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 appearance of the melt will result. A large decrease in viscosity may be more conducive to mold filling than a large gate.
The gate is a critical section of the 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% to 9% of the cross-sectional area of the runner . As for the pressure drop caused by the increased flow resistance when molding with a small gate, it can be compensated by increasing the injection pressure within a certain range. Generally speaking, when using small gate for injection molding, it 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 feature of small gates is of great benefit for thin-walled products or products with fine patterns and plastics such as polyethylene, polypropylene, polystyrene, etc. whose viscosity is sensitive to shear rate.
2, In the injection molding process, the pressure holding and shrinking phase generally continues 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 pressure holding time will continue to be relatively long, so it is possible to increase the degree of 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 will be frozen in time during the pressure holding process, so as to properly control the shrinkage time and avoid the above phenomenon.
3, Because the small gate has a small volume and fast freezing, when producing certain products, it is not necessary to wait for the entire interior of the product to be cured after the small gate is frozen. As long as the external cured layer has sufficient strength and rigidity, the product can be demolded, thereby Shorten the molding cycle and improve production efficiency.
4, In the multi-cavity unbalanced pouring system, if the small gate is used, the flow resistance of the gate to the plastic melt will be much greater than the flow resistance of the split runner, so it is possible that the melt fills the runner and After enough pressure is established, each cavity can be fed and filled at approximately the same time. Therefore, the small gate can balance the feeding speed of each cavity in multiple mold cavities, which is beneficial to the balance of the pouring system.
5, If you use a larger gate molded product, in the case of higher surface quality requirements, you often need to post-process the product with appropriate tools or machine tools to remove the gate scar, especially when the gate is too large Aggregate must also be removed by sawing, cutting, etc. However, this trouble can be avoided when using a small gate. For example, the small gate condensate can be quickly removed by hand, or it can be automatically removed by a special mold structure during demoulding. In addition, the scar after the small gate is removed is generally small, and it is generally not necessary or only needs a little trimming and polishing work. Therefore, the use of small gates is not only conducive to the separation of the condensate from the casting system, but also to the repair of the product.
However, it is worth noting that although the small gate has the above-mentioned advantages, an excessively small gate will cause a large flow resistance, resulting in a prolonged feed filling time. Therefore, some plastic melts (such as polyformate and polysulfone) with high viscosity or shear rate that have little effect on the apparent viscosity should not be formed by small gates. In addition, when molding large products, attention should also be paid to enlarge the gate cross-sectional area accordingly, and sometimes it is even necessary to enlarge the gate cross-sectional height to be close to the maximum thickness of the product in order to improve the melt fluidity. In addition to the above, for products with a large wall thickness and a large shrinkage rate, it is generally required to have enough time to shrink, so in this case, the gate cross-sectional area can not be designed too small.
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