Last updated: 24 September 2026.
Quick answer. Injection moulding makes parts by pushing molten polymer into a machined mould. When you buy from a Chinese moulder, three things decide the outcome: the tolerance framework you cite, the material and its compliance route, and whether the part needs a flammability or food-contact rating.
The standards named below come from the issuing bodies' own pages, listed at the end. How to weigh them when you write an enquiry is our editorial practice, and is labelled as such.
Injection moulding is a replication process: a mould is cut once, and every part after that is a copy of the cavity. That single fact drives every commercial and technical decision around it. The cost of the tool is paid once and then spread; the cost of each part is dominated by material, cycle time and yield.
That shape of cost structure is why moulding becomes attractive at volume and unattractive at very low volume, and why the same part is often better cut from stock on a CNC machine for a prototype and moulded only once the design stops moving. It is also why a moulder will ask about annual volume and expected order pattern before quoting a tool, and why a quotation without those two numbers is not comparable to one with them.
Plastics are not the only moulded material. Metal injection moulding appears in the same search results as polymer moulding, and it follows a different route: a feedstock is moulded and then debound and sintered, which changes the tolerances, the material options and the shrinkage you have to design around. If your part is metal and small, it belongs in that conversation rather than this one.
Four document families come up when a buyer and a moulder discuss a plastic part. They do different jobs, and confusing them is one of the most common causes of a disputed shipment.
| Standard | What the issuing body says it covers | Who publishes it |
|---|---|---|
| ISO 20457:2018 | Plastics moulded parts — Tolerances and acceptance conditions — the tolerance and acceptance framework for moulded parts | ISO, technical committee ISO/TC 61/SC 11 |
| UL 94 | Tests for Flammability of Plastic Materials for Parts in Devices and Appliances | UL Solutions |
| UL 746 series | Mechanical, electrical, thermal and flammability characteristics of polymeric materials, split into parts — UL 746A short-term property evaluations, UL 746B long-term property evaluations, UL 746C use in electrical equipment evaluations, UL 746D fabricated parts, UL 746E industrial laminates and related materials, alongside UL 746F, 746G, 746H, 746R and 746S | UL Solutions |
| ASTM D3641 | Standard Practice for Injection Molding Test Specimens of Thermoplastic Molding and Extrusion Materials | ASTM |
Two clarifications from those sources are worth keeping. UL Solutions describes the 746 series as covering mechanical, electrical, thermal, flammability and other safety-related characteristics, and describes UL 746A specifically as a framework for assessing mechanical, electrical, physical, chemical resistance and flammability properties over short durations. And ASTM D3641 is a specimen-moulding practice, not a part specification — it tells a supplier how to mould the test bars whose data it reports, which is why a material datasheet without a stated moulding practice is a weak document.
A limit on the ISO citation, stated rather than glossed. The ISO catalogue feed for ISO 20457:2018 confirms the title, the committee and the ICS classification, and records a stage 95.99 event dated 2026-08-06. The standard's own HTML page on iso.org was not retrievable from the environment used for this page, and ISO's stage codes should be read on iso.org rather than interpreted second-hand. Confirm the current status before you name the standard as a contractual reference.
A moulded part does not hold a tolerance the way a machined part does. The cavity is cut once, but the part that comes out of it is affected by shrinkage as the polymer cools, by where the material enters, by wall thickness changes, and by whether the tool is running cold at the start of a shift or hot in the middle of one. A tolerance callout on a moulded part is therefore a negotiation about what the process can hold, not a machining instruction.
This is what ISO 20457 exists for. Its subject is tolerances and acceptance conditions for plastics moulded parts, which tells you the document is as much about how a part is judged as about the number printed next to a dimension. Read it as the framework that defines which deviations are manufacturing reality and which are a defect — and then note the practical consequence: a drawing that carries a blanket tight tolerance on every dimension, including dimensions that do not matter functionally, moves cost into the tool and the yield for no functional gain.
The working alternative is to separate the two kinds of dimension. Critical features get their own callouts with the function attached; everything else is left to a general tolerance framework. That is also the pattern our tolerances page uses for sheet-metal and machined work, and it translates directly to moulded parts. When two design features are close and the process cannot separate them cleanly, the usual answer is a datum scheme agreed with the moulder rather than a tighter number, because inspection has to be able to reproduce it. How that is verified at the supplier end is covered on our quality control page.
