Rapid Tooling: Poised for Rapid Takeoff
“Rapid tooling” (RT) can mean two things. It can mean using additive technology to make a tool for short-run casting, injection molding, and the like. It can also mean using additive technology to make production tools—not just molds, but custom or short-use fixtures and assembly tools. Both RT types are becoming increasingly popular, with numerous developments happening in both. Here’s a look at some of what’s happening in RT.
Form makes function
Ultem is a tough, high-performance thermoplastic from SABIC Innovative Plastics (sabic-ip.com). It has a high strength-to-weight ratio and good compression and sheer strength. It also has an FST rating (flame, smoke, toxicity), meaning the material is safe to use in aerospace, marine, and ground vehicles. Stratasys, Inc. (Minneapolis, MN; stratasys.com) has been proving out the use of Ultem and fused deposition modeling (FDM) for metal forming (hydroforming and press brake), specifically for repairing military airplanes. Fixing the damage requires cutting out sections of sheet metal from the planes and riveting in new sections that have been properly molded to shape. In addition, upgrades, such as new electronic systems inside the plane, often require new and customized sheet metal framework. In short, explains Jeff DeGrange, vice president of direct digital manufacturing (DDM) for Stratasys, the need for sheet metal parts for repairs-in-service, spare parts, and upgrades requires a low-volume, quick-response tooling method to provide the forms to create those parts.
The plastic tools weigh about a third what the equivalent metal tool weighs. They can be built in two-thirds the time of a machined metal tool. And FDM can build forms to handle various metals—aluminum to titanium to stainless steel—of various thicknesses, typically 0.150-in. and thinner. According to DeGrange, FDM is 50% the cost of machining an aluminum metal tool. “When you have ‘aircraft-on-ground’—when you need a part before a plane can fly—time is of greater importance than overall cost. With FDM, the military is getting faster cycle time, faster delivery time, and lower cost.”
Moreover, points out DeGrange, there’s “the whole sustainability/green effort” to consider. FDM produces less material waste than conventional machining; the mold maker uses material that’s needed rather than disposing material that’s removed. In a qualitative way, DeGrange continues, “there’s less energy consumption in going from design to final tool. There are typically less transportation and warehousing costs because tools are made on-demand. And when you’re done, you can essentially throw these tools away because they’re not environmentally bad to landfill. And if you need to make them again, just open your digital files and make a new form rather than search for a form in a warehouse or in a dynamic supply chain.”
Conformal cooling channels in mold tools are typically produced via drilling. But drilling can produce only straight channels, and they can only be at right angles to each other. Such restrictions can make it difficult or impossible to run channels to the critical areas most needing cooling. The result: Heat dissipation is neither uniform across the tool nor sufficient in selected areas. Uneven cooling can result in warpage, clogged channels, lower product quality, increased waste. Slower cooling typically results in a slower cycle times between castings or mold cycles. According to EOS of North America (Novi, MI; eos.info), cooling time can account for up to 70% of injection molding cycle times.
Moldmakers have various ways to get around the problems of straight channels and location. One way is to drill numerous straight channels into the tool. Or drill a straight channel to cool a specific hotspot. Or split the mold into segments, mill half a conformal channel in each segment, and then fit and solder the segments together. This last approach is time consuming and costly. Plus, the solder sometimes doesn’t hold together as long as required by the molding process. A “non-
drilling” approach is to create an insert channel with some sort of baffle to create turbulence, which in turn quickly draws away heat.
Direct metal laser sintering (DMLS) gets around these problems. The mold designer isn’t removing material; just adding material exactly where needed. “DMLS lets designers put in cooling channels in the areas unreachable with a drill,” says Augustin Niavas, business development manager of tooling for EOS. DMLS gives tool makers “the freedom to reach these areas, to optimize cross sections and shapes, and to create an ideal cooling channel a well-defined distance to the cavity. You, the designer, can do exactly what the physics tells you to do. You don’t need to compromise.”
Two developments are helping DMLS along in conformal cooling. First material. In the past, which wasn’t so long ago, bronze was used in creating molds using DMLS. Now the DMLS material-of-choice is maraging steel. Maraging steel is heat treatable and it can be hardened.
Second, software. Current mold-design software can easily simulate conformal cooling channels as well as drilled channels. However, the software is not as good in designing those channels. Current design software only simulates cooling channels with circular cross-sections based on the conventional shape of drills. Better injection molding cycle times come from oval cooling channels. The oval shape, explains Niavas, provides a greater projected area for heat exchange on the wall of the insert even though the flow is the same flow as that through a straight channel. A ribbed oval shape has even more surface area. Continues Niavas, a designer can create and control the turbulence of the coolant “by actively choosing different cross sections and by switching between different cross sections.” Another limitation is that current design software can’t easily handle the complexity in cooling channels in terms of both the number of split channel lines and the curviness of those channels. The workaround is to design the conformal cooling channels as little tubes. EOS is already talking to some software vendors about integrating the simulation of conformal cooling.
DMLS for conformal cooling is proving to provide competitive advantage. One company reduced cycle time from 15 seconds to 9 seconds, yielding a 75% increase in productivity on a 4-bottle blow mold with DMLS inserts with conformal cooling channels. Another company, optimizing an insert to produce a child’s plastic cup, replaced a traditional insert with a DMLS-produced insert with conformal cooling. Cycle time dropped nearly 43% reduction—from 24 seconds to 13.8 seconds. “Ninety percent of the people in this company didn’t think this would work,” chuckles Niavas.
Overmolds. Mission: Possible
Vista Technologies LLC (“VistaTek,” Vadnais Heights, MN; vistatek.com) is a service bureau for rapid prototyping (RP), RT, and injection molding. Among its 1,600 customers are 3M, SPX Corporation, and Toro. VistaTek specializes in producing prototypes and complex injection molds in two to three weeks. It has “dabbled” in both additive and subtractive manufacturing. It now uses both RP/RT and CNC machining in-house. On the additive side, the company started using in 2006 an Eden500V Objet 3D printer from Objet Geometries, Inc. (Billerica, MA; objet.com). Objet’s polymer jetting technology prints horizontal layers down to 16-microns thick. These layers are built up into the RT prototypes VistaTek uses for low-volume production.
In time, VistaTek wanted to create prototypes of molded products made of multiple materials, a molding process known as “overmolding.” Typically, the finished parts combine a rigid plastic with a rubber-like elastomer. “The result is the soft-touch, non-slip surface that has become common on power tools, toothbrushes, razors, consumer electronics, medical devices, and more,” according to Objet. VistaTek bought Objet’s Connex500, a dual-material 3D printer capable of creating prototypes of overmolded parts measuring 19.3” x 15.4” x 7.9”.
Now customers get prototypes in one to two days. They can “see what’s soft, what’s tacky, what’s got friction,” says, Dan Mishek, managing director. “Now that we can do it, customers are designing more with overmolding.” For example, explains Mishek, VistaTek produced prototypes for SPX Corp. with RTV molding several years ago. The job involved a two-piece, overmolded part requiring two patterns and two molds. It took VistaTek four weeks to produce. Cost: $15,800. Recently, SPX returned with a similar job. This time, VistaTek used the Connex500. The prototypes were completed in a couple of days for about $2,800—”about the same time and cost as just the patterns used for the RTV job.” No wonder VistaTek’s overmolding tooling business has grown by 40% since using the Connex500.