Material selection is the decision that quietly sets everything else. It fixes the part’s mechanical limits, it fixes the cost per shot, and because different resins shrink at very different rates, it fixes the cavity dimensions in the steel before a single part is molded. Change the resin after tooling is cut and you are frequently paying for a new cavity.

This comparison covers six resins that account for the large majority of commercial injection molded parts, judged on the properties that matter in production rather than on a datasheet reading exercise. It also covers something most comparison articles leave out: what each material choice does to the mold and to the part price.

How to choose an injection molding material in four questions

Work through these in order and most candidates eliminate themselves.

1. What is the maximum temperature the part will see in service, including transport, storage and any cleaning or sterilization cycle? This sets the floor and removes whole categories immediately.
2. What will the part be exposed to chemically, including cleaners, fuels, oils, moisture and ultraviolet light?
3. What mechanical demand does it face, and is it stiffness, impact resistance, fatigue, or wear at a sliding surface? These are different requirements and no single resin leads on all of them.
4. Does the part need transparency, a specific appearance, regulatory approval, or a flame rating?

Cost enters last, not first. A cheaper resin that fails a service requirement is not cheaper. What cost properly influences is the choice between two materials that both meet the requirement, which is a genuinely common situation and where most money is saved.

Master comparison table

Two rows deserve more attention than they usually get: shrinkage and moldability. Both determine whether the material you selected will actually hold the tolerances on your drawing once the tool is running.

The properties that actually decide the choice

Shrinkage, and why it is a tooling decision

Every thermoplastic contracts as it cools, and the cavity is cut oversize by the expected shrinkage so the finished part lands on nominal. That compensation is baked into the steel. Semi crystalline materials such as polypropylene, nylon and acetal shrink considerably more than amorphous materials such as ABS and polycarbonate, and they shrink less predictably because crystallinity varies with cooling rate.

The practical consequences are three. High shrinkage materials make tight tolerances harder to hold, because a small variation in cooling produces a measurable dimensional change. They are more prone to warpage, because uneven shrinkage across a part pulls it out of shape. And switching between materials with different shrinkage after tooling is cut usually means re cutting the cavity, since a mold sized for ABS will not produce correct parts in acetal.

If the part carries tight tolerances, that argues for a low shrinkage amorphous material or for accepting looser tolerances and a more expensive tooling development cycle. Deciding this before the tool is quoted is far cheaper than discovering it at first article.

Moldability and what it costs at the press

Moldability covers how easily a resin fills, how much drying it needs, what melt and mold temperatures it demands, and how forgiving it is of process variation. ABS and polypropylene are easy: they fill readily and tolerate a wide process window. Polycarbonate and nylon are hygroscopic and must be dried properly, because moisture causes splay and degrades mechanical properties. PEEK requires high melt temperatures and heated tooling, which raises both cycle time and tooling cost.

A difficult material is not a reason to avoid it when the application requires it. It is a reason to expect longer cycles, tighter process control and higher scrap during startup, all of which belong in the cost model rather than arriving as a surprise.

The six resins in detail

ABS

ABS is the default for consumer housings, enclosures and cosmetic parts, and it earns that position by being easy to mold, dimensionally stable, and receptive to painting, plating and texturing. It takes a good surface finish, machines and bonds well, and has low, predictable shrinkage.

Where it fails is temperature and chemistry. Continuous service temperature is modest, and ABS is attacked by many solvents. It also has poor ultraviolet resistance without stabilization, so unprotected outdoor parts chalk and become brittle. Choose ABS for interior housings, appliance parts, electronics enclosures and anything where appearance and cost matter more than environmental exposure.

Polycarbonate

Polycarbonate is the impact and clarity option. It is genuinely tough across a wide temperature range and it is optically transparent, which is a combination few other materials offer. Service temperature is meaningfully higher than ABS.

Its weaknesses are chemical resistance and notch sensitivity. Many solvents and some cleaning agents cause stress cracking, and a sharp internal corner concentrates stress in a material that otherwise resists impact well. It is also hygroscopic and must be dried before molding. PC and ABS blends are common precisely because they trade some of the toughness for easier processing and better chemical behavior. Choose polycarbonate for lenses, guards, safety components and impact resistant housings.

Nylon

Nylon covers a family, with PA6 and PA66 the most common. It offers high strength and stiffness, good wear resistance and good resistance to fuels and oils, which is why it appears throughout automotive and industrial applications. Glass filled grades increase stiffness substantially and are extremely common.

The defining characteristic is moisture absorption. Nylon takes up water from the air, and as it does the part grows dimensionally and becomes tougher and less stiff. That means a part measured dry and a part measured after conditioning are different sizes, and any tolerance discussion has to state the moisture condition. Nylon also shrinks significantly and unpredictably, which makes tight tolerance work harder. Choose it for gears, bearings, fasteners, structural brackets and under hood components.

Polypropylène

Polypropylene is the cheapest of the six and the most chemically resistant of the commodity resins. It is light, it resists moisture almost completely, and it is uniquely capable of a living hinge, meaning a thin section that can flex repeatedly without failing. Nothing else on this list does that as well.

