|Using Thin-Wall Molding to Create Medical Devices|
GE Healthcare's Voluson E8 OB/GYN ultrasound system demonstrates how thin, lightweight walls enable greater emphasis on human factors elements such as integrated electronics.
A number of converging healthcare trends are pushing medical equipment manufacturers to design smaller, lighter, and less costly devices while also making them safer and more appealing to patients. Diverse issues contribute to these trends, such as increasing the speed of care, managing costs more effectively, and complying with stricter regulations. In terms of technology, thin-wall injection molding of engineering thermoplastics could provide a creative response to these challenges, enabling equipment makers to move forward in a variety of ways.
The thin-wall molding technique can help reduce the weight and mass of a device enhancing its portability in the hospital and at home. It can also enable more-compact designs that optimize space and are user-friendly for patients. Thin-wall molding can offer these capabilities yet still yield devices that withstand multiple cleaning regimens while maintaining an attractive appearance.
Successful thin-wall molding depends on choosing the right resins, machinery, processing, and mold design. By following best practices, OEMs can incorporate light weight yet durable thin-wall parts into their device designs.
The healthcare industry continues to struggle with tough challenges, including cost management, limited resources, and more-stringent regulatory oversight. To expedite care and reduce costs, healthcare providers are leveraging medical breakthroughs such as minimally invasive surgery to speed recovery and move care to an outpatient environment. At the same time, patients are more active and live longer than ever before. As they develop health issues that demand day-to-day attention, devices are being redesigned for ease of use at home, for travel, and for more-efficient patient and hospital-space management.
This transition from acute-care settings is driving the need for compact, portable equipment that is easy to maneuver and use, in addition to being attractive and inconspicuous. In addition, clinical equipment is being redesigned to have a smaller footprint than older versions. New versions of ultrasound, MRI and CT scanners, and anesthesia and patient-monitoring devices have become much smaller than earlier models. One equipment area that has an often-overlooked need for weight reduction is renal dialysis and IV-set components such as connectors, stopcocks, luers, and filters.
Of course with any redesign, it is imperative to maintain existing performance capabilities such as mechanical strength and chemical resistance. At the same time, more-compact durable equipment and disposable devices must meet the demands of the care environment, including chemical resistance to withstand rigorous cleaning, impact resistance for day-to-day use and rough handling, and flame retardance for safety with electrical equipment.
In addition to becoming smaller and lighter, medical equipment must meet new standards for aesthetics. Hospitals and patients want devices that are attractive and even decorative to help reduce or avoid an institutional appearance. The use of colors and textures can make equipment less formidable.
Thin-wall injection molding may be very useful for today's medical equipment housings and other components that need to be made smaller and lighter without sacrificing durability, strength, and impact resistance. The technology facilitates the design of small, complex parts and of large panels that require less material, but need to maintain mechanical and physical performance. The thin-wall approach can also help increase space for internal components, and possibly accelerate production cycle times. To achieve these benefits, it is important to meet the material processing requirements of thin-wall molding.
Material Processing Challenges
Thin-wall molding can be challenging due to several contributing factors. For example, thin-wall parts usually have length-to-thickness ratios ranging from 120:1 to more than 200:1. Filling these types of parts requires injection pressures and injection velocities far exceeding those found in conventional injection molding. In turn, these high pressures and injection velocities create additional problems, including material degradation, mold flashing, and increased melt residence time. Also, because of high fill velocities, it is almost impossible to use any type of programmed injection profile.
For injection molders, the challenges of thin-wall molding require an understanding of three key areas: material, machinery, and mold design. Fundamental to successful thin-wall processing is the understanding that these three processing areas work together as a system.
Thermoplastic resins suitable for thin-wall molding should have high-flow properties, particularly low melt viscosity. In addition, they need to be robust enough to avoid degradation from the heat generated by shear rates in excess of 50,000 sec-1.
Specialized high-flow polycarbonate (PC) resins and polycarbonate/acrylonitrile butadiene styrene (PC/ABS) blends have been engineered for a variety of thin-wall applications. Also, super-high-flow, glass-filled polybutadiene terephthalate (PBT) materials may be suited for some applications, including internal electrical components.
A simple comparison of alternatives in these three engineering thermoplastic families identifies some opportunities to enhance thin-wall molding performance while still maintaining mechanical and physical properties important for many applications, as described below.
Medical Disposables. PC is often used for medical disposables. In many cases, specialized grades are chosen for clarity, biocompatibility, sterilization capability, and resistance to impact and chemicals such as disinfectants and antiseptic agents.
