What is Additive Layer Manufacturing?
Additive layer manufacturing (ALM) is the founding principle for the majority of today’s rapid prototyping processes. A wide variety of prototyping systems apply this concept through methods that are very different. In ALM a finished part is produced from individual layers that are built up as sliced cross-sections of a 3D CAD model. Each layer is set at a user specified or process specific thickness; the finer the layer thickness, the finer the vertical or (Z) resolution of the finished prototype. The resolution capabilities of each (X,Y) or 2D cross-section layer is dependent on the minimum feature thickness as defined by the prototyping systems manufacturer.
2D CAD geometry defines a model with X and Y coordinates on a two-dimensional graph. 3D models utilize X,Y,and Z coordinates on a three-dimensional graph. 3D models for rapid prototyping can be designed in a wide variety of CAD or 3D modeling software packages, although the output is usually a .STL file. STL is a 3D file format that represents the triangulated surface geometry of a model without any other information like colors or textures. High resolution STL files may include surfaces that are comprised of thousands of triangles and can use large amounts of disk storage space. The STL files are virtually sliced by the prototyping equipment software, and sent to the equipment as a print job.
The sections are applied, stacking and bonding layer-by-layer until the model is complete. All ALM processes utilize a means of vertically stacking each layer in a sequential order, usually via an automated vertical (Z) positioning nozzle or a vertically mobile platform.
Some processes build up layers of epoxy or wax sprayed by inkjet like jet nozzles. Some use epoxy cured by ultraviolet lasers in a resin bath. Some processes extrude a bead of material in linear (X,Y) rows to build up each layer. Others utilize a powder-bed based process whereby layers of powdered material are bonded by sprayed epoxy or a focused heat-source such as laser light or electron beams. The powder bed drops down the distance of one layer thickness, and an automated rake smoothes a new layer of powdered substrate over the top to be subsequently bonded to the previous layer.
In many instances, easily removable support structures are included in the CAD model during the pre-build stages to increase stability and improve the tolerances of the finished product. In some processes the supports are made of a different material that may be easily dissolved or seperated by gentle heating. In most of the powder-bed technologies the supports are simply designed to snap off.
Depending on the properties of the layer-manufactured model it may be ready for use immediately after support-removal, or it may be subject to secondary finishing processes such as sanding, polishing, painting, or even refinement on a cnc milling machine.