Patent ID: 7913881

Claim:
For use in a viscous material dispenser, a piston comprising a. unitary body of circular cross section including: i. a spaced face and back surfaces; ii. an axial bore extending between the surfaces; iii. a face surface including a nose section surrounding said bore and an annular recess section surrounding the nose section, the recess section being perimetrically surrounded by a lip; iv. a perimetral surface extending axially from the face to the back surface; and v. the perimetral and face surfaces together defining a perimetral lip around a concave section, the lip being of tapering thickness tapering from its thickest part at a location nearest the back surface forwardly to a thin termination at a juncture of the face and the perimetral surfaces; b. an annular disc positioned in the annular recess section; c. the disc including a peripheral surface coactable with the lip to transmit comprising forces against the lip and thence against a surrounding wall of a cylinder where the piston is in use; and d. at least one elastic member positioned within said annular recess section between said body and said annular disc, the elastic member forming a gap radially inward and radially outward from the elastic member between the body and annular disc such that compression of the elastic member causes the peripheral surface to act against said lip.