Apparatus for the production of hollow bodies, especially large-diameter ducts from thermoplastic synthetic resin

An apparatus for making a duct comprising:

• an extruder having an extrusion die shaped to produce a continuous hollow profile thermoplastic strip in a thermoplastic state with a rectangular outer cross-section and a round inner cross-section;
• a coil-forming machine downstream of said extruder and comprising a coiling drum formed by a plurality of angularly equispaced coiling rollers, and means for synchronously driving said rollers in the same sense of rotation, said strip passing on to said rollers at one end and forming a helix around said rollers advancing axially toward the other end with said helix rotating in said sense of rotation;
• means for axially pressing said strip against a coil previously formed on said rollers to bond successive turns together to form the duct with the material of the axially pressed strip blending into the material of a previous turn, said coil-forming machine and said means for axially pressing said strip against a previously produced length of duct being constructed and arranged to axially advance said duct at a speed v.sub.1 and apply said strip such that the axial pressure applied to bond each turn to the previously formed turn results in an axial rate of increase in the length of said duct of v.sub.2, v.sub.1 being smaller than v.sub.2 and the difference (v.sub. 2 - v.sub.1) representing the compaction of the strip upon the application to the previously formed duct and the axial pressure applied to said strip in bonding same to the previously formed turns;
• and
• means including a duct extending through said die for passing a cooling liquid through said strip from said extruder through at least one turn of the duct formed on said coiling rollers, said coiling rollers being formed with smooth low friction peripheries at their respective one end and with braking sheaths of high friction at their other ends.