1. Extruder Screw and Barrel: This is the core area for corrosion resistance. The base material for screws and barrels is typically alloy steel like 38CrMoAlA, but the true corrosion protection comes from surface treatment processes. A common method is hard chrome plating, which provides a dense protective layer effectively isolating corrosive gases. More advanced techniques include bimetallic casting or nitriding, forming a high-hardness corrosion-resistant layer on the surface. If this used equipment previously processed PVC, carefully inspect the screw flutes and barrel inner walls for severe pitting, spotting, or signs of chrome plating peeling. Once the protective layer is compromised, the base metal corrodes rapidly, leading to reduced extrusion output and potentially black scorch marks in the product.
2. Molds (Sprue and Die): Mold runners come into direct contact with high-temperature melt, demanding exceptional corrosion resistance. The steel used for mold fabrication must exhibit excellent corrosion resistance and polishability, with common materials including H13, P20, or higher-grade stainless steel. Internal runners must be designed with streamlined, dead-angle-free geometries and mirror-polished surfaces to prevent prolonged accumulation of corrosive gases at stagnation points, which can lead to pitting corrosion. When inspecting used molds, it is crucial to open the mold and examine whether the runner surfaces are mirror-smooth, free of darkened areas or pits caused by corrosion. This directly determines the mold's remaining service life and the surface quality of the finished products.
3. Forming Molds and Cooling Water Tanks: Although this is a cooling zone, PVC volatiles may condense in the water, forming weakly acidic water. Forming molds are typically made of aluminum alloy or stainless steel. Aluminum alloy molds are lightweight and have good thermal conductivity, but their surfaces usually require hard anodizing to form a corrosion-resistant layer. If the anodized layer is damaged, the aluminum alloy will corrode rapidly in acidic water, developing white spots and powdering. Stainless steel tanks and molds offer superior corrosion resistance but come at a higher cost. Inspecting the vacuum chamber and water inlet connections of the molding mold for rust or leaks provides a straightforward way to assess the corrosion resistance of these components.
4. Traction Machine Tracks and Cutting Machine Blades: Although these components do not directly contact high-temperature molten material, they may come into contact with corrosive substances volatilizing or condensing from the pipe surface. If the chrome plating on the traction machine's upper pressure cylinder and guide rods is damaged, rusting and jamming can easily occur. The saw blades or planetary cutting blades of the cutter are typically made of high-speed steel or feature carbide-tipped inserts. They must possess adequate corrosion resistance to prevent rusting that dulls the cutting edge and compromises cut quality.