Heat Source

The heat required to melt the plastic in the barrel is developed from two sources, conductive heat from heat bands and shear heat resulting from the screw working the plastic against itself, the barrel wall and the screw surface.

Shear heat is also produced by the use of back pressure and various mixing devices near the end of the screw. Some shear heat is essential to achieve a uniform melt quality, both in temperature and viscosity.

Experience indicates that heat developed from the two sources in approximately equal amounts (assuming a shot of 30% to 70% of shot capacity) produces the best results. Incidentally, that rule is not applicable to extrusion.

Note: We suggest that you measure the hydraulic pressure or electrical energy required by the screw motor during screw recovery. Readings of 800 to 1,200 PSI suggest that the heat produced from shear energy is ranging from 40% to 60%, a desirable range. Greater pressure indicates that the screw is working very hard creating potential excessive shear. Lower readings under 800 PSI suggest that too little shear energy is being used.

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