PVC is one of the most widely used general-purpose plastics, but it is highly prone to thermal decomposition during processing, releasing corrosive hydrogen chloride (HCl) gas. This gas causes severe corrosion on mold cavities, parting lines, vents and moving components, leading to pitting, dimensional deviation and premature failure. Therefore, mold material selection for PVC must prioritize corrosion resistance, followed by wear resistance, polishability and dimensional stability.

Core Requirements for PVC Molds

PVC decomposition produces acidic gas that attacks carbon steels and common mold steels. In addition, rigid PVC often contains fillers such as calcium carbonate and glass fiber, increasing abrasive wear. Soft PVC tends to leave deposits that accelerate corrosion. Mold materials must resist HCl corrosion, maintain smooth surface, avoid rusting and ensure long service life.

Material Selection by Production Volume

For small-batch or prototype molds with production under 100,000 shots, modified corrosion-resistant pre-hardened steels such as 718H corrosion-improved grade are suitable, with hardness 30–35 HRC. They offer basic corrosion resistance and can be directly machined without heat treatment.

For medium-batch molds of 100,000–500,000 shots, martensitic stainless steels are the industry standard, mainly S136 (1.2083), STAVAX and 4Cr13 series. After quenching and tempering, hardness reaches 48–52 HRC, with excellent corrosion resistance and good polishability up to mirror finish. These materials effectively resist HCl attack and ensure stable performance for transparent or high-gloss PVC products.

For high-volume long-life molds over 500,000 shots, high-carbon high-chromium stainless steels such as S136H, 1.2316 and special corrosion-resistant hot-work steels are preferred, with hardness 50–54 HRC. They provide superior wear and corrosion resistance for continuous mass production.

For highly filled rigid PVC with severe abrasion, stainless steels with surface strengthening such as ion nitriding, hard chrome plating or PVD CrN coating are recommended to enhance surface hardness up to 800–1000 HV.

Materials for Moving Components

Ejector pins, guide pillars, slides and bushes must also use corrosion-resistant materials, including stainless ejector pins, 420 stainless steel, nitrided stainless components or SKH51 high-speed steel. Standard carbon steel or bearing steel parts will rust and seize quickly in PVC production.

Surface Treatment Enhancements

Even stainless molds benefit from surface treatments. CrN coating is particularly suitable for PVC, offering excellent corrosion resistance, low friction and anti-stick performance. Ion nitriding improves wear resistance without losing corrosion resistance. Mirror molds can be passivated after polishing to seal micro-pores.

Selection Taboos and Common Mistakes

Standard P20, NAK80 and uncoated H13 must not be used for PVC molding cavities. Using stainless steel only for cavities while keeping other parts in carbon steel leads to overall corrosion. S136 is not completely corrosion-proof; strict temperature control, reduced decomposition and proper shutdown protection are still necessary.

Summary

PVC mold material selection centers on corrosion resistance. Small batches use pre-hardened corrosion-resistant steels; medium batches use S136/1.2316 stainless steels; high-volume production uses high-hardness mirror stainless steels. All moving parts adopt stainless or treated components. Combined with proper surface treatment, molds can resist HCl corrosion and filler wear, ensuring long service life, stable dimensions and high product quality.