Injection molding gate size design
Injection molding gate design is a critical factor in determining part quality. Factors such as plastic type, part structure, and molding process must be comprehensively considered to ensure smooth melt filling of the mold cavity and effective transmission of packing pressure. Gate dimensions primarily include width, thickness, and length. Thickness has the greatest impact on melt flow and is typically 1/3-1/2 of the part’s maximum wall thickness. For example, for a 3mm wall, a gate thickness of 1-1.5mm is recommended. This ensures melt fluidity while allowing for rapid freezing after packing to prevent backflow. For plastics with higher viscosities (such as PC and PMMA), a larger gate thickness (e.g., 1/2 of the wall thickness) is recommended to reduce flow resistance. For plastics with lower viscosities (such as PE and PP), a smaller gate thickness (e.g., 1/3 of the wall thickness) can be used to avoid flash caused by excessive packing pressure. The gate width is typically 2-5 times the thickness. For example, for a 1mm thick gate, a width of 3mm is recommended. This ensures that the melt enters the mold cavity at a lower shear rate, minimizing material degradation due to shear overheating.
Different types of gates have different dimensional requirements. The appropriate gate type and dimensions must be selected based on the characteristics of the plastic part. Pinpoint gates are suitable for small and medium-sized plastic parts. Their diameter is typically 0.5-2mm, depending on the weight of the part. For example, for parts under 50g, the diameter is 0.8-1.2mm, and for parts 50-100g, the diameter is 1.2-1.8mm. A diameter that is too small can easily lead to excessive pressure loss, while a diameter that is too large can leave noticeable gate marks on the part surface. Side gates are commonly used for flat-plate parts. They are 3-10mm wide, 0.5-2mm thick, and 0.5-2mm long. For example, the side gate for a TV casing is 8mm wide, 1.5mm thick, and 1mm long. This ensures rapid melt filling and facilitates subsequent removal of gate residue. Fan gates are suitable for large, thin-walled plastic parts with widths ranging from 10-30mm and thicknesses of 0.5-1.5mm. For example, a fan gate on an automotive instrument panel can be 25mm wide and 1mm thick. By widening the gate, the melt is evenly distributed at the cavity entrance, reducing part warpage. The gate size selection must be verified through mold flow analysis to ensure that fill time, pressure distribution, and temperature field meet design requirements.
Gate size design must consider the part’s exterior quality and subsequent processing requirements to avoid cosmetic defects or processing difficulties caused by inappropriate sizing. For parts with high surface requirements (such as appliance panels), the gate should be located on a non-exterior surface and dimensioned to minimize residual gate volume. For example, a 0.8mm pinpoint gate should result in a residual height of no more than 0.3mm after molding, minimizing subsequent grinding. For parts requiring automated production, the gate size must ensure it automatically breaks during demolding. For example, a pinpoint gate for a PE part has a diameter of 1mm and a length of 1mm. This leverages the material’s toughness to automatically break the gate during ejection, improving production efficiency. The gate size must also match the runner dimensions, ensuring a smooth transition between the runner end and the gate to avoid steps. For example, when connecting an 8mm diameter circular runner to a side gate, a 5mm radius arc should be used to minimize melt flow resistance.
Injection molding process parameters and gate size must be optimized in tandem to ensure that the dimensional design can adapt to varying process conditions. When the gate size is small (e.g., a pinpoint gate with a diameter of 1mm), the melt temperature (e.g., by 10-15°C) and injection pressure (e.g., by 10%-20%) need to be increased to compensate for the increased flow resistance. For example, when a PC part uses a 1mm pinpoint gate, the melt temperature is increased from 300°C to 315°C, and the injection pressure is increased from 120MPa to 140MPa to ensure smooth filling. When the gate size is large (e.g., a side gate with a thickness of 2mm), the holding pressure and holding time need to be reduced to prevent flash caused by overfilling. For example, when a PP part uses a 2mm side gate, the holding pressure is reduced from 80MPa to 60MPa, and the holding time is shortened from 8 seconds to 5 seconds. By adjusting the process parameters, the same gate size can be adapted to the performance fluctuations of different batches of raw materials, thereby improving production stability. For example, when the raw material melt flow rate (MFR) is reduced by 10%, the same filling effect as the original MFR raw material can be maintained by increasing the melt temperature by 5°C.
Verification and adjustment of gate dimensions require mold trials, with dimensional parameters optimized based on the results. During mold trials, the melt filling sequence should be observed. If the gate size is too small, insufficient filling or excessive filling time may occur. In this case, the gate size should be increased. For example, during a mold trial, the filling time for a certain part was 2 seconds longer than the designed value. Increasing the gate thickness from 1mm to 1.2mm restored the filling time to normal. If the gate size is too large, excessive pressure retention can result, leading to flash or excessive internal stress in the part. Reducing the gate size is necessary. For example, a part with an excessively wide gate width (10mm) produced flash. Reducing the width to 8mm eliminated the flash. For multi-cavity molds, the gate dimensions of each cavity must be consistent, with an error within ±0.02mm. Test molds measure the weight and dimensions of each part. If the weight deviation exceeds 2%, the gate dimensions of the corresponding cavity must be adjusted. For example, in a mold with eight cavities, the part in cavity 3 is relatively light. Increasing its gate thickness from 1mm to 1.05mm reduces the weight deviation to within 1%. Through test molds, verification and adjustment can optimize gate dimensions and ensure consistent part quality.
