MATERIAL SELECTION GUIDE
Injection Molding Material Selection Guide: How to Choose the Right Plastic Resin (and Avoid Overspecifying)
Choosing the wrong plastic for an injection molded part costs money in two directions at once. Pick a resin that’s too weak, and you get field failures, returns, and a redesign. Pick one that’s too strong — and this is the more common and more expensive mistake — and you pay a premium on every part, often for performance the application never needed.
After 20 years specifying materials for molded parts shipped to buyers across the US, Europe, and Australia, I can tell you most cost overruns on plastic parts don’t come from tooling or labor. They come from a material decision made on instinct or copied from an old part, never revisited against what the application actually requires.
This guide walks through how we evaluate resin selection at Topworks: the requirements that drive the decision, a head-to-head look at the plastics buyers ask us about most, where overspecifying quietly inflates part cost, and how your material choice ripples directly into mold design and tooling cost — something most material guides ignore entirely.
Start With Requirements, Not With a Resin
The fastest way to overspend is to start the conversation with a material name. “We want it in polycarbonate” or “make it nylon” almost always traces back to a legacy part or a competitor’s spec, not a fresh look at the requirements.
Before naming any resin, we work through six factors. Get these clear and the material shortlist usually narrows itself to two or three candidates.
- Mechanical load. Is the part structural or cosmetic? Does it carry continuous load, take impact, flex repeatedly, or thread together with other parts? A snap-fit clip, a load-bearing bracket, and a decorative housing have completely different requirements even at the same wall thickness.
- Operating temperature. What’s the highest temperature the part sees in service — including transport, storage, and any nearby heat source? The number that matters here is heat deflection temperature (HDT), not the melt temperature. A part rated comfortably at room temperature can soften and creep at 80°C.
- Chemical and environmental exposure. Contact with oils, solvents, cleaning agents, fuels, or moisture changes the shortlist immediately. So does outdoor use — UV exposure degrades many resins unless they’re stabilized. Nylon, for example, absorbs moisture and changes dimension and stiffness as it does.
- Regulatory and compliance needs. Food contact (FDA), drinking water (NSF), medical (USP Class VI), or flame rating (UL94) requirements eliminate large swaths of the material catalog and often dictate a specific grade, not just a family.
- Aesthetics and finish. Transparency, surface gloss, color matching, and the ability to hold a textured or polished finish all influence both the resin and the mold. A part that needs optical clarity has very different options than one that will be painted.
- Volume and target cost. Annual volume and your per-part cost ceiling determine how much material premium the program can absorb. A high-volume consumer part lives and dies on resin cost per kilogram; a low-volume industrial part can justify an engineering-grade resin without moving the program economics much.
When you send these six answers with an RFQ instead of a material name, we can often recommend a resin that meets every requirement for less than the one originally requested.
The Plastics Buyers Actually Ask About
Below are the resins that cover the large majority of the parts we quote. They split into two groups: commodity plastics, which are inexpensive and cover everyday parts, and engineering plastics, which cost more but deliver mechanical, thermal, or chemical performance commodity grades can’t reach.
Commodity Plastics
Polypropylene (PP) — The workhorse. Low cost, excellent chemical resistance, and a natural living-hinge capability that makes it the default for flip-top caps and integrated-hinge parts. It’s lightweight and fatigue-resistant. The trade-offs: low stiffness, poor UV resistance unstabilized, and high mold shrinkage (typically 1.0–2.5%), which demands careful tooling.
Polyethylene (PE, HDPE/LDPE) — Even cheaper than PP, with great chemical resistance and toughness, but low stiffness and high shrinkage. Common for containers, caps, and parts where flexibility and cost matter more than rigidity or appearance.
ABS — The default for rigid cosmetic housings. Good impact strength, easy to mold to a quality finish, paints and plates well, and dimensionally stable thanks to low shrinkage (around 0.4–0.7%). It is not suitable for outdoor use without stabilization and has only moderate heat resistance.
