Copolymer POM is the default selection for injection-molded parts across automotive, electrical, and medical tooling programs, because its processing window (typical melt 190-210 °C, mold surface 80-100 °C) tolerates wider shear and temperature swings than homopolymer grades while emitting noticeably lower formaldehyde during plastication [S1].
POM grade split: homopolymer vs copolymer in tooled parts
Homopolymer POM delivers higher tensile strength and better creep resistance at continuous service temperatures up to about 100 °C, which is why it still appears in thin-wall snap-fit and gear-geometry parts that demand tight dimensional return after ejection [S1].
Copolymer grades are the workhorse inside most custom injection mold programs, including automotive connectors and medical device housings, because the wider thermal stability window reduces the risk of die-drool, plate-out, and mold deposit that would otherwise force a tool-cleaning stop every 40,000-60,000 shots on a production casting mold [S1].
When the same engineering team has to pick a grade for a different application, the POM selection for electronics: homopolymer vs copolymer decision map reference lays out the electronics-specific decision rules that can be re-applied, with adjustments, to mold-making for non-electronic parts.
Mold steel selection driven by the resin being processed
For unfilled copolymer POM running neat or in natural color, pre-hardened P20 (1.2311, ~30-32 HRC) is the standard cavity-and-core steel because it machines cleanly, polishes to A2-grade finish, and survives the 1.8-2.2% shrinkage without core shift on a properly supported mold base [S1].
When the spec calls for glass-fiber-filled POM (typically 10-30% GF for stiffness), or for any abrasive mineral-filled variant, P20 wears fast at the gate and ejector pin interface; H13 (1.2343, 48-52 HRC) or S136 (1.2083, 50-54 HRC) becomes the default to hold surface finish and gate-edge sharpness past 500,000 cycles, especially on long-run automotive connector tools [S1][S2].
A toolmaker quoting the same part in both neat and GF-filled POM should price two steel stacks: P20 for the natural grade, H13 or S136 once the resin datasheet lists a filler load above 10%, because the steel delta is typically 25-40% of cavity machining cost on a small die casting die derivative tool [S2].
Shrinkage compensation and cavity geometry
For a 100 mm nominal dimension in neat copolymer POM, the cavity must therefore be cut at roughly 101.8-102.2 mm, and the tool designer should reserve a separate shrinkage factor for each wall-thickness zone when a single part mixes 1.5 mm and 4.0 mm sections, because the thin wall will shrink ~0.2 percentage points less than the thick boss.
Draft angle is the second geometry gate: Polyplastics recommends a minimum of 0.5-1.0° per side on POM parts to keep ejection forces low and protect the surface finish on polished-cavity tooling [S1].
Cooling, cycle time, and ejector layout
POM's relatively low specific heat and fast crystallization let cycles run at 15-40 s on small connector tools and 60-120 s on thick-walled housing tools, provided the cooling channels sit within 1.0-1.5x the wall thickness from the cavity surface [S1].
Draft, ejector pin count, and under-cut release are the three ejector-side parameters that the tool maker should re-check every time a POM grade is swapped, because homopolymer and copolymer can differ by 0.1-0.3% in as-molded shrinkage even on the same tool, and the ejector return stroke is the first place that delta shows up as a flash line.
Where POM tool-making collides with metal die casting
POM injection tooling and metal die casting tooling share the same DFMEA, tooling BOM, 2D/3D drawing, and T1 sample dimension-report workflow, and Longxiang's published tooling procedure lists Order, DFMEA, Tooling BOM Form, 2D Drawing, Cutting Tooling Notice, Weekly Report, 3D drawing, and T1 Dimension Report as the eight mandatory gates regardless of the molded material [S2].
The intersection matters for hybrid parts: a metal insert (such as a threaded brass bush) overmolded with POM in the same tool shifts the steel selection away from P20 toward H13 or S136 at the insert interface, because the localized thermal mass of the metal insert elevates local cavity temperature by 15-30 °C and accelerates P20 temper softening [S2].
For non-POM work, the Holding furnace selection for telecom enclosure die cast cells reference covers the upstream thermal side of a die-cast cell, while sand casting mold processes handle the larger ferrous parts that never go through a plastic injection tool.
Limitations and what POM tooling will not solve
POM injection tools cannot replace metal casting or metal forging tooling when the end-part requires service temperatures above roughly 100 °C continuous, or when the load case calls for steel-grade mechanical properties; the plastic tool's job ends at producing the plastic component, not the metal one [S1][S4].
Inside a die casting cell, the steel tool is sized for the alloy's solidification shrinkage and thermal shock, not for plastic-shrinkage values, and mixing the two design rules in one tool leads to either short-shots (steel tool cut for plastic shrinkage) or cracked inserts (plastic tool cut for metal shrinkage) [S4].
Tool life also drops sharply on POM tools if the resin stream is left idle in the barrel past 15-20 minutes; Polyplastics explicitly warns that prolonged residence in the injection unit causes severe molecular-weight degradation and a corresponding drop in part toughness, which the tool designer cannot fix by adding ejector pins [S1].
Track, on the next RFQ, whether the resin datasheet specifies a glass-fiber or mineral filler above 10%, the as-molded shrinkage value with its test method (ISO 294-4 is the most common), and the target cycle count: those three numbers, plus cavity layout, fix whether P20 is enough or H13/S136 has to be quoted on the die casting die ticket from day one.