to six months. One- and two-part adhesive sealants can be adapted to meet many bonding requirements. Tecknit, 129 Dermondy St., Cranford, NJ 07016. Phone: 908/272-5500.

Conductive adhesives

Silicone-based adhesives and heat-sink compounds are suitable for bonding microelectric components. The adhesives provide a wide range of thermal management interface media between components and heat sinks or substrates. Formulas operate up to 250°C without losing properties, drying, or hardening. Fujipoly, P.O. Box 679, Kenilworth, NJ 07033. Phone: 908/298-3850.



Thermally conductive tapes

Thermally conductive tapes eliminate the need for liquid thermal grease and mechanical fasteners. Thermattach T413 and T414 double-sided tapes are designed to provide an effective thermal interface between components, ceramic hybrid circuits, PCBs, flexible circuits, and heat spreaders and sinks. The tapes are ionically clean and offer good thermal conductivity and bonding properties. T413 tape consists of a high-bond-strength, pressure-sensitive acrylic adhesive filled with alumina particles and applied to a fiberglass carrier. It provides good conformability to irregular mating surfaces and electrical isolation. T414 tape is similar except that the carrier is 0.001-in. Kapton MT thermally conductive polyimide film. Chomerics, div. of Parker Hannifin Corp., 77 Dragon Ct., Woburn, MA 01888. Phone: 617/935-4850.



Tacky paste flux

Tacky paste flux will not migrate and allows for easy component removal and replacement. Unlike liquid flux, the high-viscosity paste flux can be applied in the exact location required for removal of a failed component and then used to hold the new part in place before soldering. Precise areas are easily covered, minimizing cleaning, reducing flux use, and increasing productivity of the repair and rework station. The flux is packaged in dispensable 10- and 25-g syringes as well as 150-g cartridges. ESP Inc., 14 Blackstone Valley Pl., Lincoln, RI 02865. Phone: 800/338-4353.



Flexible adhesives

Flexible, photocurable adhesives are also free of solvents. Formulated for bonding, tacking, sealing, encapsulating, and potting applications, Luxtrak single-pack adhesives offer good adhesion to ABS, acrylic, PVC, and medical-grade plastics. They feature "command cure" in seconds with UV or visible light. Use of light provides increased operator safety, greater depth of cure, and cure through UV-opaque substrates. Ablestik, 20021 Susana Rd., Rancho Dominguez, CA 90221. Phone: 310/764-4600.



Polyurethane adhesive

A one-part, medical-grade polyurethane adhesive is based on a fast crystallizing polyurethane resin. Tecoflex 1-MP can be used on such substrates as polyurethane, plasticized vinyls, polycarbonates, acrylics, chlorinated SBR rubbers, and primed metals. Also available are solution grades of Tecoflex and other polyurethane resins that can be dissolved in solvent systems to create custom adhesives for specific applications. Thermedics Inc., 470 Wildwood St., P.O. Box 2999, Woburn, MA 01888. Phone: 617/938-3786.


Rubber-toughened cyanoacrylate

Designed for medical device assembly applications, a low-viscosity cyanoacrylate adhesive achieves fixture strength within 20 seconds and delivers high impact and peel strength on hard-to-bond surfaces such as metal, plastics, elastomers, and lightweight devices. Prism 4206 withstands continuous exposure to temperatures up to 250°F and offers resistance to the rigors of gamma and EtO sterilization. Available in a variety of package sizes, the USP Class VI­compliant adhesive can be dispensed manually or with the manufacturer's semiautomatic and fully automatic dispensing equipment. Loctite Corp., 1001 Trout Brook Crossing, Rocky Hill, CT 06067. Phone: 800/571-5100.

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Fighting Heart Disease with Software

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI September 1997 Column

SNAPSHOT

Suresh Gurunathan attempts to control fibrillation.

It's one thing to do research that has the potential to save lives. It's quite another to do it in graduate school. While working toward a graduate degree in biomedical engineering at Virginia Commonwealth University (VCU; Richmond, VA), Suresh Gurunathan sought to reduce the amount of voltage required to stabilize a heart experiencing ventricular fibrillation. His research involved placing three orthogonal ECG leads on the heart of a canine subject, inducing fibrillation, and writing the software to analyze the distressed heart's waveforms. He wanted to find the critical point at which a shock is most likely to defibrillate the heart.

Such innovative research, outlined in his paper, "Vector Magnitude Using Orthogonal ECG Leads during Ventricular Fibrillation Is Associated with Defibrillation Outcome," was recognized by the Association for the Advancement of Medical Instrumentation (AAMI), which honored Gurunathan with a Becton Dickinson Career Achievement Award in June. The award is given to promising health-care professionals under the age of 40 who have made significant contributions to medical devices, instruments, or systems.

"The award came as a big surprise," says Gurunathan. "I had originally sent in my thesis for consideration for the Young Investigator Award given at the AAMI conference, but instead I received this one."

Interested in both electronics and biology in India, Gurunathan received BS degrees in electronics engineering and biological sciences from the Birla Institute of Technology and Science in Pilani in just five years. He then came to the United States to finish his graduate work and gain professional experience. It was at VCU that he became involved in heart fibrillation research. "My advisor, Dr. Peng-Wie Hsia, was already researching fibrillation and encouraged me to do the same," says Gurunathan. "I designed the three-lead system and wrote the software that analyzes the heart's waveforms."

