The Problem:

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.

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The Problem:

Severe corrosion from a Halogen-free flame retardant additive completely consumes powder metallurgy feedscrews in as little as three months. This aggressive material has been introduced to meet new RoHS (Restriction of Hazardous Substances) standards established by the European Union. Extreme Coatings protective carbide encapsulation completely isolates a feedscrew from the process environment which eliminates corrosion in these applications. Feedscrews that before were quickly worn can now process for 18 to 24 months in this aggressive environment. This is a reprint of an original article that appeared on 5 June 2008 in K-Zeitung and in issue no. 6/2008 of Kunststoff-Berater

May 2008, Wiehl, Germany: Thanks to innovative feedscrew technology involving tungstencarbide (TC) coating, Rudolf Donner e.K., headquartered at Wiehl-Angfurten in Germany, is extremely well prepared for the processing of polyamides (PA) with high proportions of fiberglass. The company, having been active in the injection molding sector since 1972, predicts a growing market in this field. Specialized screws from Groche Technik, in Kalletal (Germany) (and tungsten carbide coating by Extreme Coatings St. Petersburg Florida USA) guarantee optimum plasticizing of these highly abrasive polymer blends, at the same time giving noticeably higher life times when compared with conventional screws.

Mike Donner, CEO of Donner Spritzgussfertigung e.K. sees his company as the ‘extended workbench for the automobile industry’. Working on a three shift, seven day a week basis, the company, founded some 36 years ago as a family business, processes technical polymers – mostly with high percentages of fiberglass – on 12 injection molding machines. Donner e.K. manufactures motor vehicle components as well as tracks or cable guiding crawlers, for instance for robots. According to Mike Donner, processing polymers, primarily PAs containing proportions of fiberglass between 25 and 40 percent, the standard Groche Technik feedscrew of powder metallurgically hardened steel and a bi-metallic cylinder, has proved itself optimally.

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The Problem:

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.

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