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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.

  • eodeo
    I'm excited by the advances of 3d printing. I want to get my hands on one and play :)