Defects visible on the surface of injection-molded articles include dark spots, gloss differences or fogging zones, and surface wrinkling or called orange peel. These defects usually occur near the gate or behind the sharp corners of the gate area. From the mold and molding Craftsmanship starts with two aspects and can identify the causes of these defects.
Dark spots on the product
Dark spots appear near the gate, just like dizzy solar flares. This is especially evident when producing high-viscosity, low-fluidity materials such as PC, PMMA, or ABS. In the cooled surface layer the resin is moved by the center of the resin belt. When walking, this visible defect may appear on the surface of the product.
It is often assumed that such defects occur frequently in the filling and holding stage. In fact, dark spots appear near the gate, which usually occurs at the beginning of the injection cycle. Tests have shown that the occurrence of surface slipping is actually attributable to The injection speed, more precisely the flow rate at the front end of the melt stream.
The dark spots around the gate and the dark spots that appear after the sharp corners are formed are due to the initial injection speed being so high that the cooled surface is displaced by the internal fluid. Gradually increasing the injection speed and the step injection can Customer service this defect.
Even if 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, ie the mold filling stage, the melt flow rate Begin to decline. This change in the melt flow front velocity will cause product surface defects.
Reducing the injection speed is one way to solve this problem. In order to reduce the speed of the front end of the melt at the gate, the injection can be divided into several steps, and the injection speed is gradually increased. The purpose is to obtain uniformity during the entire filling stage. The melt flow rate.
Low melt temperature is another reason for the product to produce dark spots. Increasing the barrel temperature and increasing the back pressure of the screw can reduce the chance of this happening. In addition, the die temperature is too low to produce surface defects, so increasing the mold temperature is Another possible way to overcome surface defects in products.
Die design defects will also produce dark spots near the gate. The sharp corners at the gate can be avoided by changing the radius. Care should be taken when 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 after the sharp corners of the article have been formed. For example, the sharp corners of a product are generally very smooth, but behind it are very dark and rough. This is also due to excessive The flow velocity and injection speed cause the cooling surface layer to be displaced by the internal fluid instead of slipping.
Again, it is recommended to use step-by-step injections and gradually increase the injection speed. The best method is to allow the melt to begin to increase only after passing through the sharp edge.
In areas away from the gate, sharp changes in the angle of the product can also cause this defect. Therefore, design the product to use a smoother fillet transition in those areas.
Improve gloss difference
For injection molded articles, the difference in gloss is most pronounced on the textured surface. Even if the surface of the mold is very uniform, irregular gloss may appear on the product. That is, the mold in certain parts of the product Surface effects are not reproduced well.
As the distance between the melt and the gate gradually increases, the injection pressure of the melt gradually decreases. If the tip of the product's gate cannot be filled, the pressure at that point is the lowest, so that the texture of the mold surface cannot be correctly corrected. Copy to the surface of the product. Therefore, the area where the cavity pressure 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 can also reduce the occurrence of gloss differences. For example, severe changes in the wall thickness of the product can cause irregular flow of the melt, which can cause the surface texture of the mold to be difficult to replicate on the surface of the product. Therefore, the uniform wall thickness can be designed. The occurrence of this condition is reduced, and excessive wall thickness or excessively large ribs increase the chance of gloss difference. In addition, insufficient exhaust of the melt is also a cause of this defect.
The origin of orange peel
'Orange peel' or surface wrinkling defects typically occur at the end of the flow path when high-viscosity materials are used to form thick-walled parts. During the injection process, if the melt flow rate is too low, the surface of the product will quickly solidify. With flow resistance Increasing, the melt front flow will become non-uniform, resulting in the first solidified outer material can not fully contact with the cavity wall, resulting in wrinkles.
These wrinkles become indispensable defects after curing and holding pressure. For this defect, the solution is to increase the melt temperature and increase the injection speed.