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ISO 29988-2:2018 — Plastics — Polyoxymethylene (POM) Moulding and Extrusion Materials — Part 2: Preparation of Test Specimens and Determination of Properties
Standard specimen preparation and property testing for POM materials | Injection moulding conditions | Mechanical and thermal characterization
1. Standard Test Methods for POM Materials
ISO 29988-2:2018 specifies the standard methods for preparing injection-molded test specimens from polyoxymethylene (POM) materials and the specific test procedures for determining their mechanical, thermal, and physical properties. This standard ensures that property data from different suppliers and laboratories are comparable, enabling material selection based on reliable data.
Value Proposition: Consistent specimen preparation procedures eliminate processing-induced variability, which can account for up to 30 % of apparent property differences between supposedly identical materials from different sources.
2. Specimen Preparation Conditions
The standard specifies precise injection moulding conditions for POM test specimens. Melt temperature ranges from 190-210 °C for homopolymers and 180-210 °C for copolymers. Mold temperature is maintained at 80-120 °C. Injection speed should be medium-to-fast, and holding pressure should be 60-80 % of injection pressure. These parameters are critical because POM’s crystalline morphology — and consequently its mechanical properties — is highly dependent on thermal history during processing.
| Property | Test Method | Specimen Type | Typical Range (Unreinforced) |
|---|---|---|---|
| Tensile modulus | ISO 527-2 | Type 1A | 2500-3200 MPa |
| Charpy impact (notched) | ISO 179-1 | Type 1 | 5-12 kJ/m² |
| HDT (1.8 MPa) | ISO 75-2 | 80 × 10 × 4 mm | 90-110 °C |
| Melting temperature | ISO 11357-3 | Granules | 165-178 °C |
| Density | ISO 1183-1 | Molded disk | 1.39-1.42 g/cm³ |
Critical: POM is hygroscopic. Pellets must be dried at 80-100 °C for 3-4 hours before injection moulding to achieve a moisture content below 0.1 %. Moulding with higher moisture content causes hydrolysis, resulting in molecular weight degradation and reduced mechanical properties — particularly elongation at break.
3. Property Determination and Reporting
The standard references multiple ISO test methods for property determination. Conditioned specimens must be stored at 23 ± 2 °C and 50 ± 10 % relative humidity for at least 48 hours before testing. Results are reported as arithmetic means with standard deviations. For design purposes, designers should use the lower confidence interval of the reported values rather than the mean, accounting for processing variability in production environments.
4. Frequently Asked Questions
Q1: Why must POM be dried before moulding?
A: POM undergoes acid-catalyzed hydrolysis at melt temperatures. Moisture above 0.1 % causes formaldehyde release and chain scission, reducing molecular weight by up to 50 %.
A: POM undergoes acid-catalyzed hydrolysis at melt temperatures. Moisture above 0.1 % causes formaldehyde release and chain scission, reducing molecular weight by up to 50 %.
Q2: Can test specimens be machined from moulded parts?
A: Not for ISO 29988-2 compliance. Only injection-molded specimens produced under standard conditions provide comparable property data.
A: Not for ISO 29988-2 compliance. Only injection-molded specimens produced under standard conditions provide comparable property data.
Q3: How do glass fiber grades affect property testing?
A: Reinforced grades require higher melt temperatures (200-220 °C) and specialized mould design to ensure fiber orientation consistency across the specimen.
A: Reinforced grades require higher melt temperatures (200-220 °C) and specialized mould design to ensure fiber orientation consistency across the specimen.
