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A User-Friendly Guide to Medical Plastics Selection, Part Two

Image: Hakan Tanak/Adobe Stock plastic resin forming a target
In part one of this two-part article, plastics engineering expert and industrial designer Michael Paloian discussed how application, regulatory, and physical property requirements are instrumental in the material-selection process. In part two, he discusses such business requirements as the cost and availability of resins.

You should be aware of six fundamental parameters before you specify a plastic for your next medical device. They are listed below.

  • Understand polymer classifications
    • Thermoset
    • Thermoplastic
    • Crystalline
    • Semi-crystalline
  • Define application requirements — product specifications
    • Performance
    • Safety
    • Aesthetic
    • Assembly & Service
  • Define regulatory requirements — material related
  • Identify critical physical property requirements
    • Mechanical
    • Electrical
    • Flammability
    • Thermal
    • Long term
    • Chemical
  • Define business requirements
    • Material cost
    • Material availability
    • Lead time
    • Minimum order size
    • Ease of processing
  • Seek outside assistance
    • Plastic material consultants
    • Publications (books, magazines, periodicals)
    • Internet resources
    • Molders
    • Plastic resin technical advisers

Part one of this article focused on the first four parameters — understanding polymer classifications, defining the application and regulatory requirements, and identifying the physical property requirements. In part two, we will focus on the business requirements involved in material selection and how best to fulfill them.

Business requirements

The material-selection process is not limited to technical and regulatory considerations. Material selection also must account for business-related factors listed below.

Cost constraints

The material cost can play a significant role in the decision-making process. Although material cost is not always a top priority for medical devices, it is a criterion that you must consider. It is especially significant for disposable medical devices such as single-use syringes and clamps. The cost of plastic resins cannot be ignored and is always a significant consideration during the selection process. Resin costs are highly dependent on order size. Large orders (rail cars) are sold at prices much lower than single 50-lb bags.

Availability

After a cost-effective suitable material has been identified, the next consideration is its availability in the country where it will be molded. Material grades are not always available internationally. For example, resin grades in the United States may not be available in Europe or China. In other situations, specific resins may be available in a limited supply. Sometimes resins may only be offered by one manufacturer or a small group of suppliers. These restrictions could seriously affect the decision-making process, which may shift the preference from one resin to another.

Lead time

The lead time for receiving a plastic resin shipment is an essential consideration during production startup and ongoing production. Some resins may require extended lead times, which do not coincide with production schedules. Lead times can be affected by unique colors or custom formulations that require longer production times. Custom compounded resins may extend lead times, as well. 

Order size

Large-volume railcar orders for resins command more attention from resin manufacturers than small 50-lb bags. OEM resin manufacturers typically don't ship their products directly to molders unless the order quantities match their levels. Therefore, resins are more frequently sold through distributors, catering to molders ordering a single bag of plastic resin up to multiple truckloads.

Ease of processing

Since plastic resins are converted from the raw material into a product, ease of molding is a critical consideration. Design parameters affected by the processing include tolerances, warpage, tool design, wall thickness, and gating. For example, semicrystalline materials like polybutylene terephthalate (PBT; trade name Valox) tend to shrink a lot and are very rigid. These materials, therefore, are more susceptible to warpage than amorphic materials like polysulfone. High-heat resins like PEEK require oil-cooled molds, which are designed differently than water-cooled molds. High-viscosity materials like specific grades of polycarbonate may require thicker wall sections than ABS or polystyrene.

Seeking outside help for resin options

Knowing where to go for advice or where to find information that might help you select the best plastic resin to satisfy your requirements is essential. The internet provides a wealth of information resources for plastic materials, plastics experts, industry trends, and research. The problem most people have is searching through this vast pool of information to find the optimum material for their application. There are numerous publications, such as PlasticsToday, which are excellent resources for providing an endless amount of information about the plastics industry and materials. Thousands of experts have written books about plastics processing, design, and materials. It requires your investment of time and effort to acquaint yourself with the vast amount of information about this subject. This challenge is drastically diminished when you're familiar with some of the parameters previously discussed, or if you can focus on a specific area of interest. However, other options available to you may prove to be more convenient, faster, and less risky.

Resin manufacturer/distributor application engineer

Resin manufacturers often offer customers or prospective customers free technical advice by their application engineers. Unfortunately, large resin companies like Dupont, Dow, Sabic, or Exxon are difficult for the average person to contact. They tend to focus their attention on larger customers in the automotive or major appliance industries. However, smaller resin manufacturers and resin distributors like Avient (formerly PolyOne) or Nexeo are more easily accessible. Resin distributors retain a group of application engineers familiar with a range of plastic materials manufactured by many companies. They can offer somewhat impartial information to assist you in objectively selecting the optimum resin. It's always a good idea to cross-reference information from any source for verification. An important note to keep in mind is that application engineers' recommendations are only as good as the information provided to them by you. If you've omitted some critical facts about your application, the advice may not be valid.

Plastic materials consultants

Qualified plastics materials experts can be retained for assistance in specifying a material. The benefit of retaining an independent expert for advice is their unbiased recommendations. Independent plastics materials experts are not affiliated with any resin company, enabling them to provide impartial, objective guidance. Identifying a qualified expert is the challenge. Experts can be sought out from professional organizations like SPE or recommendations from molders or even material suppliers. Experts may specialize in resins families, such as olefins, thermosets, heat-resistant thermoplastics, or elastomers. It's always good practice to verify consultants based on their recommendations, publications, and track record. Fees, accountability, and information provided must all be evaluated as part of the expert's value.

Molders

Molders can be an excellent resource of information regarding plastic materials. Some larger molders retain materials engineers on staff who are incredibly competent for recommending plastic resins. However, most molders will offer recommendations based on their experience with similar applications. Suggestions may not be based on the evaluation of a range of viable materials but more frequently on what's been used before. If the selected molder has molded similar products for other companies, his recommendations should be highly regarded. However, if there is no experience with other similar products, the information's reliability should be questioned and verified.

I hope this brief introduction to selecting a plastic material for medical devices has been somewhat enlightening and informative. The selection process is critical and complicated. It is further challenged by the growing number of new resins being introduced each year, more stringent regulatory requirements, and more demanding product requirements. I’m educated continuously and challenged with new information every day, despite my education in plastics and 40+ years of experience designing medical devices. The only way to avoid significant problems is always to remain vigilant and never assume anything. Never stop learning.

 

About the author

Michael Paloian is President of Integrated Design Systems Inc. (IDS), located in Oyster Bay, New York. He has an undergraduate degree in plastics engineering from UMass Lowell and a master's of industrial design from Rhode Island School of Design. Paloian has an in-depth knowledge of designing parts in numerous processes and materials, including plastics, metals, and composites. Paloian holds more than 40 patents and was past chair of SPE RMD and PD3. He frequently speaks at SPE, SPI, ARM, MD&M, and IDSA conferences. He has also written hundreds of design-related articles for many publications. He can be reached by phone at 516/482-2181 or via e-mail, paloian@idsys.com.

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