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INSERT MOLDING

Medical Device & Diagnostic Industry Magazine | MDDI Article Index

Originally published April 1996

Jim Vance, Jr.

President

Medical Polymers, Inc., Spencer, IN

Insert molding is an injection molding process whereby plastic is injected into a cavity and around an insert piece placed into the same cavity just prior to molding. The result is a single piece with the insert encapsulated by the plastic. The insert can be made of metal or another plastic. The technique was initially developed to place threaded inserts in molded parts and to encapsulate the wire-plug connection on electrical cords. Today insert molding is used quite extensively in the manufacture of medical devices. Typical applications include insert-molded needle hubs and luer fittings and bifurcations, as well as encapsulated electrical components and threaded fasteners. Generally, there are few design limitations or restrictions on material combinations.

There are two types of bonding that occur in insert molding, molecular and mechanical. Molecular bonding can occur when the insert material is the same as or similar to the encapsulating resin. This will yield the best results from the joint, both for physical strength and leak resistance. An example would be molding a polyurethane bifurcation to a polyurethane catheter. Mechanical bonding can take place in two ways, by the shrinking of the encapsulating resin around the insert as the resin cools, or by the surrounding of irregularities in the surface of the insert by the encapsulating resin. Although shrinkage always occurs, it is rarely sufficient to produce adequate physical strength or leak resistance of the joint. In general, when insert molding dissimilar materials, the insert should offer some means of mechanical retention such as a sandblasted, flared, or knurled surface.

Equipment. Although insert molding can be performed using a standard injection molding press, doing so can make the critical step of loading and retaining the insert in the cavity a difficult operation, and can thus place restrictions on the design of the part. However, there are some specific molding machine designs that are better suited for insert molding and offer much greater flexibility and productivity. Rotary and shuttle-table-type injection molding machines are excellent for this purpose, because they allow operators to load and unload inserts in the bottom half of one mold while actual molding takes place in another. These machines also lend themselves well to automation of the loading and unloading of inserts and parts. Other machines that offer vertical mold clamping can work well for low-volume insert molding, but are generally less productive than rotary or shuttle-table machines.

With a few exceptions, molds for insert molding are generally designed in the same fashion as are molds for injection molding. Molds for a shuttle-table press have two ejector halves, and molds for a rotary press have two to four ejector halves. Tooling costs will be higher due to the additional mold halves. The actual molding-cycle time is the determining factor for establishing the number of cavities and mold halves required. For optimum productivity the time it takes to load the inserts and unload the parts should not exceed the molding-cycle time. This consideration is also important when molding resins such as polycarbonate or PVC, for which residence time in the heated barrel of the machine is an important factor. Because the molten plastic is typically injected into the cavity and around the insert at pressures exceeding 1000 psi, when the mold is designed it is important to determine the exact location of injection gates and how the insert is to be held in place.

Materials. Like injection molding in general, insert molding can be accomplished with a wide variety of materials, including polyethylene, polystyrene, polypropylene, polyvinyl chloride, thermoplastic elastomers, and many engineering plastics. The primary factors that restrict the use of insert molding are not process related, but are determined by the strength and other properties required for the molded product.

Processing Parameters. One of the chief causes of failure in an insert-molded part is the cleanliness of the insert. It is absolutely imperative that the insert be as clean as possible prior to molding. When molding with large metal inserts, the inserts may need to be preheated to minimize the stresses caused by differential thermal expansion and contraction. When inserts are manually loaded it is important that the operator maintain a consistent cycle time.

Design Considerations. In general, the basic design rules for insert molding are the same as those that apply to injection-molded parts. However, designers should also be aware of the following elements that may affect the design of their parts:

* The material from which the insert is made.

* Pull- and compression-strength requirements of the insert from the plastic.

* Leak test requirements.

* Torque or axial forces to which the insert will be subjected.

* Voltage requirements for electrical applications.

Any or all of these elements may establish parameters that can help the designer determine what encapsulating resins will work for the application. They may also create requirements for the type of material preparation that must be performed on the insert in order to ensure proper performance of the finished part.

