Metal Prototypes and Rapid Prototyping

Metal prototypes made by rapid prototyping are becoming more mainstream as processing improvements arise.

Part complexity

If your metal prototype is composed of more than a few relatively simple pieces you are probably looking at conventional machining and metal working.  These processes might include machining, EDM, welding fabrication, laser or water jet cutting, etc.   Many prototyping shops contain all of these processes under one roof.  It will be to your advantage to spend extra time finding someone who can do all of the operations and take full responsibility for your metal prototype.

If the metal prototype contains extremely complex geometry, it is worth the effort to find a rapid prototyping vendor.

Powder Technologies for Metal Prototypes

Rapid Prototyping is based on scanned laser fusion and sintering (SLFS) using  CO2 lasers to fuse targeted areas of metal powder. The process positions the powder vertically while the laser scans and fuses the powder in horizontal slices using a solid model STL file to guide the beam position.

The part is constructed layer by layer by fusing a layer of heat-fusible powder.  The laser beam raises the temperature of the powder to the melting point fusing the powder

particles horizontally and vertically to the previously constructed layer below.  The unfused powder is undisturbed and supports the part during construction.  After the layer is fused, a new layer of powder is deposited and the process begins again.

Laser fusion and sintering bonds the metal particles just below the melting point of the metal powder.  The process can only approach full density since full melting of all particles is not possible.  The high surface energy in the powder drives the sintering process.  The sintering of metal powders is a much slower RP process.  To speed the process, the metal powder is often coated with a thin polymer film used to hold the powder in the desired shape.  After construction, the part is fired with the density increasing while sintering takes place.  The polymer binder is evaporated in an interim heating operation before placing the part in the furnace for high temperature sintering.

SLFS was the first process used to build rapid prototyping metal tooling.  SLFS processes use a CO2 laser.  Even though more laser power is available through the CO2 laser, the process is slow and the raw powder is raised in temperature to just below the fusing temperature.  A nitrogen purge in the build chamber prevents oxidation of the raw materials.  Surface finish with the SLFS method is poor and dimensional accuracy is often a problem when compared to SLS rapid prototyping.

A further improvement on the SLFS process incorporates the use of a high power Nd:YAG crystal laser.  The higher power allows the direct fusing of pure metal powder.  Powder is sprayed at the laser focal point fusing the particles together into a density form.  The fused powder also fuses to the previously deposited material below.  The process is faster than traditional SLFS, but many thermal issues, rough surface finishes and long lead times are trade offs that need to be considered.

Complex contour metal prototypes can reduce subsequent EDM finishing work and some CNC part finishing as well.  Some parts can be finished complete with no further operations.