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Die boosts productivity

A flat die with a sculpted shape has helped cut waste and increase film throughput for Pliant

A leading US manufacturer of cast film, Pliant Corp, has substantially increased productivity by replacing an existing flat extrusion die with the new Contour Die, reports supplier Extrusion Dies Industries (EDI). The die enabled the film producer to increase the throughput of an extrusion line at its Chippewa Falls, Wisconsin, plant by nearly one-third and to reduce the time and scrap involved in adjusting the gauge profiling system.

Pliant operates 26 facilities around the world and employs more than 3,250 people. It produces films for personal care, medical, food, industrial, and agricultural markets.

At the Chippewa Falls plant it uses the Contour Die to manufacture multilayer polyolefin film in gauges from 0.02-0.14mm, in widths up to 230cm, and at line speeds up to 245m/min, according to plant engineer Rex Beachem. The line is part of a custom processing operation where product change-overs take place every three or four days and colour changes typically more often.

With changes in product so frequent, the reduction in the time needed to adjust gauge profiling was a real benefit, according to Beachem, who notes that the Contour Die enables line operators to achieve target gauge several minutes sooner after startup. “The new die has dramatically reduced the amount of startup waste material between product change-overs,” he says. “A procedure that typically yielded 450-680kg of scrap now generates only 135kg, and sometimes a good bit less.”

Pliant had not anticipated the size of the increase in line speed, according to Rex Beachem. “We saw a throughput increase of 30 per cent for one of our 0.127mm products. By eliminating the thick/thin variation across the width of the film caused by die body deflection, the Contour Die reduced the tendency of the film to stick to the rubber surface of the cooling roll. The improved release made it possible to run the film faster.”

Uniform deflection

What made these improvements at Pliant possible is a design different from that of any other flat die, according to John A Ulcej, EDI’s executive vp of engineering and technology.

As with conventional dies, the manifold of the Contour is a flow channel machined between the upper and lower halves, or bodies, of the die. The body deflection cited by Beachem is caused by the pressure of the molten polymer that the extruder continuously charges into the manifold. Multiplied across the entire area of the manifold, the pressure (typically in the range of 70-280kg/m2) generates enough force to deflect heavy steel die bodies.

This deflection is non uniform in a conventional die with a standard ‘coat hanger’ manifold. The result of non uniform deflection is a tendency for centre flow to be too heavy and flow at the ends to be too light; operators need several minutes to correct for this problem in order to achieve an acceptable product. In the interval, substantial scrap is generated.

Previous designs for ‘constant-deflection’ dies solved this problem with manifolds that had straight backlines, parallel to the die exit, but they did not provide the degree of flow streamlining avail- able with a coat hanger manifold. Melt flowing through such dies stood a greater chance of encountering hang-ups or dead spots that cause polymer degradation and retard purges for colour change.

“In designing the Contour, we started with a standard coat hanger manifold and engineered the rest of the die around it,” says Ulcej. “Since there is a pressure gradient across the width of the manifold from the centre to the ends, we built in extra die body thickness where the force was greater and did the reverse where there was less force. The result is a sculpted configuration that is smaller and tapered at the ends, quite different from the block-like configuration of standard flat dies.”



Contacts

Pliant Corp
Tel: +1 866 878 6188
EDI
Tel: +44 (0) 1332 833153





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