1. air hole
Concave holes appear in the molded products, which are mainly caused by shrinkage during the cooling process of the molded products in the mold. In addition to the requirement that the material must fill the mold cavity, rapid cooling is also necessary. Specifically, it is to increase the holding pressure (secondary pressure) and reduce the set temperature of resin and mold. In addition, it also has a great dependence on the shape of the molded product. Because air holes are easy to appear in thick parts, for such products, a model design should be adopted, in which there are glue injection ports or glue flow channels around them.
2. Rough edges
This is caused by resin overflowing from the mold cavity. For injection molding of rubber, burr is normal, but it is abnormal for resin or thermoplastic elastomer TPE. The reasons are as follows: (1) Compared with rubber compound, molten resin or thermoplastic elastomer has higher fluidity and lower injection pressure; (2) contact with the mold, cool, cure and stop flowing in an instant. Therefore, resin or thermoplastic elastomer is usually not prone to burrs. As a countermeasure, first of all, it is necessary to reduce the filling amount, the holding pressure and the holding time.
In addition, it is necessary to repair the dies with poor dimensional accuracy and gaps in parting surfaces. When the projection area of the molded product is large and the clamping force is relatively lower than the injection pressure, burrs sometimes appear, so a larger molding machine must be used.
3. Flow marks
Stripes with different gloss appear on the surface of molded products. Generally speaking, in the injection molding of resin, there are: (1) narrow recording stripes; (2) Wide interval stripes of the same phase appear up and down on the surface of the molded product; (3) There are three types of relatively narrow interval stripes with ectopic phase appearing up and down on the surface of molded products. These methods can be used to solve this problem, such as increasing injection speed, mold temperature, enlarging injection port, increasing resin temperature and injection speed, increasing molding temperature, mold temperature or decreasing injection speed. It is effective to increase injection speed and mold temperature.
4. Poor demoulding
Poor releasability means that the molded product is difficult to take out from the mold or completely deformed during taking out. Sticky materials can easily cause this problem, but it can be improved by adding mold release agent into materials or coating mold release agent before molding. Such a problem easily occurs when the molded product is under-cooled (under-cured), so it is necessary to sufficiently cool the molded product. In addition, the unreasonable mold design will also become the reason why it is difficult to demould, especially in the parts which are easy to stick to the mold, such as the injection port and the injection channel. It is very effective to increase the pull-out angle of the injection port and widen the injection channel.
5. Silver stripes
The phenomenon of radial stripes with the glue injection port as the center is caused by the vaporization of moisture or volatile components in the material. Among them, the air involved in the plasticizing process or left in the mold will also lead to this phenomenon. Therefore, it is very necessary to fully dry the moisture-absorbing materials before molding and to reduce the molding temperature of the materials which are easy to produce decomposable gases.
6. Lack of glue
The phenomenon of not filling the end of the mold cavity is called lack of glue. This is mainly caused by unsuitable molding conditions, such as insufficient filling amount, but this phenomenon can also be caused by insufficient exhaust gas or unbalanced runner (multi-cavity mold) during molding.
7. Burning
It means that the parts not filled to the end and not filled with the mold cavity are aged like burning. This is mainly caused by insufficient exhaust, heat insulation compression caused by air or generated gas, and instantaneous temperature rise significantly (i.e., thermal aging on the surface of molded products). Improving the exhaust mode is a better solution, and reducing the injection speed can also be solved when the degree is light.
8. Uneven color
When thermoplastic elastomer TPE particles and dry mixture are used as the parent mixture of pigments for coloring, the phenomenon of uneven color of molded products is easy to occur. As a countermeasure, it is effective to increase the back pressure of screw and strengthen the mixing of filler.
The problems in the actual production of the method of solving problems quickly often change from an extremely simple problem (inappropriate molding temperature) to a complicated degree. Especially for the problems caused by complex factors, if we want to solve them as soon as possible, we must first collect the relevant information correctly. As shown in the table, although the surface is rough, in order to find out its specific reasons, it is necessary to know the relevant situation.
