Precision injection molding: what to specify before you ask for tight tolerances

Every molder says precision. What makes a molded part hold size is a drawing that marks what is critical, a tool cut with shrinkage in mind, a locked process and measurements that prove it. What to specify, what to ask and where it goes wrong.

Filed under Molds and tooling

Short answer: Precision injection molding means producing plastic parts that hold tight dimensional tolerances from the first shot to the last lot. It is not a machine you buy or a word on a quote. It comes from a drawing that marks which dimensions are critical, a tool cut with shrinkage in mind, a locked process, and measurements that prove capability. Spec those four things, and "precision" becomes something you can inspect.

Precision injection molding is on almost every supplier's website, and it means almost nothing until someone writes down a number. A buyer asks for "tight tolerances." The factory says yes. Six weeks later the first shots arrive and a snap fit is loose, a bore is undersize, and nobody can agree what the drawing actually required. This note is about how to avoid that: what to specify, what to ask the molder, and where precision programs usually go wrong.

What "precision" means on a molded part

There is no single industry threshold where ordinary molding ends and precision molding begins. In practice, a precision job is one where some dimensions are toleranced tighter than the general tolerance the process gives you without extra effort, and where the buyer expects those dimensions to hold across cavities and lots, not just on one good sample.

The international reference for general tolerances on plastic parts is ISO 20457. Its scope says it specifies "possible manufacturing tolerances" for plastic molded parts, covering injection molding and several related processes, and that it "allows for additional specifications in case of functional needs and requirements" using the ISO geometric tolerancing tools. In other words: a general tolerance covers most of the part, and the features that matter get their own callouts. In the US, the same job is usually done with GD&T to ASME Y14.5.

ISO 20457 also says what it does not cover: surface imperfections "such as sink marks, undesired flow structures and roughness, as well as joint lines." Cosmetic requirements need their own written standard and a sample, not a tolerance block.

Specify the critical dimensions, not the whole part

The most expensive precision drawing is the one with a tight tolerance on every dimension. It tells the toolmaker nothing about what matters, so the tool, the process and the inspection all get priced as if everything does.

A better drawing has three layers:

1. A general tolerance for every dimension that is not called out, named by standard (ISO 20457 or your own title-block note).
2. Critical-to-function dimensions, marked and toleranced individually: the bore that takes a bearing, the snap fit, the sealing face, the mating datum.
3. A datum scheme that matches how the part is used and how it will be measured. If the drawing measures from a face the part never locates on, you can pass inspection and still fail assembly.

Then mark which critical dimensions will be measured on every first article and which will be tracked in production. That list is what the molding step and the inspection plan are built around.

Shrinkage: the number the toolmaker cuts into the steel

Plastic shrinks as it cools, so the mold cavity is cut larger than the part. How much larger depends on the resin, the wall thickness, the flow path and the process, and that is where precision is won or lost.

The test standards are blunt about the limits of datasheet numbers. ISO 294-4 measures shrinkage on standard test specimens in two directions, along the melt flow and across it, because they differ. The same scope notes that its basic shrinkage figure "excludes the effects of humidity uptake," which is counted separately as post-molding shrinkage. The US method, ASTM D955, is more direct: it "can not predict absolute values in actual molded parts with varying flow paths, wall thicknesses, pressure and temperature gradients and process conditions."

What that means for a buyer:

• The datasheet shrink rate is a starting point, not a promise. The toolmaker applies a shrink factor, but the first shots tell the truth.
• Glass-filled resins shrink differently along and across the flow. A dimension that runs with the flow can come out different from one that runs across it, even on the same part.
• Some resins keep moving after molding. Nylons are hygroscopic and "will absorb or desorb moisture as a function of the ambient humidity," and their dimensions change with it. A nylon part measured dry off the press and measured after a week in a humid warehouse are not the same part. Agree when and in what condition parts are measured.

