About AFS and Metalcasting

Three Main RP Methods

By Shea Gibbs, Managing Editor

Rapid prototyping (RP) options change rapidly. New technologies become available as quickly as old ones become obsolete. But there are a few processes that never seem to go away. So, depending on the casting characteristics you desire, it’s a good idea to have a handle on the primary ways to develop a prototype when you need to try out a cast component in your design yesterday.

Pick a Process

Several methods exist to create a workable prototype, but not all of them produce an actual casting. Two industries are at work in the process—rapid prototyping and rapid manufacturing—but not all RP results in the creation of a manufactured part, and not all rapid manufacturing starts with the development of a prototype. The two fields do cross paths, though, and the results can be a quickly produced, cost-efficient casting—if you are knowledgeable about the casting capabilities of the various prototypes.

One of the most common ways to receive a casting quickly is to start with a plastic pattern, which is printed using a computer data file. Three processes remain the standard in the creation of a plastic pattern—stereolithography (SLA), selective laser sintering (SLS) and fused deposition modeling (FDM). All boast advantages and have drawbacks owing to the different materials they use and ways in which they build parts. However, all three of the processes work on the same premise—building prototypes layer by layer, caking plastic on top of plastic until the piece is complete.

Built in Stereo

SLA allows one to create solid, plastic, 3-D objects from computer-aided design (CAD) drawings in a matter of hours. The process uses an ultraviolet (UV) laser to create successive cross-sections of a 3-D object within a vat of liquid photopolymer. First, a metal platform is placed on top of a vat filled with polymer (an epoxy resin). Before the build begins, the platform is moved to a point just below the surface of the resin. As the UV laser traces the layer in the polymer, the resin begins to cure on top of the platform, solidifying the part to be manufactured. When angled parts are constructed in this way, SLA builds rely on computer-designed support structures to maintain their structure.

SLA generally is considered the RP technology that provides the greatest accuracy and best surface finish. The CAD model can be divided into cross sections between 0.002 and 0.006 in. (about 0.005 and 0.015 cm), which reduces “stepping,” the appearance of stair-like transitions between layers.

The material used also can be the least expensive. Because the structure of an SLA part is 85% hollow (owing to its honeycomb-like makeup), the machines are the most efficient when it comes to material use.

"I have seen people quote [SLA machines] as low as $0.39 per cu. in. for pure material,” said Paul Miller of 3D Systems Corp.

The build rate for SLA parts is approximately 1 cu. in./hour (2.54 cu. cm/hour), which for most parts makes it the fastest process available. It is also capable of building the largest parts available, with a maximum envelope of 25 x 30 x 22 in. (63.5 x 76.2 x 55.9 cm).

However, finished SLA parts do have their downsides. The parts can be brittle (though they have made considerable advancements over the past few years), and they can warp over time. Their surface finish, while smooth, is also somewhat tacky, as some of the material does not cure completely. Uncured SLA material can be toxic if inhaled, so ventilation is necessary when working with the process.

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.

“The resolution isn’t quite as high, because there is more stepping with wider layers,” Fischer said. FDM parts can achieve a layer thickness of 0.004 to 0.020 in. (about 0.01 to 0.051 cm), and the build rate for this process is approximately 1 cu. in./hour (2.54 cu. cm/hour) with a maximum envelope of 24 x 20 x 24 (61 x 50.8 x 61 cm).

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.{mospagebreak}

From Plastic to Metal

Once a plastic model is built, several casting methods can deliver your part. The most common process used is investment casting, but some plastics are rigid enough to use as a pattern for a sand mold, and using them as a pattern for plaster molds also is common.

Plastic prototypes can be used in much the same way a wax pattern is used in traditional investment casting. Just like wax, the pattern is submerged in a slurry repeatedly and encased in a solid ceramic shell. When the desired shell thickness is achieved, the plastic is fired out of the mold, and molten metal can be poured into the cavity left behind. This process is most valuable with parts that have highly complex geometries and require smooth surface finishes.

Plastics tend to leave some amount of ash behind when they are fired out of ceramic molds. Depending on the material used, that ash content may be limited, or it may be substantial. Each process, because of the materials they rely on, generates different amounts of ash. The ash can produce inclusion defects in the finished cast component that you receive.

The material used in a prototype investment casting also can present a structural problem for the ceramic mold. Many of the plastics used to create prototypes expand when heated, which can cause mold breakage. However, in the last five years, many SLA generators have switched from a solid plastic to a material that has a honeycomb-like pattern in the prototype. The lattice structure allows the part to collapse when the mold is fired, eliminating breakage.

Many investment casting facilities are also quite comfortable using SLS patterns, according to Miller. Because some of the prototyping machines build parts in styrene infiltrated with wax, the finished product can look and feel very similar to traditional wax investment patterns, and they can be stuck right onto a gating tree, which metalcasting facilities use when producing an investment casting. Those materials also do not expand within a ceramic pattern and have low ash content, Miller said.

If a sand cast product will suit your needs, the three devices produce parts that can be used as patterns, but each will exhibit different behavioral characteristics. While FDM parts are very durable and hold up well to repeated use, the materials can be so porous as to allow some sand grains to force through the plastic, resulting in defects. SLS parts have the advantage of emulating the surface finish of a sand mold. A grainy surface results from sintering that interlocks with sand grains. Only recently have there been SLA parts developed with enough rigidity to be used as a pattern for sand molds.  The superior surface finish of those parts makes them an attractive choice for the creation of any pattern, but they can lose their structural integrity quickly, sometimes on their first use.

Plastic prototypes also can be used in the rubber plaster mold (RPM) process for creating a rapid casting. RPM takes a plastic part and coats it in rubber. The rubber coating is then cut in half and removed from the model, creating negative images of the desired part. Plaster molds then can be created using the rubber as a pattern.

Particularly in the RPM process, designers have to decide whether the part they need quickly should have superior surface finish and accuracy, or whether those attributes can be sacrificed for the sake of speed and cost.

In the end, “the better the model, the better the casting,” said Mike Kaiser, Prototype Casting Inc., Denver, Colo. METAL