Selection of gate position during injection molding

The influence of custom plastic injection molding gate on parts and the selection of position

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. Gate position affects holding pressure. Holding pressure size. 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 (Core Shaft)

3. Tips for selecting gate position

1. Place the gate at the thickest part of the product. Pouring from the thickest part can provide better filling and holding pressure effects. If the holding pressure is insufficient, the thinner area will solidify faster than the thicker area. Avoid placing the gate at the place where the thickness changes suddenly 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 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: The gate will be cold soon after the mold cavity is filled. The dewatering gate is simple. After the dewatering gate is completed, only a small trace is left, which makes it easier to control the filling of multiple mold cavities. 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 considered as a compromise:
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 size of the gate can be determined by the cross-sectional area and the gate length. The following factors can determine the optimal gate size: The thickness of the rubber flow The amount of rubber injected into the mold cavity

When determining the gate position, the following principles should be adhered to: 1. The rubber injected into each part of the mold cavity should be as even as possible. 2. The rubber 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 location of the gate should be coordinated with all aspects. 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 gate balance methods: 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, it is necessary to 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 its gate and each corresponding mold cavity, and then the size of each gate can be successively calculated. After actual trial injection, the gate balancing operation can be completed. The position of the gate in the runner When the plastic flows into the runner, the plastic first loses heat (cools) and solidifies when it approaches the mold surface. When the plastic flows forward again, 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, the trapezoidal cross-runner cannot achieve this effect because the gate cannot be set in the middle of the runner. 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 is usually one out, but in the design of a three-plate mold or hot runner mold, multiple can be produced from one mold. 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 diameter of the nozzle and the demolding angle of the nozzle. Therefore, the large sprue mark can be reduced by reducing the size of the nozzle. 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 demolding angle cannot be less than 3 degrees. Therefore, only the nozzle length can be shortened, and the furnace 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. 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, facilitating the separation of plastic parts and minimizing the gate marks be met. The key points of gate design can be summarized as follows:
1. The gate is opened at the thicker part of the plastic part section, so that the molten material flows from the thick material section to the thin section to ensure complete mold filling;
2. The selection of the gate position should make the plastic filling process as short as possible to reduce pressure loss;
3. The selection of the gate position should be conducive to removing the air in the cavity;
4. The gate should not allow the molten material to rush directly into the cavity, otherwise it will generate a vortex and leave a spiral mark on the plastic part, especially the narrow gate is more likely to have this defect;
5. The selection of the gate position should prevent the formation of stitching lines on the plastic surface, especially in the case of circular or cylindrical plastic parts, a cold material well should be added at the molten material pouring point on the gate surface;
6. The gate position of the injection mold with a slender core should be far away from the molding core to prevent the molding core from being deformed by the material flow;
7. When forming large or flat plastic parts, a compound gate can be used to prevent warping, deformation, and material shortage;
8. The gate should be opened as far as possible 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, shape and performance of the plastic part;10. When designing a multi-cavity injection mold, consider the balance of the gate in combination with the balance of the runner, and try to ensure that the molten material is filled evenly at the same time. Gate design The gate, also known as the feed port, is the channel for the melt connecting the runner and the cavity. The appropriateness of the gate design and location selection is directly related to whether the plastic part can be injected and molded intact and with high quality. Gates can be divided into two categories: restrictive gates and non-restrictive gates. The restrictive gate is the part with the smallest cross-sectional size in the entire casting system. Through the sudden change of the cross-sectional size, the plastic melt sent from the branch channel will produce a sudden increase in flow rate, increase the shear rate, reduce the viscosity, and make it an ideal flow state, so that it can quickly and evenly fill the cavity. For multi-cavity molds, adjusting the size of the gate can also achieve the purpose of simultaneous feeding of unbalanced cavities and improve the quality of plastic parts. In addition, the restrictive gate also plays a role in early solidification to prevent the melt in the cavity from flowing back. The non-restrictive gate is the part with the largest cross-sectional size in the entire casting system. It mainly plays a role in guiding the material and applying pressure after feeding to the cavity of medium and large cylindrical and shell-like plastic parts. According to the structural form and characteristics of the gate, the commonly used gates can be divided into the following types. (1) Direct gate is the main runner gate and belongs to the non-restrictive gate. The plastic melt directly enters the cavity from the large end of the main channel, so it has the characteristics of small flow resistance, short flow process and long replenishment time. However, it also has certain disadvantages, such as large residual stress at the feed point, which causes the plastic part to warp and deform. It is difficult to remove the gate mark due to the large gate, and the mark is large, which affects the appearance. Therefore, this type of gate is mostly used for injection molding of large, medium-sized, long-flow, deep-cavity cylindrical or warped plastic parts, especially suitable for high-viscosity plastics such as polycarbonate and polysulfone. In addition, this type of gate is only suitable for single-cavity molds. When designing the gate, in order to reduce the gate area at the contact point with the plastic part and prevent the occurrence of shrinkage, deformation and other defects at this point, on the one hand, the main channel taper angle a (a=2″4 degrees) with a smaller taper should be selected as much as possible, and on the other hand, the thickness of the fixed plate and the fixed mold seat should be reduced as much as possible. Such a gate has a good melt flow state, and the plastic melt flows from the center of the bottom surface of the cavity to the parting surface, which is conducive to exhaust; this form minimizes the projection area of ​​the plastic part and the pouring system on the parting surface, the mold structure is compact, and the injection machine is evenly stressed. (2) Center gate When there is a through hole at or near the center of the bottom of a cylindrical or shell-like plastic part, the inner gate is opened at the gate, and a diverter cone is set at the center. This type of gate is actually a special form of direct gate, which has a series of advantages of direct gates, but overcomes the defects of direct gates such as shrinkage holes and deformation. The center gate is actually a ring gate for end-face feeding (introduced below). When designing, the thickness of the ring is generally not less than 0.5mm. When the area of ​​the feed gate ring is larger than the area of ​​the small end of the main channel, the gate is a non-restrictive gate; otherwise, the gate is a restrictive gate. (3) Side gate Side gates are called standard gates abroad. (I have various pictures here but I don’t have a scanner) The side gate is usually opened on the parting surface. The plastic melt fills the mold cavity from the inside or outside. Its cross-sectional shape is mostly rectangular (flat groove). Changing the gate width and thickness can adjust the shear rate of the melt and the freezing time of the gate. This type of gate can be selected according to the shape characteristics of the plastic part. It is easy to process and trim, so it is widely used. The advantages are as follows: Due to the small gate cross-section, the consumption of plastic in the casting system is reduced, and the gate is easy to remove and the traces are not obvious. Disadvantages: There are weld marks, the injection pressure loss is large, and the exhaust of deep cavity plastic parts is not good. It can also be divided into 1) fan gate 2) flat seam gate (4) ring gate The gate that uses a circular feeding form to fill the cavity is called a ring gate. Features: uniform feeding, roughly the same flow rate at all points on the circumference, good flow state, easy removal of air in the cavity, and avoidance of weld marks. The gate is designed on the core, with a gate thickness of t=0.25″1.6mm and a length of l=0.8″1.8mm; the overlapped ring gate with end-face feeding has an overlap length L1=0.8″1.2mm, and the total length L can be 2″3mm; the ring gate is mainly used to form cylindrical bottomless plastic parts, but the gating system consumes more material, the gate is difficult to remove, and the gate marks are obvious.