Methods for eliminating surface defects of injection molded products

Visible defects on the surface of injection molded products include dark spots, gloss differences or fogged areas, surface wrinkling, or what is known as orange peel. Usually, these defects occur near the gate or behind sharp corners away from the gate area. The causes of these defects can be identified by looking at both the mold and the molding process. 

Dark spots on the product 

Dark spots appear near the gate, like a dim sunburst. This is especially noticeable when producing products with high viscosity, low flowing materials such as PC, PMMA, or ABS. This visible defect may appear on the surface of the product when the cooled surface layer of resin is carried away by the central flowing resin. 

It is often assumed that this defect occurs frequently during the mold filling and holding stages. In fact, the dark spots appear near the gate and usually occur at the beginning of the injection cycle. Tests have shown that the occurrence of surface slippage is actually attributable to the injection speed, more precisely the flow rate at the front of the melt stream.

Even if the injection velocity is constant when the melt enters the mold, its flow rate can change. When entering the gate area of the mold, the melt flow rate is high, but after entering the mold cavity, i.e. during the filling phase, the melt flow rate starts to decrease. This change in flow rate at the front end of the melt flow can cause surface defects in the product. 

Reducing the injection speed is one way to solve this problem. In order to reduce the velocity of the front end of the melt flow at the gate, the injection can be divided into several steps and the injection speed can be gradually increased with the aim of obtaining a uniform melt flow rate during the entire filling phase. 

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

Mold design defects can also produce dark spots near the gate. Sharp corners at the gate can be avoided by changing the radius, so keep an eye on the location and diameter of the gate to see if the design of the gate is appropriate. 

Dark spots can occur not only at the location of the gate but also often after the formation of sharp corners in the product. For example, the surface of a sharp corner of a product is usually very smooth, but behind it is very gray and rough. This is also caused by excessive flow rates and injection speeds that cause the cooling surface layer to slip as it is replaced by the internal fluid. 

Again, a step-by-step injection with a gradual increase in injection speed is recommended. The best approach is to allow the melt to start increasing its velocity only after it has flowed over the sharp edges. 

Sharp changes in the angle of the product in areas away from the gate can also cause this defect. Therefore, it is important to design the product with a smoother, rounded transition in those areas. 

Improve gloss difference

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

The injection pressure of the melt gradually decreases as the distance of the melt from the gate gradually increases. If the distal end of the product's gate cannot be filled, then the pressure at that point is the lowest, so that the texture of the mold surface is not reproduced correctly on the product surface. Therefore, the area where the cavity pressure is highest (half of the fluid path from the gate) is the area where the least difference in gloss occurs. 

This can be changed by increasing the melt and mold temperatures or increasing the pressure while increasing the holding time can also reduce the gloss variation. 

Good design of the product can also reduce the chance of gloss variation. For example, drastic changes in wall thickness can cause irregular flow of the melt, making it difficult for the mold surface texture to be reproduced on the product surface. Therefore, designing for uniform wall thickness can reduce the occurrence of this condition, while excessive wall thickness or excessive ribbing can increase the chances of gloss variation. In addition, inadequate venting of the melt can also be a cause of this defect. 

Origin of Orange Peel

The "orange peel" or surface wrinkling defect generally occurs at the end of the runner when molding thick-walled products from high-viscosity materials. During the injection process, if the melt flow rate is too low, the surface of the product will cure quickly. As the flow resistance increases, the front end of the melt flow will become uneven, resulting in the first cured outer layer of material can not being fully contacted with the cavity wall, resulting in wrinkles. These wrinkles will become non-eliminable defects after curing and holding pressure. The solution for this defect is to increase the melt temperature and increase the injection speed.