Comparing Polyoxymethylene Quotations: What Must Be on the Spec Sheet

Procurement of polyoxymethylene (POM) is frequently conducted with quotation spreadsheets that compare only grade names, melt flow rate, and price per kilogram. A “standard acetal” from two suppliers may differ in comonomer type, thermal stabiliser package, mould shrinkage, and creep response. A comparable specification sheet must identify whether the material is a homopolymer or copolymer, the melt volume-flow rate under ISO 1133-1, the ISO 9988-1 designation block, and the test data that define moulded-part performance. Without these fields, two prices cannot be evaluated on the same technical basis. For example, a homopolymer with a melt volume-flow rate of 8 cm³/10 min at 190 °C and 2.16 kg load may show different shrinkage, creep, and chemical behaviour than a copolymer with the same flow value. The quotation must present measurable attributes rather than trade names or unstated grade claims.

On production-scale injection moulding equipment, the first consequence of an incomplete specification appears during setup. A machine with 90 tonnes clamp force running a multi-cavity gear set may show cavity-to-cavity weight variation exceeding 0.8 % if the quoted polymer type and mould shrinkage are not matched to the tool steel. Moulders observe that homopolymer POM releases from the ejector pins differently than a POM-C grade of similar flow, and the difference is not eliminated by adjusting hold pressure alone. The ISO designation block is therefore not administrative; it determines whether the supplier is offering a POM-H injection moulding grade, a POM-C extrusion grade, or an undefined reclaimed stream. A quotation that does not include the full designation cannot pass a first technical audit.

Which Melt Flow Rate and Designation Block Reject Ambiguous Offers?

The flow field on a POM quotation must be stated as melt volume-flow rate according to ISO 1133-1 at 190 °C and 2.16 kg load. A supplier that reports “high flow” or “MFR 9” without method, load, and temperature has not provided enough data for fill simulation. ISO 9988-1 defines the designation system for polyoxymethylene moulding and extrusion materials; the designation data block includes polymer type, processing method, designated property, and specification block. ASTM D6778 provides the equivalent North American classification system. The quotation should state whether the material is POM-H or POM-C, whether it is intended for injection moulding or extrusion, and which flow number appears in the designation. A flow value reported under ASTM D1238 may not be directly interchangeable with ISO 1133-1 unless the conditions are specified.

Quotations must also define moisture and processing limits at the quotation stage. A supplier may offer a POM-C with melt volume-flow rate of 30 cm³/10 min for thin-wall parts; the same material may fail in a thick extruded profile because melt stability and purge behaviour differ. Capillary viscosity data to ASTM D3835 or ISO 11443 add shear-rate information for extrusion, but the basic spec sheet must not omit the simple ISO 1133-1 flow value. In multi-source assessments, flow values are used to calculate gate solidification time and required clamp force, so omissions cannot be replaced with verbal assurances.

Mechanical data on a quotation remain incomplete if only tensile yield strength is provided. For POM gears, levers, and snap-fits, comparison must include flexural modulus, notched Charpy impact, and tensile creep data. Flexural modulus measured to ISO 178 at 23 °C on an 80 mm × 10 mm × 4 mm bar is more informative for snap-fit design than tensile modulus alone. A homopolymer POM typically reports tensile yield values between 70 MPa and 77 MPa, while a copolymer often falls between 60 MPa and 70 MPa when tested to ISO 527-2/1A. The difference is not a quality hierarchy; POM-C grades may exhibit lower mould deposit formation during long residence time. Notched Charpy impact to ISO 179-1/1eA at 23 °C commonly lies between 5 kJ/m² and 8 kJ/m², with homopolymer values near the upper end only when the mould temperature is held above 80 °C.

Creep modulus under ISO 899-1 is required for any bracket, conveyor link, or load-bearing post. A supplier quoting only short-term tensile data cannot support design at continuous loads above 20 MPa at 60 °C. Creep curves must state conditioning and test temperature because POM exhibits stress relaxation that accelerates with moisture and temperature. Published data for long-term creep of specific POM-C grades in hot water above 80 °C is limited, which is why the spec sheet must contain supplier-generated curves instead of a single value. For gears and cams, the quotation should also carry dynamic coefficient of friction and wear factor data from thrust washer testing to ASTM D3702 against hardened steel with counterface roughness below Ra 0.8 µm. Unlubricated POM against steel often reports coefficient values between 0.2 and 0.4, but the number is meaningless without surface finish and load.

Thermal, Dimensional, and Regulatory Compliance Data

Heat deflection temperature alone does not define an upper service temperature. Heat deflection temperature to ISO 75-2 Method A at 1.8 MPa typically ranges from 90 °C to 110 °C for unfilled homopolymer and some filled grades, and is lower for unfilled POM-C. The lower-load Method B value at 0.45 MPa must also be supplied when the component is exposed to standing stress. Dimensional data require both parallel and perpendicular mould shrinkage from ISO 294-4, measured on a 60 mm × 60 mm × 2 mm plaque. A quotation that reports a single shrinkage value is insufficient for tool design because POM shrinkage differs in flow and transverse directions and changes with wall thickness and mould temperature. An unfilled POM part with a dimensional tolerance below ±0.6 % cannot be approved without these two values.