LIQUID RESIN CASTING

Medical Device & Diagnostic Industry Magazine | MDDI Article Index

Originally published April 1996

James E. Snyder

Vice President

Polymer Design Corp., Rockland, MA

Pouring liquid rubber or plastic into molds, then allowing it to cure to solid form, describes the fundamentals of liquid resin casting. A technically refined version of this decades-old process is a reliable and cost-effective choice for manufacturers of sophisticated medical devices. With new developments in materials and process controls, liquid resin casting lends itself to many demanding medical ap- plications, such as cardiac pacemaker encapsulations, handheld electro-optical surgical devices, and key components of medical imagers and scanners.

Medical product developers use liquid resin casting in two principal ways: for prototyping prior to committing to high-volume production, and for ongoing low-volume production of 25­2000 units per year. The principal advantages of liquid resin casting--comparatively inexpensive tooling, short lead times for tooling and parts, mild processing conditions, and design flexibility--enable the manufacture of highly complex parts with specialized performance characteristics difficult for other technologies to duplicate. For example, unlike molding or machining, liquid resin casting is associated with mild processing conditions that allow delicate components, such as fiber optics or electronics, to be encapsulated directly into the final or near-net shape required.

New tooling starts with a model. A castable material such as silicone, epoxy, or polyurethane is poured over the model in one or more steps. The material then cures, creating a mold. (Molds may also be machined directly out of aluminum or another suitable material.) Tooling lead times are generally three to six weeks. Once the mold is finished, parts are produced by pouring a resin into it and allowing the material to cure.

For prototyping requirements, plastic patterns can often reduce initial costs and lead time when compared to metal patterns. However, manufacturers who use plastic patterns must be prepared to accept limited options for surface finish and less tolerance control. In addition, if significant design modifications are necessary, a new generation of tooling will probably be required. Production programs are better accomplished with metal patterns, which enable the manufacturer to achieve the best possible results and avoid the limitations of plastic patterns.

Equipment. Special equipment for liquid resin casting includes mixing and dispensing equipment for handling resins, degassing equipment for removing entrapped air within the resin, and ovens for curing materials. The specific equipment needed depends on the kinds of materials being processed and whether they are for a prototype or production activity. The more demanding the application, the more sophisticated the equipment required.

Materials. Thermoset resins most commonly used in liquid resin casting--epoxies, polyurethanes, and silicones--must be in liquid form at approximately room temperature for successful processing. Formulations that will satisfy virtually any application can be developed from these three basic materials types. An expanded selection of formulations also is generally available from manufacturers with a production, rather than a prototype, focus.

In general, each type of resin has its own distinct advantages. Epoxies are ideal for high temperatures (up to 450°F), or for highly corrosive applications. For example, formulations of epoxy are used when steam sterilization is essential. Polyurethanes are an excellent general-purpose material for both soft-rubber and hard-plastic applications where exceptional toughness and wear resistance are important. They are used routinely in devices where blood and patient contact is expected. Silicones are best for product applications that require rubber that is soft or of medium hardness over a broad temperature range.

The various casting resins can be compounded with fillers or reinforcements to heighten specific qualities, such as impact strength, chemical resistance, or thermal conductivity. With rare exceptions, thermoplastics are available only in injection molding or extrusion grades and cannot be processed by liquid resin casting. Other materials that cannot be processed include ABS, polycarbonate, polyethylene, and acetyl. Their physical characteristics, however, can generally be matched with elevated cure formulations of castable resins.

Processing Parameters. Little or no pressure occurs within the liquid resin casting process, but humidity should be controlled during material handling. Polyurethanes and selected curing agents for epoxies are sensitive to moisture and will react to the presence of water in the mold.

Release agents can be used on mold surfaces to facilitate part removal and are available in silicone and water-soluble formulations. If painting of parts is anticipated, water-based release agents are recommended.

Curing can occur at room temperature or at elevated temperatures, and can take anywhere from a few minutes to several days. Fast-curing compounds are generally highly reactive and generate a large amount of heat. This can cause parts to distort or discolor, and may impart residual stress if not controlled. Longer cures, which are generally performed at elevated temperatures using precisely controlled ovens, require a 6­18-hour cure schedule. This ensures superior physical characteristics for the materials processed and more predictable results.

Design Considerations. Complex parts with highly contoured surfaces and critical finish requirements are well suited to liquid resin casting. Manufacturers can achieve tolerance control of ±0.004 in./in. with silicone molds and ±0.002 in./in. using metal molds or cores to define part features. Still greater tolerance control can be achieved through the use of machining as a secondary operation.

