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How to eliminate the surface defects of three kinds of injection products?

Defects visible on the surface of injection molded articles include dark spots, gloss differences, or fogging areas, and surface wrinkling or what is known as orange peel. Usually these defects occur near the gate or behind the sharp corners of the gate area. Starting from the two aspects of the mold and the molding process, we can identify the causes of these defects.

    Dark spots appear near the gates, like dim dizziness. It is particularly noticeable in the production of high viscosity, low flow materials such as PC, PMMA or ABS. This visible defect may occur on the surface of the product as the cooled surface layer resin is carried away by the centrally flowing resin.

    It is often assumed that such defects occur frequently in the filling and holding pressure stages. In fact, dark spots appear near the gate and usually occur at the beginning of the injection cycle. Tests have shown that the occurrence of surface slip is actually due to the injection speed, more precisely the flow rate at the front end of the melt stream.

    Dark spots around the gate and dark spots after sharp corners are formed are due to the initial injection speed being so high that the cooled surface is displaced by internal fluids. Gradually increasing the injection speed and stepping injection can meet this defect.

    Even when the injection speed is constant when the melt enters the mold, its flow rate changes. When entering the mold gate area, the melt flow rate is high, but after entering the mold cavity, ie the mold filling stage, the melt flow rate begins to drop. This change in flow rate at the front end of the melt stream can cause product surface defects.

    Reducing the injection speed is one way to solve this problem. In order to reduce the speed of the melt flow front at the gate, the injection can be divided into several steps, and the injection speed is gradually increased, the purpose of which is to obtain a uniform melt flow rate throughout the filling phase.

    Low melt temperature is another cause of dark spots on the product. Increasing the barrel temperature and increasing the back pressure of the screw can reduce the chance of this happening. In addition, too low temperature of the mold can also produce surface defects, so increasing the mold temperature is another feasible way to overcome the surface defects of the products.

    Die design defects also produce dark spots near the gate. The sharp corners at the gate can be avoided by changing the radius. Care should be taken in designing the position and diameter of the gate to see if the gate design is appropriate.

    Dark spots not only occur at the gate, but also often appear after the sharp corners of the article are formed. For example, sharp corner surfaces of articles are generally very smooth, but very gray and rough behind it. This is also due to the excessive flow rate and injection speed causing the cooling surface layer to be displaced by internal fluid displacement.

    It is recommended to use step-by-step injection again and gradually increase the injection speed. The best way is to allow the melt to begin to increase in velocity only after passing through an acute edge.

    In areas away from the gate, sharp changes in the angle of the product can also cause this defect. Therefore, when designing the product, smoother fillet transitions are used in those areas.

    Improve gloss difference

    For injection molded articles, the difference in gloss is most pronounced on the surface of textured articles. Even if the surface of the mold is very uniform, irregular gloss may appear on the product. In other words, the mold surface effect of some parts of the product is not well reproduced.

    As the distance from the melt exits the gate, the injection pressure of the melt gradually decreases. If the end of the gate of the product cannot be filled, the pressure at that point is minimal, so that the texture of the mold surface cannot be properly reproduced on the surface of the product. Therefore, the area where the pressure in the cavity is the greatest (half of the fluid path from the gate) is the area where the gloss difference is least.

    To change this situation, you can increase the melt and mold temperature or increase the pressure, while increasing the pressure holding time can also reduce the occurrence of gloss differences.

    The good design of the product also reduces the chance of gloss differences. For example, a drastic change in the wall thickness of an article can cause irregular flow of the melt, causing the surface texture of the mold to be difficult to replicate to the surface of the article. Therefore, designing a uniform wall thickness can reduce the occurrence of such conditions, and an excessively large wall thickness or an excessively large number of ribs can increase the chance of a difference in luster. In addition, insufficient exhaust of the melt is also a cause of this defect.

    The origin of orange peel

    "Orange peel" or surface wrinkle defects generally occur at the end of the flow path when a thick-walled product is formed from a high-viscosity material. During the injection process, if the melt flow rate is too low, the surface of the product will rapidly solidify. With the increase of the flow resistance, the front flow of the melt will become non-uniform, so that the previously solidified outer layer material cannot fully contact with the cavity wall, resulting in wrinkles.

    After these wrinkles cure and hold pressure, they become irreversible defects. For this defect, the solution is to increase the melt temperature and increase the injection speed.

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