Water absorption to ISO 62 at 23 °C for 24 h is typically below 0.25 % for both POM-H and POM-C, but long immersion in hot water above 60 °C can produce acid-catalysed hydrolysis and molecular weight reduction. For parts exposed to fuel, cleaning agents, or sterilisation, the supplier must state results from ISO 175 immersion tests in the actual chemical at the service temperature. Autoclave cycles at 121 °C for 20 min can distort and hydrolyse some POM grades; gamma irradiation at 25 kGy can embrittle others. A quotation for medical or fluid-handling applications must state the effect of the required sterilisation cycle on tensile strength and colour, because published data for specific irradiated POM configurations is limited.

Regulatory fields must include REACH registration status for monomers and additives, RoHS recast 2011/65/EU conformity for lead, mercury, cadmium, hexavalent chromium, PBB and PBDE, and, where relevant, FDA or EU 10/2011 compliance. The statement “RoHS compliant” without the list of exemptions or the test method used for cadmium is not auditable. For food-contact applications in the United States, POM copolymer is commonly listed under FDA 21 CFR 177.2470; a homopolymer grade may require a separate food-contact verification depending on composition. For EU 10/2011, POM food-contact declarations should address specific migration limits for formaldehyde and trioxane, because these are the relevant low-molecular-weight species. A declaration that states only “food approved” without migration testing conditions, simulant, temperature, and duration does not permit compliance judgment. For electrical and electronic parts, the specification sheet should contain comparative tracking index to IEC 60112 and volume resistivity to IEC 62631-3-1. Carbon-filled or antistatic grades require explicit surface resistivity values to ASTM D257, because value shifts between batches can alter discharge behaviour.

When Homopolymer and Copolymer Prices Diverge on a Datasheet

The distinction between POM-H and POM-C should be visible in the designation block, but quotations often hide it under generic “POM”. Homopolymer grades typically provide higher crystallinity, higher short-term tensile strength, and better creep resistance at ambient temperature. However, this comes with a tighter processing window and greater formaldehyde evolution risk in a hot runner. POM-C grades often tolerate longer residence time and are less prone to centreline porosity in thick sections. A price difference of 0.15–0.35 €/kg can be explained by the specification difference, not by raw material cost alone. The table below gives typical published ranges for unfilled injection grades; these are not specification limits and must be verified from the supplier’s batch certificate.

On an injection moulding line with a 40:1 L/D general-purpose screw, POM homogenisation is sensitive to screw speed and back pressure. Running a homopolymer at barrel temperatures above 210 °C and hold pressure above 120 MPa can cause gas marks at the vent after 10 min residence time. Published processing guides recommend pre-drying only when surface moisture is visible or storage relative humidity exceeds 60 %. If drying is performed, it is commonly 80 °C for 3–4 h in a dehumidified hopper dryer. Over-drying at high temperatures creates surface oxidation and contributes to mould deposit. A supplier’s quotation should state the maximum recommended melt temperature, the allowable residence time range, and whether the grade contains a stabilisation package for hot runner systems. Residual PVC decomposition products or acid residues in a shared extrusion line can destabilise POM; purging with a polyethylene or acrylic-based compound is necessary before POM is run.

Extrusion-grade POM quotations must include shear-viscosity data, because the same melt volume-flow rate does not describe behaviour at extrusion shear rates. Spiral flow data from a specific injection machine can support thin-wall filling studies, but it is not a substitute for a capillary viscosity curve to ASTM D3835 or ISO 11443. The spec sheet should therefore separate “flowability” data for injection from “extrudability” data for profile, rod, or sheet. A quotation lacking these process windows is unsuitable for first-article tool trials. In first-article mould trials for unfilled POM-C parts with tolerances below ±0.6 %, an unrecorded parallel mould shrinkage value frequently leads to tool correction after steel cutting; the cost of reworking the tool exceeds the saving from an incomplete quotation.

The Missing Fields That Sink a First-Article Approval

A first-article approval cannot proceed when the supplier has omitted creep modulus under ISO 899-1 at 60 °C and 1,000 h, because gear tooth deflection and bracket sag cannot be predicted from short-term data. Regulatory statements must be accompanied by standard codes and specific grades; “RoHS compliant” or “FDA approved” without the applicable section is not a technical field. The quotation must also state whether the grade is lubricated internally with PTFE or silicone, because additive migration changes friction, paint adhesion, and weld strength. Published data for heavily pigmented POM under the exact creep and irradiation conditions described is limited.