Basic function of gate
The gate is the bridge of runner and the cavity, and also the last stage of injection system of injection mold
1) Enforce the molten plastic from the runner to enter each cavity at the fastest speed.
2)Once the cavity is fully filled, the gate can be quickly cooled and sealed to prevent the uncooled plastic in the cavity from returning.
How to design gate
There are no set rules to flollow on how to design the injection gate. It is done mostly based on experience, but there are two basic elements that must be considered:
- The larger the cross-sectional area of the gate, the better, and the shorter the length of the gate, the better to reduce the pressure loss when the molten plastic passes through.
- The gate must be narrow in order tomake the gate cool down easilyand prevent excessive plastic from flowing back. Therefore, the gate is in the center of the runner, and its cross-section should be as round as possible. However, the shut-off of the gate switch is usually determined by the mold
The key points of gate design
can be summarized as follows:
- The gate is opened at the thicker section of the plastic part, so that the molten plasticflows from the thick section into the thin section to ensure complete mold filling;
- The selection of gate position should make sure that the plastic filling path isthe shortest to reduce pressure loss;
- The selection of the gate position should help to eliminate the air in the cavity;
- The gate should not allow the molten plasticsto rush directly into the cavity, otherwise it will produce a swirling flow, leaving a spin mark on the plastic part, especially the narrow gate is more easierto cause such defects;
- The location of the gate should be selected to prevent theappearanceof joint lines on the plastic surface, especially in circular or cylindrical plastic parts, a cold slug well should be opened at the joint area of plastic flow on the surface of the gate;
- For the injection mold with long and thin core, the gate should be far enough so that the core won’t get deformed bye theflow of material;
- When forming large and flatplastic parts, multiple gates can be used to prevent warping, deformation andinsufficient plastic ;
- The gate be placed in a position that does not affect the appearance of the plastic part, such as the bottom of the edge;
- The size of the gate is determined bythe size& shape of plastic part and performance of the plastic material;
- When designing multiple-cavity injection molds, the balance of the gateneeds to be taken into accountwith the balance of the runner and the uniform filling of plastics nees to be ensured.
The principle of gate location selection
When determining the gate location, the following principles should be adhered to:
- The resininjected into each cavity should be as even as possible.
- Thematerial injected into the mold should maintain a uniform and stable flow at all stages of the injection process.
- Take the possible defets into consideration such as weld marks, bubbles, voids, insufficient injection and plastic spray.
- The operation of cutting the injection gate should be made as easy as possible, preferably automatic operation.
- The location of the gate should match all aspects.
Gate location selection technique
- Place the gate at the thickest part of the product, injectioning from the thickest wall can ensurebetter filling and pressure holding effect. If the holding pressure is insufficient, the thinner area will solidify faster than the thicker area. Avoid placing the gate on the surface where there is a sudden changein thickness to prevent delayed injection or fast shooting.
- If possible, inject from the center of the product. Placing the gate in the center of the product can provide equal length flow lengthwhich will affect the required injection pressure. The central injection gate makes the holding pressure uniform in all directions and avoid uneven volume shrinkage.