The host for the first SPRA meeting of the SPRA 2010-11 programme was the Department of Design, Manufacture and Engineering Management (DMEM) at the University of Strathclyde. David Cunningham, Technical Support Officer for the Rapid Prototyping and Manufacturing Facilities within DMEM, gave an overview of rapid prototyping before he and his colleague, Jim Fox, provided a tour of the facilities.
Most of the rapid prototyping techniques fall into the category of 'layered manufacture', in which a 3D CAD model is first converted to STL files as thin slices which are then used to control the different machines during the step by step build up of a solid model or prototype.
The oldest technique, stereolithography (SLA), uses a scanning laser to cure liquid resin in a bath to build up the solid model. SLA gives good accuracy and surface finish but is limited in the physical properties of the material.
Fused Deposition Modelling (FDM) in which a thermoplastic melt is extruded in a fine thread to build up the layers. This offers the opportunity to use different thermoplastics to give varying mechanical properties and colour.
The universal inkjet printing principle has been adapted to print a water-based binder onto a bed of powder, again building the model up layer by layer. This is a fast and inexpensive technique but prototypes have poor mechanical strength. However the cost of 3D inkjet printers is dropping rapidly.
The 3D inkjet prining technique has been modified in the Objet machine from Polyjet TM . The advantages include very thin layers (down to 0.016 mm) and excellent surface finish but what appealed most to the audience was the ability to modify the modulus of the material, even within one build operation, opening up opportunities to simulate soft-touch, overmoulding and multi-material mouldings. The downside of Objet is the cost of the raw materials.
David then went on to show examples of metal prototypes, produced by direct metal laser sintering (DMLS), electron beam melting (EBM) and laser engineered net shaping (LENS). These techniques appeal to the aerospace and automotive industries, where there are vast material savings compared to conventional metal removal techniques for prototypes and short run production.
The Strathclyde facility also has 3/4/5 axis milling and laser cutting for subtractive rapid prototyping. Vacuum casting, using layered manufactured masters and silicone rubber moulds, is attractive for producing up to 50 prototypes.
Layered manufacturing is now becoming a serious consideration for small production runs, with break even points as many as 5000 before conventional mass production techniques become more economic. It was stressed that components produced by rapid prototyping can reproduce the dimensional accuracy, surface finish and sometimes mechanical properties but they can fall down on other properties such as porosity, weathering, fire performance and ageing resistance.
During the tour of the facilities, not only did SPRA members and guests see many of the layered manufacturing techniques in operation but they also had demonstrations of laser measuring techniques for complex 3D objects to set up CAD models. The most impressive demonstration was stereographic projection which, in conjunction with polarised spectacles, allowed 3D viewing of CAD models, with the potential of interactive manipulation of parts, including assembly and removal.
Report by C Geddes, Hon. Secretary October 2010