The L/D ratio is the ratio of the flighted length of the screw to its outside diameter. The ratio calculation is calculated by dividing the flighted length of the screw by its nominal diameter. Although several injection molding machine manufacturers now offer a choice of injection units, most injection screws use a 20:1 L/D ratio.
The effect of changing the L/D ratio can be summarized in the following manner: the larger the ratio (longer flighted length), the:
- More shear heat can be uniformly generated in the plastic without degradation;
- Greater the opportunity for mixing, resulting in a better homogeneity of the melt;
- Greater the residence time of the plastic in the barrel possibly permitting faster cycles of larger shots.
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