Feed mechanism comprising an endless member configured as a double helix

A feed mechanism comprising:

• a rotary shaft,
• first and second cylindrical guide members mounted coaxially on said shaft for rotation therewith, the first and second guide members having respective external helical grooves thereon defining first and second feed channels,
• said helical grooves being of the same pitch and opposite hand,
• an endless flexible bearing member carried by said guide members and constrained thereby, the bearing member having a first portion extending helically along said first feed channel and a second portion extending helically along said second feed channel, the bearing member slidingly engaging the guide members along said channels,
• the second portion of the bearing member protruding from said second channel for frictional engagement with a restraining surface,
• whereby the bearing member is continuously advanced along said second feed channel and returned along said first feed channel without rotation in response to rotation of the shaft.

In combination with an extrusion head for extruding a tube of plastic material, the extrusion head having an annular die opening defining a predetermined axis, means for supporting the tube of plastic material as it is extruded from the die comprising:

• a rotary shaft extending from said extrusion head and journalled therein for rotation about said axis,
• first and second cylindrical guide members mounted coaxially on said shaft for rotation therewith, the first and second guide members having respective external helical grooves thereon defining first and second feed channels,
• said helical grooves being of the same pitch and opposite hand,
• an endless flexible bearing member carried by said guide members and constrained thereby, the bearing member having a first portion extending helically along said first feed channel and a second portion extending helically along said second feed channel, the bearing member slidingly engaging the guide members along said channels,
• the bearing member protruding from said second feed channel for frictional engagement with the tube of plastic material as it is extruded,
• whereby the bearing member is continuously advanced along said second feed channel and returned along said first feed channel without rotation in response to rotation of the shaft, thereby providing a continuously advancing support surface for the plastic tube.

In combination with an extrusion head for extruding a tube of plastic material, the extrusion head having an annular die opening defining a predetermined axis, a mold assembly comprising:

• an annular drive member journalled on the extrusion head for rotation about said axis,
• first and second cylindrical guide members extending coaxially from said drive member one within the other for rotation in unison about said axis,
• each said guide member having an internal surface providing a helical groove thereon, said helical grooves defining respective feed channels, said helical grooves being of the same pitch and opposite hand,
• an endless flexible bearing member carried by said guide members, the bearing member having first and second portions constrained by the guide members to extend helically along said feed channels, the bearing member slidingly engaging the guide members therealong,
• the bearing member protruding from the feed channel of the inner guide member for frictional engagement with the tube of plastic material as it is formed,
• means for rotating the drive member whereby to rotate said guide members about said axis,
• said protruding portion of the bearing member thereby defining a non-rotating continuously advancing mold surface for the plastic material, and
• means for expanding the tube of plastic material in engagement with said mold surface.

A shaft seal for a shaft journalled for rotation in a housing, the shaft having an extension protruding from a wall of the housing, comprising:

• first and second cylindrical guide members mounted on said shaft extension for rotation therewith, said members being disposed coaxially one within the other,
• each of said members having an external helical groove thereon defining a feed channel,
• said helical grooves being of the same pitch and opposite hand,
• an endless flexible belt carried by said guide members, the belt having respective first and second portions constrained by the guide members to extend helically along said feed channels, the belt slidingly engaging the guide members therealong,
• a cylindrical wall member mounted on said wall of the housing and extending therefrom coaxially with the guide members,
• the belt protruding from the guide channel of the outer guide member for frictional engagement with said wall member, whereby the belt is continuously advanced along one said feed channel and returned along the other without rotation in response to rotation of the shaft, successive adjacent convolutions of the belt being disposed in edge-abutting relationship thereby forming a sliding seal between the protruding portion of the belt and the inner surface of said wall member.

A rotary mandrel comprising:

• a rotary shaft,
• first and second cylindrical guide members mounted coaxially on said shaft for rotation therewith, the guide members having respective external helical grooves thereon defining first and second feed channels,
• said helical grooves being of the same pitch and opposite hand,
• an endless flexible bearing member carried by said guide members and constrained thereby, the belt having a first portion extending helically along said first feed channel and a second portion extending helically along said second feed channel, the bearing member slidingly engaging the guide members along said channels,
• the second portion of the bearing member protruding from said second channel and successive adjacent convolutions thereof being disposed in edge-abutting relationship to define a continuous mandrel support surface, and
• differential drive means for effecting rotation of said support surface in response to rotation of the shaft at a speed different therefrom,
• said differential drive means comprising a plurality of rollers frictionally engaging said first portion of the bearing member, the rollers extending parallel to the shaft, and epicyclic drive means coupled to the rollers and said shaft for rotating the rollers in response to rotation of the shaft in the direction opposite thereto.