Upside-down thinking: Engel discusses reversed airflow for cleanrooms

Helene Schöngruber and Christoph Lhota, Engel Austria, outline a new cleanroom concept with reversed airflow for high temperature applications.

High temperatures are not wanted in cleanrooms, however, in the injection moulding process they are unavoidable. Research on the influence of mould temperature by laminar clean airflow underlines the significance of this subject and lays the foundation for a new cleanroom concept with reversed airflow. The first industrial installations promise much potential for even higher cleanroom quality.

For injection moulding of thermoplastics, the resin pellets are heated in the barrel until they have reached a viscous or liquid state and are then injected into the temperature-controlled mould. The temperature of the mould is a material-specific parameter with a direct influence on the process and in particular on the cycle time. The mould temperature also influences the airflow, which is relevant to the injection moulding process in cleanrooms. The hot air radiated by the mould rises and therefore moves in the opposite direction to the cleanroom airflow which conventionally runs from top to bottom (Figure 1). As the temperature rises, the particulate load increases which puts the cleanroom quality at risk. Uneven airflow in the mould area can cause particle deposits on the parts because there is not enough clean air coming in to remove the contamination.

Influence measurable at 40°C

A thesis investigated how effectual a conventional filter fan unit or laminar flow box was on mould temperature [1]. The experiments were carried out in Engel’s cleanroom. The LMP type laminar flow modules were provided by Max Petek Reinraumtechnik and were developed in the size used specifically for injection moulding machines.

Two system conditions were used for comparative purposes: One with normal cleanroom airflow, and the other with the mould and ejector area encapsulated with an LMP. In both series of experiments, mist was added from above to the mould area to make the airflow visible for both systems. Mould heats were also held constant across both experiments.

The simple experiment established that without an additional laminar flow module the mould temperature of 40°C influences the flow of clean air through the mould area. This outcome shows how important this research work is because such a low mould temperature can only be used for very few applications.

The LMP was used to achieve an even more constant flow from top to bottom. The air velocity was set on 0.45 m/s in accordance with the EU GMP Directive. The mist tests in this encapsulated configuration were recorded in a video. The still images clearly show that a constant airflow no longer prevails and turbulence occurs from a mould temperature of 90°C and above (Figure 2). The turbulence occurs primarily directly after opening of the mould; the airflow settles again after four seconds and the flow through the mould becomes constant again.

The same measurement was repeated at a mould temperature of 140°C. Here four seconds wasn’t enough to disperse the turbulence. At this high mould temperature, the complete air in the mould area is very hot, and particles are emitted in increased number. An adequate laminar flow can only be shown again at an increased air velocity of 0.8 m/s.

Mould opening speed a further adjustment parameter

In addition to temperature, the mould opening speed also has an influence on the airflow. The airflows were investigated at opening speeds of 1100 mm/s and 220 mm/s. Tests showed that a slow movement of the mould mounting platen causes less turbulence than very fast opening of the mould. When, however, the extremes were tested, it was shown that excessively slow opening intensifies the air turbulence as the air between the mould halves heats up again during the slow mould opening. In contrast, extremely fast opening can stabilise the airflow so that the mould and injection moulded parts are constantly exposed to clean air. To depict these extreme speeds, mould 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 mould. 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 moulding processes in cleanrooms. The objective of a second thesis­ - building on the above - was to develop approaches on how to ensure a high class of cleanroom at high mould temperatures [2]. To be able to make assertions for extreme temperature conditions as well, experiments were conducted with liquid silicone rubber (LSR), not thermoplastics. A special aspect of LSR is that the material is cooled in the barrel, while significantly higher temperatures of 180°C prevail in the mould. Only at these high temperatures can LSR vulcanise and crosslink. In addition to the high mould 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 LSR 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 centre was set up in ISO Class 7 for the experiments for the thesis. Particle measurement after just a few cycles 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 mould area with an LMP in order to vaporise the silane cloud. However, the clean air was not introduced into the mould area from above, but from below. A downwards extraction, which is normal in cleanrooms, 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 mould area upwards (Figures 3). The thermal makes the mist cloud disperse and gain speed quickly.

A simulation of the experimental setup was used to corroborate the test results using ANSYS software version R16.2 Academic. The calculations confirm the good result of the tests in the technology centre and make it possible to predict the behaviour when changes in the environmental conditions occur.

Max Petek Reinraumtechnik has developed a cleanroom solution with reverse laminar flow on the basis of these results. The air is sucked upwards out of the mould area.

First industrial system built

The results of the two theses show that the influence of mould temperature on reliable cleanroom operations cannot be neglected. The laminar flow is already disturbed at mould temperatures above 40°C. A mould temperature of 110°C was established as limit for a conventional clean airflow from top to bottom (without additional laminar flow module). Both empirical measurements and simulations prove that the particle load can be minimised by reversing the flow of clean air.

Engel and Max Petek Reinraumtechnik have already implemented the results of this study industrially. The new solution has the potential to become standard for high temperature applications.

References

1. Denisa Costas, Analysis of the impact of process temperatures on the cleanroom airflow during the injecting moulding of medical grade high performance thermoplastics, thesis for the degree course in Medical Engineering at the University of Applied Sciences Upper Austria, Linz, Austria, 2015

2. Helene Schöngruber, Identification and analysis of thermal flux in the cleanroom during liquid injection moulding, thesis for the degree course in Medical Engineering at the University of Applied Sciences Upper Austria, Linz, Austria 2016