The gate is a critical section of the material flow channel in the gating system. Except for the main runner gate, most gates are the smallest part of the gating system with the smallest cross-sectional area, and its value is generally only 3%-9% of the cross-sectional area of the sub-runner.
For the plastic melt in line with the Newtonian flow law, its viscosity has nothing to do with the shear rate. Therefore, a large gate cross-sectional area can reduce the flow resistance and increase the melt flow rate. And it is beneficial to the filling and molding quality.
For most plastic melts that do not obey the Newtonian flow law, reducing the cross-sectional area of the gate may often increase the melt shear rate. The effect of shear heat will make the apparent viscosity of the melt drop greatly. This large drop may be more conducive to mold filling than large cross-section gates.
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, when small gate is used for injection molding, it has the following advantages:
There is a large pressure difference between the front and rear ends of the small gate. And it can effectively increase the shear rate of the melt and generate greater shear heat. Then, it leads to a decrease in the apparent viscosity of the melt and an increase in fluidity. That is conducive to mold filling.
This feature of small gate is suitable and good for thin-walled products or products with fine patterns. And also the plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), etc. whose viscosity is sensitive to shear rate.
In the injection molding process, the pressure-holding and feeding stage generally extends 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. Thus, it is possible to increase the orientation and flow deformation of the macromolecules. And it will also cause a great feeding stress in the product, especially near the gate. That eventually will result in warpage and deformation.
If using a small gate, adjust the volume of the small gate through mold trial or repair. That is to make the melt at the gate freeze at the right time during the pressure holding process. Then, it can properly control the feeding time and avoid the above phenomenon.
The small gate has the features of small volume and fast freezing. Therefore, when producing some products, it is not necessary to wait for the inside of the product to be completely solidified after the small gate is frozen. As long as the outer solidified layer has sufficient strength and rigidity, the product can be demolded. Thereby shortening the molding cycle and improving production efficiency.
If a small gate is used in the non-equilibrium pouring system of a multi-cavity, the flow resistance of the gate to the plastic melt will be much greater than that of the split runner multi-melt. So after the melt has filled runners and established sufficient pressure, each cavity can be filled with materials at approximately the same time.
Therefore, the small gate can balance the feed rate of each cavity in the multi-cavity, which is beneficial to the balance of the pouring system.
If using a larger gate for the molding product, with the high requirements for quality of the product surface, it is often necessary to post-process the product with appropriate tools or machine tools. So that it can remove the gate scars, especially when the gate is too large. The condensate must be removed by sawing and cutting. However, this trouble can be avoided when using a small gate.
For example, the small gate aggregate can be cut quickly by hand, or automatically cut by a special mold structure during demolding. Moreover, the scar of the cut small gate is small, and generally requires only a slight trimming and polishing work.
Therefore, the use of small gates is conducive to not only the separation of the gating system aggregates from the products, but also the finishing of the products.