A new type of flat extrusion die that differs in shape dramatically from conventional dies enables film, sheet, and coating processors to improve productivity and product quality by eliminating a longstanding tradeoff between gauge uniformity and streamlined melt flow, it was announced at NPE 2003 by Extrusion Dies Industries, LLC (EDI), which is exhibiting at Booth 4561.
Called the Contour Die because it has a tapered, ’sculpted’ shape instead of the standard block-like configuration, the new system exhibits die-body deflection that is uniform across the width of the die yet avoids the sacrifice of melt-flow streamlining incurred by earlier ‘constant-deflection’ dies, according to John A. Ulcej, executive vice president of engineering and technology. Ulcej cited these specific benefits:
. Rapid achievement of target gauge. The key advantage of uniform die body deflection is a reduction of the time required to adjust the gauge profiling system. ‘Operators can achieve target gauge several minutes sooner after startup or changes in extrusion rate,’ Ulcej said.
. Low levels of polymer degradation. ‘The superior flow of the Contour Die compared with that of earlier constant-deflection dies is particularly valuable for those running heat-sensitive polymers like PVC,’ noted Ulcej, ‘and for most polymers it provides greater assurance of good-quality product and less likelihood of lip buildup or gels.’
. Fast purges. Streamlined flow helps to speed purges for color or product changes.
. Low levels of scrap. All three improvements cited above manifest themselves in terms of reduced scrap generation.
‘We anticipate that users of the Contour Die will achieve significant reduction in scrap,’
Ulcej said. ‘With conventional dies, scrap is often the major penalty paid for non-uniform die body deflection, as well as for flow hang-ups or dead spots in the manifold. This new die reduces or eliminates both causes of excessive scrap.’
While manufacturers running heat-sensitive polymers or requiring frequent product changeovers are obvious beneficiaries of the Contour Die, Ulcej noted, ‘virtually every processor of film, sheet, or coatings can use this new system to improve productivity and quality.’
Key to Constant Deflection: ‘Engineer the Die from the Inside Out’
The manifold of a flat extrusion die is a flow channel that is machined between the upper and lower halves, or bodies, of the die and whose function is to develop uniform flow and to distribute the melt to its final product width. Die-body deflection 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, this pressure (typically in the range of 1000-4000 p.s.i., or 70-280 kg per sq. m) generates thousands of pounds of force, enough to deflect heavy steel die bodies.
This deflection is non-uniform in a die with a standard coathanger manifold (so-called because its back walls, on either side of the melt entry port, are positioned at an angle to the die exit rather than parallel to it, forming two sides of a triangle). The result of this non-uniformity is a tendency for center 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 available with a coathanger 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 color change.
‘In designing the Contour Die, we started with a standard coathanger manifold and engineered the rest of the die around it,’ Ulcej said. ‘Since there is a pressure gradient across the width of the manifold from the center 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.’
In developing the new die, EDI employed sophisticated three-dimensional engineering software and computerized manufacturing systems, including a five-axis machining center. ‘These capabilities enabled us to turn die manufacturing upside-down,’ Ulcej said. ‘Instead of starting and ending with two steel rectangles because it is easier to build conventional dies that way, we had the freedom to design the die in a way that would optimize its performance.’
For more information: sales@extrusiondies.com
