More Complex Parisons Through Use of Flexring Dies
Flexring dies for extrusion blow moulding make it possible to change the die gap at any location on the circumference of the die. As a result, production of more complex parts now becomes feasible. In addition, application of dynamic wall thickness control can be extended to small die geometries.
Blow moulded containers exhibit increasingly complex geometries. This means that parisons must be produced with appropriate, and very specific, variations in wall thickness. With Flexring technology, an interesting alternative to conventional systems for wall thickness control has been developed that permits very precise changes in parison wall thickness.
The systems employed to date for partial wall thickness control are suitable only for dies with a diameter of more than 60 mm. Moreover, the attainable variations in parison wall thickness are limited. Furthermore, these solutions are technically complex, as Heinz Groß reports in the March issue of Kunststoffe. The author has operated an engineering office that develops new production technologies for extrusion since 1992.
The newly developed wall thickness control for blow moulding dies is based on Flexring sleeves the upper ends of which utilise a multi-wall configuration. The extremely thin, nested individual walls withstand the internal pressure produced by the melt, but are still very pliable. The changes in flow channel resistance take place exactly where they are most effective, namely, directly at the end of the die.
The Flexring sleeves are extremely pliable, as a result of which they can create very large local variations in parison wall thickness. Nevertheless, there are no abrupt changes in the flow channel, since the geometry of the Flexring sleeve changes only gradually in the affected region. Because of the low mass that has to be accelerated in this system, very rapid control adjustments can be made.
Reduced production costs
With Flexring dies, the die gap can be changed at any location on the circumference. Very complex parts can be produced in this way. In addition, Flexring technology permits extension of dynamic wall thicknness control to smaller die diameters.
Optimisation of the mandrel shape can also be simplified significantly through use of Flexring technology, according to the article, since any change in a Flexring mandrel that does not yield the desired result can be corrected immediately in the next step by backing out the appropriate screw.
The above-mentioned benefits of Flexring technology thus permit production costs to be reduced. On the one hand, the system reduces material usage as a result of the improved wall thickness distribution. On the other, cycle times are shortened, since unnecessry thick regions in the blow moulded part are eliminated. It can also be expected that, in industrial use, Flexring dies will be less susceptible to problems than dies providing partial wall thickness control.
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