Curing the disposable screw syndrome

At Dinesol Plastics, a workcell running 50 percent glass-filled nylon produced this type of screw wear every four months.Dinesol Plastics’ general manager Buddy Greathouse had a problem that reared its ugly head all too often at the high-volume, 24/7 molding facility. Roughly three times a year, one of the presses at the 150,000-sq-ft plant in Niles, OH needed a screw and barrel replacement. Each time, Greathouse noticed that not only were the flights on the screw eaten away, but the compression area of the barrel was also worn.

It didn’t help that the 300-ton press was running an eight-cavity mold using 50 percent glass-filled nylon for two weeks out of every month. Before switching materials, of course, the screw had to be pulled so that a significant amount of charred material could be cleaned off. “Some of the screws were so bad,” recalls Greathouse, “we needed a grinder to get all of the degraded material off. At the time, we were using 9V screws and 10V liners.”

General Plastex worked with Dinesol as a beta site customer for its Pro-Tex wear package. A screw with hard facing applied via PTA welding, shown here, awaits final machining and carbide coating steps.He estimates that in one year, Dinesol spent more than $19,000 on replacement screws and barrels. It was time for a change. “We decided to work with General Plastex as a beta site customer for its Pro-Tex wear package. They created a custom system, including screw and barrel, that we installed in 1998. Today, there are still no visible signs of wear, and we haven’t had to pull the screw for cleaning in two years because we can now clean the screw thoroughly by purging.”

According to Oscar Toris, GM at General Plastex, the Pro-Tex system is based on a combination of commercially available bimetallic lining materials, a carbide coating, and welding materials, including a .050- to .060-inch hard facing. “It is a proprietary blend of material and process that we call full encapsulation, with the hard facing applied first via plasma transfer arc welding, and then a carbide coating applied on top.”

Switching to a wear-resistant screw and barrel from General Plastex reduced costs at Dinesol by $19,000 per year and cut downtime by 500 hours annually.Today, Greathouse runs primarily glass-filled nylon and Ultem  PEI in the same press without having to pull screws or replace them. He estimates that the company has saved about 2000 hours in downtime over the four-year period. He has also been able to redirect several maintenance personnel to operations positions because the screw no longer needs to be pulled on a regular basis.

Customers of the Pro-Tex package receive a guarantee (on new and rebuilt screws) that wear on both screw and barrel will not exceed .005 inch for 36 months. The guarantee is limited to materials with 50 percent filler and less, and operating temperatures of less than 800F. According to Toris, the package is not applicable to fluoropolymers, powdered metals, or extrusion.