Design of gate is related to size of plastic mold parts, shape of mold, plastic molding process conditions and performance of injection molded parts. However, as far as basic function is concerned, cross section of gate is small and length is short, because only in this way can requirements of increasing flow rate, rapid cooling and closing, separating plastic mold parts and minimum gate residue are satisfied.

 

1) Appearance requirements (gate marks, weld lines)
2) Product function requirements
3) Plastic molding process requirements
4) Warpage deformation of product
5) Gate is not easy to remove
6) Plastic molding process is easy to control

1) Flow length determines injection pressure, clamping force, and shortened full flow length of product can reduce injection pressure and clamping force.
2) Position of gate will affect holding pressure, pressure of holding pressure, and whether holding pressure is balanced. Keep gate away from position of product (such as bearing) to avoid residual stress. Gate position must consider exhaust to avoid accumulation occurs, do not place gate in a weaker place or in embedding to avoid misalignment.

 

Gate is a short groove with a small cross-sectional area for connecting flow path to cavity. Cross-sectional area must be small to achieve following effects:
1) soon after mold hole is injected, gate is cold junction
2) Simple water removal
3) Only after water outlet is completed, only a few traces are left
4) Make filling of multiple cavity points easier to control
5) Reduce excessive amount of filler

1) Place gate at the thickest part of product and pour it from the thickest part to provide better filling and pressure retention. If pressure is insufficient, thinner area will solidify faster than thicker area, avoiding gate being placed at a sudden change in thickness to avoid hysteresis or short shots.
2) If possible, pouring from center of product, placing gate in the center of product can provide same length of flow, length of flow will affect required injection pressure, and central pouring will make pressure in all directions uniform. Uneven volume shrinkage can be avoided.
3) When plastic flows into flow path, plastic that near die surface first heats down (cools) and solidifies. When plastic flows forward, it flows only through solidified plastic layer. Moreover, since plastic is a low heat transfer material, solid plastic forms a green layer and retaining layer that can still flow. Therefore, in ideal case, gate should be placed in cross-flow layer position to make the best plastic flow effect. This situation is most common in circular and hexagonal cross-flow paths. However, trapezoidal cross-flow paths do not achieve this effect because gate cannot be placed in the middle of flow path.
When determining location of gate, following principles should be followed:
1) Rubber compound injected into each part of cavity should be as even as possible;
2) Rubber compound injected into mold shall maintain a uniform and stable flow front at all stages of injection process;
3) Consideration should be given to possible occurrences of weld marks, bubbles, pockets, imaginary position, insufficient shot glue and glue spray;
4) Water removal operation should be as easy as possible, preferably automatic operation;
5) Position of gate should be compatible with all aspects.
There are no hard and fast rules for designing gates. Most of them are based on experience, but there are two basic elements that need to be compromised:
1) The larger cross-sectional area of gate, the better; the shorter length of channel, the better. To reduce pressure loss when plastic passes.
2) Gate must be narrow so that it can be easily cold-knotted and prevent excessive plastic from flowing backwards. Therefore, gate should be in the center of flow channel, and its cross-section should be as circular as possible. However, gate switch is usually determined by switch of module.

Size of gate can be determined by cross-sectional area and length of gate. The following factors determine optimal size of gate:
1) Rubber flow characteristics
2) Thickness of module
3) Amount of rubber injected into cavity
4) Melting temperature
5) Mold temperature

If a balanced runner system is not available, the following gate balancing method can be used to achieve goal of uniform injection moulding. 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 a different projected area, gate also needs to be balanced. At this time, to determine size of gate, it is necessary to first determine size of one gate, find ratio of its gate cavity volume, and apply this ratio to its gate and each corresponding cavity. In comparison, size of each gate can be determined successively. After actual test, balance operation of gate can be completed.

Sprue supplies plastic directly to finished product, and sprue adheres to finished product. In two-plate mold, big nozzle is usually one out, but in design of three-plate mold or hot runner mold, there can be more than one out.
Disadvantages: Formation of a stencil on surface of finished product will affect appearance of finished product, while size of stencil is determined by diameter hole of nozzle.

Therefore, large water-jet printing can be reduced, as long as size of above-mentioned nozzle is reduced. However nozzle is easy to be out of mold, and draft angle cannot be less than 3 degrees, so only length of nozzle can be shortened.
Gate selection:
Gate is connecting part of flow channel and cavity, and is also the last part of injection molding system. Its basic function is:
1) Molten plastic from flow path enters filled cavity at the fastest speed.
2) After cavity is filled, gate can be quickly cooled and closed to prevent plastic cavity from flowing back without cooling.
PS: Nozzle is a very important part of mold. After heating and dissolving, plastic is injected into mold through nozzle to form product. It is assembled in the middle of front mold (A board))

Design of gate is related to size of injection molded parts, shape of mold, plastic molding process conditions and performance of plastic mold parts. However, as far as basic function is concerned, cross section of gate is small and length is short, because only in this way can requirements of increasing flow rate, rapid cooling and closing, and separation of plastic parts and minimum gate residue are satisfied.
Main points of gate design can be summarized as follows:
1) Gate is opened in a thick section of plastic mold parts, so that molten material flows into thin section from thick material section to ensure complete filling;
2) Choice of gate location should minimize plastic filling process to reduce pressure loss;
3) Choice of gate location should be beneficial to eliminate air in cavity;
4) Gate should not be allowed to flush melt directly into cavity, otherwise it will generate a swirling flow, leaving traces of rotation on plastic part, especially narrow gate is more prone to such defects;
5) Choice of gate location should prevent formation of seam line on plastic surface, especially in the ring or cylindrical plastic mold parts, cold well should be added at melt casting of surface of gate;
6) Gate position of injection moulding with elongated core should be far from forming core, so that forming core is not deformed by flow;
7) When forming large or flat plastic mold parts, a double gate can be used to prevent warpage, deformation and lack of material;
8) Gate should be opened as far as possible without affecting appearance of injection molded parts, such as bottom of edge;
9) Size of gate depends on size, shape and plastic properties of plastic mold parts;
10) When designing multiple cavity injection moulding, balance of gates are designed according to balance of flow channels, and try to make molten materials uniformly charge at the same time.