What do we need ready in drawings, tolerances and material specs before a manufacturer will quote series production?

A manufacturer can only quote series production once four things are complete: a 3D model per part, a drawing where only the functional dimensions carry a tolerance, a material specification that goes beyond the name of the polymer, and the commercial basis of the series, meaning annual volume, programme length and how you will inspect. If one of those is missing, the manufacturer fills the gap with an assumption of his own. That assumption ends up in his price, in the steel of the tool, and in the parts you get back.

What goes into the package you send out?

Eight blocks, and they should leave in one go. A package that arrives in instalments produces quotes you cannot compare, because every supplier started costing at a different moment.

| What you send | What the manufacturer does with it | | --- | --- | | 3D model per individual part, in STEP | Basis for the tool design and for filling and cooling analysis | | Model of the assembly | Shows how the parts come together and where clearance is needed | | 2D drawing per part | Fixes which dimensions are functional and what parts are inspected against | | Material specification with grade | Drives shrinkage, cycle time and the steel grade the tool needs | | Surface finish and colour | Drives the cavity surface treatment and the draft angles required | | Annual volume and programme length | Drives cavitation and the tool life the mould is built for | | Inspection terms | Fix which dimensions are measured and when a batch is accepted | | Marking, packaging and compliance | Fix what is engraved in the steel and what travels with the shipment |

Two of those rows are the ones most often left out, and they are the two that cost the most. Programme length in years decides whether you need prehardened or hardened steel, and that difference never shows up in a piece price. The inspection terms decide what good means when the first series lands. Leave them open and your first conversation about scrap is a negotiation without a reference.

Use a simple test on the drawings. Could someone who has never spoken to you make and inspect this part without picking up the phone? Anything you would explain in a call belongs on the drawing or in the specification.

Which tolerances can a mould realistically hold?

Wider than machining, and how wide depends on your material and on where the dimension sits. For plastic parts the starting point is ISO 20457, the international successor to the German DIN 16742. It sorts dimensions into tolerance groups, where the size of the dimension and the shrinkage of the material together set what is achievable.

Three things decide how tight you can go in practice:

• The shrinkage of the chosen grade. Published values run roughly from 0.4 to 0.9% for ABS, 0.5 to 0.7% for PC, 1.0 to 2.5% for PP, and 0.5 to 3.0% for PA depending on glass content. Sources disagree with each other, so the number that counts is the one on the datasheet of your grade.
• Where the dimension sits. A dimension inside one mould half is easier to hold than one that crosses the parting line, because the second one also carries the closing accuracy of the tool.
• How many dimensions you tolerance. Every toleranced dimension gets measured, and every tight one pushes on cycle time, because the part has to cool longer in the tool to come out dimensionally stable.

A worked example on invented figures, to show why the material choice cannot wait. Take a housing 120 mm long in PP. The cavity is machined for an expected shrinkage of 1.5%, so the steel is cut at 121.8 mm. If the grade you finally approve shrinks 2.0%, the part comes out at roughly 119.4 mm, which is 0.6 mm under the drawing. On a dimension toleranced at plus or minus 0.3 mm you are out, and the fix means welding steel back in or recutting the cavity. That is why the grade, not just the polymer family, has to be fixed before machining starts.

So put a tolerance only on the dimensions that carry something: mating faces, screw bosses, the centre distance between two fixings, the dimension where two parts snap together. The general tolerance from the standard covers the rest. In practice that lands at five to ten toleranced dimensions per part. A part with many more than that has usually been drawn by someone used to machining.

What has to be in the material specification?

More than the name of the plastic. PP is not a specification, because two PP grades differ in shrinkage, flow, impact strength and colour stability. Six lines belong in there as a minimum:

• Type, grade and supplier, plus whether an equivalent alternative is allowed. If it is not, you also know straight away that the lead time of that one grade can hold up your series.
• Colour, with a reference and an agreed way of measuring it. Agree where colour is measured and what still passes, because two masterbatch lots are never identical.
• Recycled content, if you want it. This is a genuine trade-off: recyclate lowers material cost and footprint, but makes colour less predictable and mechanical properties more variable between lots. Fix the percentage and the source, or it will vary per delivery.
• What happens to the runners. Can regrind go back into the process, and at what percentage? For cosmetic and medical parts the answer is often zero, and that changes material consumption per part.
• Flammability class, if the part sits in or near electronics. UL 94 runs from HB, the lowest, through V-2, V-1 and V-0 to 5VB and 5VA, where the 5V tests use a far more intense flame. A flame-retardant grade shrinks differently from the standard one, so this is not a choice you make later.
• Requirements that come from a standard, such as EN 71-3 for toys, Regulation 10/2011 for food contact, or ISO 10993 for skin contact on a medical device. Name the standard, not the intention.

