Gate is a very critical section of material flow channel in gating system. Except for main runner type gate, most gates are the smallest part of gating system with the smallest cross-sectional area, and its value is generally only 3%-9% of cross-sectional area of runner.
For plastic melt that obeys Newtonian flow law, because its viscosity has nothing to do with shear rate, a large gate cross-sectional area can reduce flow resistance and increase melt flow rate, which is beneficial to filling and molding quality.
For most plastic melts that do not obey Newtonian flow law, reducing cross-sectional area of gate may often increase melt shear rate. Due to effect of shear heat, apparent viscosity of melt will drop significantly, which may be more conducive to mold filling than large cross-section gates.
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.

 

Generally speaking, when using small gates for injection molding, it has following advantages.
① There is a large pressure difference between front and rear ends of small gate, which can effectively increase shear rate of melt and generate greater shear heat, resulting in a decrease in apparent viscosity of the melt and an increase in fluidity, which is conducive to mold filling.
This feature of small gates is of great benefit to molding of thin-walled products, products with fine patterns and plastics whose viscosity is sensitive to shear rate such as polyethylene (PE), polypropylene (PP), polystyrene (PS).
② In injection molding process, pressure-holding and feeding stage generally continues until melt at gate is frozen, otherwise melt in mold cavity will flow back out of cavity.
If gate size is larger, packing time will last longer, so it is possible to increase orientation degree and flow deformation of macromolecules, causing a large feeding stress in product, especially near gate, resulting in final warpage and deformation of product.
If a small gate is used, it is possible to adjust volume of small gate through mold trial or mold repair, so that melt at gate will freeze in time during pressure holding process, so as to properly control feeding time and avoid above phenomenon.

 

③ Due to small volume of small gate and fast freezing, when producing some products, it is not necessary to wait for inside of product to be completely solidified after small gate is frozen. As long as outer solidified layer has sufficient strength and rigidity, product can be demolded, thereby shortening molding cycle and improving production efficiency.
④. If a small gate is used in a multi-cavity non-equilibrium pouring system, flow resistance of gate to plastic melt will be much greater than flow resistance of split runner multi-melt. Therefore, it is possible that after melt fills flow channel and builds up enough pressure, each cavity can be filled with materials at approximately same time.
Therefore, small gate can balance feed rate of each cavity in multi-cavity, which is beneficial to balance of gating system.
⑤ If a larger gate molding product is used, when surface quality of product is high, it is often necessary to post-process product with appropriate tools or machine tools to remove gate scars, especially when gate is too large. Condensate must also be removed by sawing and cutting. However, this trouble can be avoided when using a small gate.
For example, small gate aggregate can be cut quickly by hand, or automatically cut by a special mold structure during demolding. In addition, scar after small gate is removed is small, generally does not require or only a slight trimming and polishing work.

 

Therefore, use of small gates is not only conducive to separation of condensate of gating system from product, but also facilitates finishing of product.
However, it should be noted that although small gates have 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 apparent viscosity.
In addition, when forming large products, you should also pay attention to correspondingly enlarge cross-sectional area of gate. Sometimes it is even necessary to enlarge height of cross-section of gate close to maximum thickness of product to improve fluidity of melt.

 

In addition to above, for products with large wall thickness and large shrinkage, sufficient feeding time is generally required. Therefore, in this case, gate cross-sectional area cannot be designed too small.

 

Point gate, also known as pin point gate, is a gate with a small cross-sectional size, so it is also called a small gate. Point gate has many obvious advantages due to its small cross-sectional size:
1) Due to small gate size, speed of melt flowing through gate increases significantly, which increases shear rate of melt and decreases apparent viscosity of melt. At the same time, due to high-speed friction and heat generation, temperature of melt increases and viscosity decreases, which improves fluidity of melt and facilitates filling of cavity.
2) It is easy to control the solidification time of gate, that is, to ensure material replenishment and prevent backflow, ensuring product quality, shortening forming cycle, and improving production efficiency.
3) When point gate pouring system is demolded, gate and product are automatically separated, which is convenient for automation of plastic parts production process.
4) Gate marks are small, easy to repair, and appearance of the product is of good quality.
However, point gates also have some shortcomings, such as high requirements for injection pressure, complex mold structure, not suitable for plastic melts with high viscosity and insensitive to shear rate.