The Tips, Tricks, and Traps of Injection Molding Resin Selection for the Medical Industry

Each injection molding project has its own unique set of challenges, such as choosing a resin that balances high performance with the unique demands of the medical industry: compliance to FDA approval, rigorous inspection, lot traceability and more. Although there is no single “right” approach to selecting a resin for your next injection molding project, here are some tips and tricks that can simplify the process, as well as some traps to avoid.

Tip: Start the medical resin selection process with an open conversation to uncover opportunities for improvement.
Injection molded parts for the medical industry
Both parts buyer and injection molding contract manufacturer should enter this conversation without biases about which specific resin types are most appropriate because the resin that worked well on a previous project may not be appropriate for a part of a different size or shape or that will be run on different equipment. This is particularly true in the medical industry, where resins are subject to extremely complex processes and rigorous standards. However, there is an important exception to this: most medical equipment OEMs will already have research and findings on the best available resins for their applications and products, so they will often approach a contract manufacturer with a resin already chosen and approved by their legal team. Given the cost of conducting environmental studies, when a specific resin has proven successful in use for a similar product, OEMs will typically specify the resin with which they’re already familiar.

In most circumstances, the parts buyer needs to work with the contract manufacturer to identify the most critical characteristics of the finished part as soon as possible. Understanding the role the part will play in the end-product is crucial. The contract manufacturer needs to ask if the part will come in contact with food or beverages, biological samples or pharmaceuticals, harsh chemicals or petroleum products. Consider any chemicals to which the part will be exposed during use and cleaning. Take into account the thermal stresses on the part under both typical and extreme conditions.

Consider whether it needs to have inherent flame resistance or must be resistant to UV and gamma radiation. Is it available in opaque and transparent colors, particularly white and clear, common in the industry?

Part buyers must resist any temptation to withhold details about how the finished part will be used or the qualifying tests it must pass. If necessary, a parts buyer can ask the contract manufacturer to sign a non-disclosure agreement (NDA) to keep information on the project confidential, but it’s essential to have an open conversation about all the part’s requirements.

Only after this conversation takes place can the contract manufacturer start considering resin options with respect to various part performance criteria, cost, manufacturability, time to market, etc. For more guidance on what this initial conversation should cover, download a free copy of Designing Injection Molded Parts: A Handbook for Designers & Engineers.

Resin selection process for injection molded parts
Tip: Encourage parts buyers to qualify an alternative or backup resin at the same time as the primary resin is chosen.

Recent disruptions in the resin supply chain have made it obvious that it’s best to have a backup plan in place in order to manage materials risk appropriately. Any number of situations can create resin shortages, such as a producer’s difficulty in obtaining raw materials, labor disruptions, sudden price increases, natural disasters, computer or equipment malfunctions, a facility change, a forced shutdown or other governmental action, and many others.

Molding contractors should strive to optimize the primary resin chosen for whatever balance of properties is most important to the parts buyer. The alternative or backup resin might involve some level of compromise, such as a higher price, a longer lead time, or some negotiation on one or more of the desired performance characteristics.
Identifying an alternate resin eliminates the scramble to identify and source a new one if there’s a shortage of the primary choice, but most important, it presents the risk of making a less- than-optimal choice under the pressure
of time.

Tip: Start the resin selection process as early as possible in the product development cycle. The longer one waits to find the right resin, the higher the likelihood of disappointment with some element of the final part, such as part performance, resin price, price point of the part, manufacturability, etc.

Tip: Take the time to educate yourself about the characteristics of the different types and grades of thermoplastic resins available. Thermoplastic resins come in two basic classes: engineering grade resins and commodity resins. Each category includes a variety of resins, which provides flexibility to match a resin to a part based upon application, part design and moldability.

Engineering Grade Resins
• Nylon: Tough, superior thermal and chemical resistance, plus a wide variety of grades available with broad scope of physical properties.Polycarbonate: Strong, high flex modulus, good temperature range, transparent for lenses and multiple colors. Sometimes blends with polyester (PCPBT) to resist harsh cleaning agents.

• ABS: Good impact strength, superior surface quality, good colorability, good rigidity, with electroplatable grades available.

• PC/ABS: Good processability, toughness at low temperature, superior dimensional stability.

• TPE: “Soft” touch, rubber replacement materials, good tear strength and good flexibility.

• Acrylic: Transparent, good for some outdoor applications.

• Acetal: Excellent wear resistance, great for gears and high wear applications.

• Structural Foam: Excellent material to be used in where metal replacement is considered. Good weight-to-stiffness ratio.

Commodities Grades
• Polypropylene: Versatile material, variety of grades in homopolymer and copolymer classes, good fatigue resistance, excellent chemical resistance, lower cost.

• Polyethylene: Very versatile, low-cost material, variety of grades in linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE) and high-density polyethylene (HDPE), tough, weatherable, and easily processed.

• Polystyrene (PS): Available in general purpose and high impact (HIP) polystyrene, lower cost, range of impact from low to high, good clarity in general purpose (GPPS) grades, good rigidity.

• Polyvinyl chloride (PVC): Low cost, chemical resistant, can be colored, weatherable, and naturally flame retardant.

• Polyether etherketone (PEEK) Mechanical, chemical resistance, and biocompatibility. X-ray compatible, non-metal for CT/MRI scans.

• Ultem: Chemical, heat, flame resistance. UV/Gamma radiation resistance. FDA compliant.

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