How To Design The Gate Of Injection Mold?

- Mar 10, 2019-

The design of gate is related to the size, shape and mold structure of plastic parts, injection process conditions and the performance of plastic parts. However, as for the basic function, the gate section should be small and the length should be short, because only in this way can the flow speed be increased, the rapid cooling and sealing, the separation of plastic parts and the minimum residual marks of the gate be satisfied.

1. Requirements for gate location to be met

Appearance requirements (gate marks, fusion wiring)

Product functional requirements

Mold processing requirements

Warping deformation of the product

Gate capacity is not easily removed

The forming process is easy to control

2. Impact on production and function

The length of flow determines the injection pressure, the clamping force, and the shortening of the full flow length whether the product is filled can reduce the injection pressure and the clamping force.

The gate position will affect the pressure retaining pressure, the size of the pressure retaining pressure and whether the pressure retaining pressure is balanced. Keep the gate away from the stressed position of the product (such as the bearing position) to avoid residual stress. Exhaust gas must be considered in the gate position to avoid wind accumulation.

3. Tips for selecting gate position


The gate is a short groove with a small cross-section area to connect the runner to the die cavity. The cross-sectional area should be small so as to achieve the following effects:

Soon after the injection of die cavity, the gate is cold knot

Easy to remove water inlet

The drain is complete, leaving only a few traces

Make the packing of multiple die holes easier to control

Reduce overpacking

Gate position and size

(1) the gate placed in the thickest place of the product, from the thickest place into the pouring can provide better filling and pressure retaining effect. If the holding pressure is insufficient, the thicker zone will solidify faster in the thinner zone, so as to avoid placing the gate in the place where the thickness changes suddenly, so as to avoid hysteresis or short shot.

(2) if possible, from the product center into the pouring, the gate placed in the middle of the product can provide equal flow length, flow length will affect the size of the injection pressure required, the central pouring makes the pressure in all directions uniform, can avoid uneven volume contraction.

(3) when the plastic flow into the channel, plastic close to the mold surface first drop heat (cooling) and solidification. When the plastic flows forward again, it just flows through layers of plastic that have solidified there. And because plastic is a low heat transfer material, solid plastic forming a green layer and maintain the layer can still flow

Therefore, in an ideal situation, the gate should be placed in the cross-channel layer to achieve the best plastic flow effect. This condition is most common in circular and hexagonal cross-flow channels. However, trapezoidal cross-flow channels cannot achieve this effect because the gate cannot be placed in the middle of the flow channel.

When deciding the gate position, the following principles should be followed:

The injection of plastic cavity parts should be as average as possible;

The injection molding material, in each stage of the injection process, should maintain a unified and stable flow front;

It should be considered that welding marks, bubbles, cavities, empty positions, insufficient injection glue and glue spraying may occur;

Should try to make the operation of the water outlet easy to carry out, preferably automatic operation;

The location of the gate should be coordinated with all aspects.

There are no hard and hard rules about how to design gates. Most of them are based on experience, but there are two basic elements to be considered in a compromise:

The larger the cross-section area of the gate, the better, and the shorter the groove length, so as to reduce the plastic through the pressure loss.

Gate should be thin and narrow to allow cold forming and prevent excessive plastic backflow. Gate should be in the middle of the runner and its cross-section should be as round as possible. However, the gate switch is usually determined by the switch of the module.

The gate size

The gate size can be determined by cross-cutting area and gate length, and the following factors can determine the optimal gate size:

Flow characteristics of adhesive

Thickness of the module

The amount of plastic injected into the mold cavity

Melting temperature

The mould temperature

4. Gate balance

If a balanced runner system is not available, the following gate balancing method can be used to achieve the goal of uniform injection molding. This method is suitable for working dies with a large number of holes.

There are two methods of gate balancing: changing the length of the gate groove and changing the cross-section area of the gate. In the other case, the gate also needs to be balanced when the cavity has different projected area.

At this time, to determine the size of the gate, it is necessary to determine the size of one of the gate, find out the ratio of it to the corresponding cavity volume, and apply this ratio to the comparison of its gate and the corresponding cavity, then the size of each gate can be calculated. After the actual test injection, the gate balance operation can be completed

5. Direct gate or large gate

The runner supplies the plastic directly to the finished product and the runner adheres to the finished product. In the working die of two plates, the large water outlet is usually one, but in the design of the working die of three plates or hot runner, it can be more than one beer.

Disadvantages: the appearance of the finished product will be affected by the appearance of the watermark formed on the surface of the finished product.

6. Demoulding Angle of the nozzle and the length of the nozzle

However, the diameter of the nozzle is affected by the diameter of the nozzle. The ejection Angle of the nozzle should not be less than 3 degrees. Therefore, only the length of the nozzle can be shortened and the length of the nozzle can be lengthened.

Gate selection:

The gate is the connection part between the runner and the cavity, and also the last part of the injection mold feeding system. Its basic function is:

The molten plastic from the runner enters the full cavity at the fastest speed.

After the cavity is filled, the gate can be quickly cooled and closed to prevent the cavity from reflux of uncooled plastic.

PS :(the nozzle is an important part of the mold. After the nozzle is heated and dissolved, the plastic is injected into the mold to form the product, which is assembled in the middle of the front mold (plate A).

7. To summarize

The design of gate is related to the size, shape and mold structure of plastic parts, injection process conditions and the performance of plastic parts. However, as for the basic function, the gate section should be small and the length should be short, because only in this way can the flow speed be increased, the rapid cooling and sealing, the separation of plastic parts and the minimum residual marks of the gate be satisfied.

Key points of gate design can be summarized as follows:

1) the gate is opened at the part with thicker section of the plastic parts, so that the molten material flows into the thin section from the thick section to ensure complete mold filling;

2) the location of gate should be selected to minimize the plastic mold filling process to reduce pressure loss;

3) the choice of gate location should be conducive to the exclusion of air in the cavity;

4) the gate is not suitable for the molten material to be directly flushed into the cavity, otherwise it will generate whirlpool and leave swirl marks on the plastic parts, especially the narrow gate is more prone to such defects;

5) the location of the gate should be selected to prevent the occurrence of stitching lines on the plastic surface. In particular, for plastic parts with real ring or cylindrical shape, the cold material well should be opened at the fusion pouring place on the gate surface;

6) the gate position of the injection mold with slender core should be far away from the forming core, so that the forming core will not be deformed due to material flow;

7) when forming large or flat plastic parts, compound gate can be used to prevent warping, deformation and material shortage;

8) the gate should be opened in a position that does not affect the appearance of the plastic parts, such as the bottom edge;

9) the size of the gate depends on the size, shape and performance of the plastic parts;

10) when designing multiple cavity injection molding molds, the balance of gate should be considered in combination with the balance of runner, so as to achieve uniform charging of molten material at the same time.