To discuss PC, we will use two standard industry tests. The first, melt volume rate (MVR), measures the resistance to flow of a plastic material under pressure (per ISO 1133; measured in cm3/10 min). The second, notched Izod impact (at 23°C; measured in J/m), measures the impact strength of the material when subjected to a dynamic load and determines how well it will resist breaking in the presence of a notch or defect.
Table I. (click to enlarge) Comparison of standard and specialized PC and PC/ABS materials.
As shown in Table I, a standard high-flow polycarbonate may have an MVR of 23 cm3/10 min with a notched Izod impact strength of approximately 640 J/m. There are, however, copolymer alternatives that offer an MVR of 33 cm3/10 min, with an impact strength of approximately 700 J/m, thereby delivering improved flow without sacrificing product ductility. Such materials are also offered with release additives to facilitate ejection of parts from the mold. They are increasingly popular in new designs for disposable components that require low draft angles or complex geometries.
Figure 1. (click to enlarge) A comparison of copolymer PC resins and standard PC resins, demonstrating flow and ductility.
Figure 1 highlights how these copolymers compare with standard PC products. The new PCs show greater impact resistance and greater practical flow through spiral flow paths, and do so at lower ductile and brittle transition temperatures.
Equipment Housings and Electrical Components. The need for thin-wall parts extends beyond disposables into housings. In many cases, when housings are designed to be smaller and lighter, the same goals apply to internal components such as electrical connectors. Typically, materials for housings do not require clarity or biocompatibility, but they do require good aesthetics, flame retardance (FR), and resistance to standard hospital and home cleaners and disinfectants. Additional criteria may include conformance to EU directives such as WEEE for electrical and electronic devices and RoHS on the use of hazardous substances. Material selection for electrical components may call for evaluating electrical properties such as dielectric strength, volume resistivity, and arc resistance.
Common housing materials include PC/ABS (when impact resistance is a strong consideration) and PBT (when dielectric strength, heat resistance, or chemical resistance is critical). PBT materials are often selected for electrical connectors as well. When using flame-retardant polymers, thin-wall designs may be achieved through the processing capability of the material and using a wall thickness that meets UL flame-class ratings (FRs).
Not all materials offer the same potential for thin-wall molding. As shown in Table I, a standard PC/ABS resin may offer a melt flow of ~16 cm3/10 min, a corresponding notched Izod impact strength of ~550 J/m, and an FR of 94V-0 at 1.5 mm.
A spiral flow comparison. The top example demonstrates the practical flow improvements displayed by PBT alternatives.
A new PC/ABS material, however, offers a significant improvement in the FR, 94V-0 at 0.75 mm. Melt flow and impact strength are similar to standard PC/ABS.
Such capabilities afford designers the possibility to meet flame-class requirements with thin, lightweight walls without compromising impact performance or even the aesthetics of the medical device.
For example, an ultrasound system was redesigned to be one-third smaller and lighter than its previous iteration. It needed to maintain its aesthetics and chemical resistance. It also needed to meet the demands for nonchlorinated and nonbrominated flame-retardant systems for WEEE and RoHS directives. In this system, the specialized PC/ABS resin facilitated the redesign. The device was able to encompass more than 30 components, thereby easing ongoing materials purchasing and inventory management.
Replacing Thermosets. In efforts to reduce the total footprint of larger equipment such as MRI and CT scanner units, advanced PC/ABS polymers have replaced thermosets in exterior panels. These new PC/ABS materials offer a lightweight solution, allowing field service representatives to access the equipment quickly and easily.
In one example, the large panels of CT scanner equipment were thermoformed with a specialized PC/ABS sheet that exhibited good melt flow. The material provided impact strength of 500 J/m, and was rated UL 94V-0 at 1.5 mm. It also provided a nonchlorinated and nonbrominated flame-retardant system that met WEEE and RoHS requirements. In addition the PC/ABS material provided low deflection under load for improved shape retention when exposed to external forces and heat.
Designers were also able to demonstrate impact resistance and durability, and good resistance to hospital cleaning agents. Thermoplastic does not need to be painted when it is released from the mold, so secondary painting operations are usually unnecessary.
PBTs are also considered for housings, equipment work surfaces, and electrical components. These crystalline resins offer alternatives for thin-wall flame resistance (UL 94V-0 at 0.7–0.8 mm) and generally have very good resistance to a broad range of hospital cleaners.
Standard PBT and glass-filled PBT materials demonstrate a melt-flow MVR performance of approximately 11–12 cm3/10 min. New PBT materials have shown melt-flow rates of 23–27 cm3/10 min. The spiral-flow model shown in Figure 2 demonstrates the practical flow differences in the two types of PBT.
