What Is A Sprue In Injection Molding

The gate can be understood as the last “door” for the molten plastic to enter the cavity through the pouring system, and it is the feeding channel connecting the manifold and the cavity. It has two functions: firstly, it controls the flow of plastic melt into the cavity; secondly, when the injection pressure is withdrawn, it closes the cavity so that the plastic in the cavity which has not yet been cooled and solidified will not flow backward. The selection of the type of gate depends on the requirements of product appearance, size and shape constraints and the type of plastic used and other factors. The shape and size of the gate has a great impact on the quality of the plastic part. The gate is in most cases the smallest part of the runner in terms of cross-section size (except for the main flow channel type of gate), the ratio of its cross-sectional area to the cross-sectional area of the manifold is about 0.03-0.09, and its cross-sectional shape is mostly rectangular or round, with the gate step length of about 1-1.5mm. Generally a small gate is used because it has the following advantages:

Firstly, the small gate can increase the flow rate of the material when it passes through. There is a large pressure difference between the two ends of the small gate, which reduces the apparent viscosity of the molten plastic and makes mold filling easy.

Secondly, small gates can increase the temperature of the molten plastic and increase the fluidity. The friction resistance at the small gate is large, when the molten plastic passes through the gate, part of the energy is transformed into friction heat and warms up, which is good for improving the quality of thin-walled plastic parts or plastic parts with fine patterns.

Thirdly, small gate can control and shorten the time of replenishment, reduce the internal stress of plastic parts and shorten the molding cycle. In the injection, the pressure holding stage has to be continued until the condensation at the gate, the small gate condenses quickly, the replenishment time is short, which reduces the condensation orientation of the macromolecule and the condensation strain, and greatly reduces the replenishment of the internal stress. The adaptive closure of small gate can also correctly control the replenishment time and improve the quality of plastic parts.

Fourth, the small gate can balance the feeding speed of each cavity. The resistance of the small gate is much larger, only after the runner is full and has enough pressure, the cavities can be filled with similar time, which can improve the imbalance of the feeding speed of the cavities.

Fifthly, it is easy to trim the molded parts. Small gates can be removed quickly by hand. The small traces after the removal of small gates reduce the time of trimming. However, too small a gate will greatly increase the resistance to flow, prolonging the mold filling time, high viscosity of molten plastic and shear rate on the apparent viscosity of molten plastic has little effect on the small, should not be used in small gates!

The gate, also known as the feed port, it is a manifold and the cavity between the narrow opening, but also the shortest part of the role of the molten plastic into the cavity to produce acceleration, is conducive to rapid filling of the cavity, after molding the gate plastic condensation to close the cavity, to prevent the backflow of molten plastic, to avoid the cavity pressure drops too quickly, so as to produce shrinkage holes in the product or depression, molding and easy to make the pouring of condensate and product separation.