Material Selection for PVC Injection Molds

PVC (Polyvinyl Chloride) is a versatile and cost-effective thermoplastic widely used in injection molding. However, its unique properties—such as releasing mildly corrosive hydrogen chloride (HCl) gas during melting, high thermal sensitivity, and moderate flowability—pose specific challenges for mold material selection. Choosing the right material is critical to ensuring mold durability, product quality consistency, and production efficiency. This article outlines the key guidelines for selecting mold steels for PVC injection molding, covering different production scales and product requirements.

Key Requirements for PVC Mold Steels

The primary requirement for PVC mold materials is corrosion resistance and rust prevention, due to the HCl gas released during processing. Ordinary carbon steels (e.g., P20, 45#) lack this resistance and will quickly rust, pit, or darken, leading to product defects (scratches, uneven color) and mold damage.

Additional key requirements include good dimensional stability (to counter PVC’s 1.0%-2.5% shrinkage rate) and polishability (to ensure smooth melt filling and high-quality surface finish). Hardness and wear resistance are secondary unless the product contains abrasive additives like glass fiber.

Steels for Small-to-Medium Batch Production

For small-to-medium batch PVC products (≤300,000 shots), pre-hardened corrosion-resistant steels are cost-effective:

718H: Nickel-chromium-molybdenum alloy with HRC 30–35 hardness (no post-delivery quenching). It has basic corrosion resistance, good machinability, and can be polished to semi-gloss (Ra ≤ 0.8 μm), suitable for soft PVC (hoses, gaskets) and general rigid PVC (pipes, simple parts).

2316 (pre-hardened): Superior corrosion resistance to 718H, HRC 32–36 hardness, and polishability up to Ra ≤ 0.4 μm. Ideal for PVC parts with higher surface requirements (decorative parts, precision fittings).

Steels for High-Volume & Long-Run Production

For high-volume production (≥500,000 shots), through-hardened stainless steels are necessary:

S136: Martensitic stainless steel (13.0%-14.5% Cr) with HRC 48–52 hardness after quenching. It offers excellent corrosion resistance, ultra-low distortion (≤0.05 mm/m), and super-mirror finish (Ra ≤ 0.025 μm), suitable for high-precision PVC (medical accessories, transparent packaging) with service life over 1 million shots.

2316 (heat-treated): Quenched to HRC 48–52, with corrosion resistance comparable to S136 but lower cost. Suitable for high-volume PVC (pipes, valves) and meets most industrial requirements.

Steels for Transparent & Soft PVC

Transparent PVC requires ultra-smooth molds to avoid flow marks or haze; soft PVC needs anti-sticking properties:

NAK80: Pre-hardened mirror steel (HRC 38–42), achieving mirror finish (Ra ≤ 0.03 μm) without extra polishing. Good corrosion resistance to soft PVC plasticizers, ideal for small-to-medium batch transparent/soft PVC products.

High-grade S136: Best for high-end transparent PVC (optical components, medical parts), with super-mirror surface and long-term corrosion resistance.

Structural Component Selection

To optimize cost, non-cavity components can use standard materials:

Mold bases: S50C carbon steel (sufficient strength, cost-effective).

Guide pins/bushings: SUJ2 bearing steel (wear-resistant, precise).

Ejector pins: Stainless steel or SKD61 (corrosion-resistant, anti-seizing).

Runners/gates: Must match cavity material (corrosion-resistant) to ensure consistent performance.

Conclusion

Selecting PVC mold materials requires prioritizing corrosion resistance, balanced with dimensional stability and polishability. For small batches, choose 718H or pre-hardened 2316; for high-volume production, use S136 or heat-treated 2316; for transparent/soft PVC, opt for NAK80 or high-grade S136. Proper material selection ensures mold durability, stable product quality, and reduced production costs.