Causes and solutions of mold scale after injection molding of engineering plastics

1, the formation of mold scale

Almost all thermoplastic injection molding are mold deposit occurs. When the functional requirements of the final product must be incorporated into the relevant additives (eg, modifiers, fire retardants, etc.), these additives are likely to remain on the surface of the cavity during the molding process, resulting in the formation of mold scale.

There are other reasons for the formation of mold scale. The most common reasons are as follows:

a. thermal decomposition products of the raw materials;

b. Extreme shear force of the melt flow during injection molding;

c. improper exhaust;

The mold scale caused by the above is often a combination of different factors, and it is very troublesome to find out what causes and how to prevent these mold scales, and the mold scale is formed after several days.

2, the type of mold scale

1) Various additives produce a specific class of mold scale. Fire retardants react at high temperatures to form decomposition products that may form mold scales. Under the influence of excessive high temperature or extreme shearing force, the impact agent will separate from the polymer and remain on the surface of the cavity to form mold scale.

2) The pigment in the thermoplastic engineering plastic melts at a high temperature, which will lower the thermal stability of the molding material, and as a result, the degraded polymer and the decomposed pigment are combined to form a mold scale.

3) Specially hot parts (such as cores) on the mold, modifiers/stabilizers and other additives may adhere to the surface and cause mold deposits. In this case, measures must be taken to achieve better mold temperature control or to use special stabilizers.

3. The sudden occurrence of mold scale countermeasures

If the mold scale suddenly occurs, it may be due to a change in molding conditions or because of the replacement of different batches of molding materials. The following suggestions will help improve mold scale.

First, measure the temperature of the melt and visually inspect the melt for decomposition (eg, charred particles). At the same time, check whether the molding materials are contaminated with foreign substances and whether the same raw materials are used. Check the exhaust condition of the mold.

Again, check the working machine: use dye-dyed molding material (except black), close the Injection Molding Machine after about 20 minutes, remove the nozzle, connecting seat, if possible with the screw to remove, check the raw materials for charred particles, raw materials Color comparison, quickly find the source of mold.

In many cases, surprising causes of mold scale defects have been discovered. This technology is best suited for smaller injection molding machines with a maximum screw diameter of 40 mm. The elimination of mold scale also allows the quality of the part to be clearly improved when molding other materials. The above countermeasures are also applicable to the molding with a hot runner system. Mold mold scale causes defects in the appearance of injection molded parts, especially those with surface etch, which can be repaired by sand blasting.

4, mold maintenance

This mold maintenance and maintenance must be enhanced when all of the above measures are not sufficient to eliminate mold deposits.

The mold surface is relatively easy to remove at the beginning of mold formation, so the cavity and exhaust passage must be cleaned and maintained at regular intervals (for example, after each batch of molding). It is very difficult and time consuming to remove the mold after forming a thick layer of mold after a long period of time without mold maintenance and maintenance.

The sprays used for the maintenance and maintenance of injection molds mainly include: release agent, rust inhibitor, thimble oil, glue remover, mold cleaning agent, etc.

5. Suggestions for preventing mold scale

When using hot runner molding and heat sensitive materials, the melt residence time will be longer, thereby increasing the risk of decomposition of the material to form mold scale and cleaning the screw of the injection molding machine.

Larger flow channels and gates are used for forming shear-sensitive materials. Multi-point gates reduce flow distance, low injection speed, and reduce the risk of mold formation.

Efficient mold venting reduces the potential for mold formation and provides proper mold venting during the mold design phase. The automatic removal of the exhaust system, or the venting method that can easily remove the mold deposit, is the best choice, and the improvement of the exhaust system often leads to a reduction in mold scale on the mold.

A special non-stick coating is applied to the surface of the cavity to prevent the formation of mold scale, and the test should be evaluated to evaluate the effect of the coating. The titanium nitride treatment on the inner surface of the mold avoids the formation of mold scale on the mold.