What is the difference between
an RP machine and a 3D printer?

They can be used in an office environment.

3D printers are generally small and in some cases can sit on a desktop. RP machines are much larger, invariably free-standing, and often have large appetites for electric power. 3D printers are usually designed to use materials in cassettes or other enclosed means, rather than in bulk as rapid prototyping machines do. This makes 3D printers tidier to have in an office and makes it easier to change materials, if that's an available option.

3D printers make smaller parts.

Being smaller on the outside means there's less room to build parts inside. Most 3D printers are limited to making parts that will fit roughly in a cube 8 inches on a side. On the other hand, rapid prototyping machines provide a build chamber at least 10 inches on a side, and some have build chambers approaching 3 x 3 x 2 feet and larger. A smaller build envelope also means that it's not possible to make as many parts at the same time. That's lower throughput efficiency than an RP machine, but not usually a concern for the applications 3D printers are intended for.

Left to right: The Stratasys Dimension BST, Z Corporation's Z310, 3D Systems' InVision LD. (Photos courtesy of respective manufacturers.)

3D printers are easier to use.

As a consequence, they require much less or even no training at all in contrast to rapid prototyping machines. Indeed, its possible to be making parts pretty much right out of the box with some technologies. But simplicity comes at the price of flexibility. Unlike RP machines, you may not be able to adjust or select many build parameters, or change them on the fly.

3D printers are cheaper to maintain and feed.

You can expect to spend several hundreds to a few thousand dollars per year to maintain a 3D printer and keep it fed with materials, but it costs several tens of thousands of dollars each year to maintain a rapid prototyping system. Simply replacing a laser in a stereolithography machine can cost $20,000, and filling a big vat with photopolymer can easily cost more than $50,000.

3D printers are not as accurate.

Rapid prototyping machines are more accurate and in some cases produce better finishes than 3D printers today - but the difference isn't an order of magnitude. Actually, the specs are very similar for like-sized parts, and in both cases depend to a considerable extent on the geometry being produced.

Material choices are limited.

3D printers don't provide the same range of materials as RP machines. Some classes of materials such as ceramics and metals are not available at all. But it's possible to make adequately functional parts for many applications, and the materials available are certainly appropriate for concept modeling, the most frequent use of 3D printers.

3D printers are an example of the popularization and progress of a technology. It happens in almost every field. When television was first commercialized, video cameras were large, expensive, delicate and difficult to maintain instruments. They could only be used effectively in a well-lit studio environment and there was no way other than movie film to record what came out of them. And what came out of them was a low-fidelity black and white picture that made it difficult to recognize an individual person. Today, all of the functions of a TV studio fit in one hand, work flawlessly and cost little. Professional equipment is still available, of course. It provides more versatility and better picture fidelity, but most of us don't need those capabilities. So, too with copiers, computers, cellphones and now RP.

The reasons for using a 3D printer are fundamentally the same as for using RP: to verify a design; to share information across distances; to make a one-off part - and many others. Only now you can afford to do it.