Aesthetic effects such as cast-in color and surface finish are readily accommodated. Color control during casting must be carefully managed owing to the nature of batch processing. A slightly higher part-to-part variation should be expected compared to painting. Except for silicone rubber, all materials used for casting can be painted, even low-durometer polyurethanes.

There are no manufacturing restrictions on part size, which can range from a few grams to hundreds of kilograms in weight. There is no need to maintain uniform wall thickness. When using flexible silicone molds, modest undercuts can be cast in place without splitting the mold or creating side action.

Finally, mild processing conditions are especially well suited to component encapsulation, a technique that typifies the flexibility developers of precision medical devices can expect from liquid resin casting.

PRESSURE FORMING

Medical Device & Diagnostic Industry Magazine | MDDI Article Index

Originally published April 1996

Al Scovill

Vice President
Ray Products, Ontario, CA

Although comparatively few companies are familiar with pressure forming, this technique is being used increasingly to produce low- volume products that can match the aesthetics of injection-molded, high-volume parts. The process is especially suited to the production of housings and enclosures.

The pressure forming process is still relatively new. Pressure-formed items that resembled injection-molded products first appeared in the late 1950s or early 1960s, but the method has been heavily promoted only during the last 10 to 15 years. Key elements of pressure forming that are attractive to medical device manufacturers include its low initial investment requirements and its ability to create products with injection-molded aesthetics. Becton Dickinson and Johnson & Johnson were among the first medical manufacturers to use pressure forming.

In pressure forming, sheets of material are fed into ovens that heat them to the proper forming temperature. Platens are used to clamp each tool, and the sheet is forced into or over a temperature-controlled tool by shaped plugs. Air is then forced into the plug chamber, pushing the hot sheet into or over the mold.

Equipment. Heavy-duty forming equipment has aided the development of pressure forming. Electric platens, for example, can be closed and exposed to needed air pressure without "backing off" or opening and losing the air-pressure seal. Using these platens, the former can overcome the "hot strength" of rubber-modified materials and push them into sharp corners and ribs. Pressure used during the process can be 2 atm or greater, depending on the material, depth of draw, detail, and other factors. As tools get larger, pressure forming becomes relatively more economical (see Figure 1).

Most pressure forming tools are cast, fabricated, or milled of aluminum. Because mold temperature is a critical factor, temperature control apparatus is added to the completed tool. Typical additions include heaters or copper sleeves for circulating coolants such as water, oil, or antifreeze. Production of such tools usually requires approximately four to eight weeks. Depending on the complexity of their design, pressure forming tools can last almost indefinitely.

Materials. Theoretically, any thermoplastic material can be pressure formed. However, some materials are more difficult to work with than others. Polyethylene, for instance, flows easily and causes few problems for pressure formers. With vacuum alone, polyethylene can be intricately formed. On the other hand, polycarbonate, which chills quickly, can cause manufacturers to be concerned about tool design and plug assists.

Medical device manufacturers usually specify that their products should be formed of a material that passes the Underwriters Laboratories (UL) 94 V0 or 94 5V tests for flammability. The resins most commonly used in pressure-formed medical products are flame- retardant grades of acrylonitrile butadiene styrene (ABS).

In many cases, assists are used to help distribute material evenly and to coin it into sharp or narrow corners. Depending on its complexity, the design of a product's tooling may require the former to use matched heated molds and assists; otherwise, assists can be made of low-heat-transferring materials such as wood.

Processing Parameters. The most important factors in pressure forming are the input and extraction of heat. For example, ABS could be formed at very low temperatures, but this would have a drastically negative effect on its physical properties. Its impact strength could drop to as little as one-tenth of its published value and its thermal distortion temperature could fall dramatically. ABS should be processed at 325°F or more. Heat extraction is important to control warping, shrinkage, and other forming failures. After the forming cycle, most parts are removed from the mold and secondary trimming operations are performed.

Design Considerations. The heavy-gauge forming industry has changed a great deal in the past few years, particularly in the depth of draw ratios. At one time it was common practice to have a 3:1 material distribution/reduction ratio, or a 1:1 ratio (width to depth) on minor dimensions, and a draft of 5° or more was needed for deep draws. Today's advanced designs can employ 5:1 distribution/reduction ratios, a 1:1.5 width-to-depth ratio, and a draft as slight as 0° (or 1/2° for aluminum cast molds). These advances have enabled product manufacturers to use pressure forming without changing their injection molding designs.

