3D Printing - Printing Techniques
The range of materials that can now be 3D printed include ABS, polypropylene, clear acrylic in the plastics category, metals -steel, including stainless, titanium, sliver and gold, composites - ceramic and glass, plus rubber, paper, sugar, vegetable starch, wax, sand, foods, chocolate, human tissue....
Some materials are designed for a specific purpose such as a clean burn-out for the ‘lost-wax’ casting process, for dissolvable support material, transparency, biodegradability, flexibility, toughness, low cost, fine definition, whiteness, and some qualities essential for specific sectors: hyper-allergenic titanium for dentistry and prosthetics.
Print resolution can be varied from fine to course and can depend on sheet thickness and grain size. These also determine how fine and how strong the parts can be, cost and print time. If you want a colour there is a limited selection but if you want different bits of your model in different colours you currently only have one material option. And if you want multiple materials in the same print this is now possible. Methods for layering and bonding are tied into the materials being 3D printed and as the original patents run out the range will grow and choice widen. Selecting which one to use is set to get more complex.
The term 3D printing only came into common use recently and to be accurate and pernickety should only be applied to one of the processes described below. The accepted term in Industry is additive manufacture which covers all the processes below and distinguishes this method from subtractive manufacture (milling and turning). The earlier and now overall term for both is rapid prototyping.
Stereolithography (SLA) is the oldest 3D printing system and developed by Chuck W. Hull (co-founder of 3D Systems) in 1986. SLA uses a vat of liquid photo-sensitive “resin” and exposure to the UV laser light solidifies a profile in the ‘x,y, axes in the thin layer of resin flooding a platform which moves down in fine incremental units in the ‘z’ axis. Parts created this way are very accurate, require minimal post processing and are ideal for use as master patterns for vacuum casting.
PolyJet inkjet technology also works by using photo sensitive materials. These are deposited in the slice profile and immediately set and bonded to previous layers by exposure to UV light. It was developed by Objet Geometries in early 2000.
The ‘extrusion’ process (FDM - Fused Deposition Modelling) uses filaments of a material such as wax or Polycarbonate and ABS plastic blend which is deposited as a molten stream through a nozzle onto a base. The nozzle moves upwards, while also building support structures for the overhanging parts of the model. The support material is either dissolvable or easy to break off. This system is used to create functional parts of any geometry and was developed by S. Scott Crump (Stratasys co-founder) in the late 1980s and commercialized in 1990. Most DIY 3D Printers use the FDM technology.
For the ‘binding’ process powdered grains are spread thinly by a roller over the floor of the “build chamber” over which a mechanical arm rapidly moves to either fuse, glue or melt one layer of grains together in the x,y slice’s profile. Another thin layer is then rolled over the top of that (the ‘z’ axis) and so on and upwards to create the model. The excess un-bonded powder is vacuumed off and the model carefully removed for further cleaning and addition of extra bonding material to stabilise the piece to make it more robust.
‘3D Printing’ uses an ‘inkjet printing’ process to deposit a liquid binder and is a reasonably accurate term to describe this system which was developed at the Massachusetts Institute of Technology (MIT) in the late 1980s and licensed by several companies including ZCorporation. With this system full colour, multi-colour robust objects can be created.v Another ‘Wax Printing’ process is similar, with wax jetted into a profile in layers to build objects which can then be used for investment casting by coating the wax with investment plaster, burning out the wax and casting in metal.
The ‘deposition’ process is closer to an inkjet printer technique as a print head moves across the build chamber depositing the material in each of the model’s x,y profile slices. It is the build chamber’s floor that lowers incrementally to create the subsequent z axis layers to build up the object.
The ‘profiling’ process uses sheets of paper, plastic inserted one at a time into the build chamber to be cut and bonded to the previous layer to build the object which is broken out of the unbonded or sliced waste material. As paper printers produce the least expensive models they are used to prototype large forms.
The ‘sintering’ process (SLS – selective Laser sintering) has the greater range of materials as many can be granulated and laser sintered together to also fuse to the previous layer. This process which uses high powder laser was developed and patented by Dr. Carl Deckard at the University of Texas in the mid-1980s, The most common and useful material used is nylon powder and apparently adding ground glasss gives even greater strength for performance and durability. Other materials include ceramic, glass powder and metals (direct metal laser sintering) including steel, titanium, silver and gold,. These developments are very exciting for designer makers but as yet hard to justify price-wise.
The process of ‘Electron Beam Melting’ (EBM) melts metal powder in a high vacuum and is distinguished from metal sintering techniques, by producing parts that are extremely strong because they are fully dense and free of any voids. It was developed by Arcam AB.
Kits and low cost 3D printers are a huge growth sector as the instructions are open source and anyone can set up a group or company to make and replicate them. This viral DIY base is the principle behind RepRap and Fab@Home and most of these systems used the extrusion method to build the model. Although there are major challenges still to be overcome there are many sharing the work to develop and solve issues. One factor limiting the design of objects for these printers that is being sorted is the mechanism to also build structures to support the overhanging parts of the model. If the 3D printer does not include a means to print support structures you need to know how much of an overhang angle it can handle and whether your model exceeds that angle and is therefore unprintable. With so many groups working in opensource mode to improve the quality and usability this is happening fast. You too can build or buy one to sit beside your laptop and print off your models.