Common shapes of injection molding hot runner plates
The injection molding hot runner plate is the core component of the hot runner system. Its function is to evenly distribute the melt from the main channel to each gate and maintain a stable melt temperature through built-in heating elements. The shape of the hot runner plate directly affects melt flow uniformity, temperature distribution, and mold space utilization, requiring a specialized design based on the cavity layout, number of parts, and molding requirements. Common hot runner plate shapes include circular, rectangular, H-shaped, T-shaped, and special-shaped. Each shape has its own application scenarios and design characteristics. Choosing the right one can significantly improve the performance and reliability of the hot runner system.
Circular hot runner plates are suitable for symmetrical layouts of multi-cavity molds, particularly those with circumferential cavities, such as in mass-produced molds for bottle caps and small plastic parts. Their structural characteristics include a centrally located main runner inlet, with the melt delivered to each gate via radially distributed branch runners. These runners maintain consistent runner length, ensuring uniform filling across all cavities. The diameter of a circular runner plate typically ranges from 100 to 500 mm, with a thickness determined by the number of cavities and runner diameter, typically ranging from 30 to 80 mm. Internal heating elements (such as heating rods or coils) are evenly distributed along the circumference, ensuring symmetrical temperature distribution and minimizing uneven melt flow caused by temperature variations. For example, in a 32-cavity bottle cap mold, a 300 mm diameter circular hot runner plate with eight heating rods evenly distributed along a 200 mm diameter circle can maintain melt temperature differences within ±2°C at each gate, ensuring a weight variation of less than 0.1 g for each bottle cap. The advantages of circular runner plates are easy processing and good temperature uniformity. The disadvantages are low space utilization and they are not suitable for asymmetric cavity layouts.
Rectangular hot runner plates are the most commonly used general-purpose structure, suitable for molds with cavities arranged in a linear or matrix pattern, such as medium-sized products like appliance housings and automotive trim. They are rectangular in shape, with a length determined by the number and spacing of cavities (typically 200-1000mm), a width of 50-300mm, and a thickness of 40-100mm. The main runner inlet is typically located at one end or the center of the rectangle, with branch runners arranged linearly or in a branching pattern along the length, flexibly adapting to single-row, double-row, or even multi-row cavity layouts. For example, in a six-cavity automotive connector mold, a 400mm long rectangular runner plate is used. The main runner enters from one end and then branches into three branch runners, each connecting to two gates. Runner length tolerances are kept within 5mm to ensure balanced filling. Heating methods for rectangular runner plates typically include bottom-mounted heating plates or internally embedded heating tubes, offering high temperature control precision and the ability to compensate for heat loss at different locations through zoned heating. Its advantages are high space utilization and flexible layout. Its disadvantages are that when the length-to-diameter ratio is large, temperature differences are likely to occur at both ends, and the insulation design needs to be strengthened.
H-shaped hot runner plates are suitable for molds with a double-row symmetrical cavity layout, such as large, thin-walled products or paired parts (such as left and right door interior panels). Its structure is H-shaped, with the main runner inlet at the center. Transverse main channels connect to the longitudinal runners on either side. The symmetrical distribution of the runners ensures consistent melt flow paths in the left and right cavities. The transverse main channel length of an H-shaped runner plate is typically 100-300mm, and the longitudinal runner length is 50-200mm. Overall dimensions are determined by cavity spacing, with a thickness generally ranging from 60-120mm. The heating system combines central heating with independent heating on both sides. A main heating rod is installed in the transverse main channel, while independent heating tubes are installed in each of the two longitudinal runners. These independently adjustable temperatures compensate for heat loss caused by runner length differences. For example, in a two-cavity automotive instrument panel mold, the H-shaped runner plate has a 200mm-long transverse main channel and 150mm-long longitudinal runners on each side. By adjusting the heating power on both sides, the melt temperature difference at the left and right gates is controlled within 1°C, ensuring consistent shrinkage of the product. The advantages of the H-shaped runner plate are good symmetry and balanced filling, while the disadvantages are a more complex structure and higher processing cost.
T-shaped hot runner plates are suitable for molds with single cavities or those requiring center-feeding, such as long products (such as pipes and profiles) or multi-point gating for large single-cavity products. They are T-shaped, with the vertical section serving as the main runner inlet and the horizontal section as the branch runner. Two to six gates can be positioned in the horizontal section as needed, with runner length increasing from the center toward the ends. Pressure loss is balanced by adjusting the runner diameter (thicker in the center, tapered at the ends). The horizontal section of a T-shaped manifold plate is typically 150-500mm long, the vertical section 50-100mm high, and 40-80mm thick. Heating elements are placed along the horizontal and vertical runners to ensure uniform temperature throughout the runners. For example, in a washing machine panel mold with four-point gating, the horizontal section of the T-shaped manifold plate is 300mm long. A long heater rod embedded in the bottom of the runner, combined with a temperature sensor at the top, maintains consistent melt temperature at each gate. The advantages of the T-shaped runner plate are simple structure and flexible feeding method. The disadvantage is that the length of the runners at both ends is very different, and the runner size needs to be accurately calculated to ensure filling balance.
Special-shaped hot runner plates are suitable for specialized molds with complex, asymmetric, or space-constrained cavity layouts, such as large, complex parts like automotive bumpers and medical device housings. Their shape is specifically designed based on the cavity layout and mold structure, and may include curved runners, angled branches, or irregular connections to accommodate complex feeding requirements. For example, in an automotive bumper mold, where parts exceed 1.5 meters in length and have irregular shapes, the hot runner plate must be designed with an arc that mimics the bumper’s contour, with 5-8 gates and runners distributed along the arc to ensure uniform melt filling. Special-shaped runner plates vary widely in size, reaching lengths exceeding 2000mm and thicknesses ranging from 80-150mm. Machining requires a five-axis machining center to ensure precision in the complex runners. The heating system utilizes segmented, independent heating, with each branch runner equipped with a separate heating pipe and temperature control point. For example, in a seven-cavity special-shaped medical device mold, the hot runner plate is divided into three heating zones, each with independently adjustable temperatures to compensate for heat loss due to varying runner lengths. The advantage of special-shaped hot runner plates is that they can adapt to complex mold structures. The disadvantages are that they are difficult to design, have high processing costs, and complex temperature control. Therefore, they are only used under special needs.
The shape of the hot runner plate should be selected based on a comprehensive consideration of factors such as cavity layout, product characteristics, temperature uniformity, and cost. Symmetrical cavities are preferably circular or H-shaped for uniformity; linear or matrix cavities are preferably rectangular, balancing flexibility and cost; single-row or center-feed cavities are T-shaped for simplified structure; and complex asymmetric cavities require custom shapes. Regardless of the shape chosen, CAE flow simulation software must be used to analyze the melt flow and temperature distribution, optimize the runner dimensions and heating layout, and ensure that the filling time difference between each gate is within 5% and the temperature difference does not exceed ±3°C. At the same time, the hot runner plate must be manufactured from high-quality alloy steel (such as P20 and H13) and tempered to improve strength and wear resistance. The runner surface must be polished to Ra0.4μm or less to reduce flow resistance and the risk of stagnation. Reasonable shape design can maximize the advantages of the hot runner system, improve production efficiency, and ensure product quality stability.
