The gate is a critical section of the material flow cha […]
The gate is a critical section of the material flow channel in the gating system. Except for the sprue-type gate, most gates are the smallest cross-sectional area in the gating system. Generally speaking, when using small gates for injection molding, it has the following advantages.
1. In the injection molding process, the pressure-holding and feeding stage 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 packing time will last longer, so it is possible to increase the orientation and flow deformation of the macromolecules, which will cause a big shrinkage in the product, especially near the gate. Stress causes the product to eventually warp and deform. If a small gate is used, it is possible to adjust the volume of the small gate through mold trial or mold repair, so that the melt at the gate will freeze at the right time during the pressure holding process, so as to properly control the feeding time and avoid the above phenomenon.
2. If a small gate is used in a multi-cavity non-equilibrium pouring system, the flow resistance of the gate to the plastic melt will be much greater than the flow resistance of the split runner multi-melt, so it may be in the melt After the runner is filled and sufficient pressure is established, 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.
3. If a larger gate molding product is used, when the surface quality of the product is high, it is often necessary to post-process the product with appropriate tools or machine tools to remove gate scars, especially when the gate is too large. The gate aggregate must also 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. In addition, the scar after the small gate is removed is small, and generally does not require or only a slight trimming and polishing work. Therefore, the use of small gates is not only conducive to the separation of the condensate of the pouring system from the product, but also facilitates the finishing of the product.
At the same time, it should be noted that although small gates have the above-mentioned advantages, too small gates will cause great flow resistance, resulting in prolonged feeding and filling time. Therefore, it is not advisable to use small gate molding for some plastic melts (such as polyformate and polysulfone, etc.) with high viscosity or shear rate that has little effect on the apparent viscosity. In addition, when forming large products, you should also pay attention to correspondingly enlarge the cross-sectional area of the gate. Sometimes it is even necessary to enlarge the height of the cross-section of the gate close to the maximum thickness of the product to improve the fluidity of the melt.