Gate shape and size design of injection mold

Jun. 21, 2023 1083

Gates in injection molds, also called feed gates, are short, short flow paths (except for direct gates) that connect the shunt to the cavity. This is a key part of the gating system.

Its main functions are:

A, After the cavity is full, the melt will first condense at the gate to prevent its backflow.
B, It is easy to cut the condensate of the pouring system at the gate. The cross-sectional area of the gate is about 0.03 to 0.09 of the cross-sectional area of the runner. The length of the gate is about 0.5 mm to 2 mm. The specific size of the gate is usually determined empirically. Take the lower limit and gradually correct it during the test.

When the plastic melt passes through the gate, the shear rate increases and the internal friction of the melt intensifies, which increases the temperature of the stream, reduces the viscosity, improves the flow performance and facilitates filling. However, if the size of the gate is too small, the pressure loss will increase, the condensation water will accelerate, it will be difficult to shrink, and even a spray phenomenon will be formed, which will affect the quality of plastic parts.

There are several types of gates in injection molds:

1. Straight gates
Straight gates are also called center gates. The gate resistance is small and the feeding speed is fast. It is commonly used in single cavity molds to form large, deep plastic parts. It is suitable for all kinds of plastics, especially those with high viscosity and poor fluidity, such as PC and PS.
When forming shallow, flat plastic parts with straight gates, they can bend and warp. At the same time, there will be obvious gate marks on the inconvenience of removing the gate. Sometimes due to the concentration of heat at the gate, the cavity seal will be delayed and the internal stress will be large. As the source of the crack, the gate should be designed as small as possible. When molding thin-walled plastic parts, the diameter of the gate root is at most equal to twice the wall thickness of the plastic part.

2. Side gate
Side gates are also called edge gates. Its cross section is rectangular. It usually opens on the parting surface and feeds from the side of the plastic part. The gate position can be reasonably selected according to needs, and it is especially suitable for molds with multiple cavities.

3. Point gate
Point gate, also known as pin point gate, is a very small gate that is widely used in various types of shell plastic parts. When opening the mold, the gate can be pulled off by itself.
At the junction of the gate and the plastic part, in order to prevent the plastic part from being damaged when the point gate is broken, a small boss can be designed to have a small cross-sectional area, quick set, and not conducive to shrinkage. It is not suitable for thick-walled plastic parts.

4. Latent gate
Also called a shear gate, it evolved from a point gate. The point gate is used for three-plate mold, and the latent gate is used for two-plate mold, which simplifies the mold structure. The submarine gate is placed in a hidden part inside or outside the plastic part, which does not affect the appearance of the plastic part. When pushing out plastic parts, the gate is cut off, but requires a strong thrust, so it is not suitable for hard plastics.