Lose weight, cut scrap now with sequential valves

For most molders, the opportunity to reduce material use and cut scrap rates is a bonus that, in the long run, can save substantial dollars. But the big parts being produced by Cookson Plastic Molding (Mora, MN) make these cost-cutting measures a necessity.

This structural foam molder of pallets, modular material handling systems, and large custom molded parts can, on some programs, spend up to 110 lb of resin in one cycle. Material, therefore, is at a premium, and system processes have to be tightly controlled to reduce scrap.

The problem Cookson had was mold imbalance, which led to warpage and cracking. Walter Joos, mechanical engineer at Cookson, says the company was looking for a way to bring many of its parts back to within specification. “We were just looking for a way to balance an unbalanced system,” he says. The solution was a first-of-its-kind valve gate sequence control system, provided by Incoe Corp.

Cookson operates four molding facilities worldwide. At the Minnesota plant it runs about 50 presses, both standard and structural foam. The 21 structural foam machines there range from 150 to 1000 tons, 14 of which are Milacron Springfields; the rest are proprietary vertical machines manufactured in-house by Cookson.

Older presses in the company’s portfolio lack the valve gate control to modify injection profiles, and newer presses provide only limited valve sequence control. This causes problems on larger presses that are used to produce a group of heterogeneous parts in each cycle. It had become almost impossible to balance the fill portion of the cycle. As a result, some areas had to be overpacked in order to completely fill out other areas.

Overpacking

Overpacking is a problem even when a single large part is produced on these presses because the geometry of the part and obstructions in the mold frequently make it impossible to position the gates so that fill is balanced.

On one particular mold for a PE toolbox, air vent openings and multiple shutoff areas limited where the nozzles could be located. Another constraint was that nozzles could be located no closer than 6 inches from center to center.

This overpacking created stress that sometimes caused warpage or cracking during cooling, making it necessary to scrap the part. Cookson uses four molds in one machine to produce the toolbox. The shot consists of one 36-lb part, two 6-lb parts, and one 4-lb part in each cycle. The scrap rate was 10 percent. While scrap could be reground, machine time was wasted reprocessing material from out-of-spec parts.

Joos says that in an attempt to balance the fill, Cookson engineers tried restricting certain nozzles. But the nozzles that were restricted tended to freeze off, producing more scrap due to short shots. Additionally, engineers were concerned that the overpacking was adding weight to some larger parts, therefore increasing production costs.

Cutting Scrap

Controlling the nozzles was the key. Having better and more precise gate control is what Cookson wanted, but Joos says his company appeared to be running out of luck until the molder was approached by Incoe Corp. Incoe proposed that Cookson be a launch customer for the supplier’s new valve gate sequence control system. The Incoe system was designed to control the opening and closing of up to 40 pneumatic or hydraulic valve gate nozzles, and to allow all gates to be opened and closed a second time to meet final pack and hold requirements.

Incoe installed the system on the problematic mold. Cookson engineers began by setting the controller for a series of short shots designed to achieve balance of fill. For each shot, opening and closing parameters were established for each nozzle in either inches or seconds. Alternatively, a combination of linear screw position and time can be selected.

After each short shot trial, the gate opening program was adjusted to increase the fill and bring the cycle into balance. Once parameters were established to produce in-spec parts all around, the gate flow control program was stored for future recall.

In production on the floor, the operator simply retrieves the desired molding program from the controller and starts the molding job. Typically, says Joos, a competent operator can use the system and start producing acceptable parts within two or three shots. Previously it took an experienced operator eight to 10 shots, and a relatively inexperienced operator 20 to 30 shots, to achieve part acceptability.

“It’s a really big deal for the guy on the floor to just go up to the press and punch in his mold number or part number and just start producing good parts,” Joos explains. “Within two or three shots you’re saving money. It really reduces setup time.”

Ultimately, Cookson cut its scrap rate on the toolbox to about 2 percent, a decrease of 8 percentage points from the previous rate. And the efficiency of the nozzles reduced the weight of the largest part of the toolbox by 2 lb to 34 lb. This reduction in material use, when combined with the savings realized by reducing scrap, helped Cookson save some $40,000 annually on just this job.

Due Diligence

Joos says Cookson currently operates three Incoe controllers. One produces 5-by-5-ft HDPE tractor cab roofs on a 750-ton structural foam press. The system controls 20 gates in that mold. The other two systems are installed on 500-ton structural foam presses and help produce an HDPE pallet (24 nozzles), a 36-inch-wide PP storm door, the toolbox discussed above, and telecommunications cabinets molded of PPO. A fourth controller is due for delivery later this year.

On Cookson’s older structural foam presses the Incoe controller has become the de facto process controller for the whole machine. “We’re marrying it into the machine,” says Joos. “We’re controlling shot size with it and everything.” Incoe reports that the controller can be used on almost any structural foam or regular injection molding machine and comes in two models—one designed for remote hydraulic or pneumatic valve gate operation and the other for self-contained hydraulic or pneumatic valve sequencing.