What Is Gating System?
Function of the Gating System
The runner system guides molten plastic from the injection molding machine barrel to each cavity of the mold. Therefore, the structure, length, size, and connection method of the runner system all affect the injection filling effect, thus directly affecting the quality of the product. Furthermore, the design of the runner system should also consider economic efficiency, achieving rapid cooling and short cycle times.
Gating system structure
The runner system consists of four structures: main runner, branch runners, gate, and cold slug well, as shown in Figure 7-11.
Main runner (sprue)
The main runner (sprue) is the plastic channel that connects the injection molding machine nozzle to the runner system. It is the first component of the runner system.
Primary and Secondary Runners
Primary and secondary runners are plastic channels that connect the main runner to the gate of the inner mold, allowing molten plastic to flow into the inner mold. In the case of a two-plate mold, the runner is located on the parting line.
When designing flow channels, attention should be paid to their cross-sectional shape and size. There are generally four types of cross-sectional shapes for flow channels: full circle, trapezoid, modified trapezoid, and hexagon (see Figure 7-12). From the perspective of injection pressure transmission, the larger the cross-sectional area of the flow channel, the better; while from the perspective of heat conduction, the smaller the cross-sectional surface area, the better. Therefore, the larger the ratio of cross-sectional area to surface area, the more effective the flow channel. Circular and square cross-sectional flow channel designs have the highest R-values. Because circular cross-sections cool faster than square cross-sections, circular cross-sectional designs are the best. (The R-value refers to the ratio of cross-sectional area to surface area; the cross-sectional area refers to the cross-sectional area of the flow channel.)
The runner diameter is related to the flow length; the larger the diameter, the longer the flow path. At the same time, the runner should be as narrow and short as possible. Each type of plastic has a minimum runner diameter requirement; an excessively small diameter will affect the flow of the plastic in the mold cavity. The runner diameter is generally 1.0 mm thicker than the finished product level to prevent the plastic in the runner from solidifying before the finished product, thus ensuring proper pressure holding.
Gate
Gates have a significant impact on moldability and internal stress. Their appropriate form is usually determined by the shape of the part and can be broadly classified into two categories: restricted gates and unrestricted gates.
Restricted gates are narrow sections at the entrance to the mold cavity, making them easy to process and allowing for easy cutting of the part from the runner, thus reducing residual stress. This type is generally used in multi-cavity molds where multiple parts are molded in one go, as it facilitates even distribution and prevents backflow of plastic within the cavity. Restricted gates can be further categorized into side gates, overlapping gates, flanged gates, fan-shaped gates, film gates, ring gates, disc gates, point gates, and submarine gates. Unrestricted gates are where plastic is directly injected into the mold cavity via a vertical runner.
The type, location, size, and number of gates directly affect the appearance, deformation, molding shrinkage, and strength of the molded part. Therefore, the following factors should be considered in the design.
(1) Factors to consider when determining the gate shape: The gate shape affects the resin flowability within the mold cavity, the appearance of the molded part, and the material flow orientation. Therefore, when selecting the gate type, it is necessary to consider the material type or product shape and the influence of flow orientation.
(2) Factors to consider when determining the gate size:
① Plastic flow characteristics.
② Mold wall thickness.
③ Amount of plastic injected into the mold cavity.
④ Plastic melting temperature.
⑤ Mold temperature.
(3) Factors to consider when determining the gate location:
① The gate should be located at the thickest point of the plastic part's cross-section. This allows for slower cooling at the gate, facilitating the flow of molten material into the mold cavity and preventing defects such as shrinkage.
② The gate location should minimize the molten material flow path, reduce flow direction changes, and minimize pressure loss. Generally, a gate located at the center of the plastic part is more effective.
③ The gate location should facilitate the venting of gas from the mold cavity. If the molten material entering the mold cavity prematurely closes the venting system, it will be difficult for gases to escape from the cavity, affecting product quality. Venting channels should be placed at the final position where the molten material reaches the cavity to facilitate venting.