Gurunathan found that there is a critical moment during ventricular fibrillation when the heart is susceptible to defibrillation. Applying a low level of shock at this critical point, rather than a higher one (current defibrillators apply about 15 J or more) at any time during fibrillation, can stabilize the heart. "Reducing the voltage required to defibrillate a heart can reduce heart tissue damage," he says. "The system also finds the critical point for defibrillation. Current practitioners often have to repeat the process, which usually involves increasing the voltage, if the heart doesn't stabilize the first time."

Reducing the amount of energy needed for defibrillation should also help implantable cardiac defibrillator (ICD) makers reduce the size of the batteries and capacitors used in the devices, thereby reducing the size of the ICDs themselves.

Despite the accomplishments Gurunathan and his fellow researchers have made, he is careful to point out that his research is still only a scientific study. "The system we have been using isn't ready to be integrated into devices yet," he says. "The next objective is to find the exact position on the heart so we can use only one lead." His research involved placing the three leads on the x-, y-, and z-axes of the heart of a canine subject.

However, the medical community doesn't have to wait for the technology to be transferred to a device in order for the research to become beneficial. An important aspect of his research, says Gurunathan, was the information it generated about the mechanisms of fibrillation and defibrillation. "Until now the mechanisms were unknown," he explains. "My research involving real-time analysis could help explain them."

Gurunathan's research has also helped him personally--it landed him a job. Ventritex, Inc. (Sunnyvale, CA), a manufacturer of automatic ICDs, funded part of Gurunathan's research at VCU. In March he joined the company as a software engineer and is currently working on the company's next generation of ICDs.

For now Gurunathan plans to stay in the United States to gain more experience. "Biomedical software engineers in India don't work on as advanced projects as they do here," he says. But returning home to work is always something he entertains. "Maybe I'll start my own business in India, but I'll keep in touch with U.S. contacts."

Gurunathan's return home to work might do his fellow citizens a favor. If he were to bring his knowledge of ICD technology to India, he might be able to work toward making the devices available to the general public. "Right now, ICDs are only affordable for the rich," he laments. "As far as I know, no pacemakers or ICDs are made over there. If such manufacturers were there, the devices would be more affordable for the citizens of India."

Even if he remains in the United States to start a business, he plans to keep his countrymen in mind. "I could stay here and export to India," he muses. "But eventually, I want to settle there. It is my home."


Copyright ©1997 Medical Device & Diagnostic Industry

The Evolution Of Thermoforming For Medical Packaging

Medical Plastics and Biomaterials Magazine | MPB Article Index

Originally published September1997

PACKAGING

Although medical thermoforming remains an art, it is becoming more of a science every day. Technology such as computerized modeling for thermoforming mold design has brought accuracy, reproducibility, and precision to a process that previously exhibited little of these qualities. Until recently, thermoforming depended on molds that were largely handmade or cast in metal from fabricated patterns. Though relatively imprecise, these molds had one great advantage: their surfaces could be refined to any shape that the mold designer needed. Molds were sanded, carved, blended, drafted, shaped, and contoured by hand to exactly fit the product and allow for the most efficient flow of plastic. The molds could be detailed, cast as a reverse, detailed from the opposite side, and cast back to the original--all to create the exact shape required. Manual milling machines were often used to give true, square edges, and to achieve a modicum of accuracy, but the final shape was determined by the application, not by the mold-making method.

Advances in technology allow current thermoformed medical trays to accommodate fairly severe draw ratios, multiple undercuts, and radical shapes. Photo: Prent Corp.

As computer-assisted design and manufacturing (CAD/CAM) began to make inroads into the thermoforming industry, many compromises were made in the name of efficiency and timesaving. Because most of the original CAD technology was two-dimensional, the first thing to be sacrificed was the sophisticated blending of surfaces that manual mold making had allowed. For example, end mills and cutters could only be driven efficiently in straight lines, giving relatively square and sharp corners, which compromised product fit. What suffered most was formability.

The best shape for most thermoforming is a hemisphere; the further away from this configuration one gets, the less the plastic material can be persuaded to take the shape of the mold. However, recent improvements in plastics, molds, and equipment have resulted in the production of mold designs today that couldn't have been made consistently even five years ago. Many of these improvements are largely due to advancements in computerized mold and process control that have enabled thermoformed package design to be less dependent on the method of mold fabrication and more responsive to formability concerns and product function.

MEDICAL PACKAGING APPLICATIONS

Plastic medical packaging can be classified as either sterile or nonsterile, and as either disposable or reusable. This article is concerned with what is by far the largest segment: sterile, disposable medical packaging. Thermoforms are also heavily used for nonsterile medical packaging, but because quality and function are normally not nearly as critical as they are for sterile packages, mold design and processing controls for nonsterile thermoforms can generally be quite simple.

There are two basic types of sterile, disposable packages: lidded and nonlidded. Whereas both types can entail sophisticated design and processing, lidded trays require by far the most attention. This is because lidded, sterile medical thermoforms must function as a sterile barrier in addition to protecting and organizing the components within during shipping, handling, and storage.

