How to Select the Right Thermoplastic for Your Automotive Injection Molded Part

A Tier 1 supplier in Germany sent us a part drawing for an engine cover. The drawing specified PA66 GF30 — a common choice for under-hood components. But when we reviewed the full application context — continuous service temperature, oil exposure pattern, and the customer's actual tolerance requirements — it became clear that PA6 GF30 would meet every functional requirement at roughly 18% lower material cost, with better surface finish and fewer warpage headaches. The customer approved the change. The tool ran successfully.

This scenario repeats constantly in automotive mold procurement. The material callout on a drawing is often a starting point — sometimes the result of a legacy specification carried forward from a previous program, sometimes an overly conservative choice made early in development before the full production economics were understood. Understanding what each thermoplastic actually delivers — and what it costs in both material price and mold complexity — is one of the highest-leverage skills an automotive buyer or engineer can bring to a mold program.

The seven workhorse thermoplastics of automotive injection molding

Each of these materials occupies a specific performance-and-cost position. Understanding the trade-offs between them — rather than treating any one as a default — is what separates an optimized program from an over-engineered one.

How to choose: the five-factor framework we use with every customer

At Gege Mould, we evaluate material selection against five criteria before tool design begins. The weighting of each factor shifts depending on the application — a Class A exterior panel has very different priorities from an under-hood structural bracket — but every program benefits from making the trade-offs explicit.

Glass fiber reinforcement: when it helps, when it hurts

Adding glass fiber (typically 20%, 30%, or 50% by weight) to PP, PA6, or PA66 transforms the mechanical properties — and introduces a new set of challenges that mold designers must account for:

Application mapping: which material goes where

While every part warrants its own analysis, broad patterns have emerged from decades of automotive production that serve as useful starting points:

Under-hood / engine bay

• Engine covers, cam covers: PA6 GF30 (cost-optimized) or PA66 GF30 (higher heat). Both deliver the stiffness, oil resistance, and surface finish required. PA6 is often sufficient — validate peak under-hood temperature before defaulting to PA66.
• Air intake manifolds, charge-air ducts: PA6 GF30 or PA66 GF30. Glass-reinforced for burst pressure resistance. PA6 preferred where welding (vibration or hot-plate) is required — better weld strength than PA66.
• Oil pans, rocker covers: PA66 GF30 or GF50. Requires sustained oil resistance at 120–140°C. PA66's higher HDT and better creep resistance make it the safer choice here. Some programs succeed with PA6 GF30 where oil temperature is lower.
• Radiator end tanks, coolant pipes: PA66 GF30. Must resist hot water/glycol at pressure over the vehicle lifetime. PA66's hydrolysis resistance above 100°C is significantly better than PA6.

Interior / cabin

• Door panels, instrument panel substrates: PP-TD20 (talc-filled) or PP-GF20. Low cost, good impact, lightweight. The dominant solution for large interior panels.
• Center console frames, glove box housings: ABS or PC/ABS. Good rigidity, paintable, excellent appearance. PC/ABS adds heat resistance for sun-load conditions (instrument panel top surfaces can reach 110°C in parked vehicles).
• HVAC housings, ductwork: PP (unfilled or talc-filled) or ABS. PP preferred for cost; ABS where tighter dimensional control is needed for sealing surfaces.

Exterior / body

• Bumper fascias: PP-EPDM (elastomer-modified polypropylene). Excellent low-temperature impact, good paint adhesion with flame treatment, low cost. This application is dominated by PP and has been for 30 years.
• Grille surrounds, exterior trim: ABS (painted or chrome-plated) or ASA (UV-stabilized ABS variant). Good surface quality and metallization capability.
• Mirror housings, door handles: ABS, PC/ABS, or PA6 GF30 depending on structural requirements and paint strategy.

Electrical / precision

• Connectors, sensor housings: PBT GF30. The standard for automotive electrical connectors — excellent dimensional stability, good electrical insulation, fast cycling. PBT owns this space.
• Small gears, actuators, window regulators: POM. Outstanding wear resistance and dimensional stability in small, precision parts. The go-to for mechanical functional components.

The cost conversation: resin price is not the whole story

A common procurement error is selecting material primarily on resin price per kilogram. The material that is cheapest per kilogram often produces the most expensive molded part — because the mold had to be more complex, the cycle time was longer, the scrap rate was higher, or the tolerance could not be held without secondary operations.

The correct metric is total molded part cost, which includes:

• Resin cost per part: material price/kg × part weight (including runner system for cold-runner tools)
• Cycle time: faster-crystallizing materials (PBT, PP) cycle faster than slow-crystallizing ones (PC, amorphous grades). A 5-second cycle time difference on a 2-million-shot program is worth tens of thousands of dollars.
• Tool complexity: high-shrinkage materials require more complex cooling to hold tolerance. High-viscosity materials require larger gates and runners, increasing regrind or hot-runner cost.
• Scrap and rework: materials with wide processing windows (PP, ABS) produce fewer rejects than materials with narrow windows (some POM grades, high-glass-fill PA).
• Secondary operations: if a material requires painting, plating, welding, or annealing, those costs belong in the material selection equation — not as a separate line item discovered after tool delivery.