Plastic Injection Molding Material Guide

ABS (Acrylonitrile Butadiene Styrene) is the most reliable starting point for general-purpose injection-molded parts. It offers a strong balance of impact resistance, dimensional stability, good surface finish, and ease of processing. It's reasonably priced and forgiving of standard design rules. If ABS doesn't meet your temperature or chemical resistance needs, PC or PC/ABS are the logical next step.

Amorphous plastics (ABS, PC, PMMA, PEI) have a random molecular structure. They offer better dimensional stability, lower shrinkage, and typically better transparency. Semi-crystalline materials (PP, PE, PA, POM, PEEK) have ordered molecular regions that provide higher chemical resistance, better fatigue properties, and sharper melt transitions — but they shrink more and less predictably during cooling. The choice affects mold design, processing parameters, and part performance.

Nylon (PA) is hygroscopic — it absorbs moisture from the atmosphere. If molded wet, the water vaporizes during injection, creating splay marks, voids, and significant degradation of molecular weight and mechanical properties. PA6 and PA66 must be dried at 80°C for 4–8 hours before processing. Sealed packaging ("bone dry" material) extends shelf life but doesn't eliminate the need for drying once opened. The same applies to PBT, PEI, and PC to varying degrees.

Sometimes — but with limitations. Switching to a material with significantly different shrinkage (e.g., from amorphous ABS to semi-crystalline PA66) will cause dimensional shifts that may push parts out of tolerance. Wall thickness designed for one material may be inadequate for another's mechanical requirements. Minor changes (e.g., ABS to PC/ABS, or different grades of PP) are often feasible after a mold steel adjustment. Always consult your molder's DFM team before finalizing the mold design if material flexibility is important.

Many thermoplastics can be compounded into FDA 21 CFR-compliant grades for food contact: PP, HDPE, LDPE, PET, PMMA, and specific grades of POM (Delrin® FDA) and PEI are commonly used. The key is using the correct certified grade — the base polymer alone is not sufficient. BPA-containing polycarbonate is not recommended for food contact. Always obtain FDA compliance documentation from the resin supplier and specify this requirement when requesting quotes from your molder.

High-performance resins like PEEK and PPS require mold steel that can withstand elevated processing temperatures (350–400°C barrel temps), specialized hot runner systems, and higher injection pressures. This increases tooling cost and may extend lead time. Abrasive glass or carbon fiber-filled materials require hardened tool steel (H13, S136) rather than aluminum or P20, significantly increasing mold cost. Commodity resins like PP and ABS can be molded in aluminum tooling for rapid prototyping — reducing both cost and lead time substantially.

All three are thermoplastic elastomers — materials that behave like rubber but process like thermoplastics. TPU (Thermoplastic Polyurethane) offers the best abrasion resistance and mechanical properties across a wide hardness range. TPE (Thermoplastic Elastomer) is a broader category encompassing styrenic block copolymers (TPE-S/SBS, SEBS) — generally lower cost and easier to process. TPR (Thermoplastic Rubber) is an informal term often used interchangeably with TPE. For demanding applications, specify TPU; for cost-sensitive soft-touch overmolding, TPE-SEBS is typically the preferred choice.