The preferred method of sealing thermoformed trays is with a peelable lidding stock. Although paper, foil, and other media can be used for making peelable lidding, the material of choice has long been a spun-bonded olefin (high-density polyethylene filament) coated with a heat-activated adhesive. This printable lid stock is microporous and breathable under pressure, which enables ethylene-oxide (EtO) gas to be used as a sterilant: forced through the membrane under pressure, the gas kills any bacteria inside the sealed package and is then removed under vacuum, leaving all items inside the package sterile until opened. Even after other sterilization methods that did not require the use of gas were perfected--for example, gamma or electron-beam irradiation--spun-bonded sheet remained the lidding media of choice because of its toughness, lack of particulates when peeled, and resistance to moisture.

Sterile barrier thermoformed packaging has progressed in response to industry demands. In the early years, packages were little more than simple open cavities, with dividing walls to organize components. Later, as equipment and mold design evolved to handle different varieties and thicknesses of plastic, more shapes became available. Current thermoformed packaging has progressed to the point of allowing fairly severe draw ratios, multiple undercuts, and radical shapes to be formed with great success. However, two basic requirements continue to be critical concerns for the medical package end-user: the need for consistent and substantial seal flanges, and the need for even wall distribution without thin spots.

The plastic seal surface generally referred to as the flange is located at the interface between the lidding stock and the plastic. This nominally flat surface --the remnant of the original plastic sheet used to form the tray--is probably the most vital element of the sterile barrier, since its flatness and consistency can be critical to the ability to seal the lidding to the tray. Proper design considerations must be followed in order to attain a consistent flange thickness.

The second major concern with thermoformed trays is sidewall integrity. This is especially important in EtO-sterilized trays that must withstand high pressure, heat, and moisture, as well as strong vacuum during the sterilization cycle. Any holes, tears, splits, gaps, or thin areas in the body of the tray will void the sterile barrier. Once again, careful design preparation is necessary to arrive at evenly thermoformed tray walls.

VACUUM FORMING

Achieving regularity in flange and sidewall design can be a major problem, because one of the hallmarks of the thermoforming process is its inconsistency. If 10 shots are formed, all 10 will be measurably different; the key is keeping the differences to a minimum. In the basic process, called vacuum forming, plastic sheet is heated past its deflection temperature, until it becomes semimolten and sags under gravity. The sheet's ultimate shape in this instance would be a hemisphere, and the further it sags or stretches, the thinner it becomes. This sheet is then positioned over a mold, and is pulled into or over the mold by vacuum. Wherever it comes in contact with the cooler mold, the plastic will not flow as readily; so that the top of the part is thicker, and the bottom thinner--sometimes so thin as to blow out and cause vacuum to be lost. Depending on the elasticity of the sheet, its starting thickness, and the design of the mold, the part produced can vary dramatically in strength and quality.

Often, vacuum forms are made using male molds, on which the sheet forms the bottom of the tray first, and is then pulled down to form the flange plane. This gives a stronger bottom to the tray, but tends to yield a much thinner flange and sidewalls. Irregularities can be overcome with increased sidewall draft and a thicker starting gauge, but these compensations make for a larger and more expensive part (see Figure 1).

Figure 1. Examples of vacuum-formed trays, highlighting areas of design concern.

PRESSURE FORMING

Over the years, advances in technology led to the method of pressure forming, in which parts are formed within a sealed vessel--a hermetic shell containing both the tooling and the plastic (see Figure 2). In this process, the plastic sheet is prestretched through the application of positive air pressure on the side of the sheet away from the mold; the vacuum gate remains closed on the mold side of the vessel until prestretching is completed. The technique allows for a much more consistent wall thickness throughout the tray once the vacuum pulls the sheet against the mold to form its final shape (see Figure 3).

Figure 2. Pressure-forming cross section.

Plug-Assist Processing. Another processing improvement has been the use of plug assists in conjunction with pressure forming. These plugs are hobs shaped like the negative image of the mold; they permit further prestretching and preshaping of the plastic sheet on the pressure side of the vessel before vacuum is activated. This innovation has dramatically increased control over the thickness and quality of finished thermoformed parts, and has enabled processors to substantially reduce the starting thickness of the sheet needed to fashion a usable part.

Figure 3. In-line pressure-forming process.

As one can tell from the foregoing descriptions, thermoforming is not really a scientific method. The process itself incorporates numerous variables and tolerances, and can easily be affected by changes in material, equipment, temperature, pressure, and, especially, mold design. If steps are taken to minimize the variability of the first four items--that is, if material is rigorously inspected for quality and consistency, and equipment developed that can regularly operate within a narrow range of forming variations (probably via computerized controls)--then the path to thermoforming better and more consistent medical trays depends on effective part and mold design.

PART AND MOLD DESIGN

In thermoforming, part design and mold design are virtually inseparable. Unlike some other molding processes, thermoforming cannot achieve a different shape on one side of the part than on the other side: material is formed over or into a mold, and is not formed between two mold halves. Also, standard in-line or rotary thermoforming cannot predetermine material thicknesses in specific areas of the finished part, as can be accomplished, for instance, in injection molding. What thermoformers can do, however, is to target certain critical or hard-to-form areas and enhance material flow to these areas via improved mold and plug-assist design, spacing, and heat control.