Cut the tool steel-safe

On critical features, ask the toolmaker to cut "steel-safe": leave extra steel where a correction would mean removing metal rather than adding it. Taking steel off a core or cavity is a routine machining job. Adding steel back means welding or a new insert, which costs time and can leave a mark. A steel-safe first cut, followed by a correction after the first measured shots, is how most tight-tolerance tools reach size. Budget for at least one correction loop in both time and money; the mold cost note covers what those loops cost.

The tool and the process both have to be capable

The quote comparison note explains why two tools quoted for the same part can differ on several of these lines at once.

Prove it with numbers: first article and capability

A first article inspection shows the tool can make a good part. It does not show the process will keep making them. For that, critical dimensions are measured on a run of parts and expressed as process capability.

The NIST Engineering Statistics Handbook gives the standard definitions. Cp compares the tolerance width to the process spread: (USL − LSL) / 6σ. Cpk also accounts for how well the process is centered, taking the smaller of the distances from the mean to each limit, divided by 3σ. NIST's table translates a centered Cp of 1.00 into about 0.27% rejects and a Cp of 1.33 into about 64 parts per million. Many buyers write a minimum Cpk on critical dimensions into the approval; whatever number you choose, name it before the samples are made, not after.

Two practical points:

• Measure by cavity. A four-cavity tool with one weak cavity can hide inside a good average.
• Measure in the agreed condition. State the time after molding and the conditioning, especially for nylons.

Then keep a signed reference. A golden sample plus the measured report and the process sheet is what later lots are checked against, at the factory and in a pre-shipment inspection.

Where precision programs usually go wrong

• "Tight tolerance" with no numbers. If it is not on the drawing, it was not quoted.
• Every dimension critical. The price goes up and the real critical features get no more attention.
• Datasheet shrink treated as final. No time or money left for a correction loop.
• Approval on one cavity or one shot. Lot three comes from a different cavity and a different shift.
• Process changes without notice. A new resin lot, a regrind ratio, or a different press can move dimensions. Put "no change to material, tool or process without written approval" in the agreement.

How JCH handles a precision molding job

JCH Design & Manufacturing works with vetted partner factories. We don't own those factories, and we say so. On a molding job, Jack reviews the drawing for which dimensions are truly critical, translates it for the toolmaker so it is not interpreted twice, and asks for the gate, cooling and steel choices in writing before steel is cut. The tool is proven with a first article measured against the drawing before the first production shot, and the approved samples, report and process sheet become the reference for every lot after. The mold step covers how a tool is built and who keeps it.

If you have a part that has to hold size, send the drawing or a sample through the quote form. Jack will tell you which callouts drive the cost and what the molder needs to quote it properly.

Straight answers

What is precision injection molding?

It is injection molding of plastic parts that must hold tight dimensional tolerances across cavities and production lots. It depends on a drawing that marks the critical dimensions, a tool cut with the right shrink allowance, a stable process, and measurements that prove capability.

What tolerances can injection molding hold?

It depends on the resin, part size, wall thickness, tool and process. ISO 20457 sets out general tolerances for plastic molded parts and allows tighter callouts on functional features. Ask the molder to confirm each critical tolerance in writing at quote stage, before steel is cut.

Why do molded parts come out a different size than the mold?

Plastic shrinks as it cools, so the cavity is cut larger than the part. Real shrinkage depends on the flow direction, wall thickness, pressure, temperature and process, which is why ASTM D955 warns that test-specimen values cannot predict absolute values in actual parts.

What does steel-safe mean in mold making?

It means cutting critical features so that any correction removes steel rather than adds it. Removing steel is a routine machining job; adding it back needs welding or a new insert. The tool is then corrected to size after the first measured shots.

What Cpk should a precision molded part have?

Buyers choose the number, often per critical dimension. NIST notes that a centered Cp of 1.33 corresponds to about 64 rejects per million. Whatever minimum you pick, write it into the approval before samples are made and measure each cavity.

How this note was made: researched and drafted with AI assistance, checked against the sources listed below and edited at JCH Design & Manufacturing.