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

Applying standard machined-metal tolerances to a plastic injection molded part is a costly error. Injection molding involves pushing hot polymer into a cold tool; the plastic shrinks as it cools, flows differently in different directions, and reacts to variations in mold temperature and pressure.

Because of this, molded tolerance behavior is fundamentally different from subtractive manufacturing. A drawing that ignores these realities will either be quoted at an exorbitant price or returned with a long list of supplier exceptions. This guide explains how to specify critical dimensions appropriately without driving up tooling and inspection costs. (Pairs with the DFM guide and inspection standards).

Why Molded Tolerances Are Different

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

The achievable tolerance on any given dimension depends on:

• Resin Selection: High-shrink, semi-crystalline resins (like POM, Nylon, PP) are inherently harder to hold to tight tolerances than low-shrink amorphous resins (like ABS, PC).
• Fillers: Glass fibers reduce overall shrinkage but introduce directional shrinkage (anisotropy). The plastic shrinks less in the direction of flow and more across it, complicating dimensional control.
• Wall Thickness: Uneven walls cool at different rates, leading to differential shrinkage and warpage.
• Dimension Size: Larger dimensions accumulate more total shrinkage variation than smaller features.
• Tooling and Process: The precision of the mold machining, the balance of the cooling circuits, and the stability of the molding parameters all dictate how repeatably a dimension lands.

What is Realistically Achievable

Experienced buyers do not invent tolerance numbers; they use established standards. Many suppliers quote to standards such as DIN 16742 (or equivalent internal matrices). These standards assign tolerance grades based on the nominal dimension size and the specific resin family, rather than applying a blanket +/- 0.1mm across the board.

Tighter “precision” tolerances are achievable on specific, well-controlled dimensions, but they are never free. Demanding a tighter tolerance requires higher-precision tool machining, more restrictive process windows (slower cycles), and increased inspection frequency.

The Cost of Over-Tolerancing: Critical vs. General

The single most effective action a buyer can take is to separate critical-to-function dimensions from general geometry.

If a drawing tightens every dimension, the supplier must quote the tooling and process as if every feature is critical. This drives up the part price unnecessarily. Instead, explicitly call out the mating interfaces, sealing surfaces, and critical fits. Let the rest of the part fall under a sensible general tolerance block based on the material.

This principle applies heavily to warpage: demanding tight flatness on a large, thin, asymmetric part forces the molder to fight the physics of the cooling polymer. Reserve tight tolerances for the dimensions that dictate functionality.

How to Specify Tolerances Effectively

• Provide a 2D drawing with GD&T. A 3D CAD model defines the nominal shape; the 2D drawing dictates what is controlled and from what datums.
• Mark critical dimensions explicitly. Flag the few dimensions that determine part success. Reference a commercial standard (like DIN 16742) for the general tolerance block.
• Do not copy-paste machined tolerances. Re-evaluate every tight callout. Does the function truly require it, and can the selected resin physically support it?
• Discuss feasibility early. Reviewing tight dimensions with the molder before cutting steel is vastly cheaper than discovering process limitations during First Article Inspection.
• Tie tolerances to the inspection plan. The dimensions you tolerance tightly 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 answer. Achievable tolerance depends on the resin’s shrink rate, part geometry, dimension size, and tooling precision. While general tolerances are easily held, precision tolerances are possible on specific features at an increased cost. Suppliers often use standards like DIN 16742 to grade tolerances based on these variables.

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

Plastics shrink as they cool, and that shrinkage varies based on wall thickness, flow direction, and process fluctuations. Machined parts are cut from stable material. Applying machined tolerances to a molded part fights the nature of the process and inflates costs.

Should I tolerance every dimension tightly just to be safe?

No. Blanketing a drawing with tight tolerances forces the supplier to over-engineer the tool and over-inspect the production run, significantly raising the piece price. It also obscures which dimensions actually matter for assembly and function.

How does material choice affect tolerance?

Material dictates baseline shrinkage. Low-shrink amorphous resins (ABS, Polycarbonate) hold tight tolerances much more predictably than high-shrink semi-crystalline resins (Polypropylene, Nylon). Material choice must be finalized before assigning 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.