Injection Velocity and Speed Introduction

Important for the molding process is the velocity of the reciprocating screw, the axial screw speed which is only effective during the injection stage. The different responses of the hydraulic and the cavity pressure again make it clear how little information about the molding process is provided by the hydraulic pressure. Figure 3.19 shows curves of the hydraulic pressure and cavity pressure for three different screw speeds (Vs). The injection times vary inversely with screw speed. Figure 3.19 (top) shows that the hydraulic pressure rises faster with increasing injection pressure loss (△p) from filling the cavity, measured near the gate, increase with decreasing injection speed. This reflects the effect of the superimposed cooling process. The latter causes and increase in melt viscosity in the cavity during injection and a more rapid formation of a growing solid surface layer, which narrows the available flow channel. This, in turn, impedes pressure transmission and is reflected in the graph by the different levels of maximum cavity pressure. If this affects the duplication of the cavity surface (i.e., the quality of the molding), the holding pressure must be raised considerably to compensate for slow injection.

Effect of axial screw speed vs on pressure profiles mof hydraulic system and cavity

Effect of axial screw speed vs on pressure profiles mof hydraulic system and cavity

These effects are summarized in Fig.3.20, which illustrates pressure drop and cavity pressure as a function of flow front velocity. For reasons of economics (energy savings) and part quality, the highest practical injection speed should be selected.

Pressure differebtial △pc during injection and maximum cavity pressure pc max as a function of axial screw speed or velocity of flow front

Pressure differebtial △pc during injection and maximum cavity pressure pc max as a function of axial screw speed or velocity of flow front

Other important parameters affecting the molding process are, to name the most essential, the nozzle and gate geometries, the cooling and feeding time, the operation of the non-return valve, the rotational and injection characteristics of the screw drive, and the rigidity of mould and machine platens.

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