Injection Moulding Tolerances: What's Achievable and What Drives Cost
Standard injection moulding holds ±0.1–0.2 mm on non-critical dimensions. Precision tooling and process control can reach ±0.05 mm or tighter on selected features. Every 0.05 mm you shave from a tolerance specification adds cost — knowing where precision is actually required is the single most effective way to control your budget.
What tolerances can injection moulding hold as standard?
Injection moulding is a repeatable, high-volume process, but it is not a machining process. Plastic flows, cools, and shrinks — and those physical realities set a lower bound on achievable tolerance. The table below shows typical commercial ranges by tolerance grade.
| Grade | Typical application | Linear tolerance (per 100 mm) |
|---|---|---|
| Standard commercial | Non-mating, cosmetic features | ±0.15–0.25 mm |
| Fine commercial | Light assembly, snap-fit interfaces | ±0.08–0.15 mm |
| Precision | Tight fits, optical, medical | ±0.03–0.08 mm |
| Ultra-precision | Lens, connector housings | ±0.01–0.03 mm |
These ranges assume a well-maintained steel tool, process-stable conditions, and materials with predictable shrink rates. Aluminium tooling and high-shrinkage resins shift the achievable range toward the looser end.
Which standard governs injection moulding tolerances?
ISO 20457 is the primary international standard for plastic moulded-part tolerances. It defines tolerance grades from MT1 (tightest) to MT6 (broadest), referenced to nominal dimension and material group. DIN 16742 covers similar ground and is still referenced in German-supply-chain contexts.
Nordmould uses ISO 20457 as the default framework for quoting, tooling sign-off, and first-article inspection. If your drawing references a different standard, state it explicitly in your STEP file submission and the DFM review will flag any conflicts.
What physically drives tolerance in injection moulding?
Four factors determine how close to nominal a moulded part can land. Understanding them tells you where design effort pays off and where it does not.
Material shrinkage. Every polymer shrinks as it cools from melt to solid. Shrinkage rates vary widely — from 0.4–0.7 % for ABS and PC to 1.5–2.5 % for PP and POM. The mould cavity is cut oversized to compensate, but if shrinkage varies shot-to-shot (due to moisture, regrind ratio, or melt temperature drift), the final dimension varies with it.
Tool quality and rigidity. A precise EDM-finished cavity in hardened H13 steel holds its geometry over millions of cycles. A hand-finished aluminium cavity deforms slightly under clamp pressure and wears faster. Bridge-tooling tolerances are adequate for prototype and low-volume use; they are not production-grade.
Process control. Injection speed, pack pressure, melt temperature, and cooling time all influence shrinkage. A well-documented, validated process running on a closed-loop machine holds tighter tolerances than an open-loop machine running ad hoc settings.
Part geometry. Uniform wall thickness produces uniform shrinkage. Thick bosses, rib–wall junctions, and abrupt section changes create differential shrinkage, which produces warpage and dimension shift that no amount of tooling precision can fully correct.
How does material choice affect achievable tolerance?
| Material | Shrink rate | Typical achievable tolerance | Notes |
|---|---|---|---|
| ABS | 0.4–0.7 % | ±0.05–0.10 mm | Stable, widely used, good for tight fits |
| PC | 0.5–0.7 % | ±0.05–0.10 mm | Low shrink, excellent for optical/electrical |
| PMMA | 0.2–0.6 % | ±0.05–0.10 mm | Very low shrink; brittle, gate-sensitive |
| PP | 1.0–2.5 % | ±0.15–0.25 mm | High shrink, anisotropic — harder to control |
| POM | 1.5–3.5 % | ±0.10–0.20 mm | Crystalline; accurate gating and cooling required |
| TPE / TPU | 1.0–2.0 % | ±0.20–0.40 mm | Soft, post-mould relaxation adds variability |
When tight tolerances are mandatory, Nordmould may recommend switching from PP to ABS or PC to buy back 0.05–0.10 mm without increasing tooling cost.
What is the cost impact of tightening tolerances?
Tighter tolerances cost money at three points in the value chain.
Tooling. Achieving ±0.05 mm requires EDM finishing of cavity surfaces, tighter mould-base fits, and often a longer bench time for fitting and polishing. Expect a tooling premium of 20–40 % over a standard commercial tool for the same part.
