Analysis of reasons for Warpage of Plastic Injection Molding Products

With the development of the plastics injection molding industry, people have higher and higher requirements on the appearance and performance of plastic injected products. The degree of warpage deformation is one of the important indicators for evaluating product quality, and it is more and more concerned and valued by mold designers. Mold designers hope to predict the possible warpage of plastic parts during the design phase in order to optimize the design and then they can improve the efficiency and quality of injection molding, shorten the mold design cycle and reduce costs. I take this change to analyze the factors affecting the warpage of injection molded products during the design process of plastic injection molds.

Effect of the mold structure on the warpage of plastic injection products:In terms of mold design, themain factors affecting the deformation of plastic parts are pouring system, cooling system and ejection system.

Design of the gating system,the gate location, form and number of gates of the injection mold will affect the filling state of the plastic raw material in the mold cavity and then result in deformation of the plastic part. The longer the flow distance, the greater the internal stress caused by the flow and the filling between the frozen layer and the central flow layer.Conversely, the shorter the flow distance, the shorter the flow time from the gate to the flow end of the plastic injection parts, and the freezing during filling.The thickness of the layer is reduced, the internal stress is lowered, and the warpage deformation is also greatly reduced.

For a large flat plastic part, if only one center gate or one side gate is used, since the shrinkage rate in the diameter direction is larger than the shrinkage ratio in the circumferential direction, the molded plastic part will be distorted.Point gates or film gates can effectively prevent warpage. When molding by spot casting, the position and the number of gates have a great influence on the degree of deformation of the plastic part due to the anisotropy of shrinkage of the plastic.

Since 30% glass fiber reinforced PA6 is used, a large injection molded part with a weight of 4.95 kg is obtained, so that a plurality of reinforcing ribs are provided along the flow direction of the surrounding wall, so that a sufficient balance can be obtained for each gate. In addition, the use of multiple gates can also shorten the flow ratio (L/t) of the plastic, thereby making the density of the material in the cavity more uniform and shrinking more uniform. At the same time, the entire plastic part can be filled with a small injection pressure. The smaller injection pressure can reduce the molecular orientation tendency of the plastic and reduce the internal stress, thereby reducing the deformation of the plastic part.

In the design of cooling system, uneven cooling speed of plastic injection parts will also leads to uneven shrinkage of plastic injected parts. Such difference in shrinkage will lead to the generation of bending moment and the warping of injected plastic parts.

If the temperature difference between the mold cavity and core used in the injection molding of the plastic injection part is too large, the melt close to the cold mold cavity will cool down quickly, while the material layer close to the hot mold cavity will continue to shrink, and the uneven shrinkage will make the injected plastic part warp. Therefore,plastic injection mold cooling should pay attention to the cavity, core temperature tends to balance, the temperature difference between them can not be too large.

In addition to considering the plastic injected parts of the internal and external surface temperature tends to balance, should also consider the plastic parts of the same temperature on all sides, that is, when the mold cooling to try to keep the cavity, core temperature uniform throughout, so that the plastic parts of the cooling speed balance, so that the contraction of all more uniform, effectively prevent the generation of deformation. Therefore, the arrangement of cooling water holes on the die is very important. After the distance between the pipe wall and the cavity surface is determined, the distance between the cooling water holes should be kept as small as possible to ensure the uniform temperature of the cavity wall. At the same time, as the temperature of the cooling medium increases with the length of the cooling channel, the temperature difference of the mold cavity and core along the channel occurs. Therefore, the waterway length of each cooling loop is required to be less than 2m. Several cooling loops shall be provided in large molds, with the inlet of one loop located near the outlet of the other loop. For long plastic parts, the length of the cooling loop should be reduced, that is, to reduce the temperature difference between the mold, so as to maintain uniform cooling injected plastic parts.

 

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