Surface Modification of Additively Manufactured Parts for End-Use Surgical Instruments
Additive manufacturing (AM) is a fast developing industry, with the aerospace, automotive andmedical industries readily embracing the new technology. AM provides numerous benefits for designand production, however, one of the major problems preventing metal AM from overtakingconventional machining and casting processes is the high surface roughness of AM parts. In the medicalindustry, additive manufacturing is revolutionising medical devices with custom implant designs, andthere is similar potential for complex surgical tools. These specialized parts require well finishedsurfaces that are able to be easily cleaned to prevent the spread of infection as well as beingaesthetically pleasing or functional. In this thesis mass finishing and post-processing of additivelymanufactured parts were examined with the aim of producing end-use surgical instruments viaadditive manufacturing.Using the selective laser melting method, designed 17-4 PH stainless steel samples with various surfaceorientations and types were printed for investigating various surface finishing methods. 2D stylusprofilometry was used to quantify the surface roughness and a scanning electron microscope was usedto observe the sample surfaces. Energy dispersive X-ray spectroscopy was used to measure the surfaceelemental composition to investigate the contamination of the surfaces throughout post-processing.The initial surface roughness of the samples was very high (Ra=13.5±2.0μm, Wa=4.0±1.5μm,Pv=103.8±17.7μm: inclined surface).Chemical polishing methods using hydrochloric acid and hydrofluoric + nitric acid solutions were shownto be relatively ineffective at significantly reducing the surface roughness. Mechanical mass finishingprocesses, abrasive blasting and centrifugal disc finishing, were also investigated. Of the abrasiveblasting processes white oxide vapour blasting produced the smoothest surfaces (Ra=2.1±0.4μm,Wa=3.2±1.1μm, Pv=25.3±4.8μm: inclined surface), but still not comparable to machined surfaces.Centrifugal disc finishing with ceramic media reduced roughness significantly, but the external radiisignificantly increased congruently and internal surfaces were unaffected by this process. After whiteoxide blasting, centrifugal finishing for 4 hours and performing a final glass bead blast, the smoothestsurface was obtained (Ra=0.6±0.1μm, Wa=0.9±0.3μm, Pv=6.9±1.5μm: inclined surface). The order ofthese operations was also of significance as white oxide blasting after centrifugal finishing resulting inrougher surfaces. Contamination with aluminium oxide particles from white oxide blasting was able tobe removed by glass bead blasting and then using a citric acid passivation to reduce the glass particlecontamination. Wire electric discharge machining (a common process to remove AM parts from thebuild platform) of wrought Ti6Al4V and 17-4 PH stainless steel showed high amounts of copper andzinc present on the surface. Removal of these contaminants was attempted using acidic solutions.Titanium wire-cut surfaces responded only to a hydrofluoric and nitric acid solution. However, forstainless steel wire-cut surfaces, citric acid was found to reduce the levels appropriately, buthydrofluoric acid also outperformed citric acid by completely removing the contaminants.A process was determined to produce end-use surgical instruments. After printing, the parts should beremoved from the build plate via wire electric discharge machining. The supports should then bebroken and the surfaces with scaffold support attached should be machined/linished to flatten thisextremely rough surface. White oxide vapour blasting then centrifugal finishing should be used to cutdown the remaining surfaces before the parts are heat treated. After heat treatment criticallydimensioned surfaces should be machined and then the part should be glass bead blasted to removethe oxide scale and provide the final finish. A citric acid cleaning procedure then passivates the surfaceand reduces surface contaminants. When designing and manufacturing a part in this way, the processshould be adapted to the key specifications of the part and its surface.
Advisor: Woodfield, Tim; Ali, Azam
Degree Name: Master of Science
Degree Discipline: Department of Orthopaedic Surgery and Muscoskeletal Medicine
Publisher: University of Otago
Keywords: Additive Manufacturing; Surface Finishing; 3D Printing; Surgical Instruments; Surface Cleaning; Passivation; Chemical Polishing; Abrasive Blasting; Centrifugal Finishing
Research Type: Thesis