The most expensive material decision in a project is usually the one made fastest. A team picks ABS because it is familiar and cheap, the tool is cut, the parts pass inspection, and eight months later the field returns start arriving because the housing cracked at the screw boss.
Material choice is decided early, but it is paid for late. It sets the wall thickness you can use, the shrinkage the tool must be cut to, the temperatures the mold must run at, and — in most cases — whether the part survives its actual environment rather than the one it was tested in.
This guide covers the practical version of the decision: which families of plastic cover most injection molding work, what each one is genuinely good and bad at, and the three material properties that cause the majority of project failures.
Key takeaways
- Start from requirements — load, temperature, chemical exposure, appearance, and regulatory limits — then choose a material. Never the other way around.
- Seven material families cover the large majority of industrial injection molding work.
- Shrinkage, moisture absorption, and fiber orientation cause most of the failures that get blamed on the mold.
- A material substitution that looks equivalent on a data sheet can change shrinkage enough to put your dimensions out of tolerance.
- Lock the material specification — grade and supplier — before the tool is cut, not after the first samples arrive.
Start with requirements, not material names
Before looking at a single data sheet, write down five things:
- Mechanical load. Static or cyclic? Is there a snap fit, a living hinge, or a screw boss carrying torque?
- Temperature. Continuous service temperature, and the peak it will see during assembly or cleaning.
- Chemical and UV exposure. Fuels, oils, cleaning agents, disinfectants, sunlight.
- Appearance and optics. Visible cosmetic surface, texture, transparency, color consistency across batches.
- Regulatory and safety limits. Flammability ratings, food contact, medical device requirements.
These five answers usually narrow the field to two or three candidates before any cost discussion begins — and they are also the answers a molder needs when they build the tool, because several of them change shrinkage, mold temperature, and gate design.
The seven material families you will actually use
| Material | Strengths | Weaknesses | Typical applications |
|---|---|---|---|
| ABS | Easy to process, good surface finish, low cost, good impact at room temperature | Poor chemical and UV resistance; limited outdoors | Housings, cosmetic covers, consumer electronics |
| PC | High impact strength, transparent, good stiffness | Higher cost; must be dried; susceptible to stress cracking with some chemicals | Lenses, transparent guards, structural covers |
| PP | Excellent chemical resistance, living-hinge capable, low cost, low density | High shrinkage; low stiffness without fillers; needs UV stabilizers outdoors | Hinges, containers, automotive interior parts |
| PA66 (with or without glass fiber) | High strength, wear resistance, good high-temperature performance | Absorbs moisture and changes dimensions; must be dried carefully | Structural brackets, gears, under-hood parts |
| POM | Low friction, high stiffness, excellent dimensional stability | Difficult to bond or print on; poor resistance to strong acids | Gears, sliders, precision mechanical parts |
| PMMA | Optical clarity, excellent UV resistance | Brittle under impact | Light covers, display windows, optical parts |
| PEEK | Continuous service above 250 °C, excellent chemical resistance, high strength | Very expensive; requires high melt and mold temperatures and a capable machine | Aerospace, medical, semiconductor, energy |
If you are running a first project with a supplier, one of these seven is almost always the correct answer. The interesting failures happen at the edges — when a part needs two properties that no single material gives you, or when a substitution is made without re-checking the tool.
The three properties that cause most project failures
Shrinkage
Every plastic shrinks as it cools, and every material shrinks by a different amount. The tool is cut to compensate, which means the shrinkage value used at the design stage becomes a dimensional commitment.
| Material | Typical shrinkage range |
|---|---|
| ABS | 0.4–0.7% |
| PC | 0.5–0.7% |
| PP | 1.0–2.5% |
| PA66 | 1.0–1.5% |
| PA66 + 30% glass fiber | 0.3–0.6% (and different in flow vs cross-flow direction) |
| POM | 1.8–2.2% |
| PMMA | 0.3–0.6% |
| PEEK | 1.0–1.4% |
These are typical ranges quoted in supplier data sheets for unfilled grades unless noted. Always confirm the value for your specific grade — the same polymer from two suppliers can be specified differently, and glass-filled grades are direction-dependent. If the wrong value goes into the tool, no amount of process tuning brings the part back into tolerance.
