Designing for Post-Processing: A Case Study of a 3D Printed Surgical Instrument

Abstract

In this report a design is proposed for a laparoscopic gripper that can be manufactured with metal 3D printing and polished with mass finishing. The design is a continuation of the development of a laparoscopic gripper that can be 3D printed in plastic. Laparoscopic grippers 3D printed solely out of metal have not yet been presented. Laparoscopic instruments are limited in width to 5 mm, which is bordering the manufacturing limits of selective laser melting 3D printing. The use of 3D printing for medical instruments has the potential to customise instruments specific to patient, procedure, and surgeon. Metal 3D printing can produce complex parts, albeit with a high surface roughness. Post-processing is required to reduce the surface roughness. Mass finishing techniques are a group of mechanical polishing techniques, of which centrifugal disc finishing was selected due to its capability to process parts in bulk without requiring workpiece fixation. To synthesise a suitable design, the processes of printing and polishing were analysed to formulate design guidelines. The analyses were part literature study, part experimental study. The experimental study had the aim to quantify and supplement the guidelines found in handbooks and articles. Using a novel visualisation technique, the polishing of different geometries could be distilled into quantitative design considerations. Here, a marking lacquer was applied to the surface of workpieces, which remained on unpolished surfaces. In this experiment a number of features were used, which corresponded to aspects that had potential to be used in the design. The use of channels was deemed unviable for polishing at the scale of laparoscopic instruments, which required the removal of these from the design. Mass finishing polishing removed the coarse surface structure present on metal produced with 3D printing, and brought surfaces of the test pieces to 0.05 mm below their desired width. Application of the design guidelines to the laparoscopic instrument was focused on making printing and polishing compatible joints. The laparoscopic gripper has two degrees of freedom for increased manoeuvrability. The features that comprise the joint are protrusions and cut-outs, sinusoidal gear arches, and actuation cable guides. Each of these features were dimensioned with values from the guidelines. The joint design required a number of components to be split so polishing access could be guaranteed, specifically for the cable guides. This had the added benefit of having each part be orientated during printing individually. The final design is based on application of the relevant design guidelines, and has been validated using scale models for mechanical stability.