Injection Molding Tolerances: What's Realistic and What to Specify

A drawing that carries a machinist’s tolerances onto a molded part is one of the most common—and most expensive—mismatches a buyer can send to a supplier. Injection molding is a process where the plastic shrinks as it cools, flows differently in different directions, and responds to the tool and the process. That makes its tolerance behavior fundamentally different from cutting metal, and a drawing that ignores it gets quoted high, or comes back with a long list of exceptions. This guide explains what’s realistic and how to specify tolerances so the important dimensions get held without paying to control the ones that don’t matter. It pairs with the DFM guide and inspection standards.

Why Molded Tolerances Are Different

A machined part is cut to size from stable stock. A molded part is formed hot and shrinks as it cools, and that shrinkage is neither tiny nor perfectly uniform. The amount depends on the resin, the wall thickness, the direction of flow, and the process, so two dimensions on the same part can hold differently. This is why a tolerance that’s trivial for a machine shop can be genuinely hard—or expensive—for a molder, and why molding tolerances are best discussed in the language of the process rather than copied from a metal drawing.

What Drives the Tolerance You Can Hold

Factor	Effect on tolerance
Material shrinkage	Higher-shrink, semi-crystalline resins (PP, POM, nylon) are harder to hold tight than low-shrink amorphous ones (ABS, PC)
Fillers (glass)	Reduce shrinkage but make it directional, which can help one axis and hurt another
Wall thickness & uniformity	Thick or uneven walls shrink and warp unevenly, widening achievable tolerance
Dimension size	Larger dimensions accumulate more total shrinkage variation than small ones
Across vs. along flow	A dimension across the flow direction often shrinks differently than one along it
Tool & process	Tool precision, cooling balance, and process stability all set how repeatably a dimension lands

The practical takeaway: tolerance isn’t a single number you apply to a drawing. It depends on what the dimension is, where it is, and what it’s made of.

What’s Realistically Achievable

Standard injection molding holds general tolerances comfortably for most features, and tighter “precision” tolerances are achievable on specific, well-controlled dimensions—at added cost in tooling, process control, and inspection. Rather than chase a universal number, experienced buyers lean on published tolerance systems. Some suppliers quote to a standard such as DIN 16742 (or an internal equivalent), which assigns tolerance grades by dimension size and how critical the feature is, much like general tolerance tables do for other processes.

Two cautions on numbers: achievable tolerance always depends on the specific resin, geometry, and tool, so treat any general figure as illustrative and confirm it with your supplier; and tighter is not free—every tightened tolerance can add tooling precision, slower validated process windows, and more inspection.

Critical Dimensions vs. Everything Else

The single most useful thing a buyer can do is separate the dimensions that genuinely matter from the ones that don’t. A drawing that tightens everything tells the supplier nothing about priorities and forces them to quote as if all of it is critical. A drawing that calls out a handful of critical-to-function dimensions—mating interfaces, sealing surfaces, fits—and lets the rest carry a sensible general tolerance gives the supplier room to control what counts.

This is also where warpage connects to tolerance: asking for tight flatness on a large, thin, or asymmetric part is asking the process to fight the material’s nature, which is achievable sometimes but with real cost. Being honest about which dimensions are truly critical is what keeps that cost where it earns its keep.

How to Specify Tolerances Well

• Provide a 2D drawing with GD&T and datums. CAD shows shape; the drawing states what’s controlled and from where.
• Mark critical dimensions explicitly, and let the rest fall under a general tolerance block or a referenced standard.
• Don’t carry over machined tolerances. Re-evaluate each tight callout for whether molding can hold it and whether the function needs it.
• Discuss feasibility early. A quick conversation about a tight dimension before tooling is far cheaper than discovering it at first article.
• Tie tolerances to acceptance. The dimensions you tolerance are the ones that get measured at inspection, so specify with that in mind.

This is an independent buyer resource, not an engineering or metrology service. Achievable tolerances depend entirely on the resin, geometry, tool, and process—use this to specify intelligently, then confirm feasibility for your specific part with your supplier.

Buyer FAQs

What tolerances can injection molding hold?

Injection molding holds general tolerances comfortably for most features, with tighter precision tolerances achievable on specific, well-controlled dimensions at added cost. There’s no single number—achievable tolerance depends on the resin (its shrinkage), the wall thickness, the dimension’s size and orientation to flow, and the tool and process. Many suppliers quote to a tolerance standard such as DIN 16742, which grades tolerance by size and criticality. Treat any figure as illustrative and confirm with your supplier.

Why can’t molded parts hold the same tolerances as machined parts?

Because molded parts are formed hot and shrink as they cool, and that shrinkage varies with material, wall thickness, and flow direction. A machined part is cut from stable stock to a fixed size. Carrying machined tolerances onto a molded drawing usually makes a part more expensive to quote and harder to accept, because some of those tolerances fight the molding process rather than work with it.

Should I tolerance every dimension tightly to be safe?

No—that’s counterproductive. Tightening everything hides which dimensions actually matter, forces the supplier to quote as if all are critical, and raises cost without improving the part. Identify the handful of critical-to-function dimensions, tolerance those appropriately, and let everything else carry a sensible general tolerance. That focuses control—and cost—where it affects fit and function.

How does material choice affect tolerance?

A lot. Low-shrink amorphous resins like ABS and polycarbonate are easier to hold tight than higher-shrink semi-crystalline resins like polypropylene, acetal, and nylon. Glass fillers reduce shrinkage but make it directional, which can tighten one axis while loosening another. Because shrinkage is central to molded tolerance, the material is part of the tolerancing decision, not separate from it.

Evidence Box

This guide to injection molding tolerances summarizes widely documented tooling and design practice, framed for the decisions a buyer actually makes. It is educational synthesis, not an engineering specification—your supplier’s and moldmaker’s review of the specific part governs, and this page is preparation for that conversation.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, provide engineering services, or certify suppliers. Design and tooling decisions are part-specific—confirm them through your supplier’s and moldmaker’s engineering review.