Multilayer coextrusion apparatus

Details Multilayer coextrusion apparatus Multilayer coextrusion apparatus

1991-04-29

1993-06-29

Bushey, Scott

Plastic article or earthenware shaping or treating: apparatus

Means feeding fluent stock from plural sources to common...

Extrusion shaping means

264171, 4251335, 425190, 4253824, 425462, B29C 4706

Patent

active

052232760

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates generally to a coextrusion apparatus for producing a multilayer film or sheet of diverse thermoplastic materials. The apparatus includes a slit die having a slotted die opening through which the materials are extruded as a multilayer film or sheet. The die has a coathanger-type expansion chamber.
A feedblock assembly is incorporated into the die, the assembly including feed lines for conveying thermoplastic melt streams from the extruders to the feedblock inlet for forming a multilayer film or sheet as the layers combine upon exiting the feedblock, expand transversely in the chamber and extrude through the die opening.
More particularly, the invention relates to such coextrusion apparatus in which the die has a cavity for the reception of a feedblock formed as part of the feedblock assembly such that combining of the layers upon exiting the feedblock takes place within the die itself and is close as possible to the entrance to the expansion chamber. The feedblock comprises a plurality of slotted, layer distribution passages opening into the expansion chamber via a shallow entrance section, the passages comprising mutually spaced apart openings lying parallel to the slotted die opening. And, the feedblock is replaceable to accommodate selected polymer matchups, different numbers of multilayers, changes in layer thickness, layer geometry, etc.
Moreover, the invention provides for a highly effective end encapsulation of the multilayer film or sheet, and includes external adjustment to control the thickness, profile and distribution of the skin layers.
Coextrusion systems for forming multilayer film or sheets of thermoplastic materials are generally known, as shown for example in "Modern Plastics", August 1983 pages 22 to 26, and in "Plastic", August 1988, page A33. In both systems, between several extruders which each generates a polymer melt, and the slit die, a feedblock is located outside the die for combining the thermoplastic layers upstream of the die expansion chamber which is generally of the coathanger-type. Distributor pins, adjustable flow dividers, flaps, restrictor bars, or the like, are provided to control the thickness and distribution of the thermoplastic materials passing through metering channels and thus the thickness of each layer at the feedblock.
The feedblock, located upstream of the die, is generally accessible from all sides and is provided with means to control the different layer thicknesses of the polymer melts, but requires a specific feedblock for each desired layered structure from the standpoint of material and arrangement. The feedblock itself or significant elements thereof may therefore be replaced, and adjustments can be made for specific fluctuations and melt temperatures and/or viscosities of the individual layers which results in fluctuations in the layer structure of the final product.
Such adjustments to correct for fluctuations in temperature, viscosity, etc. will, of course, require significant experience on the part of the operator. Since the layer of thickness of each layer of polymer melt can be modified, the adjacent layers are always affected and consequently the entire layer structure is affected so that an unequivocal correction requires a very precise coordination of all existing possible corrections, a condition that poses continually higher demands on the experience of the operator as the number of layers increases. And, when extruding a multilayer sheet of a significant number of layers, the feedblock becomes continually more demanding and more costly.
Another drawback of the prior art coextrusion systems is that in the area of combining different polymers into the structure to be produced in the area of spreading the polymers into their final width and squeezing the layers to their final thickness in the coathanger die, the feedblock requires a channel of noticable length for reasons of stability. However, it has been demonstrated that not all layer structures of polymer melts necessarily flow strictly l

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