This customer makes valves for use with corrosive or harmful materials. These parts are large, thick-walled and made from C-PVC. Large parts necessitate a long cooling cycle which can lead to degradation of the resin waiting to be injected. C-PVC is very shear sensitive and tends to degrade easily if over heated and then stick to the screw surface. Sticking material burns and when released into parts leads to porosity and defects.
About 5,000 to 6,000 parts are made in a typical production run which then receive a post-mold machining step to complete fabrication. It not until this final step that porosity and defects are found. This customer estimates the scrap cost from degradation at $60,000 to $70,000 per year.
Their standard screw is hard faced with double and triple chrome to combat corrosive hydrochloric (HCL) acid that results from degraded PVC. Chrome is porous and HCL can penetrate to the screw surface and weaken the chrome bond. An aggressive cleaning procedure also roughens the screw surface and further loosens poorly bonded chrome. A rough screw surface leads to material build up resulting in to polymer degradation and finally increased scrap.
Scrap rates for this process vary between 15% and 30% depending on part size and the quantity of parts produced. Their standard procedure is to remove and clean the screw before beginning this process and then to purge regularly during production to reduce build up on the screw.