As a custom plastics molder and contract manufacturer serving the medical, industrial, transportation, energy/environment, computer/business, and consumer markets, Mack Molding’s customers are varied, and so are their needs. To meet these diverse requirements, Mack has vertically integrated services including designing, prototyping, supply chain management, machining, sheet metal fabrication, molding, painting, assembly, testing, and fulfillment. With this diversity of services, it is interesting to note some of the most significant variation in customer requirements comes from the resin selection process.

Material properties, processability, and cost all play key roles determining a resin’s suitability for an application. With thousands of grades to choose from, and new ones being developed to fill market niches, customers often turn to a contract manufacturer to help navigate the resin selection process.

Here are some considerations when choosing a resin.

Mechanical requirements – Parts that have specific strength requirements need to be considered when choosing a resin as different classes have various mechanical properties, including tensile strength, elastic modulus, and heat deflection. Thermoplastic resins offer a variety of strength properties that can often be modified with fillers such as glass or carbon fiber.

Chemical compatibility – Many customers are concerned about how chemicals, including cleaning solvents and process reagents, interact with their resin choice. In these cases, we consult published testing data from resin manufacturers that show a material’s performance with each chemical. At times the data may not exist and testing with specific chemicals will be requested.

Environmental compatibility – Parts that will be exposed to extreme hot or cold conditions need to be made with resins rated accordingly, making the long-term service temperature and heat deflection temperature critical performance metrics. Other conditions to consider include exposure to UV light and high humidity.

Commodity vs. performance – When possible, customer applications are paired with commodity resin grades due to the inherent cost and availability advantages of higher volume raw material production. Some projects call for specific properties – strength, heat resistance – which is where performance resins excel. Though its higher cost makes it most appropriate for low-volume, niche applications.

Amorphous vs. semi-crystalline – Amorphous resins can offer wider processing windows and improved dimensional control due to their random molecular structure. They can be transparent and are compatible with many adhesives. Semi-crystalline materials offer improved mechanical and thermal properties but can be more difficult to process.

Material shrinkage – The amount a resin shrinks during the molding process can have a huge impact on the ease of building a tool or developing a successful molding process. Try to select resins that have lower shrink rates whenever possible.

Once these factors are accounted for the list of suitable resins is typically cut down to a manageable number for consideration. The Mack team uses its knowledge of materials, coupled with supply chain management, to help make the final determination of a grade.

Mack Molding

About the author: Scott Rishell is the technical lead on several programs at Mack, developing part designs for production processes. Prior to his current role, Rishell was a program manager in Mack’s medical business, where he gained unique customer experience insights. He has a bachelor of science in mechanical engineering from the Rochester Institute of Technology. Rishell can be reached at