Third, the problems related to injection molding and its countermeasures

3.1 stomata
A pit phenomenon occurs in the molded article, mainly due to shrinkage of the molded article during cooling in the mold. In addition to requiring the material to fill the cavity, rapid cooling is also necessary. Specifically,  it is to increase the holding pressure (secondary pressure) and lower  the set temperature of the resin and the mold.
In addition, there is a great dependence on the shape of the molded article. Since  the pores are highly prone to occur in thick areas, such a product  should be provided with a model design such as a glue injection port or a  flow rubber lane around it.
3.2 Burr
This is caused by the resin overflowing from the mold cavity. For  the injection molding of rubber, it is normal to have a burr, but it is  not normal for a resin or a thermoplastic elastomer. The  reasons are as follows: (1) Compared with the rubber compound, the  molten resin or thermoplastic elastomer has higher fluidity and lower  injection pressure; (2) contact with the mold, cooling, and  solidification and termination of flow in an instant. . Therefore, a resin or a thermoplastic elastomer is generally a phenomenon in which burrs are less likely to occur.
As  a countermeasure, it is first necessary to reduce the amount of  filling, reduce the holding pressure, and shorten the holding time. In addition, for molds with poor dimensional accuracy and gaps in the parting surface, repairs are necessary. In  the case where the projected area of the molded article is large and  the mold clamping force is relatively lower than the injection pressure,  burrs sometimes occur, so a larger molding machine must be used.
3.3 Flow marks
A streak phenomenon in which gloss is different on the surface of the molded article. Generally speaking, in the injection molding of resin:
(1) a narrow strip of recording stripe;
(2) a relatively wide stripe pattern of the same phase appearing above and below the surface of the molded article;
(3) Three types of relatively narrow spaced stripes appearing on the surface of the molded article.
Solving  this problem can be solved by these methods, such as increasing the  injection speed, mold temperature, increasing the glue injection port,  increasing the resin temperature and injection rate, increasing the  molding temperature, mold temperature or reducing the injection speed. It is effective by increasing the injection speed and the mold temperature.
3.4 Poor release
The  poor mold release property means that the molded article is difficult  to remove from the mold or is completely deformed during the removal  process. Adhesive  materials can easily cause this problem, but the method of applying a  release agent to the material or applying a release agent to the mold  before molding can be improved. Insufficient  cooling of the molded article (under-curing) is also likely to cause  such a problem, and therefore it is necessary to sufficiently cool the  molded article. In  addition, the irrational design of the mold will become a cause of  difficulty in demoulding, especially in the parts that are easy to stick  to the glue injection port, the rubber feed path, etc., it is very  important to increase the extraction angle of the injection port and  widen the glue path. Effective.
3.5 silver stripes
The  phenomenon of radial streaks around the injection port is caused by the  vaporization of moisture or volatile components in the material. Among them, the air trapped in the mold or in the mold during the plasticization process will also cause this phenomenon. Therefore,  it is necessary to sufficiently dry the moisture absorbing material  before molding and to reduce the molding temperature at which the  decomposable gas material is easily generated.
3.6 lack of glue
The phenomenon of not filling the end of the cavity is called lack of glue. This  is mainly caused by the discomfort of the molding conditions such as  insufficient filling amount, but the exhaustion of the molding or the  unevenness of the flow passage (multi-cavity mold) may also cause this  phenomenon.
3.7 charred
It  means that the part that is not filled to the end and that is not  filled with the cavity has an aging phenomenon like scorch. This  is mainly due to insufficient exhaust gas, heat or compression caused  by air or generated gas, and the result of a sudden increase in  temperature (ie, heat aging on the surface of the molded article). Improving  the exhaust method is a better solution. In the case of a lighter  degree, reducing the injection speed can also be solved.
3.8 uneven color
When  the precursor mixture using the thermoplastic elastomer particles and  the dry blend as a pigment is colored, the color unevenness of the  molded article is likely to occur. As a countermeasure, it is effective to increase the back pressure of the screw and to knead the filler.