The key to consistent, high-quality thermoforming is material flow. Whatever can be done from a design standpoint to enhance material flow will result in more-consistent, more-substantial, more-reproducible, and more-economical parts. The easier it is for the material to flow into a pressure-formed tray, the less variation in material thickness there will be. When plug assists are used, the better the material flows off of the plug assist, the more consistent the sidewall thickness will be. If material freezes on the plugs--either because of faulty design, incorrect temperature, or poor choice of plug material--most of the plastic will end up on the bottom and on the flange of the tray, leaving a thin band in between. If plug timing, spacing, or clearances are incorrect, similar gauge bands will occur. For high-speed, in-line pressure forming, it can be said that trays are designed not only to fit the product, but also so that an adequate plug can be shaped for the mold.

There are three basic ingredients for a good thermoformed tray: adequate sidewall draft, adequate radii, and reasonable draw ratios. Although trays can be manufactured that do not exhibit these three qualities, they cannot be made as efficiently, as economically, or as easily as those that do. Again, the key is material flow: the easier it is for the material to flow, the more it will do so; the less work the material has to do, the more easily it will do it. For example, a tray that has straight sidewalls, square corners, sharp edges, and deep cavities will be difficult to thermoform consistently. The same-sized tray, using the same gauge of plastic, will be substantially easier to form with moderately drafted walls, rounded corners and edges, and good draw ratios. A good rule of thumb is that, whenever possible, the plastic should be stretched over no more than three times its original surface area in order to avoid severe forming problems. Thus, for a 5-in.-sq tray--a starting surface area of 25 sq in.--the sum of all surfaces within the tray, including sides and bottom, should total less than 75 sq in.

Figure 4. Plug-assist pressure forming (Step 1).

For plug-assisted pressure forming, a well-designed female tray allows for a well-designed plug assist. Because the material in this process does not touch the tray mold until the last possible instant (when vacuum is pulled), the mold has little to do with how consistently the part forms (see Figure 4). However, since the shape of the mold dictates the shape of the plug to a large extent, plug clearance should be quite uniform throughout. A rounded, drafted form allows for a plug assist with more organic shapes; this, in turn, enables the plug to stretch and distribute the plastic material more evenly, so that the material flows off of the plug instead of freezing (see Figure 5). In this fashion, efficiently shaped plugs permit material to be distributed throughout the tray in a more specific manner. When the vacuum is pulled at the end of the process, very little additional stretching takes place. This stability makes for consistent sidewalls and strong corners, which are critical to sterile barrier thermoforms.

Figure 5. Plug-assist pressure forming (Step 2).

Properly applied, plug-assisted pressure forming can also dramatically improve flange thickness consistency. Because the part is pressure formed within a sealed vessel (the sheet is clamped off from the atmosphere) and because the plastic sheet itself divides the vessel into a top and a bottom pressure area, maximum control can be obtained over the sheet. When pressure is increased from the plug side of the tool, resistance is increased from the mold side--much like pushing on a filled balloon. The air inside the mold half of the tool is compressed, which pushes the plastic sheet away from the cold cavity. The more the plug is engaged, the more the pressure against it grows, stretching the plastic.

An additional benefit is that the plastic material at the flange plane is also kept away from the cold mold, allowing it to continue to stretch and flow until, finally, vacuum is pulled, and the sheet is forced against the cooled mold (see Figure 6). Through this technique, known as ballooning, the seal flanges of the tray are kept from freezing against the mold until the process is complete. This, in turn, allows additional plastic to be pulled from the scrap areas of the sheet (between parts or around the perimeter), lending more substance and consistency to the formed part.

Figure 6. Plug-assist pressure forming (Step 3).

CAD/CAM TECHNOLOGY FOR THERMOFORMING

All of the aforementioned design considerations relate directly to CAD/CAM mold making. The advantages of computerization can be significant: accuracy, reproducibility, and cost-efficiency. These are all desirable capabilities, but they are often gained at the expense of good thermoforming mold design.

What has often taken place is the following: a part is drawn up using two-dimensional CAD, and then transferred to two-dimensional CAM for programming. Because difficult blends, contours, and shapes are not practical from a time, equipment, or training standpoint, these design features are compromised or sacrificed in the name of mold-making efficiency. Cavities are milled in aluminum, plugs milled to match the cavities as closely as possible, and parts formed. Any thin spots in the parts are detailed by upgauging the plastic thickness, thus costing the customer money. Inefficiencies in the cycle time, scrap, yield, and mold spacing required to produce adequate parts are also paid for by the customer. Compromises in product fit and tray function are ultimately dealt with by the end-user.

In this common scenario, the thermoform mold design and mold-making process have been made more efficient by computer technology, but the tray design itself has been compromised. A more sophisticated approach involves a technology known as 2 1Ž2 D, which is somewhere between two- and three-dimensional CAD design. This method requires quite a bit more training and experience to use properly, and takes substantially more time and effort--which probably explains why it is not used very often in the thermoforming industry.

