A US patent has been granted to Extrusion Dies Industries for its Contour Die flat die design that reduces the downtime for lip adjustment required with each new product run, enhances gauge profiles, and yields a uniform layer structure in coextrusion.
The Contour Die is said to solve the problem caused by the pressure of the molten polymer in the manifold or flow channel between the bodies, or halves, of any flat die. This pressure (in the range of 70-200 bar) is great enough to deflect the heavy steel bodies. In conventional dies, because the surface area that is ‘wetted’, or in contact with molten polymer, is greater at the centre than at the ends, the result is non uniform deflection. This causes a product distortion commonly called ‘clamshelling’.
While it is possible to correct the problem with the lip adjustment systems that are standard on flat dies, the frequent adjustments required when there are many small run jobs cause a substantial loss in productivity. The problem is even more serious in coextrusion, as lip adjusters cannot correct the distortions at the interface between layers.
The Contour Die has a tapered or ‘sculpted’ shape: thicker in the centre, where the bending force is greater, than at the ends. This design enables EDI to retain the triangle shaped ‘coat hanger’ manifold that is widely used in the industry because it promotes a streamlined flow of molten polymer through the die.
“Previous ‘constant deflection’ dies built by EDI and others dispensed with the coat hanger manifold in favour of a ‘broad shoulders’ manifold with a straight back wall parallel to the die exit,” explains Contour Die inventor John A Ulcej, EDI’s executive vice president of engineering and technology. “This produced uniform die body deflection, but with a sacrifice in streamlining. The corners at either end of the back wall constituted dead spots where low velocity flow or polymer hang-up could cause degradation, manifested in the finished product as gels, pinholes, and other defects.”
Another benefit of streamlined flow is said to be a reduction in the time needed to purge the die for colour or resin changes. At the same time, according to EDI, the combination of uniform die body deflection and streamlined flow provides greater leeway for increasing extruder throughput without compromising product quality.
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