Plastic Material Selection for Injection Molding

Overview

Selecting the right plastic material is one of the most consequential decisions in injection-molded product development. Material choice affects mechanical performance, appearance, mold design, production cycle time, cost, and regulatory compliance. This guide outlines key properties, common materials, and practical selection criteria.

This is general engineering guidance based on product-development experience. Always verify material properties with supplier datasheets and conduct testing for your specific application.

Key Material Properties

When evaluating plastic materials, consider:

• Tensile strength: Resistance to pulling forces
• Impact strength: Resistance to sudden impact—important for drop-tested products
• Flexural modulus: Stiffness of the material
• Heat deflection temperature (HDT): Temperature at which the material deforms under load
• Shrinkage rate: How much the part shrinks as it cools—affects mold design and dimensional accuracy
• Wall thickness capability: Minimum and recommended wall thickness for the material
• Chemical resistance: Compatibility with cleaning agents, oils, or other chemicals the product may contact
• UV resistance: Suitability for outdoor or sun-exposed applications
• Flammability rating: Regulatory requirements (e.g., UL94 V-0, V-1, HB)
• Surface finish: How well the material takes texture, gloss, or paint

Common Injection Molding Materials

Several thermoplastics are commonly used in injection molding. The following is a general overview—consult material datasheets for specific grades and properties.

• ABS (Acrylonitrile Butadiene Styrene): Good impact resistance, moderate cost, easy to mold, paintable. Widely used in consumer electronics enclosures.
• PC (Polycarbonate): High impact strength, transparent grades available, higher heat resistance. Used for lenses, durable enclosures.
• PC/ABS blend: Combines impact strength of PC with processability of ABS. Common in laptop and phone housings.
• PP (Polypropylene): Lightweight, good chemical resistance, living hinge capability, low cost. Used in packaging, consumer goods.
• PE (Polyethylene): LDPE and HDPE variants. Good chemical resistance, low cost. Used in containers, caps.
• PA (Nylon, e.g., PA6, PA66): High strength, wear resistance, but absorbs moisture. Used in gears, structural parts, fasteners.
• POM (Acetal/Delrin): Excellent dimensional stability, low friction, good fatigue resistance. Used in gears, clips, mechanical parts.
• PMMA (Acrylic): Transparent, good optical clarity, brittle. Used for lenses, displays.
• PBT: Good electrical properties, heat resistance. Used in connectors, electrical components.
• TPU (Thermoplastic Polyurethane): Flexible, abrasion-resistant. Used for overmolding, grips, protective cases.

Selection Criteria

Material selection should be driven by product requirements, not by what the factory typically uses. Consider:

• Functional requirements: What forces, temperatures, and environments will the part experience?
• Regulatory requirements: Does the product need specific certifications (e.g., FDA food contact, UL94 flammability, RoHS)?
• Appearance requirements: Is transparency needed? What surface finish and color?
• Assembly method: Will the part be snap-fit, ultrasonically welded, or solvent bonded? Material affects each.
• Overmolding: If the part involves overmolding (e.g., TPU on ABS), material compatibility is critical.
• Production volume: Some materials are easier to process consistently at high volume.

Shrinkage and Mold Design

Each material has a characteristic shrinkage rate that affects mold design:

• Shrinkage rates vary by material (e.g., ABS ~0.5-0.7%, PP ~1.5-2.5%, Nylon ~1.5-2.0%)
• The mold must be cut to account for shrinkage so that the finished part meets dimensional requirements
• Shrinkage can be anisotropic (different in flow vs. transverse direction), especially for glass-filled materials
• Changing materials after the mold is cut may require mold modification or a new mold—this is expensive

Implication: Finalize material selection before tooling begins. Changing material after mold completion can require costly rework.

Additives and Modifications

Base resins can be modified with additives to enhance specific properties:

• Glass fiber reinforcement: Increases strength, stiffness, and dimensional stability, but increases abrasion on the mold and may affect appearance
• Flame retardants: Required for certain regulatory standards
• UV stabilizers: For outdoor applications
• Colorants: Masterbatch colorants are common; ensure compatibility with the base resin
• Impact modifiers: Improve toughness for brittle materials

Additives can affect processing parameters, shrinkage, and mechanical properties. Discuss additive packages with both the material supplier and the factory.

Cost Considerations

Material cost is a significant portion of per-unit cost for injection-molded parts:

• Material price per kg varies widely—commodity resins (PP, PE) are generally less expensive than engineering resins (PC, POM, PA)
• Cycle time affects total cost—materials with faster cooling can reduce cycle time and per-unit cost
• Scrap and regrind: Runner and gate scrap can often be reground and reused, but this may affect material properties
• MOQ for material procurement: Some specialty grades have higher minimum order quantities

Use the BOM Cost Calculator to model material cost impact on your total unit economics, including scrap and MOQ overbuy.

Material Availability in China

Most common thermoplastics are readily available from Chinese material suppliers. Considerations:

• Major resin brands (e.g., SABIC, BASF, Covestro, DuPont, local Chinese brands) are generally available
• Some specialty grades may have longer lead times or limited local supply
• Material equivalence: Chinese factories may propose equivalent grades from different manufacturers—evaluate whether the alternative meets your requirements
• Verify material certifications and request material test reports (MTR) for critical applications

Common Selection Mistakes

• Choosing a material based on cost alone without considering performance requirements
• Not verifying regulatory compliance for the target market
• Changing material after mold completion without accounting for shrinkage differences
• Specifying a material grade that is unavailable or has long lead times locally
• Not testing material performance under actual use conditions (temperature cycling, drop testing, chemical exposure)
• Over-specifying—using an engineering-grade material where a commodity grade would suffice

Material selection is an iterative process that should involve engineering analysis, prototyping, and testing. Document the rationale for your selection so that future changes can be evaluated against the original requirements.