What 2 1/2 D permits is for more sophisticated surfaces and blends to be made in designated areas of the tray. For example, if the bulk of a particular tray forms easily and well, but there are corners or pockets in which the material thins excessively, these areas can be broken out of the CNC (computer numeric control) program and individually detailed. By adding more draft and radius, smoothing edges, or contouring sidewalls to fit the product properly, the designer can make the tray form and function better. However, the time involved in detailing these problem areas can be greater than that spent on the entire rest of the tray, which is why many thermoformers avoid this option.

Even fewer thermoformers make it to the next level: fully three-dimensional design and mold surfacing. Not only are equipment and training costs tremendously higher for this method, but most are not sophisticated enough as processors to take advantage of the technology. Fully 3-D CAD design allows for blended surfaces, constant corner radii, compound curves, and precise product fit--all of the things that were available 20 years ago when mold patterns were made by hand. In addition, all of the accuracy, reproducibility, and precision of computer technology can be applied to the mold-making process. When combined with 3-D CNC milling, molds offering excellent material flow, product fit, and design features can be manufactured. Furthermore, sophisticated plug assists can be designed to move material consistently and uniformly.

When a computerized mold making technology is compatible with a customer's technology, concurrent engineering becomes possible. If the customer is designing its product via CAD, wireframes of the product can be imported into the thermoformer's CAD system early in the development process. The mold can then be designed around this "virtual product," saving time. As the product changes, the mold can also be changed and updated, resulting in a very short development lead time once the product design is finalized.

CONCLUSION

Depending on the software being used, 3-D mold designs can be accomplished in a number of ways--for example, with wireframes, surfaces, or solid models. Once the basic product cavity has been determined, the key to success is to refine the shape so as to enhance material flow. With experience and forethought, a good thermoform designer should be able to contain the product securely while still allowing for adequate material flow, good draw ratios, and customer utility.

Good mold design does not necessarily equal good thermoform design, if tray function and utility are not adequate for the end-user. The computer or the CNC mill can only produce what it is asked to produce. CAD/CAM is only as good as the design itself, and will only give good results if the manufacturing process is under control. Sophistication in these areas is what the medical package buyer should look for when selecting a thermoformer.

BIBLIOGRAPHY

Seachtling H, International Plastics Handbook for the Technologist, Engineer, and User, 3rd ed, Cincinnati, Hanser/Gardner, 1995.

Plastics Handbook, edited by the staff of Modern Plastics magazine, New York, McGraw-Hill, 1995.

Don Handrow is a design team leader at Prent Corp., the custom medical thermoformer located in Janesville, WI. He is responsible for coordinating thermoforming projects from initial concept to package completion, and has participated in numerous award-winning package designs.

James Kallenbach is senior designer and product development manager at Prent, where he directs the company's design team. An industrial designer with more than 20 years' experience in thermoforming, he has designed a wide variety of thermoforms for medical applications.


Copyright ©1997 Medical Plastics and Biomaterials

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An Awards Program Dedicated to Medical Design Excellence

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI September 1997 Column

An awards program exclusive to the device industry can serve to increase the visibility not only of the winning companies, but also of the industry as a whole.

If you've been part of the medical device industry for very long, you've probably noticed the absence of an awards program dedicated to medical products. While several distinguished programs recognize medical applications, they do so by categories within horizontal industries or disciplines, such as packaging or industrial design.

This shortcoming had been the topic of much discussion among the staff of MD&DI and its publisher, Canon Communications, for several years, but our conversations had never quite resulted in a plan for starting an awards program. Last June, however, at the Medical Design & Manufacturing East show in New York, the concept became a reality.

During that event, Canon staff, together with staff and members of the Industrial Designers Society of America (IDSA), convened an informal brainstorming session. Attended by representatives of medical device companies and medical industry suppliers, the meeting focused on developing the criteria for a successful program. The meeting also verified our perception that the awards are needed, and that the potential benefits are considerable. Participants agreed that the awards can serve to increase the visibility not only of the winning companies, but also of the industry as a whole.

The result of that meeting was the Medical Design Excellence Awards, which are covered on page 21 in this issue's news section. The awards are endorsed and administered by IDSA, which will also oversee the judging of submissions. If you're familiar with IDSA's Industrial Design Excellence Awards, featured each spring in an issue of Business Week magazine, you'll know that the administration of the new awards could not be in better hands.

The Medical Design Excellence Awards will recognize design and engineering excellence in two broad categories. One is finished medical devices; that is, any product regulated as a medical device by FDA and currently available on the U.S. market. The other is components and materials intended for incorporation into a finished medical device.

I encourage you to review the advertisement and entry form for the awards program on page 131 of this issue. Submitting an entry is simple--just complete the entry form, include a slide and other visual aids, and explain how the product advances the state of the art, offers functional improvement, or contributes to cost-effective manufacture or patient care.

The deadline for submissions is January 26, 1998. First-round winners will be notified in March and invited to an awards dinner celebration in New York in June, to coincide with the Medical Design & Manufacturing East 98 Conference and Exposition. At the dinner, the grand winners will be announced.

Even if you don't intend to submit an entry, you'll want to stay informed of key events in the program, and MD&DI will help you do so. In addition to other, ongoing coverage, we'll be showcasing the first-round selections in the May 1998 issue and the grand winners in the following July issue. Profiles of the awards jurors--a select group of technical professionals from medical device and supplier companies--will appear in earlier issues of the magazine.

