Flow Adjustment in Sheet and Film Dies

The first is the flex lip adjustment. A number of bolts along the width of the die allow local closing of the final land gap. This allows fine adjustments of the extru-date thickness at discrete points. The gap can be adjusted by as much as 1 mm or more if the flex lip is properly designed. The choker bar is not used as often as the 

Methods to change the flow channel geometry in sheet dies 

By incorporating a removable lower die lip, the die can be used, for instance, for 1-mm sheet extrusion and for 4-mm sheet extrusion by changing only the lower die lip. Without the removable die lip, another sheet die would have to be used because the flex lip can only adjust over a limited range. In some automated extrusion lines, the gap of the sheet die is adjusted automatically. This is done with heat expandable die lip bolts [20], first developed by Welex in the early 1970s. A similar type of sheet die using thermal bolts is now also offered by Egan Machinery Company and a number of other companies. 

An elegant approach to thickness adjustment in sheet and film dies was developed by Gross [46]. Gross developed a system with a flexible membrane inside the die to allow rather simple thickness adjustment using low force actuators to create a smoothly curved surface without sharp angles or dead spots. This system is schematically shown in Fig. 9.16. The membrane die has been licensed by several manufacturers of extrusion dies worldwide [46]. 

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