The pouring system is changed from hot runner to normal runner
When converting a gating system from a hot runner to a conventional runner, the runner structure must first be adjusted. Hot runner components such as the nozzle and manifold must be replaced with the sprue, branch runners, and gates of a conventional runner to ensure the melt flow path is adapted to the unheated state. Hot runners typically utilize pin-point gates or direct hot nozzle feed. When converting to a conventional runner, the gate design must be redesigned based on the part structure. For example, a pin-point gate in a hot runner can be replaced with a side gate or fan gate to accommodate the lower melt temperature of a conventional runner. The runner cross-sectional dimensions must be recalculated. Conventional runner diameters are typically 20%-30% larger than those of hot runners. For example, a 6mm hot runner diameter could be 8mm after conversion to a conventional runner to compensate for the increased melt viscosity caused by the lack of heating. The runner surface roughness must be improved to below Ra1.6μm, and fine grinding should be performed to reduce flow resistance to compensate for the inability of conventional runners to maintain melt temperature through heating. In addition, ordinary runners need to be set with a reasonable demoulding slope (1°-2°) to facilitate the removal of runner condensate, while hot runners can omit this design because there is no condensate.
When converting the gating system from a hot runner to a conventional runner, the temperature control system needs to be redesigned. Since conventional runners lack heating, the barrel and mold temperatures must be adjusted to compensate for heat loss in the runners. The barrel temperature should be 10-20°C higher than during hot runner molding. For example, the barrel temperature for ABS hot runner molding is 220°C, but it should be increased to 235°C after converting to a conventional runner to ensure that the melt maintains sufficient fluidity at the runner end. The mold temperature should be adjusted according to the plastic type. For crystalline plastics (such as PP and PE), the mold temperature should be increased by 5-10°C to accelerate crystallization and reduce the cooling time of the runner aggregate. For amorphous plastics (such as PC and ABS), the mold temperature can remain unchanged or slightly lowered to avoid slow cooling of the plastic part and extended production cycle. Cooling water channels can be added near the runners to control the cooling rate of the runner aggregate. For example, an 8mm diameter cooling water channel can be installed below the branch runner, 15mm from the runner surface, to ensure sufficient strength and prevent breakage during ejection. By adjusting the temperature system, the melt flow performance of the ordinary runner can be made close to that of the hot runner. For example, after a PC plastic part is converted to an ordinary runner, by increasing the barrel temperature by 15°C, the filling time is increased from 5 seconds in the hot runner to 6 seconds, which is still within an acceptable range.
When transitioning from a hot runner to a conventional runner, additional runner slug handling design is required, including a cold slug well, a slug puller, and a slug ejection mechanism to ensure smooth separation and removal of the slug from the molded part. A cold slug well is located at the end of the sprue to collect the cold slug at the melt front. The diameter of the cold slug well should be 3-5 mm larger than that of the runner, and the depth should be 1.2 times the runner diameter. For example, if the runner diameter is 8 mm, the cold slug well should be 12 mm in diameter and 14 mm in depth. A Z-shaped or spherical slug puller is used to hold the runner slug and pull it out of the sprue during mold opening. The puller’s diameter should be the same as the cold slug well, and its head structure should match the cold slug well. Ejectors are located at runner bends or ends to eject the slug. The diameter of the ejector pins is 3-6 mm, and the number depends on the runner length. For example, a 200 mm runner should have two ejector pins, spaced 100 mm apart. For a multi-cavity mold with one die, the ejection of the runner slurry needs to be synchronized with the ejection of the plastic part. This is achieved through the linkage of the ejector plate to ensure that the slurry and the plastic part are separated from the mold at the same time. For example, the same ejector plate is used to drive the ejector pins of the plastic part and the slurry ejector pins, with the same ejection distance (such as 30mm) to enable the two to be demolded synchronously.
When transitioning from a hot runner to a conventional runner, injection molding process parameters must be re-optimized to account for increased runner resistance and accelerated melt cooling. Injection pressure should be increased by 10%-30%. For example, if the injection pressure is 100 MPa during hot runner molding, it should be increased to 120 MPa after transitioning to a conventional runner to overcome the increased runner resistance. Multi-stage injection speed control is required, with a higher speed (60-80 mm/s) used during the runner filling phase to reduce melt cooling time in the runners; a moderate speed (40-60 mm/s) used during the cavity filling phase to prevent melt turbulence within the cavity. Holding pressure and hold time should be adjusted. Holding pressure should be reduced by 5%-10% compared to hot runners to prevent flash caused by excessive runner material compression; the hold time should be extended by 10%-20% to compensate for melt shrinkage during runner cooling. For example, a hold time of 5 seconds for a hot runner can be increased to 6 seconds for a conventional runner. By optimizing process parameters, the quality of plastic parts produced by conventional runner molding can be brought close to that of hot runner molding. For example, after a PP plastic part is converted to a conventional runner, by increasing the injection pressure and extending the holding time, the deviation of the dimensional accuracy of the plastic part from that produced by hot runner molding can be controlled within ±0.02mm.
When converting a gating system from a hot runner to a conventional runner, the mold structure must be adapted and a trial run must be performed to ensure stable operation of the converted system. This mold modification involves removing the hot runner’s heating element, nozzle, and manifold, and installing the conventional runner’s inserts and ejector mechanism. The runner inserts are secured with bolts to facilitate subsequent maintenance and dimensional adjustments. After the modification, the runner seals must be inspected to ensure no leakage during mold closing. This can be verified by applying red lead powder to ensure a minimum 90% fit on the mating surfaces. During the trial run, particular attention should be paid to the runner slurry release and the quality of part filling. If slurry release is difficult, the puller pin structure should be adjusted or the number of ejector pins should be increased. If the part is underfilled, the runner or gate dimensions should be increased. For example, after converting a mold to a conventional runner, the runner slurry was found to be broken during a trial run. By adding two runner ejector pins and optimizing the puller pin head shape, the slurry release was smoother. After mold trials, the weight, dimensions, and mechanical properties of the plastic part are measured and compared with those of parts molded using a hot runner to ensure that performance indicators meet requirements. For example, the impact strength of a certain plastic part dropped from 20kJ/m² to 19kJ/m² after conversion, which is still within the standard range. Through mold modification and mold trial optimization, a smooth transition from hot runner to conventional runner can be achieved, adapting to small-batch production or cost control needs.
