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

Applying machined-metal tolerances to a plastic injection molded part is expensive. Molding pushes hot polymer into a cold tool. The plastic shrinks as it cools, flows differently by direction, and reacts to mold temperature and pressure changes.

Molded tolerance behavior is not subtractive manufacturing. A drawing that ignores that either gets quoted high or comes back with a long list of supplier exceptions. Specify critical dimensions with care so you do not inflate tooling and inspection cost. Pairs with the DFM guide and inspection standards.

Why Molded Tolerances Are Different

A machined part is cut from a stable block. A molded part shrinks, and that shrinkage is rarely uniform.

Achievable tolerance on a dimension depends on:

• Resin Selection: High-shrink, semi-crystalline resins (POM, Nylon, PP) are harder to hold tight than low-shrink amorphous resins (ABS, PC).
• Fillers: Glass fibers reduce overall shrinkage but introduce directional shrinkage (anisotropy). Shrinkage is lower along flow and higher across it.
• Wall Thickness: Uneven walls cool at different rates, which drives differential shrinkage and warpage.
• Dimension Size: Larger dimensions accumulate more total shrinkage variation than smaller features.
• Tooling and Process: Mold machining precision, cooling balance, and process stability all affect how repeatably a dimension lands.

What is Realistically Achievable

Experienced buyers do not invent tolerance numbers. They use established standards. Many suppliers quote to DIN 16742 (or equivalent internal matrices). Those standards assign grades by nominal dimension size and resin family, not a blanket +/- 0.1 mm everywhere.

Tighter “precision” tolerances are possible on specific, well-controlled dimensions. They are never free. Tighter callouts mean higher-precision tool machining, narrower process windows (often slower cycles), and more inspection.

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.