Do you know all the advantages of injection molding with small gates?

The gate is a very crucial section of the material flow channel in the injection system. Except for the main runner type gate, most gates have the smallest cross-sectional area in the injection system, which is generally only 3% to 9% of the cross-sectional area of the runner. For plastic melts that follow Newtonian flow laws, since their viscosity is independent of shear rate, a large gate cross-sectional area can reduce flow resistance and increase melt flow rate, which is beneficial for mold filling and molding quality.

However, for the vast majority of plastic melts that do not follow Newtonian flow laws, reducing the gate cross-sectional area often increases the shear rate of the melt. Due to the shear heat effect, the apparent viscosity of the melt will decrease significantly, which may be more favorable for mold filling than a large cross-sectional gate. As for the pressure drop caused by the increased flow resistance when using a small gate for molding, it can be compensated within a certain range by increasing the injection pressure. Generally speaking, using a small gate for injection molding has the following advantages.

① There is a large pressure difference before and after the small gate, which can effectively increase the shear rate of the melt and generate a large amount of shear heat, thereby reducing the apparent viscosity of the melt and enhancing its fluidity, which is beneficial for mold filling. This characteristic of small gates is particularly beneficial for thin-walled products or products with fine patterns, as well as for the molding of plastics such as polyethylene (PE), polypropylene (PP), and polystyrene (PS), whose viscosity is highly sensitive to shear rate.

② During the injection molding process, the holding pressure and packing stage usually lasts until the melt at the gate freezes. Otherwise, the melt in the cavity will flow back out. If the gate size is large, the holding pressure and packing time will be prolonged, which may increase the degree of molecular orientation and flow deformation, causing significant packing stress in the product, especially near the gate, leading to warpage of the product. If a small gate is used, the volume of the small gate can be adjusted through mold trials or modifications to ensure that the melt at the gate freezes in a timely manner during the holding pressure stage, thereby appropriately controlling the packing time and avoiding the above-mentioned phenomena.

③ Due to the small volume and quick freezing of the small gate, when producing certain products, the product can be demolded after the small gate freezes without waiting for the entire interior to solidify, as long as the outer solidified layer has sufficient strength and rigidity. This can shorten the molding cycle and improve production efficiency.

④ In a non-balanced injection system with multiple cavities, if a small gate is used, the flow resistance of the plastic melt at the gate will be much greater than that in the runner. Therefore, after the melt fills the runner and builds up sufficient pressure, all cavities can be filled and molded in approximately the same time. Thus, small gates can balance the filling speed of each cavity in multi-cavity systems, which is beneficial for the balance of the injection system.

⑤ If a large gate is used to mold products and the surface quality of the product is required to be high, post-processing with appropriate tools or machines is often necessary to remove the gate scar. Especially when the gate is too large, the gate material must be removed by sawing, cutting, etc. However, using a small gate can avoid such troubles.

For example, the small gate material can be quickly removed by hand or automatically removed during demolding with a special mold structure. Additionally, the scar left after removing a small gate is relatively small and usually does not require or only requires minimal grinding and polishing. Therefore, using a small gate not only facilitates the separation of the gate material from the product but also simplifies the post-processing of the product. However, it should be noted that although small gates have the above advantages, an overly small gate can cause significant flow resistance, resulting in an extended filling time. Therefore, for some plastic melts with high viscosity or where the apparent viscosity is minimally affected by the shear rate (such as polyformaldehyde and polysulfone, etc.), small gate molding is not advisable.

Additionally, when molding large-sized products, it is necessary to appropriately increase the gate cross-sectional area, and sometimes even the gate cross-sectional height needs to be enlarged to approach the maximum thickness of the product to improve the melt's fluidity. Besides the above situations, for products with relatively thick walls and high shrinkage rates, sufficient filling time is generally required, so the gate cross-sectional area should not be designed too small in such cases.