Mechanisms of deposit formation in injection moulding cavities and the role of tool coatings and internal release agents

A major challenge in the injection moulding industry is the formation of tool build-up and the formation of deposits. Current solutions include the use of hard coatings paired with low surface energy finishes, e.g. sol-gel coats or PTFE-based polishes, so that there is a need for more durable, scratch-resistant and PTFE-free coating solutions. Aside this technological deficit, there is also a deep lack of knowledge about the functioning principles of tool coatings and their influence on the formation of deposits.
We therefore introduced low surface energy coatings deposited by plasma-enhanced physical vapour deposition (PE-CVD), which enable to tailor hardness, Young's modulus and surface energy in a wide range.
The interactions between such coatings and molten polycarbonate (PC) were analysed to test their stability against the harsh conditions of injection moulding and to reveal the fundamental mechanisms of deposit formation. We found that interfacial chemical reactions cause deposit formation in the case of the bare steel, which were avoided by the PE-CVD coatings in presence of the internal release agents in Makrolon 2405.The PE-CVD coatings therefore shifted the interactions into a regime where deposits were formed by wetting instead.
The synergism of internal release agents and PE-CVD coatings was studied in a model system consisting of polycarbonate and pentaerythritol tetrastearate (PETS) with varying degree of esterification (hydroxyl value). The results disclosed that the role of the release agent is limited to interact predominantly with the polymer surface and to decrease its surface energy in presence of the PE-CVD coating, because of the lower surface energy of the latter.
However, the formation of a low-molecular weight weak boundary layer could not be prevented by the PE-CVD coating. This motivated to discuss the thermodynamical foundations of the observed wetting of the deposits using basic wetting theory. It was highlighted that matching the surface energy of a coating to the base polymer in a compound removes uncompensated interfacial energies so that deposits stay formally dispersed. This notion correlated with the observed reduction of deposits by the PE-CVD coatings and internal release agents and provides therefore an explanation of their functioning. Finally, we discussed the concept that an ideal surface technological solution shall decrease the interfacial energies to avoid deposits but may also decrease the individual surface energies to lower adhesion.