Insulation and positioning of injection molding hot runner plates
The insulation and positioning of injection molding hot runner plates are crucial for the proper operation of hot runner systems, directly impacting melt temperature stability, energy consumption, and mold life. As a key component for melt distribution, the hot runner plate must maintain a stable operating temperature (typically 180-300°C). However, its close contact with the mold’s fixed platen (normal or low temperature) results in significant heat loss, increasing energy consumption and potentially causing melt solidification or degradation due to temperature fluctuations. Furthermore, the hot runner plate undergoes thermal expansion at high temperatures. Improper positioning can lead to runner misalignment, material leakage, and mold deformation. For example, in the production of a PC plastic part, ineffective thermal insulation between the hot runner plate and the fixed platen resulted in heat loss of 5kW per hour and runner temperature fluctuations of ±5°C, resulting in weld marks and silver streaks on the parts. In another case, excessive clearance in the hot runner plate resulted in a 0.1mm misalignment between the runner and nozzle after thermal expansion, causing severe material leakage and requiring a four-hour production stoppage for cleanup.
The thermal insulation design of the hot runner plate requires a multi-layered structure to prevent heat transfer to the mold. This primarily involves selecting insulation materials and designing insulation gaps. Placing an insulating gasket between the hot runner plate and the fixed platen is the most common method. The gasket material must have low thermal conductivity (≤ 0.2W/(m・K) ) and high-temperature resistance (≥ 350 °C). For example, glass fiber-reinforced polytetrafluoroethylene ( 260 °C) is suitable for medium-temperature applications, while mica sheets ( 600 °C+) or ceramic fiberboard ( 1000 °C) are suitable for high-temperature engineering plastics (such as PEEK and LCP ). The thickness of the insulating gasket is typically 0.5-2mm . Excessively thick insulation can affect the stability of the hot runner plate, while too thin insulation can result in poor insulation performance. For large hot runner plates, air insulation grooves can be installed at the bottom to enhance insulation by leveraging air’s low thermal conductivity ( 0.026W/(m・K) ). A groove depth of 5-10mm and a width of 10-20mm can reduce heat loss by 30-40% . Furthermore, the outer surface of the hot runner plate can be sprayed with an insulating coating (such as zirconia ceramic coating) to further reduce radiative heat dissipation. A coating thickness of 50-100μm can reduce the surface temperature by 10-15 °C.
The positioning structure design must simultaneously ensure precise positioning of the hot runner plate and compensate for thermal expansion to prevent runner misalignment. Radial positioning typically utilizes a locating pin in conjunction with a locating hole. The locating pin should be made of a high-temperature alloy (such as Inconel 718 ) to prevent deformation at high temperatures. The clearance must account for thermal expansion. The calculation formula is: Clearance = Maximum Radial Expansion of the Hot Runner Plate × 1.2 (Safety Factor). For example , for a 300mm long hot runner plate made of P20 steel (linear expansion coefficient 12×10⁻⁶/ °C), operating at 250 °C and room temperature 25 °C, the radial expansion is 300×(250-25)×12×10⁻⁶ , ≈ 0.81mm . Therefore, the positioning clearance should be set to 0.9-1.0mm . Axial positioning requires limiting the vertical movement of the hot runner plate. This can be accomplished by using shoulder locators or stoppers. The contact area between the shoulder and the fixed platen should be minimized (to only 10-15% of the hot runner plate’s base area) to reduce the heat conduction path. For multi-cavity hot runner plates, a primary locator should be located in the center, with auxiliary locators located around the perimeter to ensure uniform movement during thermal expansion and avoid localized stress concentration.
The uniformity of the hot runner plate’s temperature field is closely related to the insulation and positioning design, requiring optimization through simulation and experimentation. Poor insulation can cause the edge temperature of the hot runner plate to be 5-10°C lower than the center, causing the melt to cool and thicken within the edge channels, impacting filling uniformity. Overtight positioning can generate additional stress due to thermal expansion, leading to channel deformation and even cracking. Finite element analysis (FEA) can simulate the temperature distribution and thermal deformation of the hot runner plate to optimize the placement of thermal insulation gaskets and the size of positioning gaps. For example, the thickness of the insulation gaskets can be increased in areas with large temperature gradients, and the positioning gaps can be increased in areas with high thermal expansion. In actual production, the temperature at each point on the hot runner plate must be monitored using thermocouples to ensure that the temperature difference is within ±3°C. For example, when producing PA66 plastic parts, a temperature difference exceeding 5°C at the nozzles of different cavities on the hot runner plate can result in a weight deviation exceeding 2%. Adjustments to the insulation structure are necessary to reduce the temperature difference to within 3°C.
Maintaining and optimizing the insulation and positioning of the hot runner plate ensures long-term stable production. Regularly inspect the thermal insulation gaskets for wear and aging. Replace any hardening or cracking, typically every 50,000 molds. Clean the locating pins and holes of the locating pins and holes to ensure a smooth fit and prevent seizures that could affect thermal expansion compensation. For high-temperature applications, apply high-temperature grease (resistant to temperatures above 300°C) to the locating gap to reduce friction and wear. If runner temperature fluctuates significantly during production, inspect the insulation structure for failure, such as adding an insulation layer or replacing higher-performance insulation materials. If leakage or runner blockage occurs, check the positioning for accuracy and, if necessary, regrind the locating surfaces to ensure the clearance meets design requirements. For example, when producing PET preforms, one company experienced frequent leaks from the hot runner plate due to misalignment. By replacing high-temperature locating pins, adjusting the clearance to 0.8mm, and cleaning the locating holes every 30,000 molds, the malfunction interval was extended to over 100,000 molds, significantly improving production stability.
