The gate design of injection molds is related to factors such as the size, shape, mold structure, injection molding process conditions, and properties of the plastic parts. But in terms of basic function, the gate section should be small and the length should be short, because only in this way can it meet the requirements of increasing the flow rate, rapid cooling and sealing, facilitating the separation of plastic parts, and minimizing gate residue.

1、 Gate position

The gate position needs to meet five basic requirements:

1. Appearance requirements (sprue marks, weld lines);

2. Product functional requirements;

3. Requirements for mold processing;

4. Warping and deformation of the product;

5. Is the gate easy to remove;

6. The molding process is easy to control.

2、 Impact on production and functionality

The flow length determines the injection pressure, locking force, and whether the product can be fully filled. Shortening the flow length can reduce the injection pressure and locking force. The position of the gate will affect the holding pressure, such as the magnitude and balance of the holding pressure. Keep the gate away from the product's stress location (such as the bearing) to avoid residual stress. The position of the gate must consider exhaust to avoid wind accumulation; Do not place the sprue in weak or embedded areas of the product to avoid misalignment.

3、 Selection techniques for gate position

1. Gate

A sprue is a short groove with a small cross-sectional area, used to connect the runner and the mold cavity. The cross-sectional area needs to be small in order to achieve the following effects:

(1) Shortly after injection into the mold cavity, the sprue immediately forms a cold junction;

(2) Easy water outlet;

(3) The water outlet is completed, leaving only a few traces;

(4) Make it easier to control the filling of multiple cavities;

(5) Reduce the phenomenon of excessive filling.

2. Gate position and size

(1) Placing the sprue at the thickest part of the product and pouring from the thickest part can provide better filling and pressure holding effects. If the holding pressure is insufficient, thinner areas will solidify faster than thicker areas. Avoid placing the sprue at locations with sudden changes in thickness to prevent hysteresis or short shots.

(2) If possible, pouring from the center of the product and placing the sprue in the center of the product can provide equal flow length. The size of the flow length will affect the required injection pressure, and the central pouring ensures uniform holding pressure in all directions, which can avoid uneven volume shrinkage.

(3) When plastic flows into the flow channel, it first cools down and solidifies near the mold surface. When plastic flows forward, only the solidified plastic layer flows through. Due to the low heat transfer properties of plastic, solid plastic forms insulation and retention layers that can still flow. Therefore, in an ideal situation, the gate should be set at the position of the cross flow layer to achieve the best plastic flow effect. This situation is most common in circular and hexagonal cross flow channels. However, a trapezoidal cross flow channel cannot achieve this effect because the gate cannot be placed in the middle of the channel.

When determining the position of the gate, the following principles should be strictly followed:

(1) The rubber material injected into each part of the mold cavity should be as evenly distributed as possible;

(2) The adhesive injected into the mold cavity should maintain a uniform and stable flow front at all stages of the injection process;

(3) Possible situations such as weld marks, bubbles, cavities, voids, insufficient glue injection, and glue spraying should be considered;

(4) Efforts should be made to make the operation of the water outlet as easy as possible, preferably automatic;

(5) The position of the gate should be coordinated with all aspects.

There are no hard and fast rules for designing gates, and most of them are based on experience. However, there are two basic elements that need to be compromised:

(1) The larger the cross-sectional area of the sprue, the better, while the shorter the length of the channel, the better, to reduce pressure loss when plastic passes through;

(2) The sprue should be narrow and narrow to facilitate easy cold forming and prevent excessive plastic backflow. Therefore, the sprue should be located in the center of the runner, and its cross-section should be as circular as possible.

3. Gate size

The size of the sprue can be determined by the cross-sectional area and sprue length. The following factors can determine the optimal size of the sprue:

(1) Flow characteristics of rubber material;

(2) The thickness of the module;

(3) The amount of adhesive injected into the mold cavity;

(4) Melting temperature;

(5) Mold temperature.

4、 Balance of sprue

If a balanced runner system cannot be obtained, the following gate balancing method can be used to achieve the goal of uniform injection molding. This method is suitable for molds with a large number of cavities.

There are two methods for balancing the sprue: changing the length of the sprue channel and changing the cross-sectional area of the sprue.

In another scenario, where the mold cavity has different projected areas, the sprue also needs to be balanced. At this point, to determine the size of the sprue, it is necessary to first determine the size of one sprue, calculate its ratio to the volume of its corresponding mold cavity, and apply this ratio to the comparison between its sprue and each corresponding mold cavity, in order to sequentially determine the size of each sprue. After actual trial injection, the balancing operation of the gate can be completed.

5、 Direct gate or large water outlet

The sprue directly supplies plastic to the finished product, and the sprue adheres to the finished product. For two plate molds, one large spout is usually used, but in the design of three plate molds or hot runner molds, more than one spout can be used.

Disadvantage: The water mark formed on the surface of the finished product can affect its appearance, and the size of the water mark depends on the small diameter hole of the nozzle.

6、 Jizui demolding angle and length

The large water mouth print can be reduced in size by reducing the size of the nozzle mentioned above. However, due to the influence of the nozzle diameter and the need for easy demolding of the nozzle, the demolding angle cannot be less than 3 degrees. Therefore, the only solution is to shorten the nozzle length.

7、 Gate selection

The sprue is the connecting part between the runner and the cavity, and also the final part of the injection mold feeding system. Its basic function is to:

1. Allow the molten plastic from the flow channel to enter and fill the mold cavity at the fastest speed possible;

After the mold cavity is filled, the sprue can quickly cool and seal, preventing the plastic melt in the cavity that has not yet cooled from flowing back.

8、 Summary

The key points of gate design can be summarized as follows:

1.The sprue is located in the thicker section of the plastic part, allowing the molten material to flow from the thick section into the thin section, ensuring complete filling of the mold;

2. The selection of gate position should minimize the plastic filling process to reduce pressure loss;

3. The selection of gate position should be conducive to eliminating air in the mold cavity;

4. The sprue should not allow the molten material to be directly injected into the mold cavity, otherwise vortex flow will be generated, leaving spiral marks on the plastic part, especially narrow sprues are more prone to such defects;

5. The selection of gate position should prevent the formation of fusion lines on the plastic surface, especially in circular or cylindrical plastic parts. A cold material well should be added at the molten material pouring joint opposite the gate;

6. The gate position of injection molds with slender cores should be far away from the core to prevent deformation due to material flow impact;

When forming large or flat plastic parts, compound gates can be used to prevent warping, deformation, and material shortage;

8. The sprue should be placed in a position that does not affect the appearance of the plastic part, such as at the bottom edge;

The size of the sprue depends on the size, shape, and properties of the plastic part;

When designing injection molds with multiple cavities, the balance of the sprue should be considered in conjunction with the balance of the runner, and the molten material should be evenly filled into each cavity as much as possible.