The polymer family you choose changes four things at once: how the material must be dried, how much it shrinks, how the tool must be built, and which compliance route the part needs. Two compliance routes come up often enough to name, and one of them is a legal act rather than a standard.
For parts that touch food — kitchenware, containers, closures, drinkware — the governing European act is Commission Regulation (EU) No 10/2011 of 14 January 2011 on plastic materials and articles intended to come into contact with food, published in the Official Journal at L 12, 15.1.2011, pages 1–89. That is a first-party citation, and it is the document a European customer will ask you about, not a trade-association summary.
For restricted substances more generally, the supplier's evidence is a declaration rather than a grade stamp, which is why our REACH and RoHS page is written around what to ask a supplier for. For parts destined for electrical equipment, the market's framework is the UL 746 series, and the material's ratings are only meaningful if the material supplier has tested it to those programmes and the moulder has actually moulded with that grade on that tool.
Cost is a factor, but be careful how you use it. Our die casting and CNC pages make the same point in metal: the cheapest material that passes the functional requirement is the right one, and the material chosen purely on unit price is the one that produces a warranty conversation later.
The fields below are our editorial practice, drawn from what makes a moulded-part quotation comparable across suppliers. None of them is a standard requirement.
| Field | Why it matters to the moulder |
|---|---|
| 3D model, plus a 2D drawing showing only the toleranced and critical features | The model defines the shape; the drawing defines what is judged |
| Annual volume and expected order pattern | Tool construction and cavitation decisions follow from this |
| Polymer family, or the functional requirement if you do not know it | Determines drying, shrinkage and compliance route |
| Critical dimensions, with the function behind each one | Lets the moulder push back on a tolerance that is not achievable |
| Surface finish and texture expectations, on a named surface | Texture is cut into the tool and cannot be added later cheaply |
| Colour and any colour-match reference | Masterbatch choice is made at tooling time, not at production |
| The compliance route: food contact, electrical, restricted substances, or none | Decides whether declarations can be issued at all |
| Assembly context: what the part fits into, and where it is inspected | Explains tolerances that look odd in isolation |
| Inspection standard and who pays for the first article | Removes the most common end-of-project dispute |
Two fields are deliberately absent from that table. Tooling and part pricing, and lead times, are not covered here, because this site does not publish figures it cannot source; ask for them per project, in writing.
Three failure modes cover most of the moulded-part problems a buyer meets, and all three are visible before the tool is cut.
Tolerancing a moulding like a machining. The first is the blanketed tight tolerance described in section 3. Its cost lands in the tool and in the reject rate rather than in the drawing, which is why it is often invisible until the second invoice.
Fixing the material after the tool exists. The second is discovering the compliance route late. A UL rating that has not been established for the actual grade, or a food-contact declaration that cannot be issued for the colourant, cannot be retro-fitted cheaply, because both are properties of the material in the tool.
Quoting without a volume. The third is a quotation that cannot be compared, because one supplier assumed a family tool running one cavity and another quoted a hardened multi-cavity tool. Both numbers are honest; they describe different projects.
If any of those three is unresolved, the right next step is not a tighter specification but a conversation with the moulder, followed by a first-article inspection under the inspection standard you named in the enquiry. When you are ready for that, you can send the project for quotation with the fields above attached.
ISO 20457:2018, titled Plastics moulded parts — Tolerances and acceptance conditions, and maintained under ISO technical committee ISO/TC 61/SC 11. Its subject is deliberately wider than a number: it covers acceptance conditions as well as tolerances, which is the part that decides how a debatable dimension gets judged.
UL Solutions lists UL 94 for the flammability of plastic materials used in parts in devices and appliances, and the UL 746 series for polymeric materials generally, described as covering mechanical, electrical, thermal and flammability characteristics. Within that series it names UL 746A for short-term property evaluations and UL 746B for long-term property evaluations, among other parts.
