In an effort to make microfluidic research more attractive and cost-effective, micromilled polymethyl methacrylate (PMMA) has gained interests as an alternative method to the conventional cleanroom-based micromolds fabrication technologies. The most enabling aspects of micromilling are flexibility on the design changes and the ability to fabricate three-dimensional structures. However, the major drawback of micromilling based micromold fabrication is the presence of burrs and tool marks on the surface after machining. High surface roughness on replicated polymer results in poor bonding strength and optical clarity. The roughness of micromilled surface strongly depends on the machining parameters such as tool size, spindle speed, feed rate, width of cut, and depth of cut. Thus, it is crucial to optimize the machining parameters to obtain a good surface finish. Although the optimal fabrication parameters are used to machine the micromold, the surface roughness of micromilled mold is still relative high compared to the surface of unprocessed PMMA.
In this paper, we first optimize the micromilling parameters of Computer Numerical Control (CNC) milling machine to achieve the best possible of surface roughness. We have optimized the machining parameters for a flat endmill with 100 μm, 200 μm, and 400 μm in diameter of spindle speed, feed rate, width of cut, and the depth of cut respectively at 18000 rpm, 20 mm/min, 30 μm, and 20 μm. Then, a method to polish the structured surface of the micromilled mold was developed using the rotary magnetic field. By modifying the CNC program language G-code, we were able to control the polishing path, polishing force and time precisely. Consequently, the burrs and tool marks are completely removed, such that the roughness of the surface is decreased from 350 nm Ra to 30 nm Ra, and 1200 nm Rz to 300 nm Rz while the profile of microstructures is not deteriorated. Finally, we demonstrate our mold fabrication scheme by building a microfluidic immunoassay device with four Quake’s valves and showed the sequential assay process successfully.