The gate is a critical section of the material flow channel in the gating system. Except for the main runner type gate, most gates are the smallest cross-sectional area in the gating system, and their value is generally only 3% to 9% of the cross-sectional area of the branch runner.
For plastic melts that obey the Newtonian flow law, since their viscosity is independent of the shear rate, a large gate cross-sectional area can reduce flow resistance and increase melt flow rate, which is more beneficial to 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 shear heat effect, the apparent viscosity of the melt will drop significantly, which may be more conducive to mold filling than a large cross-sectional gate.
As for the pressure drop caused by the increase in flow resistance when using a small gate for molding, it can be compensated by increasing the injection pressure within a certain range.
Generally speaking, the following advantages are available when using a small gate for injection molding.
①. There is a large pressure difference between the front and rear ends of the small gate, which can effectively increase the shear rate of the melt and generate a large shear heat, thereby causing the apparent viscosity of the melt to decrease, and the fluidity to increase, which is conducive to filling the mold.
This feature of the small gate is of great benefit to the molding of thin-walled products or products with fine patterns, as well as plastic molding whose viscosity is sensitive to shear rate, such as polyethylene (PE), polypropylene (PP), and polystyrene (PS).
②. During the injection molding process, the pressure holding and shrinkage compensation stage generally continues until the melt at the gate freezes, otherwise the melt in the mold cavity will flow back to the outside of the cavity.
If the gate size is large, the pressure holding and shrinkage compensation time will continue for a long time, so it is possible to increase the orientation degree and flow deformation of the macromolecules, causing a large shrinkage compensation stress in the product, especially near the gate, resulting in warping deformation of the product.
If a small gate is used, it is possible to adjust the volume of the small gate through trial mold or mold repair, so that the melt at the gate is frozen in time during the pressure holding process, thereby properly controlling the shrinkage compensation time and avoiding the above phenomenon.
③. Since the small gate has a small volume and freezes quickly, when producing certain products, it is not necessary to wait for the entire interior of the product to solidify after the small gate is frozen. As long as the external solidified layer has sufficient strength and rigidity, the product can be demolded, thereby shortening the molding cycle and improving production efficiency.
④. If a small gate is used in an unbalanced casting system with multiple cavities, the flow resistance of the gate to the plastic melt will be much greater than the flow resistance of the multi-melt in the runner. Therefore, it is possible that after the melt fills the runner and builds up sufficient pressure, each cavity can be fed and filled at approximately the same time.
Therefore, a small gate can balance the feed speed of each cavity in a multi-cavity mold, which is beneficial to the balance of the casting system.
⑤. If a larger gate is used to mold a product, when the surface quality of the product is required to be high, appropriate tools or machine tools are often required to post-process the product to remove the gate scar, especially when the gate is too large, the gate condensate must be removed by sawing, cutting, etc. However, this trouble can be avoided when a small gate is used.
For example, the small gate condensate can be quickly cut off by hand, or automatically cut off by a special mold structure during demolding. In addition, the scar after the small gate is cut off is small, and generally no or only a little finishing and polishing work is required.
Therefore, the use of a small gate is not only conducive to the separation of the condensate from the product in the casting system, but also conducive to the finishing of the product.
However, it should be noted that although the small gate has the above advantages, a too small gate will cause great flow resistance, resulting in a longer feeding and filling time. Therefore, for some plastic melts with high viscosity or shear rate that has little effect on the apparent viscosity (such as polyformate and polysulfone), it is not suitable to use a small gate for molding.
In addition, when molding large products, it is also necessary to enlarge the gate cross-sectional area accordingly. Sometimes it is even necessary to enlarge the gate cross-sectional height to close to the maximum thickness of the product in order to improve the fluidity of the melt.
In addition to the above situation, for products with thicker walls and larger shrinkage rates, sufficient shrinkage compensation time is generally required, so in this case, the gate cross-sectional area cannot be designed too small.
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