The design of the gate is related to the size and shape of the part, the mold structure, the injection process conditions, and the performance of the position. But in terms of the essential role, the gate cross-section should be small, and the length should be short because this is the only way to meet the requirements of increasing the flow rate, fast cooling and closing, easy part separation, and minimum residual marks on the gate. The following MINHUI will answer for you:
- Requirements to be met by the gate location
Appearance requirements (gate marks, fusion line)
Product function requirements
Mold processing requirements
Warpage of the product
Easy to remove the gate
Easy to control the molding process
- Impact on production and function
Flow length determines injection pressure, clamping force, and whether the product is filled. Complete flow length shortening can reduce injection pressure and clamping force.
The location of the gate will affect the holding pressure, the size of the holding pressure, whether the holding pressure is balanced or not, keep the gate away from the product stress location (such as at the bearing) to avoid residual stress, the site of the entrance must consider the exhaust to prevent the occurrence of wind accumulation, do not place the gate at the weaker or embedded part of the product to avoid deflection.
- Tips for choosing gate position
Gate
The gate is a short slot with a small cross-sectional area to connect the runner to the mold cavity. The cross-sectional area should be small to obtain the following effects.
The gate will be cold soon after the mold cavity is filled.
Easy to remove the water outlet
Only a few traces are left after the sprue is removed.
Easy to control the filling of several mold cavities
Less overfilling
Gate location and size
(1) Place the gate at the thickest part of the product. The widest part will provide better filling and pressure retention. Avoid placing the gate at sudden changes 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 provides equal flow lengths.
When the plastic flows into the runner, it is the first to cool and solidify near the mold surface. Only the solidified plastic layer flows through when the plastic flows forward again. Since plastic is a low heat transfer material, the solid plastic forms an adiabatic green layer, and the holding layer can still flow.
Therefore, ideally, the gate should be located in the cross-runner layer for the best plastic flow effect. This is most often the case with round and hexagonal cross runners. However, trapezoidal cross runners cannot achieve this effect because the gate cannot be located in the middle of the runner.
When deciding on the location of the gate, the following principles should be adhered to.
The glues to be injected into each part of the mold cavity should be as even as possible.
The rubber injected into the mold should have a uniform and stable flow front in all stages of the injection process.
Consideration should be given to the possibility of weld marks, air bubbles, pits, dips, under-injection, and spraying.
The gate removal operation should be as easy as possible, preferably automatic.
The location of the gate should be compatible with all aspects.
There are no hard and fast rules for the design of gates, most of them are based on experience, but there are two essential elements to be considered in a compromise.
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.
The gate must be thin and narrow to allow easy cooling and prevent excess plastic's backflow. Therefore, the gate should be in the runner's center, and its cross-section should be as round as possible. However, the opening and closing of the gate are usually determined by the opening and closing of the molded part.
Gate size
The size of the gate can be determined by the cross-sectional area and the length of the gate. The following factors determine the optimum size of the gate.
The flow characteristics of the compound
The thickness of the molded part
The amount of material to be injected into the cavity
Melting temperature
Mold temperature
- Gate balancing
If a balanced runner system cannot be obtained, the following gate balancing method can be used to achieve the goal of uniform injection. This method is suitable for molds with a large number of cavities.
There are two methods of gate balancing: changing the length of the gate channel and changing the cross-sectional area of the gate. In another case, where the cavities have different projection areas, the gate must be balanced.
In this case, to decide the size of the gate, one of the gates is sized, and the ratio of its volume to the volume of the corresponding cavity is calculated and applied to the comparison between the gate and the corresponding hole so that the size of each gate can be calculated successively. After the actual test injection, the balance of the gates can be completed.
- Direct gate or large watergate
The sprue supplies plastic directly to the finished product, and the sprue adheres to the finished product. The gate is usually one out of one in a two-plate mold, but it can be one out of many in a three-plate mold or hot runner mold design.
Disadvantage: The formation of spout marks on the surface of the finished product will affect the appearance of the finished product, and the size of the spout marks depends on the fine diameter hole of the nozzle. 6.
- the release angle of the nozzle, the length of the nozzle
Therefore, the significant water mouth mark can be reduced as long as the size of the nozzle, as mentioned above, is changed to smaller. But the diameter of the nozzle is affected by the diameter of the nozzle, and the water spout should be easy to get out of the mold, so the release angle should not be less than 3 degrees, so only the length of the nozzle can be shortened, and the nozzle can be lengthened.
Gate selection.
The gate is the connecting part of the runner and the cavity, and it is also the last part of the feeding system of the injection mold. Its essential function is to
Make the molten plastic from the runner enter the cavity as fast as possible.
After the cavity is filled, the gate can be quickly cooled and closed to prevent the hole from backflowing plastic that has not yet cooled.
PS: the nozzle in the mold accounts for a significant part. Heated dissolved plastic through the nozzle into the mold before forming products assembled in the middle of the front mold (A plate).
- summary
The design of the gate is related to the size of the plastic part, the shape of the mold structure, the injection process conditions, and the performance of the plastic part. But in terms of the primary role, the gate should be small in cross-section and short in length because it is the only way to meet the requirements of increased flow rate, fast cooling and closure, easy separation of the molded part, and minimum residual marks on the gate.
The main points of gate design can be summarized as follows.
(1) The gate is opened in the thicker part of the cross-section of the plastic part so that the molten material flows from the vast section into the thin chamber to ensure complete mold filling;
(2) The gate's location should be chosen so that the plastic filling process is the shortest way to reduce pressure loss.
(3) the choice of gate location should be conducive to excluding air in the cavity.
(4) The gate should not make the melt rushing directly into the cavity. Otherwise, it will produce swirling flow and leave traces of spin on the plastic part, especially since the narrow gate is more likely to have this defect.
(5) The location of the gate should be chosen to prevent the production of seam lines on the surface of the plastic, especially in the case of circular or cylindrical parts. The cold material well should be added at the melt pouring on the face of the gate.
(6) The location of the gate in injection molds with long and thin cores should be far from the heart to prevent the meat from being deformed by the material flow.
(7) When forming large or flat plastic parts, compound gates can be used to prevent warping, deformation, and lack of material.
(8) The gate should be opened 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 and shape of the plastic part and the performance of the plastic.
10）When designing multiple cavities injection molds, consider the gate's balance in combination with the balance of the flow channel, and try to fill the molten material evenly at the same time.
More details: https://minhuiglobal.com/vertical-injection-molding-machines/
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