1. Formation of mold scale

Almost all thermoplastics will have scale during injection molding. When the functional requirements of the final product require the addition of relevant additives (such as modifiers, fire retardants, etc.), these additives are likely to remain on the surface of the mold cavity during the molding process, leading to the formation of mold scale.

There are other reasons for the formation of mold scale, the most common of which are as follows:

Thermal decomposition products of raw materials;

During injection molding, the extreme shear force of the melt flow;

Improper exhaust;

The mold scale caused by the above is often a combination of different factors, and seeking to discover what causes these mold scales and how to prevent them is very troublesome, and mold scale will only form after a few days.

2. Types of mold scale

1) Various additives produce specific types of mold scale. Fireproofing agents will react at high temperatures, forming decomposition and may produce mold scale products. Under the influence of excessive high temperature or extreme shear force, impact resistant agents will separate from the polymer and remain on the surface of the mold cavity to form mold scale.

2) The melting of pigments in thermoplastic engineering plastics at high temperatures will reduce the thermal stability of the forming materials, resulting in the combination of degraded polymers and decomposed pigments to form mold scale.

3) Parts that are particularly hot on the mold (such as the mold core), modifiers/stabilizers, and other additives may adhere to their surface and cause mold fouling. In this situation, measures must be taken to achieve better mold temperature control or use special stabilizers.

The following table lists the possible causes and preventive measures for mold fouling:

2. Countermeasures for sudden mold fouling

If mold fouling suddenly occurs, it may be due to changes in molding conditions or replacement of different batches of molding materials. The following suggestions can help improve mold fouling.

Firstly, measure the temperature of the melted adhesive and visually inspect whether there is any decomposition phenomenon (such as burnt particles). At the same time, check whether the molding materials are contaminated with foreign substances and whether the same cleaning materials are used. Check the exhaust condition of the mold.

Once again, check the operating machine: use dye colored molding materials (excluding black). After about 20 minutes, turn off the injection molding machine, remove the nozzle and connection seat, and if possible, remove them together with the screw. Check if there are any burnt particles in the raw materials, compare the colors of the raw materials, and quickly identify the source of mold scale.

Surprisingly, the cause of mold scale defects has been found in many cases. This technology is most suitable for smaller injection molding machines, with a maximum screw diameter of 40mm. The elimination of mold scale also significantly improves the quality of the parts when forming other raw materials. The above countermeasures are also applicable to molding with Hot runner system.

Mold scale can cause appearance defects in injection molded parts, especially those with surface corrosion marks. The surface of the parts can be repaired by sandblasting with a sandblasting machine.

3. Maintenance of molds

When all the above measures cannot eliminate mold scale, it is necessary to strengthen the maintenance and upkeep of the mold.

The initial stage of mold surface scale formation is relatively easy to remove, so mold cavities and exhaust channels must be regularly cleaned and maintained (such as after each batch of molding production). It is very difficult and time-consuming to remove thick layers of mold scale after long-term maintenance and upkeep of the mold.

The sprays used for the maintenance of Injection mold construction mainly include: release agent, rust inhibitor, thimble oil, glue stain remover, mold cleaner, etc.

The chemical composition of mold scale is very complex, and new methods must be used and tried to remove it, such as general solvents and various special solvents, oven spray, caffeinated Lemonade, etc. Another unique way is to use rubber for cleaning model tracks.

Exhaust clearance of engineering plastics Injection mold construction

4. Suggestions for preventing mold fouling

When Hot runner molding and heat sensitive raw materials are used, the residence time of the melt will become longer, thus increasing the risk of raw material decomposition to form mold scale. Clean the screw of the injection molding machine.

When forming shear sensitive raw materials, larger runners and gates are used. Multiple gates can reduce flow distance, lower injection speed, and reduce the risk of mold fouling.

Efficient mold exhaust can reduce the possibility of mold fouling, and appropriate mold exhaust should be set up during the mold design phase. Automatic cleaning of the exhaust system or an exhaust method that can easily remove mold scale is the best choice, and the improvement of the exhaust system often leads to a reduction in mold scale on the mold.

Covering the surface of the mold cavity with a special non stick coating can prevent the formation of mold scale, and testing should be conducted to evaluate the effectiveness of the coating.

Titanium nitride treatment on the inner surface of the mold can avoid the formation of mold scale on the mold.