For plastication, the speed should be selected in such a way that the peripheral velocity of the screw does not exceed 0.3 m/s. The metering stroke should be between 1D and 3D (max. 4D). Fig. 2 shows the maximum speeds for various screw diameters.

Screw speed range

Fig. 2: Screw speed range as a function of diameter

Practical experience has shown that a 30 - 75% capacity utilization of the respective cylinder is beneficial.

If the shot volume is very low in relation to the capacity of the cylinder, the dwell time of the melt in the plasticating unit will be very long indeed. This could result in thermal damage to the melt (Fig. 3).

Breakdown of TPU melt

Fig. 3: Breakdown of the TPU melt in the processing cylinder

The back pressure required to support uniform melting will generally be 100 ± 50 bar (and the hydraulic pressure generally 5 to 20 bar).

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