Key Considerations: Plastic Part Tolerances
Summary
Tolerance on a molded plastic part is not a single number that applies uniformly across a drawing – it is a set of decisions that should track part size, material, and which dimensions actually matter to function. Specifying tolerances too loosely risks parts that do not fit or work; specifying them uniformly tight, on every dimension regardless of whether it matters, drives up tooling cost and scrap rate without improving the part. This guide covers the standards that govern plastic part tolerances, what actually determines what a mold can hold, and how to specify tolerance in a way that protects function without needlessly inflating cost.
Key Takeaways
- Standard achievable tolerances run roughly ±0.10–0.20mm below 25mm up to ±0.50mm or wider above 300mm.
- ISO 20457 is the current plastics-specific tolerance standard; ISO 2768, designed for machined metal parts, should be avoided for molded plastics.
- Crystalline plastics like PP, POM, and PA shrink more unpredictably than amorphous plastics like ABS and PC.
- Wall thickness uniformity, part size, gate and cooling design, and process stability all affect achievable tolerance.
- Tighter tolerances increase mold cost, inspection labor, and scrap risk, so they should be reserved for genuinely critical-to-function dimensions.
- Tolerance callouts should specify what the product actually requires, then be checked against what the tool can manufacture – not the reverse.
Standard Tolerance Ranges by Dimension Size
| Dimension Range | Standard Tolerance | Achievable With Design Review |
|---|---|---|
| Below 25mm | ±0.10 to ±0.20mm | ±0.05 to ±0.10mm |
| 25–100mm | ±0.15 to ±0.30mm | ±0.10 to ±0.20mm |
| 100–300mm | ±0.25 to ±0.50mm | Project-specific |
| Above 300mm | ±0.50mm or wider | Project-specific |
These figures are useful as preliminary design references, not as guarantees that apply to every part regardless of material or geometry. The achievable-with-review column reflects what a tighter process and design discipline can sometimes deliver, but confirming it for a specific part requires an actual manufacturability conversation rather than assuming the tighter numbers apply by default.
Which Standard Should Be Used?
ISO 20457 is the current plastics-specific standard for molded part tolerances, and it is the correct reference for new drawings. DIN 16742 has historical relevance, particularly in Europe, but anyone citing it should specify which edition, since tolerance values have shifted between revisions. One common and costly mistake is applying ISO 2768 to a molded plastic part – that standard was designed for machined metal parts, and using it for molded plastics creates unrealistic tolerance expectations that a mold, no matter how well designed, will struggle to meet consistently.
What Actually Determines Achievable Tolerance
| Factor | Effect on Tolerance |
|---|---|
| Material shrinkage behavior | Crystalline plastics (PP, POM, PA) shrink more unpredictably than amorphous plastics (ABS, PC) |
| Wall thickness uniformity | Uneven walls cause differential cooling, which drives warpage and dimensional variation |
| Part size | Larger parts accumulate more total shrinkage variation across their length |
| Gate and cooling design | Directly affects packing consistency and overall dimensional stability |
| Process stability | Consistent temperature, pressure, and cycle timing matter more than any single sample measurement |
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The Cost of Over-Specifying Tolerance
Tighter tolerances are not free, and the cost shows up in several places at once: more precise (and more expensive) mold-making, additional sampling and inspection labor to verify the tighter spec is actually being met, more demanding process-control requirements to hold that precision shot after shot, and a higher risk of scrap or secondary machining when a part drifts outside a needlessly tight window. The practical guidance that follows from this is straightforward but often skipped under schedule pressure: apply tight tolerances specifically to critical-to-function dimensions, and allow standard tolerances everywhere else on the part.
How to Specify Tolerance Without Overpaying for It
- Identify which dimensions are actually critical to function or fit before specifying anything tighter than standard.
- Confirm manufacturability with the molder before finalizing a tight tolerance, rather than discovering it is unachievable after the tool is cut.
- Reference ISO 20457, not ISO 2768, for anything intended to guide a plastic part’s tolerance expectations.
- Account for material behavior early, since a crystalline resin may simply not be able to hold the same tolerance an amorphous resin could on the same geometry.
- Revisit tolerance alongside wall thickness, since uneven walls will undermine even a well-chosen tolerance specification through unpredictable warpage.
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Geometric Dimensioning and Tolerancing (GPS) Basics for Molded Parts
Beyond the simple plus-or-minus dimension on a drawing, molded parts often need geometric tolerancing to fully control form, which a linear tolerance alone cannot capture.
| Control | What It Governs | Why It Matters for Molded Parts |
|---|---|---|
| Flatness | How much a surface deviates from a true plane | Warpage from uneven cooling directly affects this |
| Position | Where a feature sits relative to a datum | Core shift during molding can throw position off even when linear dimensions check out |
| Concentricity | Alignment of circular features around a shared axis | Relevant for bosses, bores, and features that must mate with a round component |
| Parallelism | Whether two surfaces or features stay equidistant | Differential shrinkage across a part can pull surfaces out of parallel |
A part can pass every linear dimension on a drawing and still fail to assemble correctly if a geometric control like position or flatness was never specified. For parts with tight mating or alignment requirements, geometric tolerancing is often more important to call out correctly than squeezing a linear dimension to a tighter plus-or-minus value.