PMMA (Acrylic) — Chosen for optical clarity and a hard, glossy surface, with good outdoor weatherability. The downside is brittleness; it cracks under impact where polycarbonate would survive.
Engineering Plastics
Polycarbonate (PC) — Outstanding impact strength combined with transparency and a relatively high HDT (~130°C). The go-to for protective transparent parts, lenses, and rugged enclosures. It costs significantly more than ABS and is prone to stress cracking with certain chemicals.
PC/ABS Blend — Balances PC’s impact and heat resistance with ABS’s lower cost and easier processing. A frequent middle ground for housings that need more durability than ABS alone but don’t justify full PC.
Nylon (PA6, PA66) — Strong, tough, wear- and chemical-resistant, with a high HDT, especially when glass-filled. Excellent for mechanical parts like gears, bearings, and structural components. Its defining quirk is moisture absorption: nylon picks up water from the air, which shifts both dimensions and properties, so it must be designed and processed with that in mind. PA66 handles higher temperatures than PA6.
POM (Acetal/Delrin) — The choice for precision mechanical parts: low friction, excellent fatigue and wear resistance, and good dimensional stability in service. Ideal for gears, bushings, clips, and snap-fits. It has high mold shrinkage (around 1.8–2.5%) and can be harder to bond and paint.
Quick Comparison
| Resin | Relative cost | Stiffness | Impact | Max use temp | Mold shrinkage | Typical use |
|---|---|---|---|---|---|---|
| PP | Low | Low | Good | Moderate | High (1.0–2.5%) | Caps, hinges, containers |
| PE | Low | Low | High | Low–Mod | High | Containers, bottles |
| ABS | Low–Mid | Moderate | Good | Moderate | Low (0.4–0.7%) | Cosmetic housings |
| PMMA | Mid | Moderate | Poor (brittle) | Moderate | Low | Clear, glossy parts |
| PC | Mid–High | High | Excellent | High (~130°C) | Low (0.5–0.7%) | Lenses, rugged clear parts |
| PC/ABS | Mid | High | Very good | High | Low | Durable housings |
| Nylon (PA) | Mid–High | High* | Good | High | Mod (0.7–1.5%) | Gears, structural parts |
| POM | Mid | High | Moderate | High | High (1.8–2.5%) | Precision mechanical parts |
*Stiffness and temperature performance rise substantially with glass fill.
Where Buyers Overspecify — and What It Costs
This is the section that saves real money. Overspecifying isn’t always obvious because the part still works; you just paid more than you had to. A few patterns we see again and again:
Specifying polycarbonate when ABS or PC/ABS would pass. PC commands a meaningful premium over ABS. Buyers reach for it out of caution, but if the part isn’t transparent and doesn’t take real impact or sustained heat, ABS or a PC/ABS blend often meets every requirement at lower cost. Reserve full PC for the parts that genuinely need its clarity or toughness.
Defaulting to glass-filled nylon for parts under light load. Glass fill dramatically increases stiffness and strength — and it also increases material cost, abrasion on the mold, and the risk of warpage and visible fiber on the surface. If the part sees modest loads, unfilled nylon, POM, or even a reinforced PP may carry it. Don’t pay for 30% glass fill the part will never use.
Using an engineering resin where a commodity grade fits. A bracket that lives indoors at room temperature under light load may not need nylon at all — a talc-filled or impact-modified PP can do the job for a fraction of the resin cost.
Over-rating temperature. Buyers frequently spec for a worst-case temperature the part never actually sees. Pinning down the real peak operating temperature — not a conservative guess — frequently opens up cheaper resin options.
The fix is straightforward: spec to the requirement, not to the safety blanket. Send us the real loads, temperatures, and environment, and we’ll tell you honestly where you can step down a grade without risking the part.
How Material Choice Drives Mold Design and Tooling Cost
Here’s what most material guides leave out: the resin you choose changes the mold itself, and therefore your tooling cost and cycle time. Material and tooling are not separate decisions.
