For plastic melts that obey Newtonian flow law, large gate cross-sectional area can reduce flow resistance and increase melt flow rate, which is advantageous for both filling and forming quality, because its viscosity is independent of shear rate. For most plastic melts that do not obey Newtonian flow law, it is often possible to increase melt shear rate by reducing cross-sectional area of gate. Due to shear heat effect, apparent viscosity of melt will be greatly reduced, but it may be more favorable for filling mold than large-section gate.

 

Gate is a critical section of gating system. Except for main channel gate, most gates are the smallest cross-sectional area in gating system. Value is generally only 3% to 9% of cross-sectional area of split runner . As for pressure drop caused by increasing flow resistance when forming with a small gate, it can be compensated by increasing injection pressure within a certain range. In general, when a small gate is used for injection molding, following advantages are obtained.
1. There is a large pressure difference between front and rear of small gate, which can effectively increase shear rate of melt and generate a large shear heat, which leads to a decrease in apparent viscosity of melt, an increase in fluidity and conducive to filling mold. This feature of small gates is for thin-walled articles , products with fine patterns and plastics that are sensitive to shear rate, such as polyethylene (PE), polypropylene (PP), polystyrene (PS), etc.
2. Due to small volume of small gate and fast freezing, when certain products are produced, it is not necessary to completely cure inside of product after small gate is frozen. As long as external solidified layer has sufficient strength and rigidity, product can be demolded, thereby shortening molding cycle and improving production efficiency.
3. During injection molding process, pressure-preserving and shrinking stage generally continues until melt at the gate is frozen, otherwise melt in cavity will flow back outside cavity. If gate size is large, pressure-holding time will continue for a long time, so it is possible to increase degree of orientation and flow deformation of macromolecule, causing a large shrinkage in product, especially near gate, resulting in warpage deformation of product. If a small gate is used, it is possible to adjust volume of small gate by trial or repair, so that melt at the gate is frozen during pressure keeping process, thereby properly controlling feeding time and avoiding above phenomenon.
4. If a larger gate is used to shape product, in the case of high surface quality requirements, it is often necessary to post-process product with a suitable tool or machine tool to remove gate scar. Especially when gate is too large, gate aggregate must also be removed by sawing, cutting, etc. However, this can be avoided by using a small gate. For example, small gate aggregates can be quickly removed by hand or automatically removed using a special mold structure during demolding. In addition, scar after small gate is small, generally does not require or only need a slight finishing work. Therefore, use of a small gate not only facilitates detachment of gluing material from product, but also facilitates trimming of product.
5. If a small gate is used in a multi-cavity non-equilibrium casting system, flow resistance of gate to plastic melt will be much greater than flow resistance of multi-melt of split runner, so it is possible that after melt fills flow path and establishes sufficient pressure, cavities can be filled at approximately same time. Therefore, small gate can balance feed rate of each cavity in multi-cavity cavity, which is beneficial to balance of gating system.

 

At the same time, it should be noted that although small gate has above advantages, too small gate causes a large flow resistance, resulting in an extended filling time of feed. Therefore, for some plastic melts (such as polyacrylates and polysulfones) whose high viscosity or shear rate has little effect on apparent viscosity, it is not suitable to use small gate molding. In addition, when molding large products, it should also be noted that cross-sectional area of gate is enlarged accordingly, and sometimes it is even necessary to enlarge height of cross-section of gate to maximum thickness of product to improve fluidity of melt. In addition to above, for products having a large wall thickness and a large shrinkage rate, a sufficient feeding time is generally required, so in this case, gate cross-sectional area cannot be designed too small.