Cycle time. Precision tolerances usually require slower injection speeds, longer pack times, and extended cooling to minimise warp. A cycle that runs at 30 seconds for standard tolerances may run at 45–50 seconds for precision tolerances — reducing throughput by 30–40 % and raising per-part cost accordingly.
Inspection and scrap. Tighter specification bands mean a higher proportion of parts fall out of tolerance. First-article inspection, in-process gauging, and CMM measurement all add to unit cost. Scrap rates rise if the process is not fully dialled in.
The practical implication: apply tight tolerances only to features that genuinely drive function — mating interfaces, snap fits, optical surfaces. Leave non-functional faces, interior walls, and cosmetic features at standard commercial tolerance. Nordmould's DFM review identifies over-toleranced drawings routinely and can save significant tooling cost before a single cavity is machined.
How does Nordmould handle tolerance-critical parts?
Nordmould treats tolerance requirements as a design input, not an afterthought. During the free DFM review, every drawing is checked against the achievable range for the nominated material and tier. If a feature is toleranced tighter than the process can reliably hold, you receive a written note with a recommended revision — before tooling begins.
For production parts with GD&T requirements, Nordmould coordinates first-article inspection (FAI) reports and can supply CMM data on request. Critical-dimension parts are run on validated, documented process settings with statistical process control (SPC) in place.
Tolerance by feature type
Not all dimensions are equally easy to control. The table below gives typical achievable tolerances by feature type for a well-controlled steel tool in ABS or PC.
| Feature type | Standard | Fine | Precision |
|---|---|---|---|
| Linear (wall-to-wall, same half) | ±0.15 mm | ±0.08 mm | ±0.04 mm |
| Linear (across parting line) | ±0.20 mm | ±0.12 mm | ±0.06 mm |
| Hole diameter | ±0.10 mm | ±0.05 mm | ±0.03 mm |
| Boss outer diameter | ±0.15 mm | ±0.08 mm | ±0.04 mm |
| Flatness (100 mm span) | 0.20 mm | 0.10 mm | 0.05 mm |
| Angular | ±0.5° | ±0.25° | ±0.10° |
Dimensions that span the parting line are always harder to hold because mould half-to-half alignment adds a variable. Where possible, move critical dimensions to a single tool half.
Frequently asked questions
What tolerance can injection moulding hold as standard? Standard injection moulding typically holds ±0.1–0.2 mm on non-critical dimensions. Nordmould reviews your drawings and flags any feature that requires tighter control before tooling begins.
Can injection moulding hold tolerances tighter than ±0.05 mm? Yes, with precision tooling, controlled process parameters, and suitable materials. Nordmould can target ±0.05 mm or better on selected features, but expect a tooling premium of 20–40 % over standard.
Which material gives the tightest tolerances? Low-shrinkage materials such as ABS and PC hold tighter tolerances than high-shrinkage semi-crystalline materials like PP or POM. Nordmould specifies the right material for your dimensional requirements during the free DFM review.
What is ISO 20457 and does it apply to my parts? ISO 20457 is the current international standard for moulded-part tolerances. It defines tolerance grades from MT1 to MT6. Nordmould uses it as the default reference for quoting and inspection.
How does wall thickness affect achievable tolerance? Thin, uniform walls cool predictably and hold tighter tolerances. Thick or non-uniform walls create differential shrinkage, sink marks, and wider dimensional scatter. Nordmould's DFM review identifies wall-thickness issues before steel is cut.
Does a harder tool really improve tolerance? Yes. Hardened P20 or H13 steel tools maintain cavity geometry over hundreds of thousands of cycles. Aluminium bridge tooling is less rigid and less wear-resistant, so tolerances widen at higher volumes. Nordmould matches tool material to your volume and precision requirements.
How much does specifying ±0.05 mm add to my part cost? Tighter tolerances require slower cycle times, tighter process control, and more frequent inspection. Combined with the tooling premium, expect per-part cost to rise 15–30 % for precision grades compared with standard tolerances.
Send your STEP file to Nordmould for a free DFM review — you will receive a written manufacturability note that includes a tolerance assessment within 2 business days.
Last reviewed: 2026-05