Canadian Plastics

New control slashes waste

By Walter Joos, Mechanical EngineerKeith Peterson, Process EngineerRoss Jackson, Process EngineerCookson Plastic Molding, Mora, MinnesotaRecently, Cookson Plastic Molding experienced unbalanced mold c...

October 1, 1999   Canadian Plastics



By Walter Joos, Mechanical Engineer

Keith Peterson, Process Engineer

Ross Jackson, Process Engineer

Cookson Plastic Molding, Mora, Minnesota

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Recently, Cookson Plastic Molding experienced unbalanced mold conditions in a structural foam molding operation when producing several different parts in a single cycle, making it necessary to overpack some areas in order to fill others. Overpacking, in turn, resulted in warpage and cracking. The problem was solved by introducing a valve gate sequence control system that independently opens and closes up to 48 machine valve gate nozzles at any point during the cycle. Installing valve gate sequence control on the operation resulted in an 80 percent reduction in scrap rate.

Cookson Plastic Molding produces pallets and custom components such as tractor cab roof linings and tool boxes using structural foam molding technology. Nitrogen and/or a chemical blowing agent is introduced into the melt stream and plasticized resin is injected to fill the mold. The blowing agent produces a cellular structure in the interior of the part.

OVERPACKING CREATES STRESS

Overpacking created stresses that sometimes caused warpage or cracking during cooling, making it necessary to scrap the part. In one typical press where the company used four molds producing a 36 pound, two 6 pound, and a 4 pound part on each cycle, scrap rate was 10%. While the material could be reground, the parts themselves had to be run through a very expensive press for a second time. In an attempt to balance the fill, Cookson Plastic Molding engineers tried restricting certain nozzles. But the nozzles that were restricted tended to freeze off, leading to additional scrap due to short shots.

GATE CONTROLLER BALANCES SHOTS

Cookson Plastic Molding engineers looked for a method of controlling the nozzles but couldn’t find any commercial product capable of controlling that many nozzles. Incoe Corp. (Troy, MI) indicated it was introducing a new gate sequence controller system. After viewing a prototype of the new controller, Cookson Plastic Molding agreed to a trial with the new controller.

Cookson Plastic Molding engineers began by setting the Incoe controller for a series of short shots in order to precisely balance the fill. For each shot, they established the opening and closing of each gate in inches or seconds. A combination of both linear screw position and time can also be selected. After each short shot they adjusted the gate opening program to increase the fill and bring it into balance. The ability to precisely control the gate opening and closing made it relatively easy to balance the fill. Once they were satisfied, they stored the gate flow control program within the control unit for future recall.

80% SCRAP REDUCTION

The engineers then turned the machine and program over to the operator. The operator loaded the program and started producing parts. In the past, it took between 8 and 10 shots for the best operators and 20 to 30 shots for less experienced operators to start up the machine. By simply recalling the program, operators of any skill level can produce good parts after only 2 or 3 shots. The new controller has eliminated scrap due to unbalanced fill conditions, reducing total scrap to only about 2%.

Cookson Plastic Molding engineers also discovered they had achieved significant weight savings. The weight of the largest part was reduced by two pounds and the weight of the smaller parts was proportionately reduced. This savings in material usage when added to the scrap reduction savings mentioned above amounted to $40,000 dollars of savings on this one job. In addition, the controller provides control over weld line placement by using machine screw linear position for precise valve gate actuation.

(Circle Reader Service No. 194 )


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