Extrusion die design guidelines for polypropylene

A common multilayer PP product is bi-axially oriented polypropylene (BOPP), produced on a line including a machine direction orientation unit, and a tenter frame for transverse orientation. This process commonly uses a three-manifold die that allows thin skin layers of copolymer to be applied on either side of a homopolymer base layer. The copolymer is used in this product so the film can be sealed, which is a requirement in packaging applications. Typically, the skin layer thickness is 5% of the total film thickness. The adhesive property of the copol3mier makes it difficult for the film to be processed through a tenter frame, because the film would stick to the tenter frame clips. The multimanifold die can be designed with narrower width flow channels for the skin layers, to keep the adhesive out of contact with the tenter clips, allowing the product to be extruded successfully. 

The flow channel in the die is designed to uniformly distribute the melted pol3nner to a desired width. It has four parts, the manifold, preland, secondary manifold, and final land. The manifold can be viewed as a pipe, which carries the material to the desired width. The preland is used to vary the resistance of the flow paths within the die. The secondary manifold is used to reduce the pressure drop within the die. The final land establishes the gauge, or thickness of the melt leaving the die. 

A basic problem with the coat hanger design is that it causes the distance from the lip exit to the back of the manifold to be longest at the 

Restrictor bars are used in most manifold designs to aid in flow tuning if the lip opening of the die is greater that 2 or 3 mm. These bars can be either fixed or adjustable, and are either profiled or bent with external adjustments to change the distribution in the flowstream. 

A typical PP die is manufactured from a P20 tool-steel billet. The flow channel is polished to 2-4 Ra, and hard-chrome plated. The exterior of the die is flash chrome plated to make die dean-up easier, and also to prevent corrosion during shipping and storage. Dies can also be built from stainless steel, which is more costly that a chrome-plated tool-steel die, and of no significant advantage, because PP is not a corrosive material. 

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Extrusion die design guidelines for polypropylene MECHANICAL DESIGN... 

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