At present, many engineering plastics have been successfully applied to mechanical parts and have achieved good results. However, the difficulty of injection molding of engineering plastics is relatively large, especially mold fouling is prone to occur. What is the cause of mold fouling after injection molding of engineering plastics, and how to solve it?
1. Formation of mold scale
Die fouling refers to a phenomenon in which attachments are deposited on the surface of the mold (see the figure below). Mold fouling occurs during injection molding of almost all thermoplastics. When the functional requirements of the final product must be mixed with related 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 reasons are as follows:
▶The thermal decomposition products of raw materials;
▶Extreme shear force of melt flow during injection molding;
The mold fouling caused by the above is often a combination of different factors, and it is very troublesome to find out what causes the mold fouling and how to prevent it, and the mold fouling will only form after a few days.
2. Types of mold scale
1) Various additives produce specific types of mold deposits. The fire retardant will react at high temperature to form decomposition and possibly produce mold scale products. Under the influence of excessive high temperature or extreme shearing force, the anti-impact agent will separate from the polymer and remain on the surface of the mold cavity to form mold scale.
2) The pigment in the thermoplastic engineering plastic melts at high temperature, which will reduce the thermal stability of the molding material, resulting in the combination of degraded polymer and decomposed pigment to form mold scale.
3) Particularly hot parts on the mold (such as mold core), modifiers/stabilizers and other additives may stick to the surface and cause mold fouling. In this case, measures must be taken to achieve better mold temperature control or use special stabilizers.
The following table lists the possible causes of mold fouling and preventive measures:
Three, the sudden occurrence of mold fouling countermeasures
If mold fouling occurs suddenly, it may be due to changes in molding conditions, or due to the replacement of different batches of molding materials. The following suggestions can help improve mold fouling.
First, measure the temperature of the melt and visually check whether the melt is decomposed (such as burnt particles). At the same time, check whether the molding materials are contaminated by foreign substances and whether the same cleaning materials are used. Check the exhaust condition of the mold.
Again, check the operating machine: use dye-colored molding materials (except for black), turn off the injection molding machine after about 20 minutes, remove the nozzle and connector, if possible, remove it with the screw, check whether the raw material has burnt particles, and proceed with the raw material Color comparison, quickly find out the source of mold fouling.
Surprising causes of mold fouling defects have 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 fouling also clearly improves the quality of parts when molding other raw materials. The above countermeasures are also suitable for molding with hot runner systems.
Mold fouling leads to defects in the appearance of injection molded parts, especially parts with surface erosion, which can be repaired by sandblasting with sandblasting machines.
Four, mold maintenance
When the mold fouling cannot be eliminated by using all the above measures, the maintenance and maintenance of the mold must be strengthened.
Mold deposits on the mold surface are easier to remove at the initial stage, so mold cavities and exhaust channels must be cleaned and maintained regularly (for example, after each batch of molding production). If the mold is not maintained and maintained for a long time, it is very difficult and time-consuming to remove the mold scale after the mold has formed a thick layer.
The sprays used for the maintenance and maintenance of injection molds mainly include: release agent, anti-rust agent, thimble oil, glue stain remover, mold cleaning agent, 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 sprays, and caffeinated lemonade. Another peculiar way is to use rubber for cleaning model tracks.
5. Suggestions to prevent mold fouling
When using hot runner molding and heat-sensitive raw materials, the melt residence time will be longer, thereby increasing the risk of raw materials decomposing and forming mold scale. Clean the screw of the injection molding machine.
Larger runners and gates are used when forming shear-sensitive materials. Multi-point gates can reduce the flow distance, lower the injection speed, and reduce the risk of mold fouling.
Efficient mold exhaust can reduce the possibility of mold fouling. Set appropriate mold exhaust during the mold design stage. An automatic exhaust system or an exhaust method that can easily remove mold deposits is the best choice. The improvement of the exhaust system often results in a reduction of mold deposits on the mold.
Cover the surface of the mold cavity with a special non-stick coating to prevent the formation of mold deposits. Tests should be conducted to evaluate the effect of the coating.
Titanium nitride treatment on the inner surface of the mold can avoid the formation of mold scale on the mold.
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