Moisture absorption
Hygroscopic materials pull water out of the air. Polycarbonate, nylon, PET, and ABS in humid conditions all do it. If the resin is not dried to the grade’s specification before molding, the water turns to steam inside the barrel and you get silver streaks, weak weld lines, and unpredictable dimensions. Typical drying conditions sit around 120 °C for three to four hours for polycarbonate and 80–100 °C for about four hours for nylon — but the data sheet wins over any general rule.
Fiber orientation
Adding glass fiber raises strength and stiffness and cuts shrinkage — but only in the direction the fibers align. Fibers align with the flow, so a glass-filled part shrinks less along the flow path than across it. That difference is the most common cause of warpage in structural parts, and it can be predicted from the gate position rather than discovered on the bench.
This is also why so many material problems end up looking like tool problems. If you are chasing a warped or short-shot part right now, the diagnostic order is worth reading in our breakdown of the most common injection molding defects.
When the material drives the whole project
Most materials in the table above run in the same mold, on the same press, with normal mold temperatures. A few do not, and those change the tooling plan rather than just the resin hopper:
- PEEK and other high-temperature resins need mold temperatures far above the norm — often in the range of 170–200 °C — which means cartridge heaters, insulation plates, and a hot oil or oil-temperature controller. Not every mold shop builds tooling for that.
- Corrosive resins, notably PVC and CPVC, attack standard tool steel and require stainless grades to avoid rust in the water lines and on cavity surfaces.
- Abrasive filled grades wear gates and flow paths quickly unless the steel hardness and surface treatment account for it.
- Transparent parts expose every polishing mark and every flow defect, which pushes both the steel choice and the polish specification up a level.
If your material is on this list, raise it before the mold is quoted rather than after. Material capability is one of the things we verify up front on new projects — you can see the range of materials our team regularly molds on our production capabilities page.
“Equivalent” substitutions that are not equivalent
Production teams substitute materials for good reasons: price, availability, a supply interruption. The problem is that “equivalent” is usually judged on the data sheet, where the two grades look close, and not on the two values that actually matter for the tool — shrinkage and processing temperature.
A substitution that shifts shrinkage by 0.2% is invisible on a data sheet and very visible on a 150 mm dimension. Before accepting any substitution on an existing tool, check three things: the shrinkage range of the replacement grade, the processing window, and whether the change affects any regulatory approval the part already carries. If the substitution happens after the tool is cut, plan for a sample run and a dimensional report — not an assumption.
Lock the material spec before the tool is cut
The material line on a purchase order should read like a specification, not a category: the polymer, the grade, the supplier, the color, and any filler content. “ABS, natural” is not a specification. “ABS, [grade name], [supplier], black, UV stabilized” is.
The reason is not bureaucracy. Each of those items changes the number the tool is cut to, and each one changes what the quote should say. If you want to see how material and tooling decisions show up on the invoice, we broke down how a mold gets quoted and what to compare line by line.
Frequently asked questions
What is the most common material for injection molding?
In industrial and consumer work, ABS, polypropylene, and polycarbonate cover the largest share. ABS dominates cosmetic housings; polypropylene dominates functional and chemical-resistant parts; polycarbonate dominates where impact strength or transparency is required.
Can I change material after the mold is built?
Sometimes, if the two materials have similar shrinkage and processing temperatures. A change in shrinkage usually means the tool dimensions are wrong, and that is a tool modification, not a material change.
How do I choose between a filled and an unfilled grade?
Choose filled grades when you need stiffness, strength, or dimensional stability at temperature, and accept that the part will be more anisotropic and more abrasive to the tool. Choose unfilled when surface finish, impact, and warp control matter more than stiffness.
Who should decide the material — me or the molder?
You own the requirement; the molder owns the process implications. Name the requirements and let the molder confirm which grades can meet them at production volume, with the tool and machine available. The decision is joint, but the responsibility for the requirements is not.
Decide the material with the tool in mind
Send us your part function, service environment, and volume. We will come back with two or three material options, the shrinkage values the tool will be cut to, and the processing implications of each — before anyone commits to steel. You can send the requirements to our engineering team here.
Author: Jack Qiao works with the engineering team at Kysen Mold, a Shanghai-based injection mold manufacturer serving overseas OEMs.
Website: www.kysenmold.com
