What to do if the adhesive mold of the injection mold is not

1. Formation of mold scale

Almost all thermoplastic injection molding processes experience mold fouling. 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

Extreme shear force in melt flow during injection molding

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

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.

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.

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.

3. 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 measures are also applicable to the forming of hot runner systems.

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.

4. 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 spray agents used for the maintenance and upkeep of injection molds mainly include release agents, rust inhibitors, thimble oil, glue stain removers, mold cleaning agents, etc.

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

5. Suggestions for preventing mold fouling

When using hot runner molding and heat sensitive raw materials, the residence time of the melt will become longer, thereby increasing the risk of raw material decomposition and formation of 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. Setting up appropriate mold exhaust during the mold design stage, automatically removing the exhaust system, or easily removing mold fouling is the best choice. The improvement of the exhaust system often leads to a reduction in mold fouling 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.