Are Investors 3-D Printing a Bubble?

It seems that every time I write about the sky-high valuations of 3D Systems and Stratasys they just keep on trekking higher (you're welcome!). There is no doubt that the 3-D printing industry will continue to grow and find new customers and applications. According to The Wall Street Journal, analysts estimate that 3D Systems and Stratasys will grow 45% and 39%, respectively, in each of the next three years. However, as more and more investors pile into the companies you have to start asking the question: "Is this a bubble?" Let's go through some valuation experiments below.