C4000 Chromium Carbide - Injection Molding, Corrosion
This small custom molder supplies injection molded parts to a Tier 1 automotive supplier. The parts are part of seat assemblies that are then delivered to the auto assembly line as Just-in-Time inventory.
These parts are right and left seat-back adjustment handles. They are made from flexible PVC with talc filler. In the spring of 2005 parts were shipped from the supplier in Michigan to the customer in Georgia for final assembly. The change in humidity caused a rust colored “blush” to form on these parts and they were immediately rejected by the customer. No similarly defective parts could be found at the supplier’s site.
Investigation into the source of contamination quickly ruled out raw materials and the focus was shifted to molding equipment. Standard procedure was to remove and clean a feed screw when operators noticed char particles in finished parts. Screws are cleaned with an aggressive steel wire wheel and scraping of the screw surface.
This 50 mm feed screw, unique to the machine manufacturer, is a 2-stage design not well suited to PVC processing. An aggressive molding process input too much shear energy to the material which resulted in HCL formation. Acid attack, the talc filler, and an aggressive cleaning procedure caused a breakdown of the chrome plating and allowed iron contamination to leach into finished products. This contaminant in the presence of moisture developed the discoloration that caused parts to be rejected.
A general purpose screw with our XC4000 Chromium carbide was installed in May of 2005. Immediately the rust “blush” ceased to be a problem. The molder tested finished parts in high humidity, per their customer’s requirements, to try and cause this defect to appear. No defects were found.
Screw cleaning, which had been required two to three times per week, was no longer an issue. The hard, dense carbide surface with a mirror finish eliminated the build up of material on the screw. Now occasional cleaning, when changing from dark to light materials, is a simple procedure.
The hard, high polish surface eliminated polymer buildup.
Less frequent cleaning was required.
Screw life will likely increase by a factor of three to five.