Tolerance Stack-Up in Multi-Part Assemblies
When a molded part mates with other components, tolerance has to be considered across the whole assembly, not just on the individual part drawing. Tolerance stack-up analysis adds up the individual tolerances of every mating dimension to determine the worst-case (or statistical) total variation at the assembly level. A part that looks reasonably toleranced in isolation can still cause an assembly fit problem if its tolerance, combined with the tolerances of the parts it mates with, exceeds what the assembly can actually accommodate. Running this analysis during design, rather than discovering a fit problem during assembly trials, is considerably cheaper to correct.
How Tolerance Connects to Other Design Decisions
Tolerance does not exist independently of other design choices – it is directly shaped by them. Wall thickness is probably the single biggest factor, since uneven walls are a leading cause of the dimensional variation that tolerance specifications are trying to control in the first place; our guide to wall thickness design covers that relationship directly. For food-contact parts, tolerance requirements often intersect with regulatory and material constraints as well; see our guide to food-grade plastic molding for how those requirements layer together.
Parts incorporating EMI shielding features often carry tighter tolerance requirements specifically at the shielding interface, which is worth planning for separately from the rest of the part; our guide to EMI shielding for molded enclosures covers that in more depth. And injection pressure settings play directly into how consistently a tool holds its tolerance shot after shot; our guide to injection pressure optimization covers the process side of that same consistency question.
Working With a Supplier on Tolerance Specification
A molder experienced in plastic injection molding should push back on an unnecessarily tight tolerance callout during design review, rather than quietly quoting it and letting the cost and schedule consequences surface later. A plastic injection molding company in Finland with real manufacturability expertise will typically flag which dimensions on a drawing are realistic to hold as specified and which ones are likely to drive unnecessary cost, before the tool is ever quoted.
Verifying Tolerance in Production, Not Just at Design Review
A tolerance specification that looked achievable during design review still needs to be verified once the tool is actually running, since real process variation – material lot differences, ambient temperature swings, minor wear on the tool over time – can shift a dimension that measured fine on the first qualified samples. Statistical process control, tracking key dimensions across a production run rather than spot-checking occasionally, is what catches that kind of gradual drift before it produces a batch of out-of-tolerance parts. This connects directly back to first article inspection and ongoing quality monitoring, which exist specifically to confirm that a tolerance holds in practice, not only on paper.
Conclusion
Plastic part tolerance works best as a deliberate, dimension-by-dimension decision rather than a single blanket number applied across a drawing. Using ISO 20457 as the reference standard, understanding how material and wall thickness affect what is actually achievable, and reserving tight tolerances for genuinely critical-to-function dimensions keeps a part both manufacturable and affordable – while still meeting the functional requirements that matter.
Frequently Asked Questions (FAQs)
What is a typical tolerance for a small molded plastic part?
For dimensions below 25mm, standard tolerance generally runs ±0.10 to ±0.20mm, with ±0.05 to ±0.10mm achievable in some cases with careful design review.
Should ISO 2768 be used for plastic part tolerances?
No. ISO 2768 was designed for machined metal parts and sets unrealistic expectations for molded plastics. ISO 20457 is the current plastics-specific standard and the correct reference.
Why do some plastics hold tighter tolerances than others?
Crystalline plastics like PP, POM, and PA shrink more unpredictably during cooling than amorphous plastics like ABS and PC, which generally makes amorphous materials easier to hold to tighter tolerance.
Does wall thickness affect achievable tolerance?
Yes, significantly. Uneven wall thickness causes differential cooling, which drives warpage and dimensional variation that can undermine even a carefully specified tolerance.
Why not just specify tight tolerances on every dimension to be safe?
Tighter tolerances increase mold cost, inspection labor, and scrap risk. Applying them everywhere rather than only to critical-to-function dimensions adds cost without improving the part.
How should tolerance be decided during the design phase?
Start from what the product actually requires functionally, then confirm with the molder what the chosen material and geometry can realistically achieve, rather than guessing at a tolerance and discovering it is unachievable after tooling begins.
What is tolerance stack-up and why does it matter?
Tolerance stack-up adds the individual tolerances of mating dimensions across an assembly to determine total worst-case variation. A part that looks fine in isolation can still cause an assembly fit problem if its tolerance, combined with mating parts, exceeds what the assembly can accommodate.
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