Does mold fouling occur after engineering plastic injection molding?
Mold fouling occurs during injection molding of almost all thermoplastics. When the functional requirements of the final product must be mixed with 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, resulting in formation of mold scale.
There are other reasons for the formation of mold scale. The most common reasons are as follows:
- ▶ Thermal decomposition products of raw materials;
- ▶During Injection Molding, the melt flows under extreme shear forces;
- ▶ Improper venting; the mold scale caused by the above is often a combination of different factors, and it is very troublesome to find out what causes the mold scale and how to prevent it, and the mold scale will only form after a few days.
(1) Various additives produce specific types of mold scale. Fire retardants can react at high temperatures to form products that decompose and potentially produce mold deposits. Under the influence of excessive high temperature or extreme shear force, the anti-impact agent will separate from the polymer and remain on the surface of the mold cavity to form mold deposits.
(2) The pigments in thermoplastic engineering plastics melt at high temperatures, which will reduce the thermal stability of the molding raw materials, resulting in the combination of degraded polymers and decomposed pigments to form mold scale.
(3) For particularly hot parts on the mold (such as the mold core), modifiers/stabilizers and other additives may adhere to their surfaces and cause mold fouling. In this case, measures must be taken to achieve better mold temperature control or use special stabilizers.
If mold fouling suddenly occurs, it may be due to changes in molding conditions or replacement of different batches of molding raw materials. The following suggestions can help improve mold deposits.
In many cases surprising causes of mold deposit defects have been discovered. This technology is best suited for smaller injection molding machines, up to 40mm screw diameter. The elimination of mold scale also significantly improves the quality of parts when molding other raw materials. The above countermeasures are also applicable to molding with hot runner systems.
Mold scale causes appearance defects of Injection Molded parts, especially parts with surface etching. The surface of the parts can be repaired by sandblasting with a sandblasting machine.
When the mold scale can't be eliminated using all the above measures, the maintenance and care of the mold must be strengthened.
It is easier to remove mold scale in the early stages of mold surface formation, so the mold cavity and exhaust channels must be cleaned and maintained regularly (for example, after each batch of molding production). If the mold isn't maintained and maintained for a long time, it will be very difficult and time-consuming to remove the thick layer of mold scale formed on the mold.
The sprays used in the care and maintenance of injection molds mainly include: release agents, rust inhibitors, ejector oil, glue stain removers, mold cleaning agents, 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, caffeinated lemonade, etc. An alternative and fancy way is to use rubber to clean model tracks.
When using hot runner molding and heat-sensitive raw materials, the melt residence time will be longer, thereby increasing the risk of raw material decomposition to form mold deposits. Clean the injection molding machine screw.
When molding shear-sensitive raw materials, use larger runners and gates. Multi-point gates can reduce the flow distance, lower the injection speed, and reduce the risk of mold scale formation.
Efficient mold venting can reduce the possibility of mold scale formation, and set appropriate mold venting during the mold design stage. An automatic cleaning exhaust system, or an exhaust method that can easily remove mold scale is the best choice. Improvements in the exhaust system often lead 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. Tests should be conducted to evaluate the effectiveness of the coating.
Titanium nitride treatment of the inner surface of the mold avoids the formation of mold scale on the mold.