Causes and countermeasures of mold fouling after engineering plastic injection

Time:2021-10-05 10:25:30 / Popularity: / Source:

1. Formation of mold scale

Mold fouling occurs during injection molding of almost all thermoplastics. When functional requirements of final product must be mixed with related additives (such as modifiers, fire retardants, etc.), these additives are likely to remain on the surface of mold cavity during molding process, leading to formation of mold scale.
Formation of mold scale 
There are other reasons for formation of mold scale. The most common reasons are as follows:
▶Thermal decomposition products of raw materials;
▶Extreme shear force of melt flow during injection molding;
▶Inappropriate exhaust;
Mold fouling caused by above is often a combination of different factors, it is very troublesome to find out what causes mold fouling and how to prevent it, and mold fouling will only form after a few days.

2. Types of mold scale

1) Various additives produce specific types of mold deposits. Fire retardant will react at high temperature to form decomposition and possibly produce mold scale products. Under influence of excessive high temperature or extreme shearing force, anti-impact agent will separate from polymer and remain on the surface of mold cavity to form mold scale.
2) Pigment in thermoplastic engineering plastic melts at high temperature, which will reduce thermal stability of molding material, resulting in combination of degraded polymer and decomposed pigment to form mold scale.
3) Particularly hot parts on mold (such as mold core), modifiers/stabilizers and other additives may stick to surface and cause mold fouling. In this case, measures must be taken to achieve better mold temperature control or use special stabilizers.

3. Countermeasures for sudden mold fouling

If mold fouling occurs suddenly, it may be due to changes in molding conditions, or due to replacement of different batches of molding materials. Following suggestions can help improve mold fouling.
First, measure temperature of melt and visually check whether melt is decomposed (such as burnt particles). At the same time, check whether molding materials are contaminated by foreign substances and whether same cleaning materials are used. Check exhaust condition of mold.
Again, check operating machine: use dye-colored molding materials (except for black), turn off injection molding machine after about 20 minutes, remove nozzle and connector, if possible, remove it with screw, check whether raw material has burnt particles, compare color of raw material, and quickly find 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. Elimination of mold fouling also clearly improves quality of parts when molding other raw materials. Above countermeasures are also suitable for molding with hot runner systems.
Mold fouling leads to defects in appearance of injection molded parts, especially parts with surface erosion, which can be repaired by sandblasting with sandblasting machines.

4. Mold maintenance

When mold fouling cannot be eliminated by using all above measures, maintenance of mold must be strengthened.
Mold deposits on mold surface are easier to remove at initial stage, so mold cavities and exhaust channels must be cleaned and maintained regularly (for example, after each batch of molding production). If mold is not maintained for a long time, it is very difficult and time-consuming to remove mold scale after mold has formed a thick layer.
Sprays used for maintenance of injection molds mainly include: release agent, anti-rust agent, thimble oil, glue stain remover, mold cleaning agent, etc.
Chemical composition of mold scale is very complex, 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, melt residence time will be longer, thereby increasing risk of raw materials decomposing and forming mold scale. Clean screw of injection molding machine.
Larger runners and gates are used when forming shear-sensitive materials. Multi-point gates can reduce flow distance, lower injection speed, and reduce risk of mold fouling.
Efficient mold exhaust can reduce possibility of mold fouling, and proper mold exhaust should be set in mold design stage. An automatic exhaust system or an exhaust method that can easily remove mold deposits is the best choice. Improvement of exhaust system often results in a reduction of mold deposits on mold.
Cover surface of mold cavity with a special non-stick coating to prevent formation of mold deposits. Tests should be conducted to evaluate effect of coating. Titanium nitride treatment on inner surface of mold can avoid formation of mold scale on mold.

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