Injection molding gate design is related to factors such as plastic part size, shape, mold structure, injection process conditions and plastic part performance. But in terms of basic functions, gate cross-section should be small and length should be short, because only in this way can it meet requirements of increasing material flow rate, rapid cooling and closure, facilitating plastic part separation and minimizing gate marks.

1. Appearance requirements (gate marks, weld lines);
2. Product function requirements;
3. Mold processing requirements;
4. Product warping and deformation;
5. Is gate easy to remove.

1. Flow length
Flow length determines injection pressure, clamping force, and whether product is fully filled. Shortening flow length can reduce injection pressure and clamping force.
2. Gate position
Gate position will affect holding pressure, size of holding pressure, and whether holding pressure is balanced. Keep gate away from stress position of product (such as bearing) to avoid residual stress; gate position must consider venting to avoid cavitation; do not place gate in weak or embedded part of product to avoid deviation.

 

Gate is a short groove with a small cross-sectional area that connects runner and cavity. Cross-sectional area should be small in order to achieve following effects:
(1) Gate will be cold soon after mold cavity is injected;
(2) Dewatering gate is simple;
(3) After dewatering gate is completed, only a small trace is left;
(4) Filling of multiple mold cavities is easier to control;
(5) Reduce phenomenon of excessive filling.

(1) Place gate at the thickest part of product. Pouring from the thickest part can provide better filling and holding pressure. If holding pressure is insufficient, thinner area will solidify faster than thicker area. Avoid placing gate at the place where thickness changes suddenly to avoid hysteresis or short shots.
(2) If possible, inject glue from center of product. Placing gate in the center of product can provide equal flow length, and size of flow length will affect required injection pressure. Central pouring makes holding pressure uniform in all directions, which can avoid uneven volume shrinkage.
(3) When plastic flows into runner, plastic close to mold surface will first cool down (cool) and solidify. When plastic flows forward again, it only flows through solidified plastic layer. Since plastic is a low heat transfer material, solid plastic forms an insulating layer, and melt in the middle can still flow.
Therefore, ideally, gate should be set at cross-flow channel layer to produce the best plastic flow effect. This situation is most common in circular and hexagonal cross-flow channels. However, trapezoidal cross-flow channels cannot achieve this effect because gate cannot be set in the middle of flow channel.

 

When determining gate position, following principles should be followed:
(1) Material injected into each part of mold cavity should be as even as possible;
(2) Material injected into mold should maintain a uniform and stable flow front at all stages of injection process;
(3) Possible weld marks, bubbles, cavities, voids, insufficient injection and spraying should be considered;
(4) Dewatering operation should be as easy as possible, preferably automatic operation;
(5) Location of gate should be coordinated with all aspects.
There are no rigid rules for gate design, and most of them are based on experience. However, there are two basic factors that must be compromised:
(1) The larger gate cross-sectional area, the better, and the shorter channel length, the better, to reduce pressure loss when plastic passes through;
(2) Gate must be narrow to facilitate cold freezing and prevent excessive plastic backflow. Therefore, gate is in the center of runner, and its cross-section should be as circular as possible. However, gate opening and closing is usually determined by mold opening and closing.

Gate size can be determined by cross-sectional area and gate length. Following factors can determine optimal gate size:
(1) Flow characteristics of rubber compound;
(2) Thickness of mold;
(3) Amount of rubber injected into mold cavity;
(4) Melting temperature;
(5) Mold temperature.

 

If a balanced runner system cannot be obtained, following gate balance method can be used to achieve goal of balance. This method is suitable for molds with a large number of cavities.
There are two ways to balance gate: changing length of gate channel and changing cross-sectional area of gate. In another case, when 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, find out its ratio compared with volume of its corresponding cavity, apply this ratio to comparison between its gate and each corresponding cavity, so that size of each gate can be successively found. After actual trial injection, gate balancing operation can be completed.

Sprue directly supplies plastic to finished product, and sprue adheres to finished product. On a two-plate mold, sprue is usually one out of one. Sprue gates are rarely used in design of three-plate molds or hot runner molds.
Disadvantages: Sprue mark is formed on the surface of product, which affects appearance of product.

Gate is connecting part between runner and cavity, and is also last part of injection mold feeding system. Its basic functions are:
1. Allow molten plastic from runner to enter cavity at the fastest speed;
2. After cavity is filled, gate can be quickly cooled and closed to prevent plastic in cavity from flowing back.