1. Requirements for gate position
1. Appearance requirements (gate marks, weld lines)
2. Product function requirements
3. Mold processing requirements
4. Product warping
5. Is the gate easy to remove
2. Impact on production and function
1. Flow length determines injection pressure, clamping force, and whether the product is filled or not. Shortening the flow length can reduce injection pressure and clamping force.
2. 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 future stress 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 (CoreShaft).
3. Tips for selecting the gate position
1. Place the gate at the thickest part of the product. Pouring from the thickest part can provide better filling and holding effects. If the holding pressure is insufficient, the thinner area will solidify faster than the thicker area. Avoid placing the gate at the sudden change of thickness to avoid hysteresis or short shot.
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. The central pouring makes the holding pressure uniform in all directions, which can avoid uneven volume shrinkage.
3. 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:
1) The gate will be cold soon after the mold cavity is filled
2) The dewatering gate is simple
3) Only a few traces are left after the dewatering gate is completed
4) It is easier to control the filling of multiple mold cavities
5) Reduce the phenomenon of excessive filling
There is no hard and fast rule for designing gates, which is mostly based on experience, but there are two basic elements that must be compromised:
1. The larger the cross-sectional area of the gate, the better, and the shorter the length of the channel, the better, to reduce the pressure loss when the plastic passes through.
2. The gate must be narrow so that it is easy to cool and prevent excessive plastic from flowing back. Therefore, the gate is in the center of the runner, and its cross-section should be as circular as possible. However, the opening and closing of the gate is usually determined by the opening and closing of the module.
3. Gate size:
The gate size can be determined by the cross-sectional area and gate length. The following factors can determine the optimal gate size:
1) Glue flow characteristics
2) Thickness of mold
3) Amount of glue injected into the mold cavity
4) Melting temperature
5) Mold temperature
When determining the gate position, the following principles should be adhered to:
1. The glue injected into each part of the mold cavity should be as even as possible.
2. The glue injected into the mold should maintain a uniform and stable flow front at all stages of the injection process.
3. Possible weld marks, bubbles, cavities, voids, insufficient injection and spraying should be considered.
4. The dewatering operation should be as easy as possible, preferably automatic operation.
5. The gate position should be coordinated with all aspects.
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 molds with a large number of mold cavities. There are two ways to balance the gate: 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 the gate and each corresponding mold cavity, and then you can successively find the size of each gate. After actual trial injection, the gate balancing operation can be completed.
5. Location of gate in runner
When plastic flows into the runner, the plastic first cools down (cools) and solidifies when it approaches the mold surface. When the plastic flows forward, 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 runner layer to achieve the best plastic flow effect. This situation is most common in circular and hexagonal cross runners. However, trapezoidal cross runners cannot achieve this effect because the gate cannot be set in the middle of the runner.
6. Direct Gate or Sprue Gate
The runner directly supplies plastic to the finished product. The runner adheres to the finished product. In a two-plate mold, the sprue gate is usually one out of one, but in the design of a three-plate mold or hot runner mold, multiple gates can be set in one.
Disadvantages: The sprue mark formed on the surface of the finished product will affect the appearance of the finished product. The size of the sprue mark depends on the fine diameter hole of the nozzle.
7. The demoulding angle of the nozzle, the length of the nozzle
Therefore, the large sprue mark can be reduced, as long as the delay of the above nozzle is reduced. However, the diameter of the nozzle is affected by the diameter of the furnace nozzle, and the sprue should be easy to demold, so the demoulding angle cannot be less than 3 degrees. Therefore, only the length of the nozzle 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 functions are:
1) Allow the molten plastic from the runner to enter the cavity at the fastest speed.
2) After the cavity is filled, the gate can be quickly cooled and closed to prevent the plastic that has not cooled down from the cavity from flowing back.
8. Summary
The design of the gate is related to the size, shape, mold structure, injection process conditions and performance of the plastic part. However, according to the above two 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, easy separation of plastic parts and minimum gate marks be met.