ROTATIONAL MOLDING

Medical Device & Diagnostic Industry Magazine | MDDI Article Index

Originally published April 1996

Mike Greer

Marketing Manager
Spin-Cast Plastics, Inc., South Bend, IN

Although the term rotational molding is unfamiliar to many, the products produced by the process are visible and familiar in a wide variety of everyday settings. Rotational molding (also called rotomolding) enables manufacturers to produce medium- to large-scale hollow plastic components ranging from 6-in.-diam containers to 20,000-gal tanks.

Recent applications of rotational molding in the medical device field include squeeze-bulb-type ear syringes, dental chairs, cases for resuscitators, medical carts, foam-filled tubs, and biomedical agitator tanks.

Rotational molding begins with a mold, a charge of resin, and a molding machine. A mold is mounted to an arm of a rotomolding machine and charged with raw material, which may be either in liquid form (such as a polyvinyl chloride plastisol) or in a dry, pulverized powder form (such as a polycarbonate).

Equipment. Rotomolding machines are classified into five categories: clamshell, independent cart, rock-'n'-roll, shuttle, and turret. Each of these offers processors an alternative manufacturing technology. Rotomolding machines may use a single arm, or may have three or four arms. The arms serve two functions: to rotate the molds on both axes in order to distribute the resin evenly and consistently along the mold surfaces, and to move the molds into one of the three stations of the machine--the oven, the cooling chamber, or the product loading and unloading station. The latest generation of equipment features onboard computers that control the arm rotation sequence, cycle time, oven temperature, and the internal and external cooling apparatus for the molds.

Molds may be fabricated of aluminum, steel, or stainless steel; electroformed from nickel; or cast from aluminum. Factors that determine what type of mold to use include the product's design, function, and aesthetics as well as the cost/benefit ratio of the mold's anticipated annual usage. Regardless of their material, all rotational molds are female cavities that do not require an inner core, similar to those used in blow molding. Unlike blow molding, however, rotomolding does not generate high pressures, and the molds used for the process therefore require only a minimum of cavity support and structure.

In rotational molding, molds act as vehicles for transferring heat to the resin, establishing the shape to be formed, and providing a cavity in which to cool the material. Compared to the molds used in injection or blow molding, those used in rotomolding are both inexpensive and quick to produce; typical lead time for production of a cast aluminum rotomold is 10 to 12 weeks.

A well-designed and properly specified mold is a key element in the successful rotomolding of a product. In fact, where other processes would require multiple molds, a properly designed rotomold can enable manufacturers to use a single tool for producing multiple parts as well as multiple-wall and multiple-layer products.

Materials. Although the number of materials suitable for rotomolding is limited compared with other molding processes, the resins that can be used still offer the medical device designer a wide range of capabilities. Acceptable resins include low-density polyethylene, high-density polyethylene, polyvinyl chloride, ethylene vinyl acetate copolymer, nylon 6, nylon 12, polycarbonate, and polypropylene. The density and melt index of these materials vary according to type as well as from vendor to vendor, but all are in an acceptable range for use in rotomolding.

Choosing the appropriate resin is frequently a function of the product's design, purpose, and anticipated work environment. For instance, rotomolded polyethylene offers significant impact resistance at low temperatures, while the nylon resins offer high-temperature resistance and stiffness.

Processing Parameters. Rotational molding is a phase-change process: its goal is to induce a physical change in the material from a dry or liquid state to a solid-wall state. This change is accomplished by inducting heat into the material via the mold. The typical temperature range of the rotomolding oven is 500° to 650°F. The precise temperature range is determined by the type of material being used in each mold.

Rotomolding offers manufacturers the capacity to produce multiple products in the same cycle. Typically, more than one mold is mounted to each arm of the rotomolding machine; so long as the time and temperature requirements of the products are similar for all molds on the arm, all can be produced at the same time.

Design Considerations. Rotomolding offers designers the ability to make use of cramped spaces by molding irregular shapes, and to enhance strength and impact resistance through the use of increased thickness in outside corners. Because rotomolding is a low-pressure process, products manufactured with this technique are relatively stress-free.

Dimensional tolerances in rotational molding are similar to those for blow molding, ranging from ±0.020 in./in. for commercial applications to as little as ±0.010 in./in. for precision applications. Most cast aluminum molds offer tolerances of about ±0.015 in./in. Processing tolerances for rotomolding are similar to those found with other processes, and can include the use of CNC-controlled routers for secondary processing of molded products.