④ The gate should be located directly opposite the core wall or a large core, allowing the high-speed molten material to directly impact the cavity or core wall, thereby changing the flow direction, reducing the flow rate, and smoothly filling the cavity. This eliminates obvious jetting marks on the plastic part and prevents melt fracture.
⑤ The number of gates should be controlled. Entering the cavity through several gates will create more weld lines.
⑥ The gate location should ensure uniform molten material feeding, with the flow path from the main runner to all parts of the cavity being the same or similar, to reduce flash and weld lines.
⑦ For plastic parts with cores or inserts, especially cylindrical plastic parts with slender cores, avoid feeding directly onto the core or insert to prevent core bending or insert displacement.
⑧ The location of the gate should avoid causing melt fracture. When a small gate is directly opposite a cavity with a large width and thickness, the high-speed molten material flowing through the gate will be subjected to high shear stress, resulting in melt fracture phenomena such as jetting and creeping. Jetting melt is prone to causing folding, resulting in ripple marks on the product.
⑨ When molten plastic is injected at high speed into the mold cavity through the gate, it exhibits a directional effect. The gate location should be chosen to avoid the adverse effects of this directional effect.
⑩ When determining the gate location and number for a mold, the flow ratio must be checked to ensure the melt fills the cavity. The flow ratio is determined by the ratio of the total flow channel length to the total flow channel thickness. Its allowable value varies depending on the melt properties, temperature, injection pressure, etc.
⑪ For flat plastic parts, warping and deformation are prone to occur because their shrinkage rates are inconsistent in different directions. Using multiple gates is much more effective.
⑫ For frame-type plastic parts, diagonally placing the gates can improve the deformation caused by shrinkage.
⑬ For ring-shaped plastic parts, the gate should be placed tangentially to reduce weld lines, increase the strength of the weld area, and facilitate venting.
⑭ For plastic parts with uneven wall thickness, the gate location should be kept as consistent as possible to avoid eddies.
⑮ For shell-shaped plastic parts, a center-mounted gate arrangement can be used to reduce weld lines.
⑯ For dome-shaped, slender, and thin-walled plastic parts, multiple gating points and process ribs can be set to guide the flow and prevent material shortage.
The above principles for selecting gating positions may lead to contradictions in application; in such cases, flexible handling based on the actual situation is necessary.
(4) Gating Balance
If a balanced runner system cannot be obtained, the following gating balance method can be used to achieve the goal of uniform injection molding. This method is suitable for molds with a large number of cavities. There are two gating balance methods: changing the length of the gating channel and changing the cross-sectional area of the gating.
When the cavity dimensions have different projected areas, the gating must also be balanced. At this point, to determine the gate size, one gate size must first be determined, and the ratio of this gate size to the corresponding cavity volume must be calculated. This ratio is then applied to the comparison of other gates with their corresponding cavity volumes to successively determine the size of each gate. After actual trial injection, the gate balancing operation can be completed.
When producing parts or more finished products from the same mold cavity, if some sections of the material are thinner, the sprue needs to be thickened. The amount of thickening depends on the size of the finished product. Generally, thicker sections of finished products flow better, resulting in normal pressure, while thinner sections flow poorer, leading to higher pressure. Therefore, if the finished products are to be filled simultaneously, the thicker sections may experience flash. To prevent flow imbalance, the runner for the thinner sections needs to be thickened to compensate for pressure loss, as shown in Figure 7-13.
Cold slug wells
Cold slug wells, also known as cold slug pits, are designed to store the cooler molten plastic material at the beginning of the filling process, preventing it from directly entering the mold cavity and clogging the gate or affecting product quality. Cold slug wells are typically located at the end of the main runner. When the branch runners are long, cold slug wells should also be installed at their ends.
Look here
For more information on other gating systems and their advantages and disadvantages, please click here.