Release time：2022-03-17
The gate in the mold of plastic home appliance shell is a key logistics channel in the gating system. Except for the sprue type gate, most of the gates are the parts with small cross-sectional area in the gating system, and its value is generally only 3% to 9% of the cross-sectional area of the shunt channel. For the plastic melt that obeys Newton's flow law, because its viscosity has nothing to do with the shear rate, a large gate cross-sectional area can reduce the flow resistance and improve the melt flow rate, which is conducive to the filling, degumming and molding quality in the injection molding of plastic home appliance shell mold.
However, for the vast majority of plastic melts that do not obey Newton's flow law, reducing the cross-sectional area of the gate may often increase the shear rate of the melt, and the apparent viscosity of the melt will be greatly reduced due to shear heat, but it may be more conducive to mold filling than large-section gates. For the pressure drop caused by the increase of flow resistance when using small gate molding, it can be compensated by increasing the injection pressure within a certain range.
General plastic home appliance shell mold adopts small gate injection molding, which has the following advantages:
1. There is a large pressure difference between the front and rear ends of the small gate, which can effectively improve the shear rate of the melt and produce large shear heat, resulting in the reduction of the apparent viscosity of the melt and the enhancement of the fluidity, which is conducive to mold filling. This feature of small gate is of great benefit to thin-walled products or products with fine patterns, as well as plastics whose viscosity is sensitive to shear rate, such as polyethylene (PE), polypropylene (PP), polystyrene (PS), etc.
2. During the injection molding process of household appliance shell mold, the pressure maintaining and feeding stage generally lasts until the melt solidifies at the gate, otherwise the melt in the mold cavity will flow back to the outside of the cavity. If the gate size is large and the pressure maintaining feeding time lasts for a long time, it is possible to increase the orientation and flow deformation of macromolecules, resulting in large feeding stress in the product, especially near the gate, resulting in warpage and deformation of the product. If a small gate is used, the volume of the small gate can be adjusted through mold trial or mold repair, so that the melt at the gate can be frozen in time during the pressure maintaining process, so as to properly control the feeding time and avoid the above phenomena.
3. In plastic mold manufacturing and injection molding, due to the small size of the small gate and fast freezing, when producing some products, the small gate does not have to wait for the internal solidification of the products after freezing. As long as the external curing layer has sufficient strength and stiffness, the products can be demoulded, so as to shorten the molding cycle and improve production efficiency.
4. If a small gate is used in the mold non-equilibrium gating system with multi-mode cavity, the flow resistance of the gate to the plastic melt will be much greater than that of the multi melt in the shunt. Therefore, after the melt fills the runner and establishes sufficient pressure, all die cavities can be supplied and filled almost at the same time. Therefore, the multi-mode cavity small gate can balance the feeding speed of each mold cavity, which is conducive to the balance of the gating system.
5. If large-scale gate molding products are used, when the surface quality of the products is required to be high, appropriate tools or machine tools are often required to carry out post-treatment on the products to remove the gate scar, especially when the gate is too large, the gate condensate must be removed by sawing, cutting and other methods. But using a small gate can avoid this trouble.
For example, small gate condensate can be quickly cut off manually during demoulding, or it can be automatically cut off with a special mold structure. In addition, the scar of the small gate after removal is small, which generally does not require or only requires a little repair and grinding work. Therefore, the use of small gate is not only conducive to pouring force, but also leads to the extension of feeding and mold filling time. However, it should be noted that although small gates have the advantages mentioned above, too small gates will cause great flow resistance, resulting in the extension of feeding and mold filling time. Therefore, for some plastic melts with high viscosity or little effect of shear rate on apparent viscosity (such as polyformate, polysulfone, etc.), it is not suitable to use small gate molding.
In addition, in the mold forming process of large plastic home appliance shell, we should also pay attention to correspondingly increasing the sectional area of the gate, and sometimes we need to increase the sectional height of the gate to close to the maximum thickness of the product, so as to improve the fluidity of the melt. In addition to the above cases, products with large wall thickness and large shrinkage generally need enough feeding time, so in this case, the gate cross-sectional area cannot be designed too small.
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