REACTION INJECTION MOLDING

Medical Device & Diagnostic Industry Magazine | MDDI Article Index

Originally published April 1996

Fred T. Wickis, Jr.

Vice President
Evergreen Molding, Greenville, SC

Reaction injection molding (RIM) creates parts by using impingement mixing to combine reactive liquid intermediates as they enter a mold. RIM differs from traditional injection molding because it forms solid parts by cross-linking or polymerization in the mold rather than by cooling. The process does not use hot mold cavities to activate the reaction; in fact, RIM molds must often be cooled because an exotherm forms when the intermediates are mixed in the presence of heat. Completed parts can often be demolded in less than 20 seconds.

RIM developed from polyurethane foam technology. The needs of the automotive market spurred major growth of the process in the United States, and this market remains the primary application for RIM. Most of the technology's advances have resulted from efforts to improve the materials and processes for use in automobile production. However, there are some significant applications for RIM in the medical device industry, including structural foam cabinet parts, wheelchair seating and structural parts, and reusable foam patient positioners.

Equipment. Equipment for RIM was first developed in Germany and is now sold by several different manufacturers. The basic elements of a RIM molding system include a conditioning system that prepares the liquid intermediates for use, a metered pumping system that ensures delivery of the intermediates in appropriate quantity and pressure, one or more high-pressure mixing heads where the liquid intermediates are combined through impingement, and a mold carrier that orients the mold as required and opens and closes it for cleaning and demolding.

Unlike thermoplastic molding, RIM uses liquids that have a low viscosity during mold filling, and fills out the part using only internally generated pressure. Consequently, molding pressures in RIM can be as little as 50 psi (compared with 5000 psi or more for thermoplastic molding), making it possible for small machines with very limited clamping force to produce even relatively large parts in large numbers. For the same reasons, RIM molds are typically much less expensive than those used in thermoplastic processing. However, RIM molds made using the criteria for traditional injection molding are often unsuccessful. Molds for RIM have unique requirements related to the low-viscosity liquids they are filled with, and molds from other processes are rarely adaptable.

Low viscosity, low mold pressure, and inexpensive mold costs make RIM attractive for short production runs and prototyping. Selection of equipment appropriate to the anticipated application is critical for successful use of RIM. Key parameters for equipment selection include the type of material to be used (e.g., foam, elastomer), suitability for the size of parts being manufactured, and the desired throughput. As development of the technology has accelerated, corresponding equipment improvements have been made. Options now available include systems that incorporate multiple mixing heads and equipment for a range of processing limitations. Equipment options include various types and sizes of mixing heads, temperature controls for both materials and molds, programmable shot time controls, and process control alarms.

Materials. The earliest use of RIM was with polyurethane, but advances with the technique now offer opportunities for use with many other materials. Depending on the intermediates used, RIM can be used to produce soft foams, rigid foams, and solid elastomers. For example, RIM technology has produced a reusable foam that would have been impossible to produce with any other technology. The flexibility of the RIM process can enable companies to resolve problems unsolved by attempts using other materials such as plastic, rubber, or even steel.

Processing Parameters. Although RIM's use of low-viscosity intermediates is an advantage for productivity, it also has some disadvantages. Handling of such reactive or hazardous raw materials requires special equipment and procedures, including spill-cleanup materials. Gowning for operators should include protective coverings, eye protection, and sometimes air filtration masks. Since some components freeze at room temperature, a temperature-controlled environment is required for their shipping and storage.

Gas bubbles can be trapped during filling, and molds can be difficult to seal, which increases flash. Such problems can normally be overcome by careful attention to materials selection, mold design and orientation, shot time, and venting. Because the low-viscosity materials generally penetrate molds, the development of mold release agents for use in RIM has been difficult. Also, the recent exclusion of certain blowing agents (such as chlorofluorocarbons and hydrochlorofluorocarbons) has created the need for extensive research to find suitable replacements.

Successful molders must use RIM long enough to learn all the peculiarities of chemical manipulation, mold manufacture, and processing parameters. These typically differ for each project, resulting in a long learning curve for the few companies that choose to offer RIM-produced products.

Design Considerations. Inserts and reinforcement materials can be readily used. Reinforcement materials may include fiberglass, scrap plastics, metal, or wood. Fillers may also be added to improve the flexural modulus of the finished product or reduce its shrink rate in processing.