Say where the part ends up, too. Outdoors, in a car, in a shower or in a cold store: that decides whether UV stabilisation is needed and whether the grade holds its properties at the temperatures it will see.

While you are fixing that grade, think about availability. A grade that only one supplier on another continent carries makes your series dependent on a single line of supply, and that is exactly where delay appears once demand picks up. Say early on whether a second, equivalent grade exists and who is allowed to approve it. Approve that second grade only when you need it and it costs you another trial and another dimensional report, at the worst possible moment.

Which choices have to be fixed before steel is cut?

The ones you cannot reverse afterwards without touching the tool. This is the conversation you have before the tooling order, not after the first trial.

• The parting line and where a witness line may show. Every tool leaves one. You point at where it is allowed to sit, or the toolmaker decides for you.
• The gate position. It drives how the part fills and whether a vestige stays visible. It is also where flow lines and gloss differences come from, as our piece on engineering decisions that impact looks describes.
• Ejector positions. Ejectors leave marks. Point out which faces must stay clean; our piece on optimising your ejection strategy covers the trade-offs.
• Draft and texture. A smooth wall already needs draft, a textured one needs more. A common rule of thumb is around one extra degree of draft per 0.025 mm of texture depth, although published recommendations range from 1 to 5 degrees. So choose the texture before the draft angles are locked, not after.
• Cosmetic surfaces. Mark on the drawing which faces are visible in the finished product, with an agreed viewing distance and lighting.
• Marking in the steel. Cavity number, material identification, a date stamp and sometimes a recycling symbol are engraved in the tool. Adding them later is possible, but it means machining a tool that is then standing still.
• How the parts are joined. Snap fit, screws, welding or adhesive: that choice drives the tolerances of two parts at once, so it is not a detail for later.

The classic scenario shows why none of this is a formality. Take a four-part housing at roughly 30,000 units a year, supplied as a clean 3D model with no cosmetic faces marked. The toolmaker then puts the ejectors where they make the most technical sense, and that can be the one face the end user ever sees. Dimensions pass, the part releases cleanly, and the steel still has to be reworked after the first samples because the marks are in view. One arrow on a drawing would have saved that round, and correcting a finished tool always costs more than a comment up front.

Have the list reviewed before the enquiry goes out, preferably as a DFM review on the injection moulded design. A manufacturability review across the whole chain takes out the wall thickness, draft and part count that otherwise come back as a surcharge in every quote. In our process that review is a fixed step before the tooling order, followed by a trial, a correction round and a second set of samples.

How do you know the package is complete?

By having it reviewed before it goes out, and by reading the quotes that come back. If one package produces two quotes that sit far apart, the gap is more often in your package than in their pricing: both parties filled in the missing information themselves, and they did it differently. Only with a complete package can you judge whether the cheapest tool is also the cheapest over the life of your product.

Walk through these points before you send:

1. A STEP of every part, plus the assembly, plus the drawings as PDF, all at the same revision. 2. Per drawing: only functional dimensions toleranced, cosmetic faces marked, material and colour in the title block. 3. A material specification with grade, supplier, colour, recycled content, regrind, flammability class and the standards that apply. 4. Annual volume, programme length in years and expected growth, so cavitation and steel grade can be chosen against them. 5. The inspection terms: which dimensions go in the dimensional report, how deliveries are sampled, and who measures. 6. Marking, packaging requirements and the documents that travel with the shipment.

The point of that list is not that everything is locked down. It is that every open choice is open on purpose. A manufacturer who knows what is still moving can allow for it in the steel. A manufacturer who does not know will decide for you, and you find out at the first samples.

Production Heroes takes products from design to series production, from feasibility and cost price to sourcing and supply chain, and supplies moulds and tooling to manufacturers across Europe, from the Netherlands and its own presence in China.

Sources: ISO 20457:2018 (Plastics moulded parts, tolerances and acceptance conditions), the international successor to DIN 16742 (2013). Shrinkage values per polymer come from publicly available material tables published by moulders and toolmakers; those sources differ from each other, which is exactly why the datasheet of the approved grade is what counts. The UL 94 classes and the more intense flame used in the 5V tests come from publicly available UL 94 documentation. The draft rule of thumb for textured surfaces comes from toolmaker and texture supplier documentation, where recommendations range from 1 to 5 degrees. EN 71-3, Regulation (EU) 10/2011 and ISO 10993 are named as standards, not as advice on which standard applies to your product. The shrinkage example uses invented assumptions.