As the first of an annual event, the 1998 awards will no doubt evolve in the coming years into something still more significant. Whether you are a participant or an observer, your comments and ideas will be part of the process. Since there's no time like the present--whether to submit your entry or offer suggestions--I urge you to contact me by E-mail or call Canon Communications' awards director Amy Allen at 310/392-5509. If you have questions about awards administration, you may also call the IDSA at 703/759-0100.

John Bethune

john.bethune@cancom.com


Copyright ©1997 Medical Device & Diagnostic Industry

Injection-Molded Polyester Medical Devices: Preventing Failure through the Proper Design of Parts and Molds

Medical Plastics and Biomaterials Magazine | MPB Article Index

Originally published September1997

PART DESIGN

Medical part quality depends on factors that include part design, resin selection, the quality and design of the molding tool, and processing conditions. Design and molding considerations, in turn, depend on the type of resin to be molded. The following general guidelines and examples are given to aid in the proper design of parts and

Precision parts such as injection-molded winged luers are among the many medical products made from polyester resins. Photo: Eastman Chemical Co.

molds for medical-grade polyesters. (The polyester resins referred to in this article were Eastar polyesters/ copolyesters and Eastalloy polymers, produced by Eastman Chemical Co., Kingsport, TN). By understanding how part design affects injection molding, tooling, and production costs, part designers and engineers can significantly reduce mold complexity. By understanding how tool design affects the processability of polyester resins, they can avoid complications during molding.

PART DESIGN PRINCIPLES

The goal of the medical part designer should be to provide a part that combines maximum functionality with minimum complexity. The following principles are useful for the designer to keep in mind when designing a part that will incorporate any thermoplastic resin. As simple as these principles are, many are often overlooked, resulting in increased mold costs, a defective final product, or premature part failure.

Wall Thickness. The most basic principle of plastic part design is that uniform wall thickness should be maintained wherever possible. Thin sections are weak structurally and difficult to fill; they can restrict flow and require increased injection pressure. Thick sections are easier to fill but difficult to cool and pack out; they are subject to increased shrinkage, and may cause sink marks, voids, and high levels of molded-in stress. When thickness variations are unavoidable, they should be designed with a gradual transition, which will help reduce the level of molded-in stress at the transition region.

Corners are often problem areas because of nonuniform thickness. If the outer radius is too small, a thick section is created in the corner; this increased thickness then causes cooling and warpage problems. If the outer radius is too large, the corner will be thinner than its adjoining walls. Besides being weak structurally, the change in thickness can serve as a flow restrictor. The best approach is to have the inner and outer radii originating from the same point, ensuring a uniform wall thickness through the corner. A good rule of thumb is to have the inner radius value 1Ž2 the wall thickness, and the outer radius 11Ž2 times the wall thickness.

Stress Concentration and Sharp Corners. Stress concentrations are areas that by the nature of their design tend to concentrate or magnify the stress level within a part. Figure 1 shows how the stress concentration factor in an inside corner will increase rapidly as the radius decreases. A good rule of thumb is to specify a minimum radius of 1/4 the wall thickness. Sharp corners should be avoided in critical stress locations such as inside corners, and at the base of ribs, bosses, and snap-fit latches.

Figure 1. The curve gives an indication of the relationship among stress, wall thickness, and corner radius for plastic parts. (Figure courtesy of SPI Plastics Engineering Handbook, 5th ed, p 323.)

Designing for Proper Ejection. Once a part is molded, ejecting it from the tool without damaging it or the tool is important. Zero degree draft is not recommended, as it can cause a part to remain in the mold, locking it up. It can also increase the cost of the mold significantly due to the additional mechanisms required for ejection.

For proper ejection, a draft of 1° per side is typical. Less than 1° is suggested only in cases where adequate cooling is supplied, the cores are short, the part walls are thick so as not to shrink tightly to the core, or sleeve ejectors are used. Sometimes side pulls can be employed on the outside of a cylindrical part so that low draft on the inside core can be easier to release. If texturing is incorporated in the mold, between 1° and 1.5° of draft for each 0.025 mm (0.001 in.) of texture depth should be added.

PART DESIGN FOR ASSEMBLY

There are many instances in which medical devices require assembling. Solvents and adhesives can often be used successfully in assembly, but such bonding agents are not acceptable for every application.

Ultrasonic welding can also be performed effectively with polyester materials, if certain design considerations are observed. Shear-type joints--which produce strong, hermetic seals--should be used. The advantages of shear joints over energy directors are that joint strengths can be doubled, crack-propagation behavior is reduced, loads are more evenly distributed during use, and joint flash can be controlled. In some limited situations, energy directors may work satisfactorily with polyesters; in most situations, however, the assembly will not retain enough toughness for the application.

General guidelines for successful shear-joint design include (1) interference of between 0.20 to 0.30 mm (0.008 to 0.012 in.) between the mating parts; (2) weld depths of 11Ž4 times the nominal wall thickness for maximum joint strength; and (3) a minimum radius of 0.76 mm (0.030 in.) to minimize stress risers that may crack via flex-fatigue during welding.

