The gate design 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 as far as the basic function is concerned, the gate section should be small and the length should be short, because only a small gate can meet the requirements of increasing the flow rate, rapid cooling and sealing, facilitating the separation of plastic parts and the minimum gate residue.
- Requirements to be met by the gate location
Appearance requirements (gate marks, weld lines)
Product functional requirements
Mold processing requirements
Warpage of the product
The gate volume is not easy to remove
Type process is easy to control
- Impact on production and function
The flow length determines the injection pressure, clamping force, and the shortening of the full flow length when the product is not filled 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 product's stressed position (such as the bearing) to avoid residual stress. The gate position must consider exhaust to avoid wind accumulation. If it happens, do not place the gate on the weaker or embedded part of the product to avoid misalignment.
- Tips for choosing gate location
Gate
The gate is a short groove with a small cross-sectional area to connect the runner and the cavity. So the cross-sectional area is OK. The purpose is to obtain the following effects:
Soon after the cavity is injected, the gate will be cold
Simple dewatering
The drain is complete, leaving only a few traces
Make the filling of multiple cavities easier to control
Reduce too much filler
Gate location and size
① Place the gate at the thickest part of the product, and pouring from the thickest part can provide better filling and pressure holding effects. If the holding pressure 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, pouring from the center of the product, and placing the gate in the center of the product can provide equal length flow. The size of the flow length will affect the required injection pressure. The central pouring makes the holding pressure uniform in all directions. Avoid uneven volume shrinkage.
③ When the plastic flows into the runner, the plastic is the first to reduce heat (cooling) and solidify when it approaches the mold surface. When the plastic flows forward, only the solidified plastic layer flows through. Also, because plastic is a low heat transfer material, solid plastics can still flow when forming an insulating green layer and a retaining layer.
Therefore, under ideal circumstances, the gate should be set at the position of the cross-runner layer to make 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.
When determining the gate location, the following principles should be adhered to:
The rubber injected into each part of the cavity should be as even as possible;
The rubber material injected into the mold should maintain a uniform and stable flow frontline at all stages of the injection process;
Consideration should be given to the possible occurrence of weld marks, bubbles, cavities, vacant positions, insufficient glue injection and glue spraying, etc.;
The operation of the dewatering port should be made as easy as possible, preferably automatic operation;
The location of the gate should match all aspects.
There are no hard and fast rules for the method of gate design, mostly based on experience, but there are two basic elements that must be compromised:
The larger the cross-sectional area of the gate, the better, and the shorter the length of the channel, the better, in order to reduce the pressure loss when the plastic passes through.
The gate must be narrow in order to facilitate cold formation 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 gate switch is usually determined by the switch of the module.
Gate size
The gate size can be determined by the cross-sectional area and gate length. The following factors can determine the best gate size:
Rubber flow characteristics
The thickness of the module
The amount of rubber injected into the cavity
Melting temperature
Model catch
- 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 type of squid, when the mold cavity has a different projected area, the gate also needs to be balanced.
At this time, to determine the size of the gate, one of the gate sizes must be determined first, and the ratio of it to the volume of the corresponding cavity is calculated, and this ratio is applied to the gate and each corresponding cavity. In comparison, the size of each gate can be obtained one after another. After the actual test injection, the balance operation of the gate can be completed.
- Direct gate or large nozzle
The sprue directly supplies plastic to the finished product, and the sprue adheres to the finished product. In the two-plate mold, the large nozzle is usually one out, but in the design of the three-plate mold or the hot runner mold, it can be more than one beer.
Disadvantages: the formation of nozzle marks on the surface of the finished product will affect the appearance of the finished product, and the size of the nozzle mark lies in the small diameter hole of the nozzle.
- The ejection angle of the nozzle, the length of the nozzle
Therefore, the large nozzle mark can be reduced, as long as the size of the above-mentioned nozzle is reduced. However, the diameter of the nozzle is affected by the diameter of the nozzle, and the nozzle must be easy to be ejected from the mold. The ejection angle cannot be less than 3 degrees, so only The length of the pump can be shortened, just use a longer pump.
Gate selection:
The gate is the connecting part of the runner and the cavity, and also the last part of the injection mold feeding system. Its basic functions are:
Make the molten plastic from the runner enter the filling cavity at the fastest speed.
After the cavity is full, the gate can be quickly cooled and closed to prevent the plastic from returning to the cavity that has not yet cooled.
PS: (The nozzle occupies a very important part of the mold. The heated and dissolved plastic is injected into the mold through the nozzle to form a product, and it is assembled in the middle of the front mold (A plate))
- to sum up
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 as far as the basic function is concerned, the gate section should be/and the length should be short, because only in this way can it meet the requirements of increasing the flow rate, rapid cooling and sealing, facilitating the separation of plastic parts, and the minimum gate residue.
The main points of gate design can be summarized as follows:
1) The gate is opened in the thicker section of the plastic part, so that the melt flows from the thick section into the thin section to ensure complete mold filling;
2) The selection of gate position should make the plastic filling process the shortest to reduce pressure loss;
3) The selection of the gate position should help to remove the air in the cavity;
4) The gate should not allow the melt to rush directly into the cavity, otherwise it will produce swirling flow, leaving swirling marks on the plastic part, especially the narrow gate is more prone to such defects;
5) The location of the gate should be selected to prevent the occurrence of stitching on the plastic surface, especially in the circular or cylindrical plastic parts, and the cold material well should be opened at the melt pouring on the surface of the gate;
6) The gate position of the injection mold with a slender core should be far away from the molding core, so that the molding core is not deformed by the flow of material;
7) When forming large or flat plastic parts, in order to prevent warping, deformation, and lack of material, multiple gates can be used;
8) The gate should be opened as far as possible in a position that does not have the appearance of the stew, 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-cavity injection molds, consider the balance of the gate in combination with the balance of the runner, and try to make the molten material uniformly filled at the same time.