Everyone thought that Under Armour's first 3D-printed sneaker released in March was a glorified marketing tool. Guess again, as the red-hot footwear and apparel maker is about to prove it could be a potentially lucrative business. The company will launch its second 3D-printed sneaker -- likely in the training category -- by the end of the summer, a source close to the matter confirmed to TheStreet. That release may be followed by several others before the end of the year.