The trade-offs are stiffness and temperature: it is comparatively soft, it creeps under sustained load, and its service temperature is limited. It also shrinks heavily, so tight tolerances are difficult. Choose polypropylene for containers, closures, living hinge parts, chemical exposure applications and high volume components where material cost dominates.

POM (acetal)

Acetal is the precision mechanical material. It is stiff, dimensionally stable in service, has a low coefficient of friction and excellent fatigue resistance, which makes it the standard choice for gears, cams, bushings, latches and small moving components.

Two things limit it. Shrinkage is the highest on this list, so cavity sizing and cooling design matter more than usual and thick sections are prone to voids. And acetal does not bond or paint easily, so it suits functional internal parts rather than cosmetic ones. Choose it wherever a part slides, rotates or has to spring back reliably over many cycles.

PEEK

PEEK sits in a different category. It withstands continuous service temperatures far above the others, resists almost all chemicals, holds strength at temperature, and performs in demanding medical, aerospace and semiconductor applications. Where it is specified, it is usually because nothing else works.

It is also by a wide margin the most expensive material here, and it requires high melt temperatures and heated tooling, which pushes up cycle time and tooling cost. Specifying PEEK where a lower cost engineering resin would meet the requirement is one of the more expensive mistakes in material selection. Choose it when temperature, chemical exposure or regulatory requirements genuinely leave no alternative.

Filled and modified grades

Base resins are only the starting point. Most engineering applications use a modified grade, and the modification changes both properties and processing.

• Glass fiber reinforcement raises stiffness and strength substantially and reduces shrinkage, but it makes the material more brittle, more abrasive to the tool, and prone to fiber orientation effects that cause directional warpage.
• Mineral fill improves dimensional stability and reduces cost, with a smaller stiffness gain than glass.
• Flame retardant grades are required for many electrical and electronic applications and typically reduce impact strength and complicate processing.
• Ultraviolet stabilized grades are necessary for any part with outdoor exposure, and adding stabilization later does not repair a part already specified without it.
• Impact modified grades trade some stiffness for toughness and are common where a base resin is close but slightly too brittle.
• Medical and food contact grades carry the regulatory documentation those applications require, and the approval attaches to the specific grade, not to the resin family.

Glass fill deserves particular caution on tooling. Abrasive filled resins wear steel over time, especially at gates, which affects the tool steel choice and the maintenance schedule. That belongs in the tooling conversation at design stage, not after the first hundred thousand shots.

Choosing by application requirement

Where a part needs two different behaviors in one piece, such as a rigid body with a soft grip, the answer is often a process rather than a single material. Overmolding with soft touch materials combines a rigid substrate with an elastomer, and moulage par insertion encapsulates metal components in plastic where threads or conductive paths are required.

What the material decision does to the mold and the part price

• Cavity dimensions are cut for a specific shrinkage rate, so changing resin families after tooling normally means re cutting steel.
• Cycle time varies by material. Materials needing high mold temperatures or long cooling raise the cost per part on every shot for the life of the tool.
• Gate and runner sizing depends on melt viscosity, so a material change can require gate modification even when the cavity is unaffected.
• Abrasive filled resins accelerate tool wear and influence the tool steel specification and maintenance interval.
• High shrinkage materials require more attention to cooling channel layout, which is a tooling design cost rather than a per part cost.
• Drying requirements add equipment and handling time at the press for hygroscopic materials such as polycarbonate and nylon.

This is why material selection should be settled before tooling is quoted rather than after. A resin decision made late is a tooling change, and tooling changes in hardened steel are the most expensive corrections in the entire process.

Common material selection mistakes

• Selecting on tensile strength alone, when stiffness, impact behavior or fatigue resistance is what the application actually demands.
• Ignoring moisture absorption on nylon and then writing tolerances that cannot be held across normal humidity conditions.
• Specifying a high performance resin where an engineering grade would meet every requirement, which raises material and tooling cost for no functional gain.
• Overlooking ultraviolet exposure on parts that will sit outdoors or near a window, then discovering embrittlement in the field.
• Assuming a datasheet value applies to your part, when published figures come from standard test specimens rather than your geometry, gate position and cooling conditions.
• Changing resin after tooling is cut without checking whether the shrinkage difference invalidates the cavity dimensions.

The last one is the most costly and the most avoidable. Confirming the resin before the tool is cut, or at minimum agreeing which alternates are dimensionally compatible, removes the risk entirely.

Taking the material decision into production

Material and geometry are decided together rather than in sequence. Wall thickness, rib proportions and gate location all interact with the resin’s shrinkage and flow behavior, and a design that works in one material may not work in another. Our guide to rib and gusset design guidelines covers the geometry side of that relationship, and many of the issues in our article on material related molding defects trace back to a mismatch between resin and part design rather than to the press.

For very small components, material behavior changes again because flow lengths, cooling rates and gate proportions do not scale down neatly. Our comparison of micro molding applications covers where those differences matter.

Elite Mold Tech molds ABS, polycarbonate, nylon, polypropylene, POM, PEEK and TPU through its services de moulage par injection de plastique, and reviews material selection against part geometry and tolerance requirements during quoting rather than after tooling is committed. Send a part model and the service requirements to Elite Mold Tech for a recommendation that accounts for shrinkage, tolerance and tooling implications together.

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