How to solve the side wall dent of injection molde

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How to solve the side wall dent of injection molded parts

"dent" is caused by local internal shrinkage caused by gate sealing or lack of material injection. The depression or micro depression on the surface of the code product of injection molding punch is an old problem in the process of injection molding. Generally, dents are caused by the local increase of shrinkage rate caused by the increase of wall thickness of plastic products. They may appear near the external sharp corners or at the sudden change of wall thickness, such as the back of bulges, stiffeners or supports, and sometimes appear in some unusual parts. The root cause of dents is the thermal expansion and cold contraction of materials, because the thermal expansion coefficient of thermoplastic is quite high. The degree of expansion and contraction depends on many factors, among which the performance of plastics, the testing equipment most needed by the textile industry: large and minimum temperature range and mold cavity pressure are the most important factors. The size and shape of injection molded parts, as well as cooling speed and uniformity are also influencing factors

the amount of expansion and contraction of plastic materials during the molding process is related to the thermal expansion coefficient of the processed plastic, which is called "molding shrinkage". As the molded part cools and shrinks, the molded part loses close contact with the cooling surface of the mold cavity. At this time, the cooling efficiency decreases. After the molded part continues to cool, the molded part shrinks continuously, and the amount of shrinkage depends on the comprehensive action of various factors. The sharp corners on the molded part cool the fastest and harden earlier than other parts. The thick part near the center of the molded part is the farthest from the cooling surface of the cavity and becomes the last part to release heat on the molded part. After the material at the corners solidifies, with the melt near the center of the molded part cooling, the molded part will continue to shrink. The plane between the sharp corners can only be cooled on one side, and its strength is not as strong as that of the material at the sharp corners. The cooling shrinkage of the plastic material at the center of the workpiece pulls the relatively weak surface between the partially cooled and the sharp corners with a greater degree of cooling inward. In this way, dents are produced on the surface of the injection molded parts. The existence of dents indicates that the molding shrinkage here is higher than the shrinkage of its surrounding parts. If the shrinkage of the molded part in one place is higher than that in another, then the reason for the warpage of the molded part. The residual stress in the mold will reduce the impact strength and temperature resistance of the molded parts. In some cases, the generation of dents can be avoided by adjusting the process conditions. For example, during the pressure maintaining process of molded parts, the mold cavity is additionally injected with plastic materials with better durability to compensate for molding shrinkage. In most cases, with the gradual awakening of the international market, the gate is much thinner than other parts of the parts. When the molded parts are still very hot and continue to shrink, the small gate has been cured. After curing, the pressure maintaining has no effect on the molded parts in the mold cavity

the shrinkage of molded parts made of semi crystalline plastic materials is high, which makes the dent problem more serious; The molding shrinkage of amorphous materials is low, which will minimize dents; Filling and maintaining reinforced materials have lower shrinkage and less possibility of dents

thick injection molded parts have a long cooling time and will produce large shrinkage. Therefore, large thickness is the root cause of dents. Attention should be paid in the design. Thick wall parts should be avoided as much as possible. If the thick wall cannot be avoided, they should be designed as hollow, and the thick parts will smoothly transition to the common wall thickness. Using a large arc to replace the sharp corners can eliminate or minimize the dents near the sharp corners. (Liu Zhaolin)

development of multi-layer PET bottles

since DuPont company successfully developed pet two-way extension technology to PET bottles as carbonated drinks 30 years ago, it has attracted great attention. In particular, the processing methods and applications of PET bottles have developed rapidly

1. In order to achieve high barrier performance, PET bottles are made of two materials into a 5-layer structure. Two injection molding machines a and B and one barrier resin injection molding machine C are mainly used for successive injection molding to obtain PET bottles with 5-layer structure of two materials. In order to ensure the accuracy of the quality, injection volume and injection accuracy of PET bottles, special equipment must be used. AsAb company of Japan and kortee company of Japan have produced pet bottles with 3 layers of 2 materials, PET bottles with 3 layers of 3 materials and PET bottles with 5 layers of 3 materials

2. In recent years, in order to improve the quality of multi-layer bottles, bottles with absorption structure in multi-layer bottles have been developed. The main function is that after the oxygen in the outside air passes through the multi-layer bottle, the oxygen in the air will be absorbed by the oxygen absorption layer in the multi-layer bottle, preventing the oxygen in the air from entering the bottle and affecting the oxidation and deterioration of the contents in the bottle. The oxygen absorption layer in the multi-layer bottle does not affect the transparency, molding plant performance and recycling performance of the bottle

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