Visible defects on the surface of injection molded products include dark spots, gloss differences or fogging areas, and surface wrinkles or so-called orange peels. These defects usually occur near gate or behind sharp corners away from gate area. Starting from both mold and molding process, reasons for these defects can be found.

 

Dark spots appear near gate, like a dim solar halo. It is especially obvious when producing products with high viscosity and low fluidity materials, such as PC, PMMA or ABS. When resin of cooled surface layer is taken away by resin flowing in the center, such visible defects may appear on the surface of product.
It is generally believed that such defects frequently occur in filling and holding pressure stages. In fact, dark spots appear near gate, usually at the beginning of injection cycle. Experiments have shown that occurrence of surface slip is actually due to injection speed, more precisely flow speed at the front end of melt flow.
Dark spots around gate and dark spots that appear after formation of a sharp corner are caused by initial injection speed being too high, and cooling surface is displaced by internal fluid. Gradually increasing injection speed and step-by-step injection can overcome this defect.
Even if injection speed of melt is constant when it enters mold, its flow speed will change. When entering gate area of mold, melt flow rate is very high, but after entering mold cavity, that is, during mold filling stage, melt flow rate begins to decrease. This change in flow velocity at front end of melt flow will cause surface defects of product.
Reducing injection speed is one way to solve this problem. In order to reduce speed of front end of melt flow at gate, injection can be divided into several steps and injection speed is gradually increased. Purpose is to obtain a uniform melt flow rate throughout filling stage.
Low melt temperature is another reason for dark spots on products. Increasing barrel temperature and increasing screw back pressure can reduce chance of this phenomenon. In addition, temperature of mold is too low to produce surface defects, so increasing mold temperature is another feasible way to overcome surface defects of product.
Mold design defects can also produce dark spots near gate. Sharp corners at gate can be avoided by changing radius. Pay attention to position and diameter of gate during design to see if design of gate is appropriate.
Dark spots not only occur at gate location, but also often appear after sharp corners of product are formed. For example, sharp corner surface of product is generally very smooth, but behind it is very dark and rough. This is also caused by fact that cooling surface layer is replaced by internal fluid and slips due to excessively high flow rate and injection speed.
It is again recommended to use step-by-step injection and gradually increase injection speed. The best way is to allow melt to only start to increase in velocity after it has flowed over sharp edge.
In area away from gate, sharp changes in angle of product can also cause this defect. Therefore, when designing products, use smoother rounded transitions in those areas.

For injection molded products, difference in gloss is most obvious on surface of textured products. Even if surface of the mold is very uniform, irregular gloss may appear on product. In other words, mold surface effect of some parts of product is not well reproduced.
As distance of melt from gate gradually increases, injection pressure of melt gradually decreases. If far end of gate of product cannot be filled, then pressure there is the lowest, so that texture of mold surface cannot be correctly copied to surface of product. Therefore, area where cavity pressure is the highest (half of the fluid path from the gate) is area where gloss difference appears the least.
To change this situation, you can increase melt and mold temperature or increase pressure, while increasing holding time can also reduce difference in gloss.
Good product design can also reduce chance of gloss differences. For example, drastic changes in wall thickness of product can cause irregular flow of melt, which makes it difficult for surface texture of mold to be copied to surface of product. Therefore, design of uniform wall thickness can reduce occurrence of this situation, excessive wall thickness or excessive ribs will increase chance of gloss difference. In addition, insufficient venting of melt is also a cause of this defect.

"Orange peel" or surface wrinkle defects generally occur at the end of runner when a high-viscosity material is used to mold thick-walled products. During injection process, if melt flow rate is too low, surface of product will quickly solidify. As flow resistance increases, front flow of melt will become uneven, so that the first solidified outer layer material cannot fully contact cavity wall, resulting in wrinkles.
These wrinkles will become inevitable defects after curing and holding pressure. For this defect, solution is to increase melt temperature and increase injection speed.