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Plastic Prototyping: Cut, Mold, or Build Part I

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By Brad Cleveland, Protomold

Three Ways to Make a Horse

I’ve heard attributed to various sculptors—Michaelangelo more than others—the suggestion that the way to make a statue of a horse is to start with a block of marble and chisel away everything that isn’t a horse. Several years ago, at The Minneapolis Institute of Art, there was a larger-than-life sculpture of a horse. It was assembled, improbably enough, of garage sale junk: old garden tools, toasters, golf clubs, and the like, but if you relaxed your eyes it was definitely a horse. And years ago, I can remember buying one of my kids a souvenir somewhere, a cast-on-the-spot plastic horse, which, for a fistful of quarters, was injection molded before your eyes and delivered, warm but solid, to a hopper on the front of the machine.

The reason I mention all these horses is that they represent all the ways I know of making a prototype in plastic (or in most any other material for that matter). You can carve your finished product out of a larger block of material, you can build it up, or you can mold it in place. Obviously each of these processes has its advantages, or it would have long ceased to exist, so in these series of articles, we’re going to discuss process.

Artists have their own reasons for choosing their media and methods, but for industrial designers, there are three significant concerns in the production of a prototype: quality, delivery, and cost. I list them in that order because that is the order in which the engineers we polled ranked the importance of the three variables. I’m guessing that for production, they might have ordered them differently, but prototyping has its own priorities. In this issue we will discuss each of these variables in some detail.

As will be mentioned elsewhere, Protomold (which, as you know, does rapid injection molding) now has a sister division. First Cut Prototype produces fast, high quality prototypes by direct CNC machining parts from solid stock. Together, the two make up Proto Labs Inc. In order to keep up with all of this change, we’ve expanded the scope of The Journal to address our newly expanded operations.

Together, Protomold and First Cut Prototype account for two of the three prototyping processes I mentioned: cutting and molding. But there are important offerings of a third type: building up or additive processes. These include stereolithography, 3D printing, selective laser sintering (SLS), and fused deposition modeling (FDM). We will discuss these along with our own processes in the following articles.

	Cut, Mold, or Build
Part I	Three Ways to Make a Horse
Part II	Comparative Strength and Finish
Part III	Comparative Speed
Part IV	Comparative Cost