Enough injection quantity: the weight of product and the number of mold cavity determine the “injection quantity” and select the appropriate “screw diameter”.
For stability purpose, the injection volume required for more than 1.35 times of the product, which means the product weight should less than 75% of the injection amount.
Good injection: plastic determine “screw compression ratio” and “injection pressure” and other conditions.
Some plastic requires a higher injection pressure and appropriate screw compression ratio for a good molding. So when choosing screw, it should considered about the demand of injection pressure and the problems of compression ratio. In general, smaller diameter screw provides higher injection pressure.
High injection speed: confirm the injection speed.
Some product requires a high injection speed to stabilize the injection molding, such as ultra-thin wall product. In this case, it is necessary to make sure that whether the injection speed and injection rate are enough or not, and whether accumulator and closed-loop control are necessary or not. Generally speaking, in the same condition, a screw in the plastic injection molding machine with high injection pressure has a lower injection speed, on the contrary, a screw with low injection pressure has a higher injection speed. So when choosing the screw diameter, injection volume, injection pressure, and injection rate (injection speed) should be cross considered.