A second tooling machine is out-fitted with a specific part. The CNC machine isn't capable of performing the task. A strong electric current is used to make the mold with the second tooling machine. The plastic cools and hardens. The mold is easily persuaded to release the product.
Take a tour of the Injection Molding Company, and the actual process as it unfolds at https://www.youtube.com/watch?v=VHYouINymiY&index=1&list=PL2JWDcbnTw4X2ZA05oNsIsxpVz2gZ1LS6.