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The critical nature of fill

June 7, 2001

4 Min Read
The critical nature of fill

If it were possible, Joe Hoepfl would turn back the clock. He could save Snap-on Tools the cost of replacing one of the plates on a three-plate mold if he had only known then what he knows now. Instead, he learned a valuable lesson in the use of moldfilling analysis software that he will not forget. 

As a principal engineer for the $2 billion Snap-on Tools in Kenosha, WI, Hoepfl designs hand tools such as screwdrivers, pliers, and wrenches. About three years ago, Snap-on implemented a program to include soft-touch handles on a number of its products, including a line of screwdrivers called Soft Grip. The screwdriver handles were to be made by overmolding an elastomer over a polypropylene substrate. 

Never having worked with two-shot molding, Hoepfl enlisted the aid of Santoprene TPE manufacturer Advanced Elastomer Systems LP (Akron, OH). "AES offered us the ability to look at our initial design and make recommendations," says Hoepfl. "Together, we analyzed the product design, but we didn't look at the tooling." 

Unsuccessful Iterations 
Based on analysis of the part using Moldflow software, Snap-on had an eight-cavity, three-plate family tool built, which included components of two different sizes. Molding technicians next began to debug the tool in a molding cell designed for this project. The cell, located at the company's captive molding plant in Johnson City, TN, consisted of a 300-ton press for the elastomer and a 200-ton machine for the polypropylene—both from Engel—with an Engel five-axis robot tasked with transporting PP parts to the TPE machine. 

It was at this point that Hoepfl's nightmare began. The parts would not fill properly. "We had voids, short fills—all sorts of problems," he recalls. Along with the toolmaker, the plant's technicians used trial and error in their attempts to fix the process, but with only limited success. "We fell into a hole and spent considerable time trying to get out of it ourselves before we went back to AES for help," Hoepfl adds. 

When Snap-on did approach AES, the TPE firm's Goran Pesevski, senior marketing technical service rep, suggested using Moldflow's tool analysis module. 

Sizing Down 
So what did these analysis results reveal? "We had a recommendation that was quite different from conventional thinking regarding our gate and runner system in our TPE mold," Hoepfl says. "We needed to make the runners considerably smaller." 

The reason for this, explains Pesevski, were the two different volumes required to fill the tool. "To do that, you have to choke one runner down more than the other to balance the fill pattern," he says. "We sized the runners down more than 50 percent." Using the molding analysis software removed the iterative troubleshooting process, he adds, and replaced it with a quick and accurate solution. 

Unfortunately for Snap-on, finding out the optimal runner sizes after the tool was already built meant cutting new steel for the TPE mold's third plate. Hoepfl states that, in addition to the cost of the plate, the company lost valuable time during this debugging phase. "Trial and error costs you money," he laments. 

Yet, Hoepfl has learned from this experience. He drove the efforts for Snap-on engineering to purchase its own copy of Moldflow, and the company has used it in a number of other projects involving soft-grip products. "As far as I'm concerned, it's an important design tool, just as important as our CAD system," he asserts. "It's that important to plastic design." 

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