What Is the Typical Tolerance for Injection Molding?

Your CAD model says 25.00 mm. Your first molded parts come back at 24.93 mm. Is that a problem, or is it normal?

The answer depends on the tolerance you called out on your drawing. It also depends on whether that number was realistic for plastic.

Cost plays a part too. The tightest numbers on your drawing can raise your mold price. Tightening a callout past what your part needs adds tooling cost with no benefit to the end user.

So, what is the typical tolerance for injection molding? Below, we give you the numbers and the factors that change them. Then we show you how to spec your drawing so parts fit without overpaying.

We start with the three tolerance classes: commercial, standard, and fine. Next, we cover how material, part design, and tooling and process control move those numbers. Last, we walk you through how to write tolerance callouts on your drawing.

The typical tolerance for injection molding is about ±0.005 inch (±0.127 mm) on small, well-controlled features. Molded tolerances are often grouped into three levels:

• Commercial: about ±0.010 inch (±0.25 mm) for non-critical features
• Standard: about ±0.005 inch (±0.127 mm) for most functional parts
• Fine: about ±0.002–0.003 inch (±0.05–0.08 mm) with tighter tooling and process control

Some precision molds can hold ±0.001 inch on a few select features. That number does not apply across a whole part. Tolerances also loosen as parts get larger. Material shrink, wall thickness, and mold quality all change what is possible for your part.

What Is an Injection Molding Tolerance?

A tolerance is the allowed range around a dimension on your drawing. Say your print calls for 25.00 mm ±0.13 mm. Any part that measures between 24.87 mm and 25.13 mm passes. The 24.93 mm part from earlier would be in spec.

Plastic shrinks as it cools in the mold. That means a molded part never matches your CAD model exactly. A tolerance tells your molder how much difference the part can handle and still work.

There are two main kinds of tolerance:

• Dimensional tolerance controls size, such as length, width, or hole diameter.
• Geometric tolerance controls shape and location, such as flatness, roundness, or position.

Tolerances are set per feature, not per part. A snap-fit tab may need a tight callout. The outside edge of a cover may not. You decide where precision matters, and the mold is built around those choices.

Typical Injection Molding Tolerances: Commercial, Standard, and Fine

Molded tolerances are often grouped into three classes. Each class fits a different kind of feature on your part.

You may see ±0.001 inch quoted as possible. That number is real, but only on a few select features. It takes precision tooling and close process control. It is not something you should expect across a whole part.

Size matters too. A ±0.005 inch callout is realistic on a small feature. On a long dimension, the same number gets much harder to hold. Tolerances widen as dimensions grow because larger features shrink more in total.

Industry standards give you a shared language for these numbers. ISO 20457 covers tolerances for molded plastic parts. It replaced older German standards for molded parts, including DIN 16901.

Some features need more than molding can give. Tolerances tighter than about ±0.025 mm (±0.001 inch) usually call for precision machining after molding. For those features, the part is molded close to size and then machined to final spec.

How Material Affects Injection Molding Tolerances

Those numbers are starting points. The first thing that moves them is the plastic itself.

Every resin shrinks at its own rate as it cools. Semi-crystalline resins, such as polypropylene and nylon, tend to shrink more than amorphous resins, such as ABS and polycarbonate. Low-shrink resins are easier to hold to tight numbers. High-shrink resins give you a wider range to plan around.

Nylon has one more quirk. It absorbs moisture, so parts can swell after molding. A nylon part that measures right on day one may change size later. If your fit is tight, plan for that before you pick the material.

*These are general reference ranges. Grade, fillers, and processing change the real values. Always check the resin maker’s data sheet for your exact grade.

How Part Design Affects Tolerances

Material sets the range. Your part design decides where in that range you land.

A few design choices make the biggest difference:

• Wall thickness: Even walls shrink evenly and warp less. See our wall thickness guidelines.
• Draft: Draft angles make a feature slightly larger at one end than the other. Your callout should account for that. See our draft angle guidelines.
• Ribs, bosses, and holes: Features formed around core pins are harder to hold than flat surfaces.
• Parting line: Features that cross the parting line need looser tolerances than features in one mold half.
• Large, flat parts: Big flat surfaces are the hardest to keep flat as they cool. Learn why large parts are harder to mold.

If a tight feature sits in one of these tricky spots, a small design change can often help. It is easier to adjust the part now than to fight it in the mold later.

How Tooling and Process Control Hold Tolerances

The same part design can hit or miss its tolerances depending on the mold and the process behind it. That is why your choice of molder matters.

Steel vs. aluminum molds: Hardened steel molds resist wear better than aluminum molds, so they hold their size over more cycles. Aluminum works well for prototypes and short runs. For long runs with tight callouts, steel keeps your parts in spec longer.

Mold layout also plays a part. A multi-cavity mold makes several parts per shot, but parts from each cavity can differ slightly. That adds cavity-to-cavity variation you have to plan for.

Repeatable parts come from steady cooling and stable process settings. A well-placed cooling layout helps each part shrink the same way. Locked-in settings keep shot 10,000 close to shot 10.

Before production starts, we run molds through staged trials known as T0, T1, and T2. The first trial checks how the mold moves and functions, and the next ones check real parts. Every trial is documented, and every change to the tool or process is tracked. We sign off a mold only after it meets the criteria set at the start of your project.

Our ISO 9001:2015 certified quality system keeps that same discipline from one run to the next. Your tenth order should measure like your first.

How to Specify Tolerances on Your Drawing

Knowing what’s possible is half the job. The other half is writing it on your drawing the right way.

Use this checklist before you send your print out for quotes:

1. Add a general tolerance note. Cover most features with one note, such as an ISO 20457 tolerance grade. This keeps your drawing clean and your intent clear.
2. Call out tight tolerances only where they matter. Save them for critical-to-function features, like mating surfaces, snap fits, and sealing edges.
3. Remember that tight numbers add up. Tight callouts can require a more precise tool, more mold trials, and more careful measurement. Loose callouts on non-critical features keep your costs down.
4. Plan a second step for features beyond molding. Mold the part close to size, then machine the critical feature to final spec.
5. Get a design review before the steel is cut. Changes cost far less on paper than in a finished mold. Ask for a DFM review before tooling.

A careful drawing gets you accurate quotes and parts that fit the first time.

Tight-Tolerance Injection Molding in Utah

When your part has tight callouts, it helps to have every step under one roof. At our shop in Logan, we handle design review, mold making, molding, and machining in-house. Your drawing, your mold, and your parts stay with one team from start to finish.

Here is what that means for your tolerances:

• Realistic numbers from the start: Standard molding holds about ±0.005 inch. For precision applications, ±0.002 inch or better is achievable.
• One place to fix problems: If a feature runs out of spec, the mold and the process are both right here.
• Machining when molding isn’t enough: Parts move from the press to our own machine shop, with no shipping between steps.

Our ISO 9001:2015 certified quality system backs every run. We work with companies across Utah and even beyond that need local, reliable parts.

Ready to see what your part can hold? Learn more about our custom plastic injection molding or call us at (435) 774-9090 to get started!