Solution to the generation of spots in the production of flexible PVC using chrome plated screws.
I have the following three options for dealing with the problem of spotting on chrome plating:
I. Upgrade the screw plating (fundamental measures):
1. Chemical nickel-phosphorus (Ni-P) plating: HCl corrosion resistance is better than chromium, no cracks (recommended thickness ≥ 50μm).
2. Composite plating: bottom nickel plating + surface chrome plating (reduce crack exposure) or nickel plating + PTFE (anti-adhesion).
3. Sprayed tungsten carbide (WC-Co): high hardness and complete corrosion resistance (high cost, suitable for long-term production).
ii. Formulation and process optimization:
1. Replacement of sulfur-containing stabilizers: switch to methyltin or rare earth stabilizers (sulfur-free).
2. Control the melting temperature: under the premise of ensuring plasticization, reduce the temperature to 180-190℃. The production of soft PVC at this temperature is completely feasible, but must meet:
✅ Formulation: low polymerization resin + high efficiency plasticizer (≥ 50phr) + methyl tin stabilizer + composite lubrication system
✅ Equipment: optimize the screw structure (compression ratio of ≤ 2.5:1) + strengthen the cooling of the feed section
✅ Process: Appropriate increase in rotational speed and backpressure, add ACR processing aids
3. Shorten the downtime: downtime> 30 minutes Need to clean the screw with LLDPE and apply high-temperature antirust grease.
iii. Adjust the process parameters:
1. Reduce the processing temperature (reduce decomposition under the premise of plasticization).
2. Avoid long time holding downtime, use LLDPE or special cleaning material to clean the screw before downtime.
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