1. Appearance requirements (gate traces, weld lines)
2. Product functional requirements
3. Mold processing requirements
4. Warpage of product
5. Gate volume is not easy to remove

1. Flow length (FlowLength) determines injection pressure, clamping force, and shortening of full flow length when product is not filled can reduce injection pressure and clamping force.
2. Position of gate will affect holding pressure, size of holding pressure, and whether holding pressure is balanced. Keep gate away from future stress position of product (such as bearing) to avoid residual stress. Gate must be vented to avoid accumulation of wind. Do not place gates on weak or embedded parts of product to avoid coreshaft.

1. Place gate at the thickest part of product, and pouring from the thickest part can provide better filling and pressure-holding effects. If holding pressure is insufficient, thinner areas will solidify faster than thicker areas. Avoid placing gates where thickness changes suddenly to avoid hysteresis or short shots.
2. If possible, pouring from center of product and placing gate in the center of product can provide an equal flow length. Size of flow length will affect required injection pressure. Central pouring makes holding pressure in all directions uniform. Uneven volume shrinkage can be avoided.
3. Gate: Gate is a short groove with a small cross-sectional area to connect runner and mold cavity. Cross-sectional area is so small in order to obtain following effects:
1) Shortly after cavity is injected, gate is cold
2) Water outlet is simple
3) After water outlet is completed, only a few traces are left
4) Make filling of multiple cavities easier to control
5) Reduce phenomenon of excessive filler
There are no hard and fast rules for method of designing gates, and most of them are based on experience, but there are two basic elements that must be compromised:
1. The larger cross-sectional area of gate, the better, and the shorter length of channel, the better to reduce pressure loss when plastic passes through.
2. Gate must be narrow, so as to be easy to freeze and prevent excess plastic from flowing back. Therefore, gate is in the center of runner, and its cross-section should be as circular as possible. However, gate switch is usually determined by switch of molded part.
3. Gate size:
Size of gate can be determined by cross-sectional area and length of gate. Following factors can determine optimal size of gate:
1) Rubber flow characteristics
2) Thickness of molded part
3) Amount of rubber injected into cavity
4) Melting temperature
5) Die temperature

 

When determining gate location, following principles should be adhered to:
1. Glue injected into each part of cavity should be as even as possible.
2. Compound injected into mold should maintain a uniform and stable flow front in each stage of injection process.
3. Possible occurrence of welding marks, bubbles, pockets, virtual positions, insufficient glue injection and glue spraying should be considered.
4. Operation of water outlet should be made as easy as possible, preferably automatic operation.
5. Location of gate should be coordinated with all aspects.

If a balanced runner system cannot be obtained, following gate balance method can be used to achieve goal of uniform injection molding. This method is suitable for molds with a large number of mold cavities. There are two methods of gate balance: changing length of gate channel and changing cross-sectional area of gate. In another case, that is, when mold cavity has different projected areas, gate also needs to be balanced. At this time, to determine size of gate, it is necessary to first determine size of one of gates, and find ratio of it to its corresponding cavity volume, apply this ratio to comparison between its gate and each corresponding cavity, then size of each gate can be calculated successively. After actual test injection, gate balancing operation can be completed.

When plastic flows into runner, plastic first cools down (cools) and solidifies when it is close to mold surface. When plastic flows forward, only solidified plastic layer flows through it. And because plastic is a low heat transfer material, solid plastic forms insulating green layer and retaining layer can still flow. Therefore, ideally, gate should be set in the position of cross-flow channel layer to make the best plastic flow effect. This situation is most common in circular and hexagonal cross-flow channels. However, trapezoidal cross runner cannot achieve this effect, because gate cannot be set in the middle of runner.

Runner directly supplies plastic to finished product. Runner is adhered to finished product. In the two-plate mold, large nozzle is usually one cavity. But in the design of three-plate mold or the hot runner mold, there can be multi-cavity.
Disadvantages: Forming a nozzle mark on the surface of finished product will affect appearance of finished product. Size of nozzle mark depends on small diameter hole of nozzle.

Therefore, large nozzle mark can be reduced by reducing size of nozzle. However, diameter of nozzle is affected by diameter of nozzle, nozzle should be easy to be ejected from mold, and draft angle should not be less than 3 degrees. So only length of nozzle can be shortened, and furnace nozzle can be lengthened.
Gate selection: Gate is connecting part of runner and cavity, and it is also last part of injection mold feeding system. Its basic functions are:
1) Make molten plastic from runner enter filling cavity at the fastest speed.
2) After cavity is filled, gate can be quickly cooled and closed to prevent plastic from returning to cavity that has not yet cooled down.

Design of gate is related to size, shape and mold structure of plastic part, injection process conditions and performance of plastic part. However, according to basic functions of above two sentences, gate section is small and length is short, because only in this way can it meet requirements of increasing flow rate, rapid cooling and sealing, easy separation of plastic parts, and minimum gate residue.

 

Main points of gate design in picture can be summarized as follows:
1. Gate is opened at thicker section of plastic part, so that molten material flows from thick section into thin section to ensure complete mold filling;
2. Selection of gate location should make plastic filling process the shortest to reduce pressure loss;
3. Choice of gate location should be conducive to removing air in cavity;
4. Gate should not make melt go straight into cavity, otherwise a swirling flow will occur, leaving a swirl mark on plastic part, especially narrow gate is more prone to this defect;
5. Selection of gate location should prevent seam lines from forming on the surface of plastic, especially in the case of circular or cylindrical plastic parts, a cold well should be added at melting point on the surface of gate ;
6. Gate position of injection mold with a slender core should be far away from molding core, so that molding core is not deformed by flow of material;
7. When forming large or flat plastic parts, a double gate can be used to prevent warping, deformation and lack of material;
8. Gate should be opened as far as possible in a position that does not affect appearance of plastic part, such as bottom of edge;
9. Size of gate depends on size, shape and performance of plastic part;
10. When designing an injection mold with multiple cavities, balance of gate should be considered in combination with balance of runner, and molten material should be charged evenly at the same time as much as possible.