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

Applying CNC-machined metal tolerance blocks (such as a blanket ±0.1 mm or ±0.005 in across an entire print) to a plastic injection molded part is one of the fastest ways to inflate tooling and piece-part quotes. Molding pushes molten polymer into a cavity where it cools, shrinks anisotropically, and responds to pack-pressure gradients and ambient humidity.

Molded tolerance behavior is fundamentally different from subtractive machining. A drawing that ignores polymer shrinkage either gets priced with heavy risk margin or comes back with a long list of supplier exceptions. Specify critical-to-function dimensions selectively and pair this guide with the DFM guide and part inspection standards.

Why Molded Tolerances Are Different

A machined part is cut from a stress-relieved solid block. A molded part shrinks volumetrically by anywhere from ~0.2% to over 2.5% depending on the resin family, and that shrinkage is never completely uniform across a complex geometry.

Achievable tolerance on any molded feature depends on six interacting variables:

• Resin Selection: High-shrink, semi-crystalline resins (POM, Nylon, PP, PE) exhibit wider dimensional dispersion than low-shrink amorphous resins (ABS, PC, PC/ABS, PEI/Ultem).
• Reinforcement & Fillers: Adding glass or carbon fibers reduces overall flow-direction shrinkage but introduces strong directional anisotropy—shrinkage along the fiber flow orientation is much lower than shrinkage transverse to flow; see what changes with glass-filled plastics.
• Tool-Bound vs. Parting-Line Dimensions: Dimensions formed entirely within one piece of steel (cavity-only or core-only) repeat much more tightly than dimensions that cross the parting line or rely on moving slides and lifters.
• Wall Thickness Uniformity: Uneven walls cool at different rates, creating internal stress and differential shrinkage that manifest as warpage.
• Nominal Dimension Size: A 200 mm housing span accumulates far more absolute thermal and shrinkage variation than a 10 mm boss diameter.
• Process & Cooling Control: Scientific molding controls (decoupled cavity-pressure transfer, tightly regulated coolant loops, and consistent resin drying) narrow shot-to-shot scatter.

What Is Realistically Achievable

Experienced buyers and moldmakers reference established plastics tolerance standards—most commonly ISO 20457 (which superseded DIN 16742)—rather than applying a blanket ±0.1 mm title-block tolerance across the board. ISO 20457 assigns a Tolerance Grade (TG1 through TG9) based on the polymer’s viscosity/shrinkage profile, whether the dimension is tool-bound or crosses the parting line, and the nominal feature size.

Tighter “precision” tolerances (such as TG3/TG4 or sub-±0.05 mm on small features) are achievable on specific critical dimensions, but they carry real engineering cost: steel-safe grooming loops after T1, scientific process monitoring, single-cavity or low-cavitation tools, and CMM fixture inspection on every lot.

The Cost of Over-Tolerancing: Critical vs. General

Separate critical-to-function dimensions from general geometry.

If every dimension is tight, the supplier quotes tooling and process as if every feature is critical. Piece price rises for no functional gain. Call out mating interfaces, sealing surfaces, and critical fits. Let the rest fall under a sensible general tolerance block for the material.

This matters heavily for warpage: tight flatness on a large, thin, asymmetric part forces the molder to fight cooling physics. Reserve tight tolerances for dimensions that dictate function.

How to Specify Tolerances Effectively

• Provide a 2D drawing with GD&T. The 3D CAD defines nominal shape; the 2D drawing states what is controlled and from which datums.
• Mark critical dimensions explicitly. Flag the few dimensions that determine success. Reference a commercial standard (such as DIN 16742) for the general tolerance block.
• Do not copy-paste machined tolerances. Re-evaluate every tight callout. Does the function need it, and can the selected resin support it?
• Discuss feasibility early. Reviewing tight dimensions with the molder before steel is cut is cheaper than finding process limits at First Article Inspection.
• Tie tolerances to the inspection plan. Tight dimensions are the ones the supplier must measure during inspection. Specify them with measurement method in mind.

Buyer FAQs

What tolerances can injection molding hold?

There is no single number. Achievable tolerance depends on resin shrink, geometry, dimension size, and tooling precision. General tolerances are routinely held; precision tolerances on specific features cost more. Ask the supplier which standard they quote to (often DIN 16742) and map your critical dimensions to that grade before RFQ.

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

Plastics shrink as they cool. Shrinkage varies with wall thickness, flow direction, and process. Machined parts are cut from stable material. If a drawing carries machined callouts onto plastic, require a feasibility review before award or expect price and exceptions.

Should I tolerance every dimension tightly just to be safe?

No. Blanket tight tolerances force over-engineered tooling and over-inspection, raise piece price, and hide which dimensions matter for assembly. Mark critical-to-function only; put the rest under a general block. Reject quotes that treat every feature as critical when your drawing never said so.

How does material choice affect tolerance?

Material sets baseline shrinkage. Low-shrink amorphous resins (ABS, polycarbonate) hold tight tolerances more predictably than high-shrink semi-crystalline resins (polypropylene, nylon). Finalize material before you lock final tolerances.

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; verify achievable tolerances through your supplier’s engineering review.