For a quarter of a century, 3D printing has been a staple of various manufacturing processes, but during most of that time, the process has been used for prototyping.

But here's a shock, the use of 3D printing for the production of final products now accounts for more than a quarter of the $2.2 billion spent last year on 3D printing products and services worldwide.

And the trend isn't just represented by the recent creation of an all-metal 3D printed gun, it's part of a ten-year long transformation of the industry according to a Wohlers Report.

The report says the manufacturing of finished production parts rose to 28.3% of all monies spent last year on 3D printing products and services, and as recently as 2003, those parts accounted for only 3.9% of revenues.

The use of 3D printing, or additive manufacturing as it's referred to at the upper echelons of the technology, is taking off in a number of market segments like manufacturing dental crowns and bridges, orthopedic implants, and jewelry.

At the bleeding edge of the growth curve sits the aerospace industry. Boeing, among others, uses additive manufacturing throughout their production processes to make environmental control systems for use in military and commercial aircraft, and GE Aviation is soon to use 3D printed fuel nozzles as components of their advanced LEAP engine. In fact, GE Aviation plans to 3D print some 40,000 nozzles every year if all goes to plan.

And at one point, the use of 3D printed parts as final products will surpass prototyping applications.

Tim Caffrey, a senior consultant at Wohlers Associates says the ratio of prototypes to production parts can clock in at 1:1,000.

"The money is in manufacturing, not prototyping. The opportunity for more commercial production activity from additive manufacturing is immense," Caffrey said.

Caffrey is the principal author of Wohlers Report 2013. That annual report covers aspects of 3D printing, including its history, applications, processes, manufacturers, and materials, and it seeks to document the entire range of the market, in this, its 18th year of publication.

Wohler's adds that they expect the 3D printing industry to continue double-digit growth over the next few years and that the sale of 3D printing products and services will reach $6 billion worldwide in the next four years. Wohlers Associates says the entire industry will eclipse the $4 billion mark some time during 2015.