Controlled depth micro-abrasive waterjet milling of aluminum oxide to fabricate micro-molds containing intersecting free-standing structures

Abstract

Traditionally, photolithography and soft-lithography methods have been used to fabricate micro-molds for casting polymeric microfluidic chips. Because of the time and cost involved, there has been motivation to fabricate microfluidic micro-molds using other techniques. A previous study utilized stainless steel masks cut using abrasive waterjet micro-machining (AWJM) together with abrasive slurry jet micro-machining (ASJM) to mill micro-molds in Al6061 which would allow casting of polymeric microfluidic chips with intersecting micro-channels. However, the deflected slurry jet from the mask edge struck the substrate at oblique incidence and led to two undesirable effects: a trench along the edges of the raised structures representing the channels, and an undercut of the mask. Both effects were linked to the propensity for ductile materials to erode more rapidly at oblique incidence than perpendicular. The present paper demonstrates that these effects can be greatly reduced by using AWJM to create molds in more brittle materials which erode in the opposite fashion, i.e., more rapidly at perpendicular than oblique incidence. To demonstrate this, AWJM was used to machine masks from aluminum oxide and subsequently mill micro-molds with raised intersecting free-standing structures into aluminum oxide substrates. It was found that for molds of identical depth, the undercut and the undesirable erosion in aluminum oxide molds could be reduced by five and four times, respectively, when compared to molds machined in Al6061 using the previous AWJM/ASJM hybrid technique. It was also found that much deeper molds containing intersecting raised free-standing structures of up to 435 μm in height could be made in aluminum oxide, while still maintaining an acceptable surface quality (1.51 μm Ra and 2.47 μm Wa), undercut (25 μm), and undesirable erosion (3%). Finally, since the technique involved only AWJM, it was much more convenient than the previously utilized hybrid AWJM/ASJM.