Material Progress toward Better Molded Parts

Originally Published MDDI March 2003

Molding

New molding materials and additives offer many properties that can enhance medical product quality.

by William Leventon

For manufacturers seeking molding materials with specific properties, there is good news. Offerings now on the market are clear and soft. They stand up to chemicals and high temperatures. They're biocompatible and biodegradable. There are also new material-modification products for molders to consider. These products can reduce melt viscosity, improve flow, add material properties, and even improve the shape of finished devices.

Materials Checklist

What material properties or processes are molders now seeking?According to Bill Torris, director of project engineering at Unimark Plastics (Greenville, SC), "Medical molders like Unimark need materials that we can use to validate our components and devices. The material suppliers have helped this process by controlling the quality of their products. 
They are controlling product variables like melt-flow rate, additives, and mechanical properties very closely. Lot-to-lot repeatability is critical for us to achieve the manufacturing rates and tolerances that we must maintain."

Torris further explains that, "There have also been advances in materials like polycarbonate that make processing easier and more repeatable. The material suppliers have also improved their additive packages so that we can better control our product quality before and after sterilization. Poststerilization part dimensions and mechanical properties are critical for medical devices."

A materials checklist for device molders would generally show that materials must have a diverse set of desirable properties to satisfy most product engineers. These properties include:

• Clarity. Clear materials let users see what's happening inside devices.
• Chemical resistance. Chemical-resistant materials protect devices against cracking and leakage.
• Processability. Materials should be easy to process, both in injection molding machines and secondary manufacturing operations such as bonding, welding, and sterilization.
• Biocompatibility. Medical device materials must meet biocompatibility standards that ensure patient safety. These standards apply to devices that come in contact with a patient's body fluids. Among such products are IV sets and procedural trays.

According to Jim Roma, product engineer at B. Braun Medical Inc. (Bethlehem, PA), all these characteristics can be found in Makrolon, a polycarbonate that the firm has used to replace Isoplast thermoplastic polyurethane. B. Braun uses Makrolon to make disposable valve components and end adapters on IV sets. Although Isoplast offers clarity and chemical resistance, it is more difficult to process than Makrolon, Roma says. The company also found that Isoplast turns red during radiation sterilization. "People perceive it as dried blood, so we backed away from using radiation with that material," Roma notes. The sterilization problem was solved by using Makrolon, which can be irradiated without discoloration.

However, although Makrolon costs less than Isoplast, it's more expensive than other polycarbonates. In addition, Roma found that Makrolon will craze, but the cracks won't propagate all the way through the material. "It will look like it's cracked, but it won't leak," the material supplier told him.

Because Makrolon requires high processing temperatures and pressures, B. Braun is evaluating an alternative that might be easier to mold. Cyrolite Med 2 can be processed at lower temperatures than polycarbonates like Makrolon, according to Cyro Industries (Rockaway, NJ), which developed the acrylic-based multipolymer compound for the production of thin-walled parts and other difficult-to-mold components.
Med 2 is intended to provide good alcohol and lipid resistance in medical components, such as filter housings, luers, connectors, valves, pumps, and catheter adapters. The material also offers strength, ductility, and clarity. It yellows slightly under normal gamma sterilization, but shows no deterioration, according to Cyro.

Clear Alternatives

Device makers seeking materials that provide clarity may want to consider an amorphous cycloolefin copolymer called Topas. "If you put a part made of Topas next to a part made of glass, it's hard to tell the difference just by looking at them," says Gil Reich, vice president of sales and marketing for The MedTech Group, a New Jersey molding firm.

The Topas formulation is designed to offer biocompatibility, heat resistance, and a moisture barrier. But Reich says its main purpose is to replace glass, which is very difficult to process. The material is also an alternative to styrenes and polycarbonates for the production of syringes and other devices. Reich adds, however, that Topas costs twice as much as polycarbonates and roughly three times as much as styrenes.

Like Topas, Zeonex is a cyclic polyolefin that offers ultra-high transparency and good chemical resistance. Zeonex and Topas "are the first clear materials that are resistant to almost all the chemicals that polycarbonate is not resistant to," notes Kristy Johnson, senior research engineer for Phillips Plastics Corp. (Hudson, WI). Johnson says the polyolefins are well suited to devices that will be exposed to polar chemical agents, such as acids, bases, alcohols, and ketones. On the other hand, the materials are not resistant to nonpolar solvents such as hexane and toluene.

Applications for Topas and Zeonex include syringe bodies and blood-handling devices. But the materials have some limitations, according to Johnson. She explains that Topas and Zeonex have much less impact strength than polycarbonate. They can also yellow under gamma or electron-beam sterilization.

Getting the Feel

While Topas and Zeonex provide certain visual characteristics, other materials are chosen for the feel they give a product. For example, materials such as Versaflex, Pellethane, and Santoprene add a "soft touch" to disposable handheld devices and limited-use instruments, says Scott Wolf, engineering manager at Scientific Molding Corp. (Somerset, WI). The materials can also help healthcare workers get a grip on wet devices. "If you're wearing gloves, devices can be slippery when they've got body fluids on them," Wolf notes. "These materials give you a little more tactile feel against latex gloves."

