China Plastic Molds: Material Selection and Key Processing Points for POM Injection Molds

POM, also known as acetal or polyoxymethylene, is a high‑performance crystalline engineering plastic widely applied for gears, sliding parts and structural connectors. It features high hardness, low friction coefficient and excellent wear‑resistance. Nevertheless, high molding shrinkage, abrasive melt and trace acidic substances released during thermal decomposition bring strict requirements for injection molds. Improper steel selection or machining details will cause rapid cavity wear, rust spots, product surface scratching and unstable dimensional tolerance during mass production. Mold manufacturers in China need to match steel grades, surface treatment and structural processing according to production volume and product requirements to achieve stable long‑run molding of POM components.

1. Mold Steel Grade Selection for POM Injection Molds

Steel selection should fully consider POM’s abrasive property and weak corrosive gas generated in molding process. For small‑batch trial production within tens of thousands of shots, pre‑hardened steel such as P20 with hardness from 28HRC to 32HRC is acceptable. It controls mold cost for simple non‑appearance parts, yet surface strengthening treatment must be implemented to prevent early wear. For medium‑volume production up to hundreds of thousands cycles, 718H steel at 33‑38HRC stands as a practical option. It delivers good polishing performance, and further improves wear resistance after nitriding treatment, covering most general‑purpose POM structural components.

For large‑scale continuous mass production especially wear‑critical POM gears and sliding fittings, corrosion‑resistant mirror steel S136 is highly recommended. After quenching, S136 reaches hardness of 48‑52HRC, resisting acid corrosion from decomposed POM melt and supporting high‑gloss polishing. NAK80 is another alternative for high‑precision appearance‑demanding articles. SKD61 can be adopted together with nitriding for enhanced surface hardness in heavily abrasive working conditions. Ordinary non‑hardened steel is not suitable for POM mass‑production molds, as cavity surfaces will suffer abrasion quickly and trigger frequent quality fluctuations.

2. Suitable Surface Treatment Processes for POM Molds

Proper surface treatment improves anti‑corrosion, wear‑resistance and demolding performance beyond raw steel performance. Gas nitriding is widely used for inserts, sliders and ejector pins. Nitriding layer thickness shall be controlled between 0.05 mm and 0.1 mm to avoid layer peeling under repeated thermal cycling. Polishing quality directly influences POM demolding. Crystalline POM tends to stick and scratch, so forming cavities require fine mirror polishing to eliminate tool marks and sanding traces. Deep cavities, thin ribs and undercut zones need extra careful polishing work to reduce friction between molded plastic and mold steel.

Hard chrome plating can be applied on partial insert components for extra wear protection, but overly thick plating should be avoided for fear of peeling under continuous high‑temperature cycles. S136 after precise polishing may skip electroplating procedures. Its inherent corrosion resistance can handle trace acid gas released by POM melt, and unnecessary plating will damage mirror surface finish and hurt product appearance.

3. Core Machining and Structural Key Points

POM owns typical molding shrinkage ranging from 1.5 % to 2.5 %, varying with material grades and molding parameters. Cavity dimensions shall reserve sufficient shrinkage allowance during machining. Extra grinding allowance should be kept for complex‑geometry products to adjust cavity size after trial molding. Venting system deserves special attention, because fast‑solidifying POM melt easily creates burning marks, bubbles and short shots when trapped air cannot escape. Vent slot depth is controlled at 0.015‑0.03 mm on parting lines, insert joints and melt end positions. Vent width shall be enlarged appropriately to prevent blockage by plastic residues.

Ejection mechanism must be carefully processed. POM parts turn rigid and brittle after cooling. Uneven ejection force will result in whitening or cracking. Ejector pins and blocks maintain smooth surface finish with reasonable fitting clearance to prevent sticking. Additional ejection positions are arranged for deep‑cavity articles to distribute ejection stress. Runner and gate design adopts short flow path to cut pressure loss. Gate size cannot be too small, otherwise excessive shear heat will trigger POM thermal decomposition. Tunnel gates and point gates must be finely polished. Cooling channels distribute evenly around mold cavity to stabilize mold temperature within 60‑90℃. Balanced mold temperature reduces shrinkage fluctuation and local rust risk.

4. Assembly and Routine Maintenance Requirements

During mold assembly, fitting clearances for inserts and sliders need strict control. Excessive gaps produce flash while insufficient gaps cause seizure during movement. Every moving component must slide smoothly without jamming after assembly. Do not machine cavity dimensions to final size in one pass; keep allowance for adjustment according to actual shrinkage data from trial runs. In daily production, clean plastic residues and vent slots regularly. Trace corrosive decomposition products of POM accumulated on cavity surfaces will induce rust if left untreated. Apply anti‑rust protection thoroughly for long‑term shutdown. Avoid hard impact on nitrided or plated surfaces, once protective layers get damaged, corrosion will spread rapidly and shorten overall mold service life.

Summary

POM injection mold design and manufacturing must cope with high shrinkage, abrasive melt and slight corrosive decomposition products. Steel grades are selected based on production batch size. Corrosion‑resistant mirror steel serves mass‑volume projects, cooperating with reasonable nitriding and polishing treatments. Manufacturers focus on shrinkage allowance, venting layout, ejection structure and gate runner details in machining phase. Assembly clearance control plus standardized daily maintenance reduce cavity corrosion and abrasion. Well‑executed mold making work helps factories avoid scratching, burning and dimensional drifting defects, supporting consistent mass‑production of high‑quality POM molded parts.