Material Selection for Injection Molding: PA, PP, ABS, POM, TPE Compared

Choosing the right plastic is one of the earliest and most consequential decisions in an injection molding project. It determines not only the mechanical properties of the part, but also tooling costs, per-unit price and processing parameters. Changing material after tooling is built is expensive — in the worst case, a new mold is required.

This article compares the five most important injection molding plastics by the criteria that matter in practice: strength, temperature resistance, shrinkage, cost and typical applications.

The Materials at a Glance

For enclosures and mechanical parts in hardware development, five plastic families dominate. Each has specific strengths and limitations that directly affect design decisions.

PA (Polyamide / Nylon) – The All-Rounder for Mechanical Parts

• Tensile strength: 70–85 MPa (PA6), 80–90 MPa (PA66)
• Temperature resistance: Continuous up to 80–100 °C, short-term up to 150 °C
• Shrinkage: 1.0–2.0 % (high — account for this in design!)
• Material cost: €3.50–5.00/kg
• Notable: Moisture absorption (2.5–3% for PA6) changes dimensions and mechanical properties

Typical applications: Gears, clips, enclosures with snap-fit closures, cable ties, bearing bushings. PA is the first choice when impact strength and wear resistance matter.

Practical tip: PA6 absorbs moisture, PA66 less so. For parts with tight tolerances, either use PA66 or factor moisture swelling into the design. Glass-fiber-reinforced PA (PA-GF30) reduces shrinkage to 0.3–0.7% and significantly increases stiffness.

PP (Polypropylene) – The Cost-Effective Standard

• Tensile strength: 30–40 MPa
• Temperature resistance: Continuous up to 100 °C, short-term up to 140 °C
• Shrinkage: 1.0–2.5 % (high, direction-dependent)
• Material cost: €1.50–2.50/kg (cheapest standard material)
• Notable: Living hinge effect at thin web connections (lifetime > 100,000 cycles)

Typical applications: Consumer product housings, packaging, flip-top caps with living hinges, lightweight enclosures without high mechanical demands. PP is the material of choice when per-unit cost is the primary concern.

Practical tip: PP has low surface energy — bonding and printing are difficult. For visible surfaces, plan either plasma pre-treatment or in-mold labeling (IML).

ABS (Acrylonitrile Butadiene Styrene) – The Classic for Enclosures

• Tensile strength: 40–55 MPa
• Temperature resistance: Continuous up to 85 °C
• Shrinkage: 0.4–0.7 % (low — good dimensional accuracy)
• Material cost: €2.50–3.50/kg
• Notable: Excellent surface quality, easily printable and coatable

Typical applications: Electronics enclosures, appliance housings, prototypes (also in 3D printing), visible parts with high surface finish requirements. ABS is the industry standard for enclosures that need to look good.

Practical tip: PC/ABS blends combine ABS surface quality with polycarbonate impact strength. For enclosures that must survive drops (e.g. power tools), PC/ABS is often the better choice.

POM (Polyoxymethylene / Delrin) – The Precision Material

• Tensile strength: 60–70 MPa
• Temperature resistance: Continuous up to 90–110 °C
• Shrinkage: 1.8–2.5 % (high, but very uniform)
• Material cost: €4.00–6.00/kg
• Notable: Low friction coefficient (0.1–0.3), excellent sliding properties, very good dimensional accuracy after run-in phase

Typical applications: Gears, sliding bearings, precision parts, springs, parts with friction contact. POM is the first choice for moving parts that must run without lubrication.

Practical tip:POM cannot be easily bonded — use clips, screws or ultrasonic welding for connections. Formaldehyde depolymerization at processing temperatures (> 210 °C) requires good ventilation in the mold.

TPE (Thermoplastic Elastomers) – The Flexible Partner

• Tensile strength: 5–25 MPa (depending on hardness)
• Temperature resistance: Continuous up to 80–100 °C
• Shrinkage: 1.5–3.0 % (high, strongly dependent on wall thickness and hardness)
• Material cost: €4.00–8.00/kg
• Notable: Shore A hardness from 20–90, bondable to rigid substrates (2-shot injection molding)

Typical applications: Seals, grips, soft-touch surfaces, cable glands, vibration dampers. TPE is almost always used as a second shot onto a rigid material (PP, ABS, PA).

Practical tip: Not every TPE bonds to every rigid material. Adhesion depends on chemical compatibility: TPE on PP works well, TPE on PA only partially. Always perform an adhesion test before tooling is manufactured.

Head-to-Head: Which Material for What?

Material selection depends on three factors: mechanical requirements, environmental conditions and budget. Here is a decision guide:

• Enclosure with high surface quality: ABS or PC/ABS
• Mechanical parts with wear: PA (GF-reinforced) or POM
• Cost-sensitive, moderate requirements: PP
• Moving parts without lubrication: POM
• Seals, grips, soft-touch: TPE (as 2K component)
• Humid environment or outdoor: PA66 or PP (both more moisture-resistant than PA6)

Shrinkage: The Underestimated Cost Factor

Shrinkage determines whether the part has correct dimensions after demolding. PA6 with 1.5–2% shrinkage means: A 100 mm part becomes 98–98.5 mm. The toolmaker must mill the mold larger — the shrinkage rate is calculated as the "mold dimension."

Problems arise with uneven shrinkage. PP and PA shrink differently along flow direction versus perpendicular to it. This causes warpage — the part bends. Glass-fiber-reinforced materials worsen the effect because fibers align in flow direction.

Practical tip: For warpage-critical parts, run a mold flow simulation (Moldflow) before tooling. Cost: €500–2,000. This saves expensive mold corrections (€2,000–10,000).

Material Cost Comparison

Material cost is only part of the total calculation. A more expensive material can be cheaper if it increases tool life or eliminates secondary operations:

• PP: €1.50–2.50/kg — Cheapest material, but low strength
• ABS: €2.50–3.50/kg — Good value for enclosures
• PA: €3.50–5.00/kg — Higher cost, but mechanically superior
• POM: €4.00–6.00/kg — More expensive, but no post-processing needed for sliding surfaces
• TPE: €4.00–8.00/kg — Varies widely by type and Shore hardness

Note: Prices refer to standard grades. Specialty additives (UV stabilizers, flame retardants, color masterbatches) add €0.50–2.00/kg.

Tool Wear: Which Material Needs Which Mold

Not every material stresses the mold equally:

• PP, ABS: Standard mold steel (1.2343/1.2344), tool life 500,000–1,000,000 shots
• PA, PA-GF: Wear-resistant steel (1.2379) for glass-fiber-reinforced variants. Glass fibers act like abrasive — without hardened steel, gates and cavities erode after 100,000–200,000 shots
• POM: Standard steel suffices, but formaldehyde corrodes uncoated surfaces. Chrome-plated or nickel-coated cavities extend tool life
• TPE: No significant wear, but low viscosity requires precise parting line to avoid flash

When to Make the Material Decision

Before tooling is built. Changing material after tooling is possible but expensive:

• Same process, similar shrinkage: Often possible without tool modification (e.g. ABS → PC/ABS)
• Different shrinkage rate: Cavities must be reworked (€2,000–10,000)
• Higher processing temperature: May affect cooling channels and heaters (€5,000–15,000)

Recommendation: Make the material decision together with the CAD design. Wall thicknesses, draft angles and ribs are designed specifically for the chosen material. A DFM audit before tooling verifies that the material matches the design.

Anton Steenken

B.Eng. · Hardware R&D Engineer · Founder of engineer your idea