The materials available for injection molding include plastics and metals as well as hybrid materials and additives and fillers. The range of materials can be overwhelming, so it is often best to work with your injection molding provider.
Begin by defining the core characteristics needed for a part.
Plastics consist of groups of polymer chains. And each chain composition is different between different types of plastics, resulting in a range of properties available among this type of material. Be aware that while two plastics can look the same, they can deliver major differences in performance.
Plastics fall into two categories: Thermoset and thermoplastic.
Thermoset basically means that once the plastic has gone through a chemical reaction, it is not reversible. Thus, these plastics cannot be reground and repurposed. The chemical reaction creates a strong bond among the polymer chains. The benefits of thermoset plastics are their high strength and suitability for high temperature applications. Thermoset plastics include epoxy, phenolic, polyurethane, and silicone.
Thermoplastic means that the chemical change that this material undergoes is reversible. Thus, these materials can be repurposed.
This material comes in two varieties—semi-crystalline and amorphous. One advantage of this type of plastic is that heat softens the pellets and makes it easy to use in injection molding.
Semi-crystalline materials are a hybrid of amorphous and crystalline structure, so they have random sections of crystalline structures. Suitable products made from this plastic include bearing surfaces and living hinges. They offer good chemical resistance, but they can shrink and warp more than amorphous plastics. Semi-crystalline plastics include acetal, HDPE, LDPE, nylon, PBT, PET, and polypropylene.
The polymer chains in amorphous plastics are random and entangled. This chain arrangement delivers strength so they are often well suited for structural applications. However, they can be susceptible to stress fractions. Amorphous plastics include ABS, acrylic, polycarbonate, and PVC.
Many of these materials can be altered for specific characteristics through the use of additives and fillers. Additives can alter the color, for example. Or, they can protect a part from UV radiation, or improve lubrication, deal with static, or deliver antimicrobial properties.
Engineered materials often consist of fillers, such as carbon fiber, and made for specific applications. One example is to add fiberglass to polycarbonate to enhance strength.