Selection Specification of Anti-Corrosion Steel for Rigid PVC Accessories of Chinese Injection Molds

During the injection molding production of rigid PVC accessories in domestic mold factories, high-temperature molten PVC will continuously decompose hydrogen chloride gas. When combined with water vapor in the mold and production environment, it forms acidic corrosive substances that erode mold cavities, cores, runners, gates and cooling systems. Many domestic mold manufacturers tend to adopt ordinary pre-hardened steel such as P20 and 718H to control costs. However, these conventional steels lack acid resistance and are prone to pitting corrosion, rust accumulation and surface carbonization after continuous PVC production, resulting in product yellowing, black spots, surface roughness and high mold maintenance rates. Combined with the actual processing conditions, domestic steel supply standards and mass production requirements of China’s injection molding industry, this paper systematically sorts out standardized anti-corrosion steel selection principles for rigid PVC molds to ensure mold stability, prolong service life and control comprehensive production costs.

1. Basic Selection Principles of Anti-Corrosion Mold Steel for Rigid PVC

Corrosion resistance to hydrogen chloride is the primary criterion for PVC mold steel selection, followed by polishing performance, wear resistance and thermal stability under long-term thermal cycling. All molding parts that directly contact PVC melt, including cavities, cores, gate inserts and runner blocks, must use mold steel with a chromium content of more than 13%. Ordinary pre-hardened steel and carbon structural steel are strictly prohibited for molding surfaces, because their loose metal microstructure cannot resist acid ion penetration, which will cause rapid corrosion and irreversible mold damage.

In accordance with the cost control habits of domestic mold projects, differentiated material selection shall be implemented for different mold components. Anti-corrosion stainless steel is used for melt-contact molding parts, while common mold base steel can be used for non-contact structural plates to balance performance and cost. In terms of smelting standards, ESR electroslag remelting steel is preferred for key molding components. ESR steel features low impurity content, uniform carbide distribution and stable microstructure, which effectively improves corrosion resistance and high-gloss polishing performance, fully adapting to the continuous and high-frequency production mode of domestic injection molding workshops.

2. Matching of Steel Grades and Hardness for Domestic Mainstream Working Conditions

For small and medium-batch rigid PVC products with matte surface and low filler content, domestic 4Cr13 stainless steel is the most cost-effective conventional material. After vacuum quenching, the hardness is controlled at HRC 48–52, and the pre-hardened state is HB 280–320. This steel has sufficient domestic supply, excellent machinability and moderate anti-corrosion performance, suitable for conventional PVC pipe fittings and common structural accessories with a mold life of 100,000 to 450,000 shots.

For medium-volume production and medium appearance requirements, 1.2316 ESR steel is widely used in domestic PVC mold manufacturing. With a chromium content of approximately 16%, its acid corrosion resistance is significantly better than 4Cr13. The heat-treated hardness is stabilized at HRC 45–50. It is applicable to household appliance PVC accessories and structural parts, supporting stable mass production of 500,000 to 1,200,000 shots.

For high-gloss appearance, high-precision and high-volume PVC products containing mineral powder or glass fiber fillers, S136 ESR / S136H is the preferred high-grade anti-corrosion steel. With pre-hardened hardness of HB 290–330 and quenched hardness of HRC 48–52, it can achieve mirror polishing effect and maintain stable anti-corrosion performance under long-term high-temperature production. It is the most versatile high-standard steel for high-quality rigid PVC molds in China, with a service life of 800,000 to 1,500,000 shots.

For harsh working conditions such as non-stop continuous production, high-filler formula and transparent PVC products, 1.2083 ESR steel is selected. It has higher chromium purity and stronger resistance to chloride ion corrosion, with heat-treated hardness of HRC 50–54, balancing excellent anti-corrosion and anti-wear properties.

3. Standardized Selection Rules for Mold Components

In domestic mold manufacturing, component classification selection must be standardized to avoid overall cost increase while ensuring anti-corrosion capability. All molding functional parts including cavities, cores, gate sleeves and runner inserts must adopt professional anti-corrosion stainless steel. Sliders, lifters and moving inserts that are easy to accumulate PVC overflow shall prefer anti-corrosion steel; if restricted by cost, ordinary steel substrates must be treated with hard chrome plating with a thickness of 0.03–0.05 mm, and re-plating is mandatory after grinding and repair.

Ejector pins and sleeves directly contacting plastic products must use anti-corrosion materials or electroplated anti-corrosion treatment. Mold base plates, top plates and non-mold-surface structural parts do not contact melt, so ordinary domestic pre-hardened steel can be used with regular rust prevention maintenance.

4. Heat Treatment and Factory Inspection Specifications

All anti-corrosion stainless steel for PVC molds must adopt vacuum quenching and double tempering processes to stabilize hardness and eliminate internal stress, preventing microstructure deformation that reduces corrosion resistance. Welding repair on molding surfaces is strictly prohibited, because welding thermal deformation will destroy the original anti-corrosion structure and form local corrosion-prone areas.

Incoming inspection must verify steel material certificates, chromium content, smelting process and hardness uniformity. The hardness deviation of a single mold module shall not exceed HRC 3 to ensure consistent anti-corrosion performance of the entire mold.

Conclusion

The anti-corrosion steel selection of domestic rigid PVC injection molds must be based on localized production conditions, batch requirements and product grades. Adopting differentiated stainless steel matching, standardized heat treatment and component classification application can fundamentally solve mold corrosion problems caused by PVC acid decomposition. Reasonable steel specification selection not only improves product appearance quality and mold stability, but also reduces frequent mold maintenance, which is the core technical guarantee for long-term stable mass production of domestic PVC injection molding projects.