Charles L. Thomas

Rapid tooling using SU-8 for injection molding microfluidic components

In this work we present an injection molding tool fabricated using current micromachining techniques. The process was used to fabricate micro fluidic channels in a plastic substrate with depths of approximately 27 micrometers . The process can easily be altered to form channels of varying depths ranging from a few microns to approximately 100 micrometers . The tool was made using a photosensitive epoxy (SU-8) on silicon. Complex two-dimensional micro channel / chamber shapes and intersections are also achievable because the process for defining SU-8 is lithography. Two polymers were successfully used for injection molding the channels, clear rigid polycarbonate and opaque flexible polypropylene. Plastic replications of the inverse pattern of the SU-8 tool were made with the described process. Fabrication time of the tool was approximately 30 minutes and survived without failure, 22 shots for polypropylene and eight shots for polycarbonate (Lexan). Some deformities of the channels were observed and were more pronounced in the PP channels. Channel height was increased by 2-3 micrometers due to a ridge that was formed due to shear forces generated during the release stage of the process. The channel width shrunk approximately 7.9% maximum for polypropylene after release from the mold.

Solidification sensing for closed loop control of injection molding hold time

Hold time for injection molding is currently controlled open loop, using a fixed time delay set by the machine operator. Insufficient hold time leads to inconsistent moldings, while excessive hold time increases the cost of the part. We have developed new sensors that monitor the solidification of polymer in the mold. A closed loop strategy is demonstrated where the hold time is automatically controlled based on feedback from a solidification sensor. Using this strategy, the hold time is set once in reference to the sensor signal, providing the minimum proper hold time for each part under changing processing conditions.

Computer-aided design/computer-aided manufacturing skull base drill

The authors have developed a simple device for computer-aided design/computer-aided manufacturing (CAD-CAM) that uses an image-guided system to define a cutting tool path that is shared with a surgical machining system for drilling bone. Information from 2D images (obtained via CT and MRI) is transmitted to a processor that produces a 3D image. The processor generates code defining an optimized cutting tool path, which is sent to a surgical machining system that can drill the desired portion of bone. This tool has applications for bone removal in both cranial and spine neurosurgical approaches. Such applications have the potential to reduce surgical time and associated complications such as infection or blood loss. The device enables rapid removal of bone within 1 mm of vital structures. The validity of such a machining tool is exemplified in the rapid (< 3 minutes machining time) and accurate removal of bone for transtemporal (for example, translabyrinthine) approaches.