why and where to set gates for injection molding
The gate is a short groove with a small cross-sectional area to connect the runner and the cavity. Therefore, the cross-sectional area should be small in order to obtain :
1) Place the gate at the thickest part of the product, and injection on the thickest part can provide better filling and pressure holding . If the holding pressure is insufficient, the thinner area will solidify faster than the thicker area. Avoid placing the gate at a sudden thickness change section to avoid hysteresis or short shots.
2) If possible, set the gate in the center of the product. Placing the gate in the center of the product can provide equal length flow. The flow length will affect the required injection pressure. The central gate makes the packing pressure uniform in all directions,also can avoid uneven volume shrinkage.
3) When the melt flows into the runner, the flowing front is the first section to reduce heat (cooling) and solidify when it approaches the mold surface. When the plastic flows forward, the molten material only flows through the solidified plastic layer. And because the plastic raw material is a low heat transfer material, the solid plastic forms an insulating layer but still keeps the plastic flowing inside.
Therefore,the ideal gate should be set at the position of the cross-runner layer to achieve the best plastic flow. This is most common in circular and hexagonal cross flow channels. However, the trapezoidal cross runner cannot achieve this, because the gate cannot be set in the middle of the runner.
When determining the gate location and variety , the following should be adhered to:
There are no fast rules for the method of gate design, mostly based on experience, but there are two basic factors that must be considered:
The larger the cross-sectional area of the gate: the shorter the length of the gate is, the better it is to reduce the pressure loss when the plastic passes through.
The gate must be narrow so that it is easy to freeze 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, the gate solidified time is usually determined by the size of the gate.
The gate size can be determined by the cross-sectional area and gate length. The following factors can determine:
- Soon after the cavity is injected full, the gate will be solidified
- Simple demoulding
- The part ejection is completed, leaving only a few traces
- Makes the filling of multiple cavities easier to control
- Reduce excessive runner scrap
Gate location and size
1) Place the gate at the thickest part of the product, and injection on the thickest part can provide better filling and pressure holding . If the holding pressure is insufficient, the thinner area will solidify faster than the thicker area. Avoid placing the gate at a sudden thickness change section to avoid hysteresis or short shots.
2) If possible, set the gate in the center of the product. Placing the gate in the center of the product can provide equal length flow. The flow length will affect the required injection pressure. The central gate makes the packing pressure uniform in all directions,also can avoid uneven volume shrinkage.
3) When the melt flows into the runner, the flowing front is the first section to reduce heat (cooling) and solidify when it approaches the mold surface. When the plastic flows forward, the molten material only flows through the solidified plastic layer. And because the plastic raw material is a low heat transfer material, the solid plastic forms an insulating layer but still keeps the plastic flowing inside.
Therefore,the ideal gate should be set at the position of the cross-runner layer to achieve the best plastic flow. This is most common in circular and hexagonal cross flow channels. However, the trapezoidal cross runner cannot achieve this, because the gate cannot be set in the middle of the runner.
When determining the gate location and variety , the following should be adhered to:
- The melt plastic via gates to each cavity should be as even as possible;
- The plastic material injected into the mold cavity should maintain a uniform and stable flow fronts at all stages of the injection process;
- The possibility of weld marks, bubbles, cavities, vacant positions, short shot and jetting should be considered;
- The removal of the gate scrap should be made as easy as possible, preferably automatic operation;
- The location of the gate should match all aspects.
There are no fast rules for the method of gate design, mostly based on experience, but there are two basic factors that must be considered:
The larger the cross-sectional area of the gate: the shorter the length of the gate is, the better it is to reduce the pressure loss when the plastic passes through.
The gate must be narrow so that it is easy to freeze 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, the gate solidified time is usually determined by the size of the gate.
The gate size can be determined by the cross-sectional area and gate length. The following factors can determine:
- Plastic material characteristics
- The thickness of the plastic part
- The amount of material injected into the cavity
- Melting temperature
- Mold temperature
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