Visible defects on the surface of injection molded products include dark spots, gloss differences or atomized areas, and surface wrinkles or what is known as orange peel. These defects usually occur near the gate or behind sharp corners away from the gate area. The causes of these defects can be found out from both the mold and the molding process.

Dark spots on the product

Dark spots appear near the gate, like a dim halo. This is especially evident in the production of products with high viscosity and low flow materials, such as PC, PMMA or ABS. This visible defect may occur on the surface of the product when the cooled surface layer resin is carried away by the center flowing resin.

It is generally assumed that such defects occur frequently in the mold filling and pressure holding stages. In fact, dark spots appear near the gate and usually occur at the beginning of the injection cycle. Experiments show that the occurrence of surface slippage is actually due to the injection speed, more specifically the flow speed at the front of the melt flow.

Dark spots around the gate and after the formation of sharp corners are due to the initial injection speed is too high, and the cooling surface is driven by the internal fluid displacement. This defect can be overcome by gradually increasing the injection speed and step-by-step injection.

Even though the injection speed is constant when the melt enters the mold, its flow rate will change. When entering the mold gate area, the melt flow rate is high, but after entering the mold cavity that is the mold filling stage, the melt flow rate begins to decrease. This change in the flow velocity at the front end of the melt flow will cause surface defects of the product.

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

Low melt temperature is another reason for the product to produce dark spots. Increasing the temperature of the barrel and increasing the back pressure of the screw can reduce the probability of this phenomenon. In addition, the temperature of the mold is too low will also produce surface defects, so increasing the mold temperature is another feasible way to overcome the surface defects of the product.

Mold design defects will also produce dark spots near the gate. The sharp corner of the gate can be avoided by changing the radius. Pay attention to the position and diameter of the gate during the design to see if the design of the gate is suitable.

Dark spots will not only occur at the gate location, but also often appear after the formation of a sharp corner of the product. For example, the surface of the sharp corner of the product is usually very smooth, but behind it is very gray and rough. This is also caused by the excessive flow rate and injection speed, which causes the cooling surface layer to slip and be replaced by the internal fluid.

Again, a step-by-step injection with a gradual increase in the injection speed is recommended. The best approach is to allow the melt to begin increasing in speed only after it has passed over the sharp edge.

Sharp changes in the Angle of the product away from the gate can also cause this defect. Therefore, the product should be designed with smoother rounded corner transitions in those areas.

Improve the luster difference

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

The injection pressure of the melt gradually decreases as the distance from the gate gradually increases. If the far end of the gate of the product cannot be filled, then the pressure there is the lowest, so that the texture of the mold surface cannot be correctly copied to the surface of the product. Therefore, the area with the greatest pressure in the mold cavity (half of the fluid path from the gate) is the area with the least difference in gloss.

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

products can also reduce the chance of luster differences. For example, drastic changes in the wall thickness of the product can cause irregular flow of the melt, making it difficult for the mold surface texture to be copied to the product surface. Therefore, a uniform wall thickness can reduce the occurrence of this situation, while too large wall thickness or too large rib will increase the chance of luster differences. In addition, insufficient melt venting is also a cause of this flaw.

The origin of orange peel

"orange peel" or surface wrinkling defects generally occur at the end of the flow channel when forming thick wall products with high viscosity materials. In the injection process, if the melt flow speed is too low, the surface of the product will be quickly cured. With the increase of flow resistance, the flow at the front of the melt will become uneven, resulting in the first solidified outer material can not fully contact the cavity wall, resulting in wrinkles.

These folds will become irreducible defects after curing and holding pressure. For this defect, the solution is to increase the melt temperature and increase the injection speed.

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