Mold opening speed a further adjustment parameter
In addition to temperature, the mold opening speed also has an influence on the airflow. The airflows were investigated at opening speeds of 1100 mm/s and 220 mm/s. The tests showed that a slow movement of the mold mounting platen causes less turbulence than very fast opening of the mold. When, however, the extremes were tested, it was shown that excessively slow opening intensifies the air turbulence as the air between the mold halves heats up again in the long time of mold opening. In contrast to this, extremely fast opening can stabilize the airflow so that the mold and injection molded parts are constantly exposed to clean air. To depict these extreme speeds, mold opening times of 12 and 3 seconds were investigated. The optimal opening speed for the purposes of cleanroom reliability depends in each case on the manufacturing process and the mold. In practice, however, the flow effects cannot always be considered adequately when it comes to setting the opening speed. The medical technology sector is also subject to strong cost pressure and cycle time is a decisive factor in profitability.
The challenge of liquid silicone rubber
The previous experiments established an important baseline for further consideration of injection molding processes in cleanrooms. The objective of a second thesis was, building on the above, to develop approaches on how to ensure a high class of cleanroom at high mold temperatures . In order to be able to make assertions for extreme temperature conditions as well, further experiments were not conducted with thermoplastics, but with LSR (liquid silicone rubber). A special aspect of liquid silicone rubber is that the material is, unlike thermoplastics, cooled in the barrel, while significantly higher temperatures of 180°C prevail in the mold. Only at these high temperatures can LSR vulcanize and crosslink. In addition to the high mold temperatures, a further complicating factor is the fact that LSR outgasses during processing. At high temperatures silane is released, which can be seen as a cloud with the naked eye. These volatile components of the liquid silicone rubber are an additional contaminate to the cleanroom and in the course of production can quickly exceed the limit defined for the respective class of cleanroom. The cleanroom in the ENGEL technology center was set up in ISO Class 7 for the experiments for the thesis. Particle measurement after just a few cycles already showed an excessively high concentration of particles with a diameter of 0.5 µm.
A first approach to solve this problem consisted of encapsulating the mold area with an LMP in order to vaporize the silane cloud. Unlike normally, however, the clean air was not introduced into the mold area from above, but from below. The extraction downwards normal in cleanrooms up to this point in time was to be used to remove the silane particles. Although this experimental setup was unsuccessful, a lower concentration of particles was measured than in the previous measurement, even though it did not yet conform to the requirements of the cleanroom class ISO 7.
Simulation confirms empirical research
The idea of reversing the airflow was then implemented consistently in a second step. The clean air was not only passed from bottom to top, but the mist cloud was also sucked out of the mold area upwards (Figures 4 and 5). The thermal makes the mist cloud disperse and gain speed quickly.