Only if the end product's market requires one, but for parts inside electrical equipment the UL 746 series is the expected framework and UL Solutions describes preselection flammability programmes that follow UL 94 for flame spread and extinguishing behaviour. Small polymeric component materials are covered by a separate standard, UL 1694.
They apply to the regulated uses, not to moulding as such. For plastic materials and articles intended to come into contact with food, the governing act is Commission Regulation (EU) No 10/2011 of 14 January 2011, published in the Official Journal at L 12, 15.1.2011, pages 1–89. For restricted substances in general, the supplier's evidence is a declaration, which our REACH and RoHS page sets out.
Through a moulding practice rather than the part drawing. ASTM D3641, Standard Practice for Injection Molding Test Specimens of Thermoplastic Molding and Extrusion Materials, is one such document; when a supplier quotes material data, ask which practice was used to mould the specimens, because the data describes the specimen as much as the material.
All sources retrieved 24 September 2026. This page is an independent reading of the sources listed; the official pages themselves are the specification.
| Fact used on this page | Source |
|---|---|
| S1 — Title, committee and classification of the tolerance standard cited, and the catalogue event recorded against it: ISO 20457:2018 is titled Plastics moulded parts — Tolerances and acceptance conditions; the catalogue entry records technical committee ISO/TC 61/SC 11, ICS 83.140.01, and a stage 95.99 event dated 2026-08-06 | ISO, official catalogue feed for ISO 20457:2018 (retrieved 2026-09-24; direct HTML page returned a bot challenge and was not retrievable) https://www.iso.org/contents/data/standard/06/80/68097.detail.rss |
| S2 — The list of UL standards relating to polymeric materials, with their exact titles, plus the IEC standards based on UL standards: UL 94 — Tests for Flammability of Plastic Materials for Parts in Devices and Appliances; UL 746A — Polymeric Materials, Short-Term Property Evaluations; UL 746B — Polymeric Materials, Long-Term Property Evaluations; UL 746C — Polymeric Materials, Use in Electrical Equipment Evaluations File Review Guidelines; UL 746D — Polymeric Materials — Fabricated Parts; UL 746E — Polymeric Materials — Industrial Laminates, Filament Wound Tubing, Vulcanized Fibre, and Materials Used in Printed Wiring Boards; UL 1694 — Tests for Flammability of Small Polymeric Component Materials; and IEC 60695-10-2, IEC 60695-2-10 and IEC 60695-2-11 described as IEC standards based on UL Standards | UL Solutions, official site — Standards for Evaluating Plastics (retrieved 2026-09-24) https://www.ul.com/resources/standards-evaluating-plastic-products |
| S3 — Scope of the UL 746 series, the parts it is divided into, the scope of UL 746A, and the flammability test programmes: UL 746 covers mechanical, electrical, thermal, flammability and other safety-related characteristics, and is divided into UL746A, UL746B, UL 746C, UL 746D, UL 746E, UL 746F, UL 746G, UL 746H, UL 746R and UL 746S; UL 746A is described as the standard for polymeric materials — short term property evaluations, providing a framework for assessing mechanical, electrical, physical, chemical resistance and flammability properties over short durations; UL Solutions describes two primary preselection flammability test programmes, one on flame spread and extinguishing behaviour following UL 94, and lists testing of dimensional stability at elevated temperatures, density and moisture content in polymers, and scratch resistance of surfaces | UL Solutions, official site — Understanding the UL 746 Series of Standards (retrieved 2026-09-24) https://www.ul.com/services/understanding-ul-746-series-standards |
| S4 — Title and subject of the test-specimen moulding practice cited: ASTM D3641, Standard Practice for Injection Molding Test Specimens of Thermoplastic Molding and Extrusion Materials | ASTM, official store — D3641 (retrieved 2026-09-24) https://store.astm.org/d3641-24.html |
| S5 — Identity and publication details of the EU act cited for plastics in food contact: Commission Regulation (EU) No 10/2011 of 14 January 2011 on plastic materials and articles intended to come into contact with food, published in the Official Journal OJ L 12, 15.1.2011, pp. 1–89 | EUR-Lex, official EU law database (retrieved 2026-09-24) https://eur-lex.europa.eu/eli/reg/2011/10/oj |