Piston

摘要

In a medical injection device an ampoule containing a fluid to be injected has a resilient piston closing on one end of the ampoule, which resilient piston is moved forward by a plunger. A membrane closes the other end of the ampoule. An injection needle projecting through the membrane of the ampoule and into the ampoule is mounted prior to use. To protect the pointed end of the needle projecting into the ampoule from being damaged by the resilient piston when the ampoule is about to be empty and the piston is moved close to the membrane, the resilient piston has a central part of the front wall lowered relative to the rest of the front wall to provide a central well in the front wall.