Be Selective

SLS creates 3-D objects by fusing powdered materials with a laser. A wide variety of particulate materials can be used if they are coated with a thermal binder. To create a part, a thermoplastic powder is spread by a roller over the surface of a build cylinder. A piston in the cylinder then moves down one layer at a time to accommodate a new layer of powder. The powder delivery system is essentially opposite in function to the build cylinder. Here, a piston moves upward incrementally to supply a measured quantity of powder for each layer. A laser beam is then traced over the surface of this tightly compacted powder to selectively melt and bond it to form a layer of the object. The process is repeated until the entire object is fabricated.

The fabrication chamber is maintained at a temperature just below the melting point of the powder so that heat from the laser need only elevate the temperature slightly to cause sintering, thus greatly speeding up the process. After the object is fully formed, the piston is raised to elevate it. Excess powder is simply brushed away and final manual finishing may be conducted. No supports are required with this method because overhangs and undercuts are supported by the solid powder bed. However, it may take a considerable length of cool-down time before the part can be removed from the machine. Large parts with thin sections may require as much as two days of cooling time. The build rate for SLS is between 0.25 to 1 cu. in./hour (0.635 to 2.54 cu. cm/hour), and the largest parts that can be made through the process are 22 x 22 x 30 in. (55.9 x 55.9 x 76.2 cm).

The advantages of SLS parts are owed mostly to its flexibility of material use. Powders can be inexpensive, produce high yields and offer faster part finishing. However, SLS powders also lead to some drawbacks—surface finishes and accuracy are not as good as those of SLA, due to the grainy way in which the powder is sintered. Like SLA plastic models, SLS pieces also have some tendency to warp over time, though generally they are not as apt to do so.

Take a Deposition

The FDM method offers greater strength and a wider range of materials than other processes. The process is fairly fast for small parts on the order of a few cubic inches or those that have tall, thin structures, but it can be slow for parts with wide cross sections. To build a part using this method, a plastic filament is unwound from a coil and supplies material to an extrusion nozzle. The nozzle is heated to melt the plastic and has a mechanism that allows the flow of the melted plastic to be turned on and off. The nozzle is mounted to a mechanical stage which can be moved both horizontally and vertically. As the nozzle is moved over a table in the required geometry, it deposits a thin bead of extruded plastic to form each layer.

"It’s kind of a hot glue gun on steroids,” said Fred Fischer, product manager for Stratasys, Eden Prairie, Minn.

The plastic hardens almost instantaneously after being squirted from the nozzle and bonds to the layer below. The entire system is contained within a chamber, which is held at a temperature just below the melting point of the plastic. The finish of parts produced by FDM has been greatly improved over the years but isn’t quite on par with SLA parts.

Fused deposition modeling prototypes, like the one pictured at top, are rigid enough to be used as patterns for sand molds, producing working castings like this C355 aluminum aircraft fuel pump

One of the greatest advantages of FDM machines is their behavior in the workplace, Fischer said—they are clean, quiet and environmentally friendly. They are also inexpensive, particularly on the front end. Thus, FDM machines can be an attractive option when considering purchasing a rapid prototyping machine.

Speed Sells

The true speed of each of the big three rapid prototyping systems is a relative matter. Each is affected somewhat differently by varying geometries. FDM machines falter when it comes to crossing long horizontal distances but are comparable to SLS and SLA machines with small parts. SLS machines build parts faster than any other method on the vertical axis, but SLAs are widely considered to be the fastest overall method of producing a prototype. Yet another factor plays into the speed of the machines, though—the number of parts required. Here, SLA and SLS machines have a leg up on FDMs once again.  Whereas an FDM machine must extrude plastic for one part at a time, the other two methods can build multiple parts during the same vertical pass. SLA machines can do so with several parts sitting side by side, and SLS systems can take it a step further, building multiple parts side by side and stacked one on top of another.  The greater the volume, the greater the advantage for sintering.

Yet another consideration is the wall thickness desired.  Here, too, SLS gains an advantage with greater volume—the thicker the walls, the better it performs. FDM systems are the least productive when it comes to thick walls and often should not be considered.