The design of the gate is related to the size, shape, mold structure, injection process conditions and performance of the plastic parts. But in terms of basic functions, the gate section should be small and the length should be short, because only in this way can the requirements of increasing the flow rate, rapid cooling and closing, facilitating the separation of plastic parts and minimizing the gate marks be met.
1. Requirements that the gate position needs to meet
Appearance requirements (gate marks, weld lines) Product function requirements Mold processing requirements Warping and deformation of the product Is the gate easy to remove? The molding process is easy to control
2. Impact on production and function
Flow length determines the injection pressure, clamping force, and whether the product is filled or not. Shortening the flow length can reduce the injection pressure and clamping force.
The gate position will affect the holding pressure, the size of the holding pressure, and whether the holding pressure is balanced. Keep the gate away from the force position of the product (such as the bearing) to avoid residual stress. The gate position must consider exhaust to avoid wind accumulation. Do not place the gate at the weaker or embedded part of the product to avoid deviation.
3. Tips for choosing the gate position
Gate
The gate is a short groove with a small cross-sectional area, which is used to connect the runner and the mold cavity. The cross-sectional area should be small in order to achieve the following effects:
The gate is cold soon after the mold cavity is filled. The simple dewatering gate is completed, leaving only a few traces, making it easier to control the filling of multiple mold cavities and reducing the phenomenon of excessive filling
Gate position and size
① Place the gate at the thickest part of the product. Pouring from the thickest part can provide better filling and pressure holding effects. If the pressure holding is insufficient, the thinner area will solidify faster than the thicker area. Avoid placing the gate at a sudden change in thickness to avoid hysteresis or short shots.
② If possible, pour from the center of the product. Placing the gate in the center of the product can provide equal flow length. The size of the flow length will affect the required injection pressure. Pouring from the center makes the pressure holding pressure uniform in all directions, which can avoid uneven volume shrinkage.
③ When the plastic flows into the runner, the plastic close to the mold surface first cools down (cools) and solidifies. When the plastic flows forward again, it only flows through the solidified plastic layer. Because plastic is a low heat transfer material, the solid plastic forms an insulating layer and a retaining layer that can still flow.
Therefore, ideally, the gate should be set at the cross-flow channel layer to achieve the best plastic flow effect. This situation is most common in circular and hexagonal cross-flow channels. However, the trapezoidal cross-flow channel cannot achieve this effect because the gate cannot be set in the middle of the flow channel.When determining the gate position, the following principles should be adhered to:
The rubber injected into each part of the mold cavity should be as even as possible; the rubber injected into the mold should maintain a uniform and stable flow front at all stages of the injection process; possible weld marks, bubbles, cavities, voids, insufficient injection and spraying should be considered; the dewatering operation should be as easy as possible, preferably automatic operation; the gate position 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 elements that must be compromised: The larger the gate cross-sectional area, the better, and the shorter the channel length, the better, to reduce the pressure loss when the plastic passes through.
The gate must be narrow so that it is easy to cool and prevent excessive plastic backflow. Therefore, the gate is in the center of the runner, and its cross-section should be as circular as possible. However, the switch of the gate is usually determined by the switch of the module. Gate size
The size of the gate can be determined by the cross-sectional area and the gate length. The following factors can determine the optimal gate size:
Rubber flow characteristics Module thickness Rubber amount injected into the mold cavity Melting temperature Tooling temperature
4. Gate balance
If a balanced runner system cannot be obtained, the following gate balance method can be used to achieve the goal of uniform injection molding. This method is suitable for tooling with a large number of mold cavities.
There are two gate balance methods: changing the length of the gate channel and changing the cross-sectional area of the gate. In another case, when the mold cavity has different projected areas, the gate also needs to be balanced.
At this time, to determine the size of the gate, you must first determine the size of one of the gates, find out its ratio to the volume of its corresponding mold cavity, and apply this ratio to the comparison between its gate and each corresponding mold cavity, and then you can find the size of each gate in turn. After actual trial injection, the gate balancing operation can be completed.
5. Direct gate or large gate
The runner directly supplies plastic to the finished product, and the runner adheres to the finished product. In a two-plate mold, the large gate is usually one out of one, but in the design of a three-plate mold or a hot runner mold, it can be one out of many.Disadvantages: The gate mark formed on the surface of the finished product will affect the appearance of the finished product, and the size of the gate mark depends on the fine diameter hole of the nozzle.
6. Demolding angle of nozzle, nozzle length
Therefore, the large sprue mark can be reduced by reducing the size of the nozzle mentioned above. However, the nozzle diameter is affected by the nozzle diameter, and the sprue should be easy to demold, so the demolding angle cannot be less than 3 degrees, so only the nozzle length can be shortened, and the nozzle can be extended.
Gate selection:
The gate is the connecting part between the runner and the cavity, and it is also the last part of the injection mold feeding system. Its basic function is:
Let the molten plastic from the runner enter the cavity at the fastest speed. After the cavity is filled, the gate can be quickly cooled and closed to prevent the plastic that has not cooled down from flowing back into the cavity.
PS: The nozzle occupies a very important part in the mold. The heated and molten plastic is injected into the mold through the nozzle to form the product, which is assembled in the middle of the front mold (A plate).
7. Summary
The design of the gate is related to the size, shape, mold structure, injection process conditions and performance of the plastic part. But in terms of basic functions, the gate section should be small and the length should be short, because only in this way can the requirements of increasing the flow rate, rapid cooling and closing, facilitating the separation of plastic parts and minimizing gate marks be met.
The key points of gate design can be summarized as follows:
1) The gate is opened at the thicker part of the plastic part section, so that the molten material flows from the thick material section to the thin section to ensure complete mold filling;
2) The selection of the gate position should make the plastic filling process as short as possible to reduce pressure loss;
3) The selection of the gate position should be conducive to removing the air in the cavity;
4) The gate should not allow the molten material to rush directly into the cavity, otherwise it will generate a vortex and leave a spiral mark on the plastic part, especially the narrow gate is more prone to this defect;
5) The selection of the gate position should prevent the formation of seams on the plastic surface, especially for circular or cylindrical In plastic parts, a cold well should be added at the molten material pouring point on the gate surface;
6) The gate position of the injection mold with a slender core should be far away from the molding core to prevent the molding core from being deformed by the material flow;
7) When forming large or flat plastic parts, a compound gate can be used to prevent warping, deformation, and material shortage;
8) The gate should be opened as far as possible in a position that does not affect the appearance of the plastic part, such as the bottom of the edge;
9) The size of the gate depends on the size, shape and performance of the plastic part;
10) When designing a multi-cavity injection mold, consider the balance of the gate in combination with the balance of the runner, and try to ensure that the molten material is evenly filled at the same time.
