Colors, screws, and changes

The 1000 or so standardized colors used by Silhouette in making eyeglass frames are cataloged in sliding wall racks. Besides reducing color change times, Engel’s Marathon screws reduce the risk of nonhomogenous distribution of mica- and aluminum-based special-effect pigments.Sometimes new products do not follow the exact route envisioned by their creators. Users of the product often surprise the maker. Though this could be a problem, that is certainly not the case with Engel’s Marathon Screw.

IMM remembers the introduction of the Marathon screw line in 2000 when Engel stressed the longer service life to be expected with these carbide-coated screws. Other benefits of the Marathon screws mentioned at the time included plasticating capacity 5 to 8 percent greater than hardened steel screws, reduced torque requirements for plastication, the ability to reduce melt temperature by about 5 deg C, and a narrowing of the residence time profile. These benefits were mostly attributed to a more slippery surface, which provides a low level of friction resistance as the material moves through the barrel.

One of the other benefits of this new screw design was faster color changeover times, which stood to reason if the material slid through faster. As those who frequently change color know, it involves a lot more than flipping a switch, especially in terms of time. If the Marathon screw could indeed save color change time, it would be a real benefit to those companies that make many and/or frequent color changes. A couple of Engel’s customers in Austria who fit that description put it to the test.

Quality Up, Time Down
Eyeglass frame manufacturer Silhouette has a product range broad enough to call for molding 25 different materials, mostly varieties of nylon, in about 1000 colors. The coloring primarily is done via powder masterbatches. Because of the variety of colors and the widely varying and ever-shifting demands of the fashion market, the company normally deals with batch sizes of 50 pieces and sometimes fewer. One of Silhouette’s production units, comprising nine molding machines, performs an average of 20 color changes per shift. That means about 60 color changes in 24 hours, not to mention two or three changes of material.

The screw in back is an Engel Marathon model after the carbide coating has been applied by a flame-spraying process. The screw in the front is also a Marathon, which, after finishing, looks much like a standard hardened metal unit.Silhouette started testing the Marathon screw on one of those machines using a 25-mm unit. Because the initial results after three months were so positive, the company then equipped all nine machines with Marathon screws ranging from 18 to 25 mm. After nine months of use on all of the machines, Silhouette was saving about 30 percent on both time and material for each color change, or the capacity equivalent of an additional 2.7 injection machines.

In addition to the time savings, Silhouette found that the screws helped with quality when using the mica- and aluminum-based pigments that create special effects in the company’s sunglasses. Prolonged residence time raises the risk of nonhomogenous mixing of the pigment particles. The Marathon screw reduced that risk notably, which cut the need for costly visual control and allowed for a higher degree of process automation.

Manufacturing Uptime
Trodat is the Austrian market leader in rubber stamp and marking systems. Processing mainly ABS, PS, POM, and TPE, the company has to cope with about 2500 color changes annually on its 45 injection molding machines. The reason here, too, is small batch sizes. Trodat made a systematic study of the material feeding systems and all the individual components involved in the plastication process.

Peter Baldinger (left), development manager for plasticating units at Engel and Karl Ebenberger, works manager of Trodat, are shown next to some of the latter's many stamping products. Changing screw type and some other processing parameters helped reduce color change time by 30 percent.After a series of tests, the company determined the optimum combination of screw profile, mixing torpedo, and shutoff system. Plastication components such as the cylinder flange and the nozzle were redesigned to improve melt flow, in addition to switching to the Marathon screws. The result is that Trodat dropped its average color change time from 40 to 25 minutes. Multiply the 15-minute time savings by the 2500 color changes, and that means Trodat now enjoys an additional 625 hours of uptime per year.

Engel is currently making Marathon screws in diameters up to 105 mm, although its Steyr, Austria facility has the machinery to make screws as large as 170 mm in diameter and 5m long. More than 1000 Marathon screws are already in use and Engel says they can be used on any injection machine.