(Reuters) – 3D Systems Corp cut its earnings forecast for 2013 as it spends more on research and expanding its retail presence, sending its shares down 7 percent in premarket trading. The largest listed 3D printer company said it expects full-year adjusted earnings of 93 cents to $1.03 per share, down from its previous forecast of $1.05 to $1.20. The company, however, raised its revenue forecast for the year to $500 million-$530 million from its previous forecast of $485 million-$510 million, and reported better-than-expected third-quarter results. …