The injection rate refers to the amount of melt ejected from the nozzle per unit time. The following formula indicates:
Where: qz ― injection rate, cm3;
Vs ― actual injection volume, cm3;
tz-injection time, s;
D-inner diameter of barrel, cm;
vz ― injection speed, cm / s.
The injection speed vz is the moving distance of the screw or plunger per unit time, which is expressed by the following formula:
In the formula: S ― maximum injection stroke of the screw (or plunger), cm; tz ― injection time, s.
Relationship between injection volume and injection rate and injection time
The size of the injection rate will directly affect the quality and production efficiency of the product. Low injection rate, slow injection speed, long injection time, difficult to fill the mold cavity with the melt, the product is prone to quality problems such as cold joints, uneven density, large internal stress, etc. The rate is too high, the speed is too fast, and the material flows through the nozzle, etc. It is easy to generate a lot of frictional heat when it is burnt, discolored or degraded. At the same time, high-speed injection, the gas in the mold is often too late to be exhausted, and the inclusion of the material affects the apparent quality of the product, resulting in silver streaks and bubbles.
Therefore, the injection rate should be selected according to the properties of the plastic, the shape of the product, the process conditions, and the characteristics of the mold.