Precision tools Elaborate machining concept for microfluidic sample carrier mould

Highly complex contour areas required the smallest cutter diameters with 5-µm accuracy on 384 measuring fields, and extremely long machine runtimes: The Spe-Chip microfluidic sample carrier mould was in a league of its own. The toolmaking department at Erwin Quarder Systemtechnik in Espelkamp in Germany's Eastern Westphalia managed the project — with support from Moldino.

Spe-Chip, which sounds like a James Bond movie title is a challenging project in which Erwin Quarder Systemtechnik played a significant role in its toolmaking implementation. The roots of today's globally active group of companies with a total of 1,100 employees lay in toolmaking and were founded in 1971 by Erwin Quarder in Espelkamp in Germany's Eastern Westphalia. The group's core competencies are product development, injection moulding tool technology, the establishment of automated production, and the worldwide production of plastic hybrid components for automotive applications. This also includes MID technology (Molded Interconnect Devices) and cleanroom production. Toolmaking has been part of Erwin Quarder Systemtechnik, employing a staff of 530 at the Espelkamp site. The moulds are used both by Quarder itself and by third-party customers.

Spe-Chip is a disposable microfluidic sample carrier made of transparent plastic. The 96 sample channels, each of which has four measuring chambers, enable the automated measurement of particle concentrations in liquids down to molecular size. However, given that the Spe-Chip method is based on UV light absorption measurement, the optical properties of the sample carrier are critical. “Milling with a tolerance of plus 5 µm is one thing. But milling at 384 different points — now that was a real challenge for us,” says Jakob Kehler, CAM programmer in the tool shop at Erwin Quarder Systemtechnik, who is also in charge of the milling department. “Due to the complexity and dimension of the contour areas, we had to mill almost entirely with minimal diameters below 1 mm.”

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The mould was designed for the injection-compression moulding process. Specifically, this low-pressure process offers advantages for lighting applications. The cavity pressure is distributed more evenly here, minimising any tension in the plastic. With the design in place, the first step was determining how long the machining process would take. That is why a small test workpiece with only six sample channels plus the measuring fields was milled initially. On the one hand, to demonstrate whether it would work, and on the other hand, to extrapolate the overall machining time. The test was an immediate success thanks to the support of Moldino. That is because around two and a half years ago, shortly before the start of the Spe-Chip project, Erwin Quarder initiated a significant expansion of HSC machining, particularly the high-precision hard milling. This meant that a large proportion of die-sinking EDM was to be replaced with milling. Modlino provided intensive support during the transition period.

Coordination of complex processes

Projections showed that the Eastern Westphalians would need 200 to 300 hours for each half of the mould, in other words, an extremely long time. The company wanted to avoid tying up its machine capacity on this project for that long, so inquiries were made with production service providers. However, everyone who saw the component was immediately skeptical. After all, this would have meant that a machine would be continuously assigned to the project for up to two months. So the decision was made to manufacture the Spe-Chip mould in Espelkamp. This was done in blocks so the machine would also be available for other orders.

According to the Production50 manufacturing concept developed by Moldino, the first step was determining the basic strategy. Then the suitable milling cutters were selected in stages based on the machining progress. The EHHR milling cutter with polygon geometry, the EPDRF/EPDRE Torus milling cutters, the EPDBE, and the EPDBPE/EPDBEH ball nose end mills in the diameter ranges 0.2 mm to 1 mm were mainly used. What was important here was to precisely coordinate the complex machining sequences, the respective moulds, and the mostly very long runtimes. Here, the advantage is that a few different moulds follow one another.

For this reason, the Moldino process optimiser decided to start with small moulds from the outset. A total of three different diameters were used for roughing: The smallest diameter was 0.6 mm for finish roughing, a double-edged Torus milling cutter EPDRE with 4 mm effective length. To produce homogeneous allowances for finishing, the surfaces were first pre-finished.

Keeping added value in-house

An average roughness value of Ra 0.2 µm to 0.5 µm was specified for all 384 measured surfaces, i.e., glossy surfaces. Minimal protective surfaces were applied at these points. This provides a small additional allowance for the subsequent production of high-gloss surfaces. Several different procedures were tested beforehand using the reference workpiece. The best result was realized with the EHHR cutter with a diameter of 1 mm: The polygon milling cutter enabled excellent surfaces in flat areas. The trick was to set the tool at a minimum of 0.1 degrees to reach the gauge block. These glossy surfaces were milled at the very end with minimum lubrication.

Finally, the Eastern Westphalians point to the subject of sister moulds, which was crucial because of the 384 measuring fields: The Röders machine features a unique measuring device so that all moulds have the same length. This was the only way to maintain the 5-µm tolerances. A total of 710 hours were required to mill this complex mould. “Despite the many challenges, the Spe-Chip project was extremely successful, and without the tools and know-how from Moldino, we would never have managed it,” says Jakob Kehler, drawing a positive conclusion. “In the past, this kind of mould would have been vertically eroded with graphite, and the rough functional surfaces would then have been polished by hand at the cost of around 14,000 euros by a service provider. We would never have achieved the high precision we can attain today through milling.”

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