The gate in the injection mold, also known as the feed gate, is a short and short flow path connecting the diverter and the cavity (except for the direct gate). This is a key part of the gate control system. Its main functions are:
① After the cavity is full, the melt condenses at the gate to prevent it from flowing back.
② It is easy to cut off 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 0.5mm to 2mm. The specific size of the gate is usually determined by experience. Take the lower limit value and then 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, thereby increasing the temperature of the material flow, reducing the viscosity, improving the flow performance and facilitating filling. However, if the gate size is too small, the pressure loss will increase, the condensation water will accelerate, the shrinkage will be difficult, and even the spraying phenomenon will form, affecting the quality of the plastic parts.
There are several types of gates in injection molds:
(1) Straight gate
The straight gate is also called the center gate. This gate has low resistance and fast feeding speed. It is usually used in single-cavity molds to form large and deep plastic parts. It is suitable for various plastics, especially plastics with high viscosity and poor fluidity, such as PC and PSF.
When forming shallow and flat plastic parts with straight gates, bending and warping will occur. At the same time, the inconvenience of removing the gate will have obvious gate marks. Sometimes due to the concentration of heat at the gate, the cavity sealing will be delayed and the internal stress will be large. As the root of the crack, the gate should be designed to be 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
The side gate is also called the edge gate. 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, especially suitable for molds with multiple cavities.
(3) Point gate
The point gate is also called the pin-point gate. It is a very small gate and is widely used in various shell-type plastic parts. When the mold is opened, 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 breaks, a small boss can be designed. The point gate has a small cross-sectional area, solidifies quickly, and is not conducive to shrinkage. It is not suitable for thick-walled plastic parts.
(4) Submarine gate
Also called shear gate, it is evolved from the point gate. The point gate is used for three-plate molds, and the submarine gate is used for two-plate molds, which simplifies the mold structure. The submarine gate is set in a hidden part inside or outside the plastic part, which does not affect the appearance of the plastic part. When the plastic part is pushed out, the gate is cut off, but a strong thrust is required, so it is not suitable for hard plastics.