TOOL-DESIGN PRINCIPLES

For any molding process, proper tool design is an essential part of a quality operation. A well-designed, well-built tool made from durable materials and incorporating good cooling and venting will last longer and require fewer repairs than a tool of lesser quality. It will also increase the quality of the finished parts, decrease scrap, and shorten cycle time.

Mold Cooling. Good cooling is absolutely critical in molds that are designed to run polyester resins. Such resins are likely to stick to hot (>49°C (120°F)) surfaces in the mold. Good cooling design and practices will reduce cycle times, prevent sticking, and aid in part ejection.

Figure 2 shows a suggested layout of drilled cooling lines in the mold. The cooling line spacing should be 21Ž2 to 3 times the diameter between lines, and 11Ž2 to 2 times the diameter away from the surface of the part. Uniform placement of cooling lines, as illustrated in the figure, will help ensure that the part is cooled equally and adequately.

Figure 2. Suggested layout of drilled cooling lines in a mold.

Proper cooling is especially important in mold cores. The processing window and part performance can be greatly enhanced by following good core-cooling principles. Although it may be initially more expensive to place proper cooling in the cores of a tool, it pays off in the long run every time a part is made. There are many methods of achieving proper core cooling: baffles, bubblers, high-conductive alloys, and circular cooling channels around cavity and core inserts. The specific cooling method is not important as long as it is capable of providing good, uniform temperatures throughout the core geometry.

Baffle Configuration. Figure 3, which shows a typical water baffle, illustrates some considerations in core cooling. The water flows all the way into the core, ensuring uniform, complete heat removal. It is critical to bring water to the end of the core pin to allow optimum cooling at the innermost section of the core region.

Figure 3. Typical water-baffle configuration.

When cutting a baffle coolant drop, designers should make it larger in diameter than the feeder tube in order to prevent a flow obstruction from occurring. The cross-sectional area of each side of the drop channel should equal the area of the feeder tube. In a typical baffle tube assembly, a water channel is drilled into the area to be cooled. An intersecting water channel provides water flow that is diverted up one side of the baffle and down the other.

Sprue Design. Because polyester-based resins tend to stick to hot mold steel (>49°C (120°F)), some specific design guidelines are suggested for sprues. In many cases, a thick sprue is the hottest and most difficult area of the tool to cool.

Many molders are successfully using the type of high-conductivity sprue bushing shown in Figure 4. Made from a high-conductivity copper alloy, the bushing contains a hardened 420-stainless-steel nozzle seat to insulate it from nozzle heat and to provide wear resistance. This construction is effective in reducing sprue sticking, increasing sprue rigidity, and cutting cycle time. With this sprue bushing, a standard sprue taper of 4.2 cm/m (0.5 in./ft) has been found to be acceptable for good heat transfer. Installation of these sprue bushings in new molds--or when modifying existing molds to process polyesters--is strongly suggested.

Figure 4. Cross section of popular high-conductivity sprue bushing.

A maximum sprue length of 82.5 mm (3.25 in.) is suggested. To aid ejection, the sprue should be polished in the draw direction. A generous radius should be included at the junction of the sprue and runner system to avoid breakage during ejection. An ejector pin, rather than an air poppet valve, should be placed under the sprue puller, since an air poppet could cause a hot spot and impede cooling.

Upper and lower cooling-line circuits are suggested around the sprue to aid in cooling (see Figure 5). The sprue bushing should be assembled with a slight 0.005-mm (0.2 mil) interference fit to ensure good heat transfer from the bushing into the mold plate.

Figure 5. Sprue design featuring upper and lower cooling-line circuits and slight interference fit for cooling/heat transfer.

Runner Design. Runner systems should be flow-optimized. They should be large enough to deliver the resin to the gate with a low pressure drop and minimal shearing, but small enough to avoid excessive regrind. The most common errors in runner designs are oversizing and inclusion of sharp corners.

Common runner-design guidelines for other engineering polymers also apply to polyesters. A cold-runner system should be designed for smooth, fully balanced material flow. Generously radiused transitions are suggested to reduce resin hang-up and shearing. Cold slug wells are useful in trapping frozen material at the flow front, and should be sized 11Ž2 times larger than the diameters of the runners they are attached to. Runners should always be vented generously.

Regarding runner geometry, trapezoidal and rectangular runner systems are not optimum, since most of the material flow takes place in the circular cross sections indicated by dark shading in Figure 6, and the rest of the runner configuration is not used efficiently. Round runners are best, because they deliver the melt to the cavity with the least amount of dead space. However, they require machining both halves of the mold across the parting line. Typically, a compromise is reached with the half-round approach. A draft angle of 5° on the flat sides of the runner is suggested to ensure good ejection. The bottom of the runner should be fully radiused.

Figure 6. Runner design options. Flow efficiency increases as the cross section approaches a circular shape.

Gate Size and Location. When possible, designers should gate into the thickest section of the part. Several problems can occur if a part is gated in a thin section. These problems include high material shear, which can cause degradation; increased injection pressures; and difficulty in packing out the thicker sections.

