Injection hot runner plate design
The design of the injection molding hot runner plate is the core guarantee of the hot runner system’s performance, directly determining the uniformity of melt distribution, temperature stability, and overall mold efficiency. As a key component connecting the main channel to each hot nozzle, the hot runner plate must simultaneously meet the multiple requirements of fluid mechanics, thermodynamics, and structural mechanics: it must ensure that the melt flows smoothly in the runner with minimal pressure loss, while also ensuring consistent temperatures at each gate and sufficient strength to withstand injection pressure. The design process requires comprehensive consideration of factors such as cavity layout, plastic properties, and molding parameters. Through precise calculations and simulation analysis, the structure must be optimized to avoid problems such as unbalanced filling, local overheating, or mold deformation caused by improper design.
The runner design of the hot runner plate is key to ensuring uniform melt distribution and must adhere to the principle of “equal distance and equal cross-section.” The runner layout should match the cavity distribution, ensuring that each branch runner has consistent length, diameter, and turning angle to minimize pressure and temperature differences at each gate. For example, for a mold with a symmetrical four-cavity layout, the runner should be designed as a cross-shaped, symmetrical structure, with branch lengths within a 2mm tolerance and diameter deviation within ±0.1mm. The runner diameter should be calculated based on the plastic viscosity and flow rate. Low-viscosity plastics (such as PE) can use a φ6-8mm runner, while high-viscosity plastics (such as PC) should be increased to φ8-12mm to reduce flow resistance. Runner bends should have a large radius (radius no less than 1.5 times the runner diameter) to avoid melt stagnation and pressure loss caused by right-angle turns. Additionally, the runner inlet and outlet should have a transition taper to ensure smooth melt entry and exit, minimizing turbulence and shear heating.
Temperature control design is the basis for the normal operation of the hot runner plate, and it is necessary to achieve uniform and precisely controllable temperature over the entire area. The selection and arrangement of heating elements directly affect the temperature distribution. Common heating methods include built-in heating rods, heating tubes, and external heating plates. Built-in heating rods are suitable for small and medium-sized runner plates, and are arranged symmetrically along both sides of the runner with a spacing of 15-25mm to ensure uniform heat transfer; large runner plates should use heating tubes arranged around the runner, and cooperate with insulation layers to reduce heat loss. Temperature sensors need to be installed in key locations (such as the end of the runner and near the gate), and each heating zone is equipped with an independent temperature controller to achieve a temperature control accuracy of ±1°C. For easily degradable plastics (such as PVC), chrome plating (thickness 0.01-0.02mm) is required on the runner surface to reduce the risk of melt adhesion; at the same time, an overheating protection device is set to automatically cut off the power when the temperature exceeds the set value by 5°C to prevent plastic decomposition.
The structural strength of the hot runner plate must withstand the dual loads of injection pressure and temperature to avoid deformation or cracking. The runner plate is typically made of pre-hardened alloy tool steel (such as P20 or 718H), with a hardness of HRC 30-35, ensuring both processability and sufficient compressive strength. Plate thickness should be calculated based on the number of cavities and injection pressure. For small, single-cavity runner plates, the thickness can be 30-50mm, while for large, multi-cavity runner plates, the thickness should be increased to 60-100mm. If necessary, reinforcing ribs should be installed in non-runner areas to increase overall rigidity. High-strength bolts (such as 12.9-grade hexagon socket bolts) should be used to connect the runner plate to the mold plate, with bolt spacing no greater than 150mm to ensure uniform tightening force. In addition, a thermal insulation gasket (such as asbestos board or mica sheet, 0.5-1mm thick) should be placed between the runner plate and the fixed mold plate to reduce heat transfer to the mold, thereby minimizing mold temperature fluctuations and energy consumption.
The installation and positioning of the hot runner plate must ensure precision and stability to avoid performance degradation due to assembly errors. The manifold plate and hot nozzle should be positioned using a stopper, with a clearance of 0.01-0.02mm to ensure coaxiality. A locating ring should be used for the connection to the main runner bushing, achieving a positioning accuracy of H7/h6. Guide posts should be positioned around the manifold plate to align with the guide holes in the template to prevent deflection during installation. The length of the guide posts should exceed the thickness of the manifold plate to ensure proper guidance during assembly. For large manifold plates, support posts should be installed at the bottom, evenly distributed around the edges and areas of concentrated stress. The height tolerance of the support posts should not exceed 0.01mm to prevent bending and deformation of the manifold plate due to its own weight or injection pressure. Furthermore, the manifold plate should have vent holes and lifting holes. The vent holes should have a diameter of 6-8mm and be located at the highest point of the runner to exhaust air and volatiles. The lifting holes should be symmetrically positioned to ensure stable lifting.
Hot runner plate design optimization requires continuous iterative improvement through CAE simulation and mold trials. Flow and temperature simulations are performed in the early stages of the design process using software such as Moldflow. This analyzes the melt’s pressure distribution, temperature field, and shear rate within the runners, identifies potential stagnation zones or hotspots, and optimizes the runner structure in advance. During the mold trial phase, the fill time, pressure curve, and part weight of each gate are monitored. If a discrepancy exceeds 5%, the runner dimensions or heating power are adjusted. For example, during a mold trial of a six-cavity mold, a 10% weight discrepancy was detected between the two sides of the part. Simulation analysis revealed asymmetric runner lengths, which were then adjusted to within 3%. After long-term use, runner wear should be assessed based on changes in part quality (such as flash and material shortages). When the runner surface roughness exceeds Ra1.6μm, the runner plate should be repolished or replaced. Through continuous design optimization, hot runner plates can maximize production efficiency and material utilization while ensuring part quality.