Shrinkage sets the cavity dimensions. Every resin shrinks as it cools, and the mold cavity must be cut oversized to compensate. High-shrinkage semi-crystalline resins like PP, PE, and POM (often 1.5–2.5%) shrink far more than low-shrinkage amorphous resins like ABS and PC (around 0.5%). Higher and less predictable shrinkage means tighter tolerances are harder to hold and the tool may need adjustment after first samples — a real cost factor on precision parts.
Abrasive and corrosive resins demand tougher tool steel. Glass-filled and flame-retardant grades are abrasive and wear out soft tool steel quickly. To protect tool life and your investment, these materials call for hardened steel or surface treatments, which raises the upfront mold cost but prevents premature wear over a long production run.
Flow behavior affects gating and runners. Different resins flow differently. A stiff-flowing or thick-walled part may need larger gates, more gates, or a hot runner system to fill cleanly — each of which influences mold complexity and cost.
Material affects cycle time, which affects per-part cost. Semi-crystalline resins generally need more cooling time than amorphous ones at the same wall thickness. Longer cycle time means fewer parts per hour and a higher molding cost per piece — an ongoing cost that compounds over the life of the program, often outweighing a small difference in resin price.
The practical takeaway: tell your molder the candidate material early, ideally at the quoting stage. A resin chosen in isolation can quietly add cost to the tool and to every cycle. Chosen together with the mold design, it can do the opposite.
A Note on Food-Grade and Medical-Grade Parts
If your part contacts food, drinking water, or the human body, compliance overrides everything above. You’ll need a specific certified grade — FDA-compliant for food contact, NSF for potable water, or USP Class VI / ISO 10993 for medical — not just a generic resin from the same family. Documentation, traceability, and resin lot control matter as much as the polymer itself. Flag these requirements at the very start of the conversation, because they shape both material and process.
Common Mistakes to Avoid
- Naming a resin before defining the requirements
- Copying material spec from a legacy part without revisiting whether it still fits
- Guessing a conservative worst-case temperature instead of measuring the real one
- Ignoring moisture absorption when specifying nylon
- Choosing the resin and designing the mold as two separate, disconnected decisions
- Specifying glass fill the part’s loads don’t justify
Frequently Asked Questions
What is the most common plastic used in injection molding?
Polypropylene and ABS are among the most widely used. PP dominates caps, containers, and living-hinge parts thanks to its low cost and chemical resistance, while ABS is the default for rigid cosmetic housings because it molds to a clean finish and is dimensionally stable.
How do I choose between ABS and polycarbonate?
Choose ABS for rigid cosmetic parts that don’t need transparency, high heat resistance, or extreme impact strength — it costs less and molds easily. Choose polycarbonate when the part must be transparent, take significant impact, or withstand higher temperatures. A PC/ABS blend often serves as a cost-effective middle ground.
Does the plastic material affect mold cost?
Yes, significantly. Material shrinkage influences how the cavity is cut and how tightly tolerances can be held; abrasive grades like glass-filled resins require hardened tool steel that raises mold cost; and the resin’s cooling behavior affects cycle time and therefore per-part cost over the production run.
What is overspecifying and why does it cost me money?
Overspecifying means selecting a resin with more performance than the application requires — for example, full polycarbonate where ABS would pass, or glass-filled nylon for a lightly loaded part. The part still works, but you pay a material premium on every unit, sometimes for the entire life of the program.
How does moisture affect nylon parts?
Nylon absorbs moisture from the air, which changes both its dimensions and its mechanical properties. Parts must be designed and processed with this behavior accounted for, and tight-tolerance nylon parts require particular care in both molding and conditioning.
Get a Material Recommendation With Your Quote
The best material decision is made with your molder, not in isolation. Send us your part design along with the real operating conditions — mechanical load, peak temperature, chemical and environmental exposure, any compliance requirements, and your target volume — and we’ll recommend the resin that meets every requirement at the lowest sensible cost, along with how it affects your tooling.
At Topworks, material selection is part of the DFM review we run before quoting, not an afterthought. If you’re at the sourcing stage and unsure which resin fits, that’s exactly the conversation we’re here to have.