For small medical parts, such as luers or needle hubs, gates should be from 0.762 to 1.27 mm (0.030 to 0.050 in.) in diameter. Larger gates may be required for bigger parts. For polyester resins, tunnel gates are commonly used.

If polyesters are to be molded in tools designed for other resins, it may be advantageous to change the gate size to account for differences in viscosity. In general, polyester-based resins may require larger gate sizes than some other resins with lower viscosities. Typically, gate diameters for efficient processing should be 1Ž2 to 2Ž3 times the wall thickness of the part.

HOT-RUNNER SYSTEMS

Hot-runner systems are becoming more common in applications that require the use of polyester materials. When designed properly, these systems can eliminate sprue and runner regrind, mold with lower pressures, and reduce cycle times.

Hot-runner systems should be designed up-front using reputable vendors who have experience with polyester resins. Good hot-runner systems will not have holdup spots in the manifold or gate areas. They will also be designed to avoid sharp corners, extremely small gates, and other high-shear areas. In general, polyesters are more sensitive to shear and thermal conditions than many other resins, and the hot-runner system should be selected with this difference in mind.

Excellent thermal control and good cooling at the gate location are critical for molding polyesters. The mold should be designed so that heat is quickly removed from the gate, since, if the gate does not cool properly, drooling, sticking, or stringing may occur. Steel that is heated as part of the hot drop should not directly contact the part, but should be insulated from the cooled portion of the mold.

For hot-drop gate cooling, separate cooling loops with individual flow and temperature controls are advisable. The additional control is very useful in debugging and optimizing gate appearance and performance. If possible, a water-jacketed insert should be used to remove heat from the gate area.

Hot Drops. Hot "probe" systems, like the one shown on the left of Figure 7, are very common. These sometimes work quite well for processing polyesters, and sometimes don't. Though results with this type of equipment are difficult to predict, generally the more crystallizable types of polyesters do not function well in these systems.

Figure 7. Varieties of hot-drop runner systems: a "probe" system (top), a system in which the melt is completely enclosed in a heated tube (center), and a valve-gate system (bottom).

Polyesters have better molding success using a drop that has the melt completely enclosed in a heated tube, as illustrated in the center image of Figure 7. In this design, the drop is insulated from the gate opening and mold. With such a system, the plastic in the hot drop is 100% melted, resulting in lower pressures and reduced degradation or crystallinity. The gate area still requires excellent cooling.

If at all possible, a valve-gate system should be used. This setup has several advantages: the melt channel is externally heated, and the mechanical valve gate ensures good gate appearance, even when a large gate diameter is used. In non-valve-gated systems, the gate size is often kept very small to reduce gate vestige, which can mean higher material shear through the gate and increased pressure to fill the part. With valve-gated systems, these problems can be avoided.

CONCLUSION

When creating medical devices, designers should maintain a relatively uniform wall thickness throughout the part to ensure quality filling and part performance. A radius should be applied to all sharp corners of the part wherever possible to reduce the likelihood of premature failure due to stress concentration. A minimum draft angle of 1° per side is suggested to help slide the part off the mold during ejection.

Additional consideration should be given to part design when ultrasonic assembly is required. Shear-joint designs are preferred over energy directors when part fabrication requires the welding of polyester resins. These joints provide stronger welds, reduce crack propagation, and facilitate a more evenly distributed load during end use.

Quality mold cooling is of utmost importance in the design of tools for molding polyester resins. Maintaining a mold surface temperature of 38° to 49°C (100° to 120°F) is necessary to prevent material from sticking to the hot mold surface and causing ejection problems. Adequate use of cooling channels, baffles, bubblers, or high-conductivity metals throughout the mold is suggested.

The gate of the part should be located in its thickest section whenever possible. Because of their higher viscosity, polyesters may require larger gate sizes if they are molded in tools designed for other resins. If the mold design includes a hot-runner system, drops that completely enclose the melt in a heated tube have been particularly successful with polyesters, with valve-gated systems performing the best.

By following these general guidelines, design engineers can be successful in achieving proper part and tool design for injection-molded polyester medical devices.

BIBLIOGRAPHY

Dowler B, "Thermoplastic Polyesters" in "Guide to Medical Plastics," Med Dev Diag Indust, 16(4):54­56, 1994.

Dowler B, "Tool Surface Enhancements: The Extra Edge in Injection Molding?", Inj Mold, supplement, November 1993.

Eastman Polymers Design Guide for the Medical Industry, PPM-104A, Kingsport, TN, Eastman Chemical Co., 1995.

Medical publications on Eastman Chemical Co.'s Internet Web site (http://www.eastman.com/ppbo/medical/online/contents.shtml).

Processing Guide for the Medical Industry, PPM-4, Kingsport, TN, Eastman Chemical Co., 1993.

Proper Tool Design for Eastar and Eastalloy Amorphous Polymers, PP-7A, Kingsport, TN, Eastman Chemical Co., 1995.

Vance M, and Dowler B, "Using Thermoplastic Polyesters in Medical Devices," Med Plast Biomat, 2(2):20­32, 1995.

Melanie L. Jones is a technical service representative at Eastman Chemical Co. (Kingsport, TN). Her principal service areas deal with part and tool design for injection-molded medical applications, medical chemical-resistance testing, and secondary operations for amorphous copolyesters.


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