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Origins, introduction to and my perspective of the FreeForm™ modelling system. Written in 2002
Context - Realization
RP additive systems
Adding material
Self supporting
Additional support
RP subtractive systems
CNC Machining - Manufacture and multiple copies
Adding Material
In principal this is where material is added to create the form intended, taking a model and creating horizontal slices systematically achieves this. Each slice is methodically placed, firstly to a base
plate and then added to the previous layer until the file is completed. The piling up of the slices gives the form its height whereas the slices give the form its length and breadth. This method is
generally referred to as 3D printing.
Materials include: - paper, powder, resin, wax, plastic etc. Material growth is due to extruding, gluing, fusing, combining or sintering.
Additive processes have a table, or bed on which the work is placed. It is the bed that moves down that allows for the growth of the model. The printing activity is usually positioned and anchored to give
the maximum amount of accuracy. The printing has two axes to control and the bed one.
The disadvantage of this process is that because it is layered the surface of a curved model can show contours giving a wood grain effect.
Material properties vary and some need special help while they cure or set. In principal this means that some materials will need supporting during production and others will be self-supporting. The
self-supporting ones are faster and more economic but in general are less accurate.
The processes that need external support can produce accurate detailed models, but where the model needs support, it can leave witness marks, or texture on the surface of the model. These systems can be
very good for making front and back models, where the detail is uppermost and a model is assembled after being made.
Self-supporting
Paper
The LOM process uses rolls of paper to create the layers, a cutter cuts out the cross section, during the 'printing' operation the excess, or waste paper is cut into small sections making it easier
to remove on completion of the printing. Each layer is glued to the previous creating a solid model that can be sanded and worked on. Large models can also be made this way.
Plaster and Starch
The Zcorp
system uses powder as the layer and glues the particles together when printing the cross section. This system is very fast but generally gives a grainy texture to the surface; the surface can be worked on and lightly smoothed. There are two types of powder that this machine uses, the first is starch based and the second is a much finer plaster based material. The plaster-based material is the most commonly used and has the additional capability of being printed with colour.
The starch material can be used as a direct former in metal casting and can be burnt out during the pouring process. This technique was used to create the joints on Anthony Gormley's sculpture situated
outside the Millennium Dome in London.
The plaster material can be used to cast from, to make case moulds. Special glue can be applied to this material to make it stronger and less porous.
This 3D printing system is self-supporting and as such creates a model quickly and is uniform in surface texture.
SLS Selective Laser Sintering
This process uses powder, a carbon dioxide laser beam scans over the powder to the shape of the model's cross section and heats the grains so that they melt on the outside and stick together (sinter). The
base plate moves down slightly, and the next layer of powder is spread across the surface by a rotating roller. The powder supports the object as it is made. (Similar system to the Zcorp above) At the
end of the build process, the entire cake of powder, sintered and un-sintered, is allowed to cool down and lifted out of the machine revealing the sintered model. Accuracy is very good for this system
Objects made by SLS using nylon powder have a powdery white appearance. The surface is absorbent and marks easily, but otherwise the objects are fairly robust. SLS models can be made in wax, sand, or
steel powder as well as the most commonly used nylon.
Additional support
Wax
3D Systems Thermojet
Printer uses wax as the forming material, which is printed by heating wax and depositing it onto the cross section. With this system a support structure is created by the software and added to the process, it works and appears like scaffolding. This supporting material can be removed after the work has been made, but does leave a witness mark.
Thermoplastics
FDM Fused Deposit Modelling
Here the object is made by passing a continuous thread of material through a narrow, heated nozzle that is moved over the base plate. As the thread passes through the nozzle it melts, only to harden again
immediately as it touches (and sticks to) the layer below. For certain shapes, a support structure is needed, and a second nozzle provides this. At the end of the build process, the support structure is
broken away and discarded, freeing the object. FDM method produces models that are physically robust. Wax can be used as the thread material, but generally models are made of ABS plastic. Models may have
a fairly rough surface finish when taken out of the machine, but they can easily be cleaned up. ABS fibre is available in a range of bright primary colours. Alternatively, models can be painted.
Photopolymers
SLA
Stereo Lithography uses a vat of liquid resin in which sits a moveable base plate. A laser is focussed on to the surface of the liquid and draws out the cross section. The laser light rapidly cures and solidifies the liquid forming the solid model. The base plate is lowered and the process continued until the model is complete. This system requires a support structure that is added to the model.
This system is accurate and produces a non-porous pattern. Layer thickness is 0.1mm SLA was the first process to become established commercially. Resin models are delicate and need to be cured and stored
very carefully in a dry atmosphere, if they are not to soften and distort. The resin is expensive; the cost of a model is reduced if it can be made hollow.
Objet
The Objet printer uses a multi jet head that moves back and forth over the build area to print a full cure photopolymer, firstly on to the build tray and subsequently the growing model and support
material. UV lighting cures each 20-micron layer before the next layer is applied. Support material is removed by water jet and the models are ready for immediate use. This process produces a good
surface, with little evidence of stepping due to the small layer thickness. It is also durable.
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