How to design gate of an injection moulding die?

Time:2019-11-20 09:39:42 / Popularity: / Source:

Mold manufacturing process 
Design of gate is related to size of plastic part, shape of injection moulding die, injection process conditions and performance of plastic part. However, in terms of basic functions, gate section is small and length is short, because only in this way can requirements of increasing flow rate, rapid cooling and closing, separation of plastic parts and minimum gate residue are satisfied.

1. Requirements for gate location to be met

· Appearance requirements (gate marks, weld lines)
· Product feature requirements
· Mold manufacturing process requirements
· Warpage of product
· Pouring capacity is easy to remove
· Mold manufacturing process is easy to control

2. Impact on production and function

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

3. Tips for choosing a gate location

Mold manufacturing process 


Gate is a short groove with a small cross-sectional area for connecting flow path to cavity. Cross-sectional area is so small that purpose is to achieve following effects:
· Soon after cavity injection, gate will be cold
· Simple water removal
· Only traces are left, leaving only a few traces
· Make filling of multiple cavities easier to control
· Reduce excess filler

Gate location and size

1 Place gate at the thickest part of product, and pour it from thickest part to provide better filling and holding effect. 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 the center of product, placing gate in the center of product can provide same length of flow, size of flow will affect required injection pressure, and central pouring will make pressure in all directions uniform to avoid uneven volume shrinkage.
3 When plastic flows into flow path, plastic first heats down (cools) and solidifies near die face. 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 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:
· Rubber injected into each part of cavity should be as even as possible;
· Injecting compound of mold, a uniform and stable flow front should be maintained at all stages of injection process;
· Consideration should be given to possible occurrences of weld marks, bubbles, pockets, imaginary position, insufficient shot glue and glue spray;
· It should be as easy as possible to make water removal operation easy, preferably automatic operation;
· Location 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:
· The larger cross-sectional area of gate, the better, and the shorter length of channel, the better pressure loss during plastic passage.
· 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 in mold manufacturing process.

Gate size

Size of gate can be determined by cross-sectional area and length of gate. Following factors determine optimal size of gate:
· Compound flow characteristics
· Thickness of module
· Amount of rubber injected into cavity
· Melting temperature
· Tool temperature

Balance of gate

injection moulding die 
If a balanced runner system is not available, following gate balancing method can be used to achieve goal of uniform injection molding. This method is suitable for injection moulding die with a large number of cavities.
There are two ways to balance gate: changing length of gate channel and 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 of gates, find ratio of its gate cavity volume, 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.

Direct gate or large nozzle

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

Draft angle of nozzle, length of nozzle

Therefore, large water-jet printing can be reduced, as long as size of above-mentioned nozzle is reduced. However, diameter of nozzle is affected by diameter of nozzle. Nozzle should be easy to demoulding, and draft angle cannot be less than 3 degrees, so only length of nozzle can be shortened in mold manufacturing process.
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:
· Allow molten plastic from flow path to enter filled cavity as quickly as possible.
· After cavity is filled, gate can be quickly cooled and closed to prevent cavity from reflowing from uncooled plastic.
PS: nozzle is a very important part of injection moulding die, heating and dissolving plastic is injected into injection moulding die through nozzle to form product, assembled in the middle of front mold (A board))


injection moulding die 
Design of gate is related to size of plastic part, shape of injection moulding die, injection process conditions and performance of plastic part. However, in terms of basic functions, gate section is small and length is short, because only in this way can requirements of increasing flow rate, rapid cooling and closing, 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 part, 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 parts, cold well should be added at melt casting of gate surface;
6) Gate position of injection moulding die 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 parts, a double gate can be used to prevent warpage, deformation and lack of material in mold manufacturing process;
8) Gate should be opened as far as possible without affecting appearance of plastic part, such as bottom of edge;
9) Size of gate depends on size, shape and plastic properties of plastic part;
10) When designing multiple cavity injection moulding die, consider balance of flow channels to consider balance of gates, and try to make molten materials uniformly charge at the same time.

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