New soft-touch material formulations offer improved performance, including the ability to withstand limited sterilization. For example, Wolf says, some of the materials can hold up to high continuous-use temperatures, making them suitable for autoclaving.

Some medical device makers like the softness of polylactic acid (PLA), according to Len Czuba, president of Czuba Enterprises Inc. (Lombard, IL), which provides medical product design services. Environmentalists certainly have a soft spot for PLA, which is a biodegradable polymer made from cornstarch. “We're starting to use it in devices to reduce the dependence on petroleum-based polymers,” Czuba says.

Czuba thinks PLA can be a good choice for disposable products, including drug-delivery systems. The plastic can also be used to make implants that will dissolve and be resorbed into the body over time. On the downside, PLA is more expensive than conventional plastics.

To reduce processing expenses, some companies are developing thermoplastic elastomers (TPEs) to replace thermoset materials, which are more difficult to mold. "With TPE, you save a lot of processing time and cost," Czuba says.

In hospital settings, devices made from TPEs are not likely to break when dropped. The materials can also replace controversial natural-rubber latex in tubing, catheters, connectors, seals, and gaskets.

In many cases, TPEs provide the same material advantages as thermoset elastomers, Czuba points out. They also offer stronger bonding characteristics than thermoset materials. But TPEs are limited in some areas, including heat and chemical resistance.

Taking the Heat

To make devices that will be exposed to intense heat, some manufacturers use polyether etherketone, or PEEK. PEEK is a crystalline engineering thermoplastic that can retain its flexural and tensile properties at very high temperatures. In addition to offering excellent mechanical and electrical properties, the material provides good fatigue and chemical resistance.

MedTech uses PEEK to mold several components used in orthopedic devices. "PEEK gives you mechanical characteristics comparable to those of metal," Reich says. "It also gives you radiolucency, which you can't get from metal." Doctors can x-ray through radiolucent PEEK parts, he adds.

For implantable devices, manufacturers can use a special version of the material called PEEK-Optima. Johnson says this type of PEEK is toxicologically safe and highly resistant to chemicals. It can also withstand all common sterilization methods without a serious loss of mechanical properties.

According to Johnson, molders can use PEEK-Optima to make long-lived implants that won't interfere with x-rays, magnetic resonance imaging, or computed tomography. Applications include prosthetic hips, spine cages, bone screws, dental implants, and cardiovascular rotors.

Like PEEK, Radel was developed to withstand chemical exposure and high temperatures. MedTech uses the transparent polymer to make containers for electronic components. Housed in Radel containers, these expensive devices can withstand repeated sterilizations in hospital settings.

Unfortunately, Radel and PEEK can be expensive. Reich says they can cost up to eight times more than other plastics. They are also difficult to process, so MedTech must mold them in specially modified machines that run as hot as 750°F—hundreds of degrees higher than the temperatures used in conventional molding operations.

Foam for Better Molding

Manufacturers experiencing molding problems may benefit from the MuCell process, which is used to produce microcellular foamed plastics. Invented at the Massachusetts Institute of Technology, MuCell is licensed by Trexel Inc. (Woburn, MA), which is developing and commercializing the technology.

During molding operations, MuCell equipment injects nitrogen or carbon dioxide into a polymer to create evenly distributed, uniformly sized microscopic cells throughout the material. This reduces part weight, but Trexel downplays this seemingly important result.

"People associate foaming with weight reduction," says Dan Szczurko, Trexel's vice president. "But we generally tell people to keep weight reduction under 10%. If you do that with most engineered resins, your material properties will be equal or close to what you get with solid materials."

Rather than focusing on weight reduction, molders are encouraged to consider MuCell's process and product quality benefits. Trexel claims, for example, that MuCell reduces polymer viscosity by up to 50%. This can reduce melt temperatures by as much as 140°F, while maintaining material flow in the mold. Lower melt viscosity also enables molders to reduce injection pressures by up to 50%, according to the company.

In turn, lower pressures in the mold cavity mean lower clamp tonnages. "We have customers who have gone from 700-ton presses to 300-ton presses," Szczurko reports. "Others have gone from 300-ton presses to 100-ton presses. This is a huge savings."

MuCell can also reduce molding cycle times. Among other things, this is a result of internal gas pressure, which eliminates pack-and-hold time; a reduction in the part mass that must be cooled; and decreased viscosity, which reduces shear heating, shortening cooling time. The time savings can cut cycle times by up to 50%, according to Trexel.

In a normal molding process, the press keeps pushing plastic into the mold, putting a great deal of stress on the part. By contrast, gas-filled foam expands to take on the contours of the mold in the MuCell process. "The expanding cells pack the plastic in the mold, so you have much less stress on a MuCell part," Szczurko explains. By reducing so-called "molded-in stresses," he says, MuCell produces flatter parts than those made by conventional molding operations.

