Dispenser for viscous fluids

A dispenser for a viscous fluid comprising:

• a barrel having a cylindrical inside wall defining a chamber for receiving, containing and discharging a viscous fluid;
• a nozzle at one end of said barrel, said nozzle having an open end spaced axially from said barrel for passing said fluid into said chamber and for discharging said fluid therefrom;
• a cylindrical piston head in said barrel axially movable in said chamber, said head having an external diameter substantially equal to the internal diameter of said inside wall of said barrel to grip the same frictionally and thereby to seal said fluid in said chamber, said piston head having a closed wall facing said nozzle for pushing said fluid to discharge the same from said chamber;
• and
• means in said chamber near said nozzle defining a plurality of shallow, fine, circumferentially spaced passages between said piston head and said inside wall of said chamber, said passages comprising a multiplicity of short shallow narrow grooves in said inside wall of said barrel, said grooves being fine enough to pass air entrapped between said fluid and said closed end of said head while preventing said fluid from passing therethrough, said passages extending axially of said chamber near said nozzle a distance not exceeding twice the axial length of said piston head at said inside wall of said barrel, said distance being not less than 102% of said axial length of said piston head, to pass said entrapped air rearwardly beyond said piston head, said head having sufficient hardness to prevent said head from extending laterally into said passages to avoid obstructing the same and thereby permitting said entrapped air to pass therethrough, said hardness of said head being sufficient to prevent said fluid from leaking rearwardly of said piston at all positions of said head in said barrel, even when said head is advanced axially under pressure in said chamber to discharge said fluid from said nozzle.