Alternative RP Methods
By MODERN CASTING Staff
Several prototyping processes can provide the tools needed to make a rapid casting without developing a model through stereolithography (SLA), selective laser sintering (SLS) or fused deposition modeling (FDM). One of them is the old-fashioned way—hard work. With the right amount of personnel and equipment, patternmakers can CNC machine a metal pattern, and a prototype can be poured within two or three weeks. But some shops have developed unique ways to produce a viable, temporary mold for short run pours in even less time.
A Different Plastic Pattern
The simplest way to shave hours off of the mold building process is to stick with CNC machining patterns but switch to a different material. Patterns machined out of plastic, while not suitable for sustained production, can be assembled from computer data far more quickly than metal parts.
A variety of plastic tooling materials can be used to make metalcasting patterns. Because the material is soft and easily machined, creating tooling out of plastic can be accomplished up to 50% faster than tooling up with metal, according to Ronald Gustafson, Clinkenbeard and Associates, Rockford, Ill.
“You could use, for example, a 450 ren board material,” he said. “There are probably 50 [possible materials] out there.”
The pliability of the material under the CNC cutting tool offers a tradeoff, though. Ren board materials are not sturdy enough to hold up to long manufacturing runs.
“If you were going to make 1,000 [castings] with metal tooling, you’d probably get between 100 to 200 [with plastic],” Gustafson said. “The short run really shines in development work, or it could be a bridge tool while you’re producing your metal tooling.”
End users across the board call upon plastic tooling, said Gustafson; however, the process can be most effective for those customers who anticipate significant and costly changes in a product’s design before it is sent to a production run. The pattern can be sent back to the drawing board and altered slightly before another pour is conducted.
The Mold Machine
Another option is to skip the tooling process altogether by having sand molds and cores machined directly. Working from computer aided design (CAD) data and using a diamond headed cutting tool, Clinkenbeard utilizes a process that starts with a solid block of cured, nobake sand and finishes with a fully functional mold or core.
According to Reg Gustafson, project manager, the company’s process can turn out machined pieces of sand with even higher complexity than can be achieved with the use of a traditional pattern or corebox, making them capable of producing extremely complex castings. The process also replicates the long-term production run exactly. The same sand composition that will be used when and if a prototype needs to be manufactured in bulk is used to create the sand blocks that are machined into cores and molds. The binders will appear in the same percentages that they will in the metalcasting facility.
“Customers find that sometimes they will prototype in one method, and when they go into production, the molds and cores are made differently,” Ronald Gustafson said. “And it turns out that they have issues in the casting that they did not find in the prototype. It’s due to using different materials.”
This method of rapid manufacturing is most useful when a casting requires a large number of cores and is very complex, said Gustafson. However, since the painstakingly machined molds and cores are discarded like scrap after the casting is poured and cooled, the process Clinkenbeard uses is only useful for very limited runs—as low as two to three.
And the process of cutting a mold out of sand is sometimes more difficult than machining metal, according to Reg Gustafson. The sand mixture must be entirely uniform, lacking both soft spots and hard spots, for the CAD data to translate to a properly formed mold or core. Soft spots can result in irregular mold and core faces, and hard spots can lead to equipment damage, which explains the need for the ultra-hard, diamond- tipped cutting tool.
Molds Can Be Self-Sintered
In a similar process to sand mold machining, molds and cores can be created without ever tooling up through a combination of laser sintering and mold milling.
Sintering most often is used to create plastic prototypes, using a laser to cement powders together layer by layer until a full, 3-D object is formed. But the process also can be used to put together small, complex cores and, less frequently, molds.
“We have two processes in house,” said Thomas Becker, sales manager, ACTech, Freiberg, Germany. “If the stuff is small and highly complex, we use the sintering process. If it is large and simple, we use direct mold milling.”
To apply SLS to cores and molds, the user need only switch from a thermoplastic powder to a chemically bonded sand capable of being fused with a laser. The core or mold is then built layer by layer until it is ready for casting.
The maximum envelope that an SLS machine offers is 22 x 22 x 30 in. (55.9 x 55.9 x 76.2 cm), which can be easily eclipsed by a normal sized mold. So, the second half of this duo, direct mold milling, comes into play on anything larger. Milling involves carving large, simple shapes out of the two mold halves separately, creating the full mold without building a tool.
The collective use of sintering and milling exhibits many of the same benefits and drawbacks as the process Clinkenbeard uses. Because molds and cores are created without tooling, they are destroyed each time a casting is poured and shaken out, making the process useful only for very low production. However, the process is able to closely replicate the way a casting will eventually be made in a metalcasting facility.
“We make a casting the same as you would in a normal foundry,” said Becker. “The chemistry and processes are the same as in a foundry. It’s as good or bad as the casting is.”
Put the Metal to the Metal
Direct metal printing will sound familiar to anyone with a limited knowledge of additive layer prototyping. If you can print a prototype layer by layer using plastic as a base or print a core using chemically bonded sand, why not skip the middle-material and go directly to metal?
Laser sintering, meet powdered metal alloys. To begin the process, a steel platform is positioned under a movable laser, and a roller passes over the platform, depositing a layer of powdered alloy as thin as 20 microns—almost 10 times thinner than an SLS machine can handle.
“There is an expectation that you’re going to hold tighter tolerances,” said Greg Morris, COO of Morris Technologies, Cincinnati.
The laser is directed by computer data to trace out the shape of the first layer of the object, and then the process repeats itself. When the roller is finished rolling and the laser is finished tracing, you have a completed metal object.
The printed metal piece can be used in one of a few ways, depending on the CAD data that was initially fed to the sintering machine. Either it can be used as a pattern, which would be capable of production runs nearly as long as machined patterns (depending on the alloy), or you have built an already functional prototype from the ground up.
Currently, five different alloys are available for direct metal printing: direct metal, which is bronze-based; direct steel, a low carbon steel; direct steel H-20, a tool steel-like alloy; 17-4 stainless steel; and a cobalt-chromium alloy, the strongest of the group.
“Everything we do is used in a functional way,” said Morris. “Cobalt-chromium is good for aerospace because of its strength and resistance to heat. It is used functionally in some very extreme environments.”
When and if you find that you need a cast prototype faster than you can say stereolithography, a lot of options exist. Whether you go the more traditional route of the plastic pattern—SLA, SLS or FDM—or use an alternative method, carefully consider the capabilities of each before going in rapid fire.
“There are some processes that are only good for certain applications,” said Ronald Gustafson. “In each method, there’s probably a shape or category that would shine. But they’re not for everything. Like burning out SLA patterns. There’s only a select market for it.”