MuCell-equipped molding machines can also make thinner parts than conventional equipment can produce, Szczurko adds. Because of the low viscosity of foamed resins, he explains, molding machines can push them into extremely thin mold sections that would be inaccessible to high-viscosity nonfoamed plastics.

In addition, the MuCell process can produce thinner parts than other thermoplastic foaming processes. According to Trexel, these processes produce relatively large cells that prevent thin-wall formation. But MuCell's microcellular foam contains much smaller voids. This enables molders to make parts with cross sections as thin as 0.02 in.

MuCell equipment can now be retrofitted into installed injection molding machines. The equipment is also available as an option to be added on some new molding machines. In either case, the equipment may be limited by its high cost. "I'm disappointed that it's so expensive," says Czuba, who often works on low-volume products. Generally speaking, Szczurko believes a molding press should be running about half the time to justify the cost of MuCell equipment.

Glass Cuts Viscosity

For molders who can't afford MuCell, there may be a less expensive way to get some of the same advantages. VitroCo (Santa Ana, CA) has developed a patent-pending technology aimed at reducing the high viscosity of polymeric liquids. The technology is based on a natural amorphous aluminosilicate glass, which the company processes into an additive called Vitrolite.

In molds, the addition of Vitrolite can reduce polymer viscosity as much as 40%, according to the company. Reduced viscosity lets molders use lower processing temperatures without affecting molding speed or quality. "A polymer will flow as if it's much hotter than it actually is," says George Taylor, director of the firm's medical division.

Taylor explains that by improving polymer flow, Vitrolite will normally reduce cycle times by 20 to 35%. Improved flow also makes it easier to fill problem molds such as those for very thin or intricate parts, he adds.

The company has found that its additive can also improve the properties of molded materials. For example, says Taylor, polyolefins combined with Vitrolite have shown improved stiffness and heat deflection. Taylor believes the materials may be benefiting from the lower temperatures and pressures of Vitrolite-assisted molding operations.

Taylor also claims that Vitrolite improves the dispersion of additives, reducing the required quantities of expensive agents such as pigments. "We've reduced pigment loads 40 to 60%, while still producing the same color intensity," he says.

VitroCo's tests show that Vitrolite is inert, nontoxic, and safe for use in medical devices. The additive produces a haze in plastics, however, making it unsuitable for devices that require clear material.

As of now, no medical device manufacturers are using Vitrolite in production. But Taylor points to test runs conducted by several medical molders, in which Vitrolite increased production 20 to 30% while maintaining physical properties. He believes the additive will yield the greatest benefits in high-volume production of products such as disposable medical devices. 

Helpful Additions

Besides Vitrolite, device manufacturers can benefit from several other new materials that can be added during the molding process. For example, Johnson of Phillips Plastics describes an EMI shielding agent that can be molded directly into a plastic housing, reducing the number of device malfunctions caused by signal interference. She adds that firms manufacturing these agents are trying to make them more visually appealing. This would allow users to improve product appearance while providing shielding.

For certain applications, there is a new class of antimicrobial agents that eliminate unpleasant product odors, preserve materials, and improve hygiene. At present, Johnson says, these additives aren't being targeted specifically at the medical device industry. But they could be used in handheld medical devices as an additional safeguard against the spread of bacteria.

In addition, Johnson is monitoring progress in the development of nanomaterials, ultra-small-scale fillers consisting of ceramics, carbon tubes, and calcium carbonate, among other substances. These nanoscale fillers can be evenly distributed in a plastic resin, adding or improving properties in special high-tolerance molded devices, such as a lab-on-a-chip device.

Processes to Match New Materials

Unimark's Torris agrees that the device molding industry has many new materials and processes available to improve product quality and reduce costs. He explains that his company uses newly developed materials to add sealing capabilities to components and is working on soft touch finishes for its devices. One new process it uses is micromolding, Torris adds. "We can now mold products that are smaller and have tighter tolerances than previous manufacturing techniques allowed. Another manufacturing advancement is multimaterial molding." 

Unimark is also developing technologies for insert molding of cannulae. Torris says, "We are working closely with our press and assembly equipment manufacturers on increasing the number of products we can produce using new processes. Unimark has used new technologies to eliminate assembly steps and reduce costs. The materials and technologies have allowed for components and devices that were not previously possible."

Conclusion

Today's molding materials offer medical device makers a plethora of useful properties. The materials can also be enhanced by a variety of additives that improve both products and the molding process. 

What changes lie ahead for the molding industry, and what challenges must be addressed? Torris speculates that, "the next major advance for medical molders is becoming a total solution provider. Molders and device manufacturers must be able to produce components, produce final assemblies, and distribute the finished product to the end-users." 

He adds that Unimark is integrating the advances in materials, machines, and processes to make better products from both a manufacturing standpoint and a regulatory standpoint. "We integrate technologies in mold labeling, multimaterial molding, insert molding, assembly, and packaging into one assembly line so that the finished device is the end product. This allows for better lot control, inventory management, and product traceability,”" says Torris. 

Copyright ©2003 Medical Device & Diagnostic Industry