After the materials leave the feedblock and enter the die, they are no longer confined individually within steel channels. They are in intimate contact with each other and in the fluid state. They remain segregated because plastics, when melted, are part of a family of fluids characterized as being "non-Newtonian".
Unlike Newtonian fluids, such as water or mineral oils which would readily co-mingle if discharged together, plastics will remain separate and distinct.
The uniformity of the layers in the final film is a function of how carefully the primary rule of this type of coextrusion is followed. That rule is "adjoining materials MUST have a reasonable rheological match".
Rheology is the flow performance of a given plastic material. It is graphically represented in the form of viscosity versus shear rate, which is the fingerprint of a specific material in terms of how it will flow when melted and pressurized.
The rule simply states that the "fingerprint" of adjoining materials must be similar if layer uniformity of the final product is expected. The less the match, the less the uniformity that will result. When layer distortion occurs it happens in the die, not the feedblock, during the transition or distribution in shape from the square incoming stream to the wise thin film shape.
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