Plastic Material Selection for Injection Molding: Engineering Guide to ABS, PC, PA, POM & PEEK

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Choosing the correct plastic material for an injection-molded part is one of the most consequential engineering decisions in product development. Material selection determines not only how the part performs in service, but also how the mold must be designed, what steel grade is required, and what processing parameters will be used. This guide covers the key material properties, a comparison of the most common engineering plastics, and how material choice feeds back into mold design.

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1. The 5 Most Important Material Properties

When evaluating plastics for injection molding, five properties dominate the decision:

• Tensile strength & impact resistance — Determines structural load-bearing capacity. PA66-GF30 offers 180 MPa tensile strength vs PP homopolymer at 35 MPa.
• Heat deflection temperature (HDT) — The temperature at which the material deforms under load. Critical for automotive under-hood and electronics applications.
• Mold shrinkage rate — The percentage by which the part shrinks as it cools. Ranges from 0.5% (PC) to 2.5% (PP). Cavity dimensions are calculated as: Cavity size = Part size × (1 + shrinkage rate).
• Chemical resistance — Resistance to solvents, fuels, cleaning agents, and UV exposure determines material suitability for outdoor or industrial environments.
• Flow rate (MFI) — Melt Flow Index determines how easily the material fills the mold. Low MFI materials require higher injection pressure and may not fill thin-wall features.

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2. Common Engineering Plastics Compared

Material	Shrinkage (%)	HDT (°C)	Tensile Strength (MPa)	Key Advantage	Typical Application
PP (Polypropylene)	1.5–2.0	90–110	30–40	Low cost, chemical resistance, fatigue life	Consumer products, packaging, automotive trim
ABS	0.4–0.7	80–100	40–55	Good surface finish, easy processing	Electronics housings, toys, appliances
PC (Polycarbonate)	0.5–0.7	120–135	55–65	Optical clarity, high impact strength	Lenses, covers, safety equipment
POM (Acetal/Delrin)	1.8–2.5	100–110	65–70	Low friction, dimensional stability	Gears, bearings, precision mechanisms
PA66 (Nylon 66)	1.0–2.0	200–250 (GF)	80–180 (GF30)	Heat resistance, strength	Automotive under-hood, connectors
PEEK	0.5–1.0	160–170 (neat)	100–200 (GF30)	Chemical resistance, high temp	Medical, aerospace, semiconductor

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3. How Material Choice Affects Mold Design

• Shrinkage compensation — High-shrinkage materials (PP, PE, POM) require cavity oversizing of 1.5–2.5%. Errors in shrinkage allowance cause parts that are consistently over or under nominal dimension.
• Mold temperature — Crystalline polymers (PA, POM, PPS) require elevated mold temperatures (60–160°C) to achieve proper crystallization and dimensional stability. This impacts cooling time and energy consumption.
• Venting requirements — Materials with high gas content (ABS, PC) or low flash threshold (PA) require precise vent sizing to prevent burn marks without causing flash.
• Gate design — High-viscosity materials (PC, PEEK) require larger gate dimensions and shorter flow paths. Low-viscosity materials (PA, PP) can fill thin walls and long flow lengths but require tighter vent gaps.
• Steel grade — Abrasive materials (GF-filled grades, PPS) accelerate cavity wear and require H13 or harder steel. Corrosive materials (PVC, fluoropolymers) require S136 stainless steel.

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4. Filled & Reinforced Grades

Adding fillers and reinforcements to base resins dramatically changes both part performance and processability. Common additives include:

• Glass fiber (GF10–GF50) — Increases stiffness and reduces warpage. 30% GF in PA66 triples tensile strength but requires harder mold steel and increases anisotropic shrinkage.
• Carbon fiber (CF) — Higher stiffness-to-weight ratio than GF. Highly abrasive — carbide or H13 steel cavities with PVD coating are recommended.
• Talc & mineral fillers — Improve HDT and reduce warpage in PP. Less abrasive than fiber reinforcement. Common in automotive door panels.
• PTFE / MoS2 (dry lubricants) — Reduce coefficient of friction in POM and PA grades for self-lubricating bearing applications. No additional lubrication required.
• Flame retardants (FR) — Required for UL 94 V-0 rated parts. Halogenated FR grades require corrosion-resistant mold steel; halogen-free FR grades are preferred for RoHS compliance.

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5. Material Selection Checklist

• Define the operating environment: temperature range, chemical exposure, UV exposure, mechanical loads
• Specify regulatory requirements: FDA food contact, USP Class VI medical, RoHS, UL94 flammability
• Determine part wall thickness — this constrains MFI requirements and gate sizing
• Calculate estimated production volume — this affects whether material cost per kg is a dominant factor
• Consult with your mold manufacturer to confirm shrinkage allowance and mold temperature requirements before finalizing material

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Material selection is most effective when done in close coordination with the mold designer. At BuildMold, our DFM analysis includes material-specific shrinkage calculations, gate sizing recommendations, and steel grade selection based on the chosen resin — ensuring the mold is optimized for the material before a single line of CNC code is written.