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.
Astonishingly high continuous resilience “We are experiencing significantly low levels of wear and tear with the powder metal”, explains Mike Donner. The screw, with its 35 mm diameter, is installed in a 75 ton injection molding machine. Predominantly switch shutters are produced on this unit, parts which have to withstand high temperatures and high levels of wear and tear. These components weigh approximately 75 g with a cycle time of 20 seconds. “Above all, we have been pleasantly surprised with the resilience levels of the screw at continuous operation. The screw has now been installed for the last five years”, so Donner explains. Generally, only the non-return valve check rings have to be exchanged now and then. This standard unit has been operating for a number of years with extreme reliability. Donner explains that these switch shutters are produced by the million. More than 100,000 units leave the Donner workshops every month. “The injection molding machine used is now over 10 years old. Powder metal has been running like clockwork for the last five years – almost entirely without down time. Up till now, no maintenance has been necessary. Even the cleaning process, essential in color or material change, has been totally unproblematic”, explains Mike Donner.
The powder metal is superbly suited when using polymers with conventional proportions of fiberglass, i.e. for switch shutters. However, powder metal does reach its limits when it comes to particularly aggressive and corrosion triggering polymer blends – as for the manufacture of elements for the cable guiding crawlers. As demonstrated by Mike Donner, powder metal screws processing such materials would have the appearance of an orange skin after just four to six weeks. “In other words, they are scrap!” A new cylinder/screw set, costing a few thousand Euros, will last eight weeks maximum. After that, variations in the process parameters become apparent and the melt cushion slides downwards; in addition, wear increases steadily. This becomes apparent on the product: deviation from the target dimensions combined with possible streaking. The search for new screw solutions began. According to Donner, Groche Technik offered the ideal answer, Extreme Coatings tungsten-carbide encapsulation. From then on, tungsten carbide (TC) coated screws have been installed processing PA polymers with up to 45 percent fiberglass, manufacturing components for cable guiding crawlers. “Our customer substituted a proprietary polymer without informing us. In addition to the high fiberglass content this polymer contains a unique halogen-free flame retardant additive that multiplies the effects of abrasion and corrosion within the cylinder/screw set.”
A 35 mm TC encapsulated screw was installed in an 800-kN DEMAG ergotech Extra 80-310 injection molding machine in September 2006 on a trial basis. Anticipation was high and the obtained result exceeded all expectations. The cylinder/screw set withstands the highly aggressive flame retardant without difficulty. The production of the crawler elements runs dayin day-out and only permits cycle times of 20 seconds with a component weight of 30 g. Reject levels run below two percent with tolerances, i.e. precision of produced component within the region of 100th of a millimeter. Mike Donner again: “At the last change of the non-return valve check ring some six months ago, the screw was still in very good condition.” This means that that the TC encapsulated screw has now been in operation for more than 15 months. This prolongs the average three-month life span of a conventional powder metal screw by a factor of five. It goes without saying that this level of quality comes at a price. A TC encapsulated screw adds roughly 50% more than the cost of a standard powder metal screw. “This investment has been absolutely worthwhile”, Mike Donner summarizes. Apart from the significantly higher life span, the cleaning process of the screw is much simpler. “We are committed to deliver maximum product quality. This is only achievable with the application of state-of-the-art technology, well trained employees, and verified quality control; as far as we are concerned, all this goes without saying, as our customers already insist on those parameters.” At Donner, the boss, together with three of his colleagues, take care of quality control. Mike Donner considers fiberglass enhanced PA polymers to be a lasting growth market. “These materials are becoming ever more sophisticated. It therefore makes sense to invest in TC protected cylinder/screw sets when installing new injection molding machines”, emphasizes Mike Donner. This is the ultimate as far as sophisticated materials are concerned and also allows to process polymers with less demanding production processes. “Groche screws with Extreme Coatings TC encapsulation are the very best available in today’s market. We are all set to meet all new challenges,” Mike Donner is convinced.