The gate is a very important material flow channel in the pouring system. Except for the main channel gate, most gates are the smallest cross-sectional area in the pouring system, and the value is generally only 3%-9% of the cross-sectional area of the runner .

For plastic melts that obey Newtonian flow laws, since the viscosity is independent of the shear rate, a large gate cross-sectional area can reduce the flow resistance and increase the melt flow rate, which is more favorable for mold filling and molding quality. For most plastic melts that do not obey the Newtonian flow law, reducing the gate cross-sectional area often increases the melt shear rate. Due to the effect of shear heat, the melt appearance will result. A large decrease in viscosity may be more conducive to mold filling than a large-section gate. As for the pressure drop caused by increasing the flow resistance when molding with a small gate, it can be compensated by increasing the injection pressure within a certain range. Generally speaking, the use of small gates for injection molding has the following advantages.

① There is a large pressure difference between the front and back ends of the small gate, which can effectively increase the shear rate of the melt and generate a large shear heat, resulting in a decrease in the apparent viscosity of the melt and enhanced fluidity, which is beneficial to Mold filling. This characteristic of small gates is suitable for thin-walled products or products with fine patterns and plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS) whose viscosity is relatively sensitive to shear rate, Are of great benefit.

② In the injection molding process, the pressure holding and shrinking phase generally needs to continue until the melt at the gate is frozen, otherwise the melt in the mold cavity will flow back out of the cavity. If the gate size is large, the packing time will last longer, so it may increase the orientation and flow deformation of the macromolecules, causing large shrinkage in the product, especially near the gate The stress causes the final warpage of the product. If a small gate is used, it is possible to adjust the volume of the small gate through trial or repair, so that the melt at the gate is frozen in time during the pressure holding process, so as to properly control the time of feeding and avoid the above phenomenon.