What Doesn't Make Sense to 3D Print?

The Bay Lights Project

In the past year, 3D printers have been used to make parts for NASA spacecraft, to patch holes in human heads and to create (more or less) functional handguns.

As the once-exclusive technology becomes increasingly affordable and accessible, more businesses and consumers are turning to 3D printing to solve a variety of problems.

But the technology still has its limitations. Earlier this year the city of San Francisco launched the Bay Lights Project, in which the city covered the San Francisco Bay Bridge with 25,000 LED lights.  The design team considered 3D printing the 50,000 clips they'd need to attach the lights, but eventually decided it would be better for them to make the clips using injection molding —a mechanical production technique that involves filling a custom-made metal mold with melted plastic.

Gian Pablo Villamil, who worked as a design consultant on the project, said there were three reasons for this: the cost to 3D print at high volume, the quality of the finished product and the quality of printing materials.

High volume

The real strength of 3D printing lies in creating a small amount of highly complex objects, not a large amount of simple objects.

That's because the way 3D printers build objects by layering filament from the bottom up makes it very easy to create detail.

"If you have an object that fits in a 4-inch by 4-inch by 4-inch cube, it doesn't cost any more to make it intricate than to make it simple," said Villamil, because all the complexity is in the digital design, not built into a physical mold or sculpted by hand, so when the printer is laying down the material it's just as easy to follow complex directions as simple ones. "That's a huge difference with other processes [such as] injection molds [where] complexity costs money."

However, when it comes to the size of the product, 3D printing may be at a disadvantage. As an object's size increases, the cost to 3D print it rises much faster than the cost to produce it mechanically, all other things being equal. This is because 3D printers build objects from the ground up, layer by layer, so they need to build supports for tall or overhanging sections as they go to keep the object from falling over.

The size of the order also plays a role in determining the best means of production.

"In real-world manufacturing right now, low-volume-high-complex applications justify 3D printing. High volume-low complexity tends to favor traditional manufacturers," Villamil said.

"Usually there's a threshold for where it's cheaper to 3D print, and then we hit a certain number and it becomes cheaper to mass produce," said Alyssa Reichental, a spokeswoman for 3D printing company 3DSystems. 

The Bay Lights project sat in the gray area between those two extremes, Villamil said: the clips were custom-designed but simple enough that injection molds could easily produce the form. And 50,000 is too tall an order to hand-make but far too few to factory-produce.

The team members needed an efficient mode of production for their midsize order, and found that they could afford to buy five to 10 Replicators from Makerbot to produce the clips. However, they realized that injection molding would be even cheaper.

Even though the injection molding process required expensive, custom-made molds, the cost of the molds spread over the almost 50,000 created clips was cheaper per item than printing each one on the MakerBots.

The caveat was that the team had to be certain of their design before they created the molds. A digital model can be tweaked and printed as many times as you'd like and the cost will never change, but changing a mold means scrapping the old mold and building a new one.

Material Costs and Quantities

Quality of materials

Material costs are the primary reason why 3D printing might not be the best method for production, according to Erol Gunduz, a digital artist and professor at NYU's Center for Advanced Digital Applications.

"Where it becomes cost prohibitive is basically the cost of the mold materials," he said. "A lot of these materials are … not archival. They won't last.  Over time [most of these materials will] slump due to gravity, or they'll [turn] yellow."

For the Bay Lights project, Villamil and his co-workers realized that the Makerbot prototypes they looked at print in a material called ABS, which doesn't have the longevity they needed.

"We needed a material that could stand up to outdoor exposure, and most of the plastics that work with MakerBots and other 3D printers aren't really made for outdoors," said Villamil, explaining that many 3D printing materials degrade under ultraviolet light.

By contrast, the injection molding contractors were able to make the clips in a UV-resistant polycarbonate.

Further, Villamil said that the quality of 3D-printed products still isn't quite at the level of manufactured products. 3D printers may be able to produce more complex designs, but not without imperfections in the finished product. The extruder (the nozzle that applies the printing material) can get clogged, or the material might dry in an incorrect way. Factors such as room vibrations or high humidity also cause deformations.

The future

The Bay Lights project went live in March and will stay up through March 2015. Villamil now works for Autodesk, a 3D design company that creates software used in 3D printing and other applications such as animation and architectural design.

Even in the few months since the Bay Lights team decided to pass on 3D printing, Villamil has already seen improvements.

"I think five years ago at any volume it would have been prohibitive to think of 3D printing," he said. "[Today] the cost of print has come down quite a lot. The quality, even though it's variable, is getting really, really good. I think that's a matter of time before [3D printing] can match [mechanical production] on quality. And the final point, materials, is where there needs to be a lot of effort in 3D printing now."

3D printing is also becoming a game-changer for products that fall somewhere between high and low volumes.

 "People tend to go to low volume-high complexity and do it by hand, or high volume-low complexity and get it made in China. But that middle space has always been very difficult to explore economically because [in those cases] manufacturing processes are too expensive and handmade custom processes are too slow. 3D printing is actually opening up that space, so you're looking at things like limited runs [of things like] toys or jewelry or art projects. There's a lot of stuff on Kickstarter that is enabled by this technology."

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