Injection speed If the mold is well vented, the faster the injection speed, the less obvious the weld line will be. A common problem is that when the mold is poorly vented, technicians reduce the injection speed, which affects weld line quality. If a weld line is formed by two streams of material “colliding” together, it will usually be stronger and less obvious. Likewise, the shorter the injection time, the more consistent the viscosity of the material becomes, making it easier to improve the weld line through holding pressure. Maintaining a consistent viscosity during the injection process will help minimize pressure drop across the cavity, resulting in more consistent cavity pressure during the packing phase.
Melt temperature: The plastic melt temperature should be maintained at a high level to ensure that the plastic melt at the weld line is tightly bonded together. If the melt temperature is too low, the weld line becomes brittle because there is not enough attraction between the molecules. Fountain flow describes the flow pattern from the inside out as the melt fills the mold walls. The advantage of fountain flow is that the high-temperature material flow always reaches the flow front first, which will supplement the welding line with higher-temperature molten material. (See the picture below), the molten material on both sides of the mold cavity will gradually cool down as it travels injection.
Mold flow analysis results show that the melt temperature during the filling process is high and low. The key is that the temperature should exceed a certain level so that the weld lines are well “bound” together. The image below is an enlarged view of the weld line, showing the grooves formed by the flow fronts being welded together.
Holding pressure There should be sufficient holding pressure when the weld line is formed. Higher holding pressure will force the materials at the weld line to be tightly combined and provide strong bonding force. Products with sufficient pressure retention have stronger weld lines. Verify the holding pressure according to the process sheet, taking into account the necessary reinforcement ratio. If there is a defect in the weld line that is not strong enough, try increasing the holding pressure and then check whether the defect is improved Injection Molding.
Higher mold temperature helps form a stronger weld line. At higher mold temperatures, the shrinking plastics bond more easily and the weld lines are less obvious. High mold temperature can make the molecular chains at the weld line more tightly entangled. Increasing mold temperature can improve weld line appearance and strength. If weld line issues are present, try mold testing at a higher mold temperature. There are several rapid mold heating systems on the market that can provide very high mold temperatures. This technology can eliminate weld lines from the surface and increase the strength of weld lines. If weld lines are a critical defect, rapid mold heating technology may be the best solution.
