11 kinds of surface defect defects and treatment methods for TPE injection molded parts

1, sticky phenomenon:

The surface of the soft thermoplastic elastomer is more susceptible to stickiness than the resin.

Reason: It is mainly caused by aging to produce low molecular weight polymer and plasticizer and stabilizer to migrate to the surface. In either case, it is relatively easy to determine the substance related to this by analyzing the sticky component by means of analytical methods such as infrared spectroscopy (IR).

The stickiness is mainly due to the fact that the molding temperature is too high and the polymer is thermally decomposed to form a low molecular weight substance. Although the set temperature of the molding machine itself does not reach the thermal decomposition temperature, the shear heat generation in the molding process sometimes causes it to temporarily reach a high temperature.

Solution: As a countermeasure, it is very effective to reduce the molding temperature and low shear, and purify the inside of the hydraulic cylinder of the molding machine with nitrogen. Further, when the molding machine is suspended, the rubber is left in a molten state for a long time in the cavity of the molding machine, and sometimes it is sticky due to heat aging. In addition, articles used at high temperatures are prone to stickiness. Therefore, the selection of the stabilizer and the type of softener and the determination of the amount are very important.

2. Whitening phenomenon:

It means that the compounding agent such as a stabilizer migrates to the surface of the molded article, and its surface appears white as if it were sprayed with powder.

Reason: It is mainly caused by excessive compounding of stabilizer or incompatibility with polymer. Incompatibility can be solved by adding relevant compatibilizer. Stabilizers with good polymer compatibility should be selected or the amount of stabilizer should be controlled to the optimum range.

Secondly, it is also very effective to switch to a stabilizer with a relatively high molecular weight. In addition, there are stabilizers that function by migrating to the surface of the molded article. For example, a lubricant, an antistatic agent, etc., for such a stabilizer, it is necessary to select a stabilizer which is less likely to be whitened even if it migrates.

When the thermoplastic elastomer is used in a general environment, there is little whitening problem. However, in the case of high temperature, humidity, and outdoor long-term use, it is very necessary to add heat-resistant stabilizer (anti-aging agent) and weathering stabilizer to improve durability. necessary. Especially at high temperatures, the choice of stabilizer is also very important because it is highly susceptible to migration.

3. Aging phenomenon:

The mechanical properties of the product are significantly reduced, and the appearance quality is deteriorated.

Reason: Compared with inorganic materials and metal materials, the heat resistance and ultraviolet resistance of polymer materials are poor, causing aging of products. Most of the products have a markedly reduced mechanical properties due to aging, and the appearance quality is deteriorated.

Treatment method: By adding a stabilizer such as heat resistance and weather resistance, by adding a UV absorber or a light stabilizer, the occurrence of aging can be suppressed to some extent. Problems related to injection molding and their countermeasures.

4, air holes:

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.

5, raw edges:

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 reason is:

Compared with rubber compounds, molten resin or thermoplastic elastomer has higher fluidity and lower injection pressure;

It is in contact with the mold and cooled, and it can be solidified and stopped at 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.

6, 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:

a narrow strip of recording stripe;

a relatively wide stripe pattern of the homo phase appears above and below the surface of the molded article;

There are three types of relatively narrow stripe-like ectopic phases appearing above and below the surface of the molded article.

Solving this problem can be solved by adding pure monomer resin, increasing injection speed, mold temperature, increasing the 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.

7, 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.

8, 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.

9, lack of glue:

It means that the end of the cavity is not filled. 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.

10, uneven color:

When the precursor mixture of the thermoplastic elastomer particles and the dry blend is used as a pigment, the color unevenness of the molded article is liable to occur, and the mixing is uneven or the bonding is not good;

As a countermeasure, a suitable coupling agent or compatibilizer is used. It is effective to increase the back pressure of the screw and to knead the filler.

11, burnt:

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 charring. 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).

Labor method: Improve the exhaust method, and reduce the injection speed in a lighter degree to solve this problem.