An extrusion tool for producing sheets with an upper and lower outer wall (100, 102) and containing cavities bounded by dividers (101, 103) has a flow splitter to divide the main polymer flow into two streams. One of the streams is diverted to by-pass the other stream and in this region a second polymer stream is introduced by its side. An extrusion tool for producing sheets with an upper and lower outer wall (100, 102) and containing cavities bounded by dividers (101, 103) has a flow splitter to divide the main polymer flow into two streams. One of the streams is diverted to by-pass the other stream and in this region a second polymer stream is introduced by its side. The streams then rejoin to co-extrude the required profile in the main polymer (40), with the second component (50) appearing on the inside of the outer wall (100). An Independent claim is also included for producing multi-layered thermoplastic sheets with cavities by co-extrusion, one outer layer being coated on the inside with a layer of another polymer (50).
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