P. Koch¹*, O. Barth¹, M. Jehle¹, R. Streller¹, R. Zengerle^1,2 and M. Rombach^1
1Hahn-Schickard, Freiburg, 79110, Germany
²University of Freiburg, Laboratory for MEMS Applications, Freiburg, 79110, Germany
*Philip.Koch@hahn-schickard.de

Introduction
Microthermoforming is an enhanced variant of blister technology[1,2] adapted and improved at Hahn-Schickard to produce microfluidic cartridges e.g for diagnostic purposes.[3,4] These cartridges feature a great variation in aspect ratios ranging from microchannels (μm range, see Figure 1 d) to large chambers (mm range). Apart from material variations, geometric aspects are one of the major challenges in process automation. Here we present the adaption of the manual sheet-by-sheet process to a fully automated “roll-to-sheet” variothermal replication process to produce structured films as a low cost alternative to injection molding.

Process and Material
The thermoplastic polymer film is pulled from feeding roll in the process chamber (see Figure 1 a) over the metal tool (Figure 1 b), which subsequently is closed and evacuated followed by heating (130 - 165°C*) and application of air pressure (2 - 5 bar*) to pre-form the polymer film. Afterwards the final forming step is initialized by increasing the air pressure (≥ 7.5 bar*) and heating (≥ 180°C*) followed by cooling (to 50 - 80°C*) and air-pressure supported de-molding (≥ 1.5 bar*) as well as cutting of the structured polymer film into single sheets (see Figure 1 c). Overall cycle time ranges from 2 - 8 minutes / per part.

Conclusion and Outlook
This technology allows for fast development cycles and highly precise replication of small and medium numbers of parts at low cost (compared to injection molding) from various tools. Turn-around times from final design to the first thermoformed parts are possible within approx. 2 weeks with low tooling costs (< 6,000 €). This enables the scale up of functional prototype production e.g. for pre-clinical studies of diagnostic point-of-care applications.[5,6,7]

* depending on the polymer film type and thickness (e.g. COP (ZEONOR® films), PC (Makrofol®, Covestro AG) or custom-made COC 6013/8007 bilayer film (Tekni-Plex, Inc.))

Referenzen
[1] J. L. Throne; Technology of Thermoforming, 1996, Auflage: 1, 1-898
[2] P. Schwarzmann; Thermoformen in der Praxis, 2016, Auflage: 3, 1-519
[3] M. Focke et al; Lab Chip, 2010, 10, 1365-1386
[4] M. Focke et al; J. Micromech. Microeng. 2011, 21, 115002
[5] M. Rombach et al; Analyst, 2020, 145, 7040-7047
[6] M. Schulz et al; Lab Chip, 2020, 20, 2549-2561
[7] S. Hin et al; Processes 2020, 8, 1677