With tens of thousands of types of plastic to choose from, material selection can be a daunting exercise. Selecting a material for a medical application introduces yet more complexity to the process.
There are tens of thousands of types of plastic when you factor in the various additives and fillers used to fine-tune the raw material and achieve the flexibility, hardness, color, and other properties required by an application. That alone makes material selection a seemingly daunting exercise. Selecting a material for a medical application introduces yet more complexity to the process.
It’s important to begin the material selection process early in the development cycle, Paloian told PlasticsToday. How early depends on the product. “If we’re talking about product conception, you should start thinking about material selection no later than 10 or 20% of the time from when product design begins,” says Paloian. “But if you’re doing engineering design, it should happen almost immediately. The reason for that is because material selection will affect how your product is assembled, and the assembly features will affect how the product will be designed.” Paloian cites the example of parts that will need to be bonded. “If you’re designing something with materials that don’t easily bond together, you’ve got a problem,” explains Paloian. “If you are designing snap fits, on the other hand, you want to be sure the material will perform [as intended]. The snap might not be adequate to retain the part.”
A regular contributor to PlasticsToday, Paloian recently wrote an article, “Injection molding design fundamentals: Snap-fits for plastic parts,” that goes into some detail on this topic. For example, when designing an annular snap lock, he notes that its performance “is highly dependent on the materials of both mating parts, the wall thicknesses, and the amount of interference.” For torsional snap locks, on the other hand, the “stressed torsional portion of the lock must be designed to flex within the elastic working stress of the material while inducing enough forces to perform its desired function,” writes Paloian, who is president of Integrated Design Systems Inc.
Speaking more generally on the materials selection process, Paloian recommends conducting a comprehensive review of how the product will be used to avoid some fairly easily avoidable pitfalls. You should look at cleanability, since medical environments are using increasingly harsh chemicals to prevent hospital-acquired infections, doing drop tests, and so forth.
Another aspect is assembly and service. “If you’re assembling something with screws, for instance, and the screws are not properly washed, residual oil could remain. Say you’re using polycarbonate. You assemble the part and it’s working fine, but in six months to a year your part is stress cracking prematurely. That’s because there is an incompatibility between the residual oil on the screw and the material. I have heard of this happening,” says Paloian.
Another possible failure point involves sterilizability. “Manufacturers often will publish statistics on how many autoclaving cycles a device can withstand. But that can be misleading,” says Paloian. The device should be tested under the stresses and conditions it will experience in the real world. “Taking a plaque of plastic and putting it in a steam sterilizer under no stress will give you different results than taking that same part and testing it under the stress of environmental conditions,” notes Paloian.
“Don’t assume anything. Ask a lot of questions. Do research and reach out to experts before you start specifying plastics,” Paloian suggests. “Reach out to molders, OEMS, resin suppliers and, better yet, distributors who are experts in material selection and let them help to guide you.”