Entire 3D printing market space looks overvalued

DDD SSYS XONE PRLB and VJET: All these companies are in the 3D printing space. DDD has a short ratio of 28.9%. They've seen several recent IPO's. The entire market segment seems over valued at this point. 3D printing is suppose to be a super hot space but the trajectory of these stocks have been crazy. Some of these companies trade at 20 times sales. Reminds me of all the nano-technology stocks in 2004. Most of them went belly up in a couple of years.