====== A synopsis of the FDM process ======

======What is FDM?======

FDM stands for Fused Deposition Modeling, a name invented in the late 80’s as the first rapid prototyping processes were being developed.  It consists of building a model additively layer by layer by depositing a thin filament of melted plastic via a precisely calibrated orifice mounted on a computer controlled XY movement.  The width of the deposited plastic line is fine, so the mass of the model is built up in many successive passes.

The model is created on a base which is also computer controlled to index downward one layer thickness as each layer is completed.  Each successive layer fuses to the preceding layer as the thermoplastic material is dispensed in a hot, semi-liquid state, then quickly cools down and solidifies.  The entire model consists of many fine parallel layers (as if it was a topographic map) in the vertical (Z) direction.

The resolution and precision of the process varies with the machine and the type of material employed.  The finest that can be had currently is a layer thickness of about 0.125mm, the “standard” layer thickness on less expensive machines is twice that at about 0.25mm.  The line width of each pass is approximately twice the layer thickness.  Precision is around +/- 0.2mm  overall for smaller parts.

The process, like virtually all other rapid prototyping processes, needs a 3D CAD model in the form of an STL file.  Virtually all 3D CAD programs are capable of producing these.  This model is then “sliced” horizontally to form the layers that correspond with the machine’s layer thickness, and the contour curves that are thus formed are followed by the machine head depositing the plastic, forming the outside surface.  The spaces between the outer surfaces (the thickness of the model) is then filled in with a series of parallel passes.  The machine then indexes to the next layer.

The materials that can be employed are thermoplastics, most commonly a compound called ABS (Acrylonitrile Butadeine Styrene), but PC (Polycarbonate), a PC/ABS blend, and Polyphenylsulfone are also available.  FDM can also be done with a wax material.

To allow hollow sections and overhanging parts, the process automatically generates supports.  These can either be special “break-away” style supports of the same material as the model, or in some cases, supports of another material.  Newest technology allows the ABS support material to be “water soluble”, allowing relatively easy removal (nevertheless requiring a heated ultrasonic tank, a mild chemical solution and some time).  It is also possible to create structures with a solid skin and a honeycomb interior, resulting in a lighter part with less material use.

The advantage of the process is that it can create good sized strong, non-brittle parts of considerable complexity without a lot of difficulty if the limitations are respected. The process is also clean, quiet and does not require liquid resins.  The main disadvantages of the process are that it is slow (average of 10cc model volume/hour), not extremely precise, and that the material is quite expensive (at around CHF 400.- /kilo).  There is also post-process finishing to consider, the surface finish not being extremely fine, the models need sanding in order to have a good surface for painting or molding.

Even small structures can take a number of hours to build, larger objects may take days and even weeks to finish.  However, during this time, no human intervention is needed, the machines can safely work in an office environment as well as lights-out, 24/7 without danger.  The larger machines do produce a significant amount of heat, and thus need a well ventilated area to avoid overheating.  [Mitch 2006-17-02]