Molders may experience reductions in injection pressures of 20–50% using the new PBTs, compared with standard counterparts. There is also the opportunity for better hydrostability performance. These capabilities not only facilitate thin-wall molding but also help reduce system costs through the potential for reduced scrap, low tool maintenance, and lower material cost, compared with some alternative resin families.
GE Healthcare's BrightSpeed and BrightSpeed Select CT Scanner equipment employ a specialized PC/ABS sheet with good melt strength for thermoforming the large panels.
For internal components such as connectors, glass-filled PBTs may be considered because of their electrical performance capabilities. As noted above, similar benefits may accrue to the design and manufacturability of electrical components when selecting new PBT resins for thin-wall parts. Therefore, when designers are required to produce very small components that can pass electrical tests such as those found in UL 746C, Polymeric Materials Use in Electrical Equipment Evaluation, new material products offer additional options with which to work.
Lowering the Carbon Footprint. With the growing concern for the environment, some manufacturers are looking for materials that have a lower carbon footprint. Some new specialized PBT resins offer the opportunity to reduce post-consumer waste, CO2 emissions, and energy consumption while retaining the expected performance level.
Plastics manufacturers are introducing engineering thermoplastic resins that are evolving in response to marketplace trends that call for smaller, lighter devices while maintaining desirable performance capabilities. Once the right material has been chosen for the medical device or equipment, it is time to consider molding equipment.
Thin-wall molding places unusual stresses on injection molding machines. Machines that have been designed to handle thin-wall parts can typically supply injection pressures above 30,000 psi. Compared with traditional molding machines, thin-wall machines require much higher clamping forces to compensate for the increased injection pressure. Typically these range from 5 to 8 tn/sq in., compared with 3–5 tn/sq in. in conventional molding. Thin-wall-capable machines usually also have accumulator-assisted clamps to accommodate fast cycle times.
In addition to high-pressure capability, injection velocity is very important in filling thin-wall parts. A fast fill speed that avoids freeze-off before the cavity is filled and packed is vital for successful thin-wall molding. Thin-wall parts often need injection fill speeds greater than 3 in./sec, and many jobs may require filling at 8 in./sec.
Another consideration in thin-wall molding is total melt residence time. It is important to properly size the injection barrel capacity to the part or to the total shot volume. Too large a barrel capacity prolongs material residence time and thus increases the risk of polymer degradation. The relationship to the shot size is important when selecting barrel size. Generally, the shot size should be at least 40% of the barrel size—a common minimum for engineering resins.
The final component of the thin-wall molding system is the mold.
Tooling for thin-wall molding must be robust enough to withstand high injection pressures and velocities. A well-designed thin-wall mold must be more robust than a conventional tool or it can deflect during high-pressure injection. Deflection can cause flash and premature mold wear. Steels harder than P20 should be considered. In addition, thicker cavity-support plates and additional support pillars will help counteract mold deflection. Preloading the support pillars in the center of the tool by 0.004–0.005 in. may help.
Mold cooling is a crucial part of good thin-wall mold design. Good cooling design practices include nonlooped cooling lines in the core and cavity blocks; larger cooling lines to increase flow through the tool (rather than relying on lower coolant temperatures); and, where necessary, inserts made of special high-conductivity metals for faster heat transfer.
Part ejection is another crucial concern for thin-wall parts. Thin walls and ribs can deflect more on ejection than thicker parts, and therefore are more easily damaged. Thin-wall tools need more and larger ejector pins than conventional tools. As a guide, a thin-wall tool usually has double the number of ejector pins, and pins are larger than those found on a conventional tool. Many thin-wall parts require blade- or sleeve-style ejectors on ribs and bosses to help avoid damage to these features.
Part gating can be a critical aspect of thin-wall processing. Because of the high velocities used in thin-wall filling, high shear rates at the gate areas may cause increased temperature and shear stress on the resin. It is important to make gate sizes large enough to minimize pressure drops yet still freeze off fast enough not to hinder the faster cycle times gained from thin-wall molding. A computer-aided mold-filling analysis can help find the optimum balance between residence time and pressure drop.
The success of thin-wall molding is in the interaction of material, machine, and mold. It is important that all three of these components be optimized together to achieve the best results. These efforts will help to ensure high-quality parts and will result in systems that retain the advantage of the improved properties of next-generation resins. Successful thin-wall molding can help medical device manufacturers achieve their high-level goals of producing compact, lightweight, durable, and attractive products to meet today's healthcare challenges.
Greg Tremblay is senior project engineer at SABIC Innovative Plastics (Pittsfield, MA). He can be reached at email@example.com.