Proto Labs, which manufactures custom parts for prototyping and short-run production, has filed for an IPO of up to $100 million. Jefferies and Piper Jaffray are leading the underwriting. Proto Labs intends to list on the NASDAQ under the symbol PRLB. Its net proceeds from the offering are currently expected to be used for general corporate purposes, including working capital. Proto Labs' existing shareholders include North Bridge and Private Capital Management.