Meaning
Coextruded thermoplastic sheet configurations provide a method for optimising material cost by sandwiching a recycled or filled core between two thin layers of virgin polymer. In this arrangement, an a-b-a multi-layer sheet utilizes the outer a-layers to maintain surface finish, sealability, and food contact compliance, while the thicker b-layer incorporates regrind or less expensive resin.
Coextrusion Structure
Feedblock and multi-manifold die systems divide the melt flow to achieve the desired layer ratios during the extrusion process. When running an a-b-a multi-layer sheet, the thickness of each outer skin layer normally represents five to fifteen percent of the total structure.
Material Distribution
Symmetric cooling across both faces of the extrudate prevents differential shrinkage and the subsequent warpage of the finished part. For an a-b-a multi-layer sheet, mismatches in the melt flow index of the chosen skin and core resins can induce interfacial instability, leading to visible wave-like defects in the sheet surface. Careful adjustment of die temperature and extrusion speed helps to balance the shear stresses at the layer boundaries.
Defect Mitigation
Regular gauging and ultrasonic layer thickness measurement ensure the integrity of the barrier layers. If the skin layer of an a-b-a multi-layer sheet becomes too thin, the recycled core material may break through, resulting in aesthetic blemishes or localized mechanical failures in thermoformed parts.
