If 3-D printing stocks have been plagued by anything, it's the belief that there's a limited market for iterative manufacturing of everyday consumer products. That may be changing, thanks to Normal.

A New York City start-up, Normal plans to sell customized earphones for $199 each. Stratasys' (NASDAQ:SSYS) $45,000 Fortus 250mc printers comprise the assembly line. Might this be the beginning of a long-awaited movement to print more low-end devices?

Not exactly an everyday opportunity
As is often the case with start-ups -- or at least sustainable start-ups -- founder Nikki Kaufman created Normal to solve her own problem. According to a launch press kit, her search for a custom-fit pair of earphones ended with "an exhaustive process that involved a visit to the doctor's office, uncomfortable silicon molds, a three-week wait, and a $2,000 price tag."

Normal's process seems simple enough. Users download either the iOS or Android smartphone app and then take a picture of each ear, using a coin as a reference point for the final mold, which is 3-D printed and fitted with premium audio to complete the product.

Experience at Quirky, the inventive consumer products company she co-founded, "exposed" Kaufman to "leading advances in 3-D printing, production, and manufacturing," the press kit says. Applying a similar approach to Normal must have seemed, well, normal.

Your next gadget will be printed
While it's not entirely clear how fast others will take to Normal's 3-D printing-powered model for mass customization, most agree that the market is growing fast. Wohlers Associates says overall growth accelerated to 34.9% last year. Canalys pegs the market expanding by 45.7% annually from now till 2018, and consumer products will help drive gains.

"Advances in technology are yielding faster print times and enabling objects to be printed in greater combinations of materials, colors, and finishes. Crucially, prices are also falling, making the technology an increasingly feasible option for a broad variety of enterprise and consumer uses, restricted only by computer aided design competencies and printer availability -- both of which are set to improve significantly," Canalys senior analyst Tim Shepherd said in a press release.

The Foolish bottom line
What to do if you're an investor? I'd advise owning a basket of 3-D printing stocks with the potential to drive mass production of consumer products. Certainly Stratasys fits the bill, having worked with Adidas, Stanley Black & Decker, and now Normal. I'd also take a careful look at 3D Systems (NYSE:DDD), which has long talked up the potential for the consumer side of the 3-D printing market and enjoyed notable successes prototyping for the likes Timberland.

Neither stock is cheap -- each trades for more than 10 times sales, a hefty premium -- but as market leaders they're also the most likely to cash in on the movement toward massively customized, 3-D printed products. I've added both to my CAPS portfolio as a result.