Choosing the Right Plastic for Injection Molding

How do you identify the right resin for your plastic injection molded part? Beyond a certain plastic family, what grade, brand and characteristics do you need or want? On the other hand, if you already have selected a specific material, are there any alternative or equivalent resins available? If so, what are the differences and similarities, the pros and cons, the costs and benefits?

The base materials used for plastic injection molding generally are referred to as thermoplastic polymers. There are multiple broad families of plastic (e.g., ABS, Polypropylene, Nylon), and various grades with differing qualities within each family. Also, there are a myriad of brands of the same material family produced by different plastic manufacturers. Further, numerous additives exist that can be included with the base material in order to add or enhance a certain quality. Common examples include adding a UV additive to protect against harm caused by sunlight and using glass fiber to act as a reinforcing agent.

One of the first steps in choosing the right plastic for your part or component is to determine all of the applicable requirements. Legally speaking, any relevant statutory or regulatory requirements need to be identified. In the U.S., these can include federal and state laws, agency regulations, industry standards, and others.

Next, as a practical matter, what characteristics does the resin need to have? Some of the most frequently referenced properties of plastic used for injection molding include heat resistance, chemical resistance, impact resistance, weather resistance, UV resistance, moisture resistance, flame retardant, toughness, hardness, flexibility, stability, elasticity, strength, weight, mass density (or specific gravity), and clarity.

Once materials meeting the requirements are found, research can be done to identify any possible alternative or equivalent resins. On UL’s Prospector materials database (one of the most widely used, but subscription based), an alternative materials search will show numerous similar resins ranked on the correlation between the specified grade and the alternates regarding many different characteristics, including those mentioned previously and others relating to moldability. Moreover, if one digs into the details a little deeper, it may be seen that a possible substitute is even closer to the original material than reported, although Prospector did not have data for a certain material quality.

Armed with some potential candidates, the material suppliers (manufacturers and/or distributors) should be contacted regarding pricing, availability, lead times, and for their recommendations regarding any other possible substitutes you may not have uncovered. After receiving and analyzing all of that information, the next step would be to obtain some material from the supplier and then to use it to produce some sample parts. Different grades of material can be purchased and used to make samples in order to compare and contrast the properties of each resin. However, trying to use different material family types with the same mold can be problematic, due to the dissimilar rate of shrinkage between plastics, among other factors.

Generally speaking, the preferred way to determine the right plastic for any injection molded part, component or product is to have everyone associated with the program collaborate on the matter. At a minimum, supply chain personnel, engineers, mold builders, and injection molders all should be included. Each will bring his or her own unique perspective and, hopefully, add value to the discussion. Taking a joint, inclusive, multidisciplinary approach – where the different departments within a company and the customer and supplier all work together – should result in the identification and utilization of the right material for the job.


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