Introduction: The gate is a critical section of the pouring system, except for the mainstream type gate, most of the edge gate is the smallest part of the pouring system, its value is generally only 3% to 9% of the cross-sectional area of the manifold.
For the plastic melt which obeys Newtonian plastic flow law, the large gate cross-sectional area can reduce the flow resistance and increase the melt flow rate because its viscosity is not related to the shear rate, which is more favorable for mold filling and injection molding quality.
For most of the plastic melts that do not obey the Newtonian flow law, the use of reduced gate cross-sectional area is often likely to increase the shear rate of the melt, which, due to shear heat, will lead to a significant decrease in the apparent viscosity of the melt, but may be more conducive to mold filling than a large cross-sectional injection molding gate.
As for the pressure drop caused by the increased flow resistance when using small tunnel gates, it can be compensated by increasing the injection pressure within a certain range.
Generally speaking, when small gates are used for injection molding, they have the following advantages.
1. The large pressure difference between the front and rear ends of the small gate can effectively increase the shear rate of the melt and produce a larger shear heat, which leads to a decrease in the apparent viscosity of the melt and an increase in fluidity, which is conducive to mold filling.
This feature of small gates is of great benefit to thin-walled products or products with fine patterns, as well as plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), etc., whose viscosity is sensitive to shear rate.
2. In the injection molding process, the pressure-holding and shrinkage phase generally continues until the melt freezes at the gate, otherwise the melt in the injection mold cavity will flow backward outside the cavity.
If the gate size is larger, the holding pressure and shrinkage time will be longer, so the orientation and flow deformation of macromolecules may increase, which will cause great shrinkage stress in the product, especially near the gate, and lead to warpage of the product in the end.
If a small gate is used, it is possible to adjust the volume of the small gate through injection mold trial or mold repair, so that the melt at the gate can be frozen at the right time during the pressure-holding process, thus properly controlling the shrinkage time and avoiding the above phenomenon.
3. Because of the small volume of the small gate and fast freezing, it is possible to produce some products without waiting for all the internal curing of the products after the small gate freezing, as long as the external curing layer has sufficient strength and rigidity, the products can be demolded, thus shortening the injection molding cycle and improving the production efficiency.
4. In a non-equilibrium casting system with multiple cavities, if a small gate is used, the resistance of the gate to the flow of the plastic melt will be much higher than that of the melt in the manifold, so it is possible that each cavity can be fed and filled at approximately the same time after the melt has filled the manifold and built up enough pressure.
Therefore, small edge gates in multiple cavities can balance the feed rate of each cavity and contribute to the balance of the pouring system.
5. If a large gate is used for injection molding products, in case of high surface quality, appropriate tools or injection molding machine tools are needed to post-process the products to remove the gate scars, especially when the gate is too large, the gate condensate must be removed by sawing or cutting. However, this trouble can be avoided when small gates are used.
For example, small gates can be removed quickly by hand or automatically by special injection mold structure during demolding. In addition, the scar after the small gate removal is small, and usually no or only a little rimming and polishing work is needed.
Therefore, the use of small gates not only facilitates the disengagement of the condensate of the casting system from the products but also facilitates the trimming of the product.
However, it should be noted that despite all the above advantages, a small gate can cause great resistance to flow and lead to longer injection mold filling time.
Therefore, some high viscosity or shear rates on the apparent viscosity of the molten plastic melt (such as polycarbonate and polysulfone, etc.), are not suitable to use small gate molding.
In addition, when injection molding large products, attention should also be paid to enlarging the gate cross-sectional area accordingly, sometimes even the height of the gate cross-section needs to be enlarged to close to the maximum thickness of the product to improve the flowability of the melt.
In addition to the above cases, for products with larger wall thickness and shrinkage rate, it is generally required to have sufficient shrinkage time, so in this case, the gate design cross-sectional area should not be designed too small.