What are the advantages of small gate injection molding?

Introduction

Except for the main flow gate, most of the gates are the smallest part of the pouring system, with a cross-sectional area of only 3% to 9% of the cross-sectional area of the manifold.

For the plastic melt obeying Newtonian flow law, since its viscosity is not related to 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 of the plastic melts that do not obey the Newtonian flow law, the use of reduced gate cross-sectional area is often likely to increase the shear rate of the melt, which will lead to a significant decrease in the apparent viscosity of the melt due to the sheer heat effect.

As for the pressure drop caused by the increased flow resistance when using small gates, it can be compensated by increasing the injection pressure within a certain range.

Generally speaking, when small gates are used for injection molding, they have the following advantages.

1. The large pressure difference between the front and rear ends of the small gate can effectively increase the shear rate of the melt and produce a larger shear heat, which leads to a decrease in the apparent viscosity of the melt and an increase in fluidity, which is conducive to mold filling.

This feature of small gates is very beneficial for thin-walled products or products with fine patterns, as well as plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), etc., whose viscosity is sensitive to shear rate.



2. In the injection molding process, the pressure-holding and shrinkage phase generally continues until the melt freezes at the gate, otherwise, the melt in the mold cavity will flow backward outside the cavity.

If the gate size is larger, the holding pressure and shrinkage time will be longer, so it is possible that the orientation and flow deformation of macromolecules will increase, causing great shrinkage stress in the product, especially near the gate, resulting in warpage and deformation of the product.

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 they melt at the gate can be frozen at the right time during the pressure-holding process, thus properly controlling the shrinkage time and avoiding the above phenomenon.

3. Because the volume of the small gate is small and freezing is fast, so when producing some products, it is not necessary to wait for all the internal curing of the products after the small gate freezing, as long as the external curing layer has enough strength and rigidity, the products can be demolded, thus shortening the molding cycle and improving the production efficiency.

4. If the small gate is used in the non-equilibrium casting system of multi-cavities, the resistance of the gate to the flow of the plastic melt will be much larger than that of the melt in the manifold, so it is possible that each cavity can be fed and filled at approximately the same time after the melt has filled the manifold and built up enough pressure.

Therefore, small gates in multiple cavities can balance the feeding speed of each cavity and help to balance the pouring system.

5. If a large gate is used for molding products, in case of high surface quality, appropriate tools or machine tools are needed to post-process the products to remove the gate scars, especially when the gate is too large, the gate condensate must be removed by sawing or cutting. However, this trouble can be avoided when small gates are used.

For example, small gates can be removed quickly by hand or automatically by a special mold structure during demolding. In addition, the scar after the small sprue removal is small and usually requires no or only a little trimming and polishing work.
 

Conclusion

Therefore, the use of small gates not only facilitates the separation of the condensate from the product in the pouring system but also facilitates the trimming of the product.

However, it should be noted that despite all the above advantages, a small gate can cause great resistance to flow and lead to a longer filling time. Therefore, for some high viscosity or shear rate on the apparent viscosity of the plastic melt (such as polycarbonate and polysulfone, etc.), are not suitable to use small gate molding.

In addition, when molding large products, attention should also be paid to enlarging the gate cross-sectional area accordingly, sometimes even the height of the gate cross-section needs to be enlarged to close to the maximum thickness of the product in order to improve the flowability of the melt.

In addition to the above cases, for products with larger wall thickness and shrinkage rate, it is generally required to have sufficient shrinkage time, so in this case, the gate cross-sectional area should not be designed too small.