Method and apparatus for continuously extruding an expandable thermoplastic resin composition onto an elongated preform

Method for extruding a thermoplastic composition onto an elongated article to provide a layer of foamed thermoplastic composition thereon, comprising the steps of

• feeding the composition into an input end of an extruder housing containing a conveyor screw,
• adding cross-linking agents to the thermoplastic composition before such composition is fed into said input end of said extruder housing,
• conveying said composition along a conveyor screw and simultaneously compressing and heating said composition,
• providing while not compressing the composition a pressure retention chamber along a portion of the conveyor screw having a higher pressure than at other portions along the screw and maintaining the higher pressure substantially without pressure drop in said pressure retention chamber,
• injecting an inert gas at the higher pressure into said pressure retention chamber,
• extruding the resultant mixture about the elongated article.

An improved apparatus for extruding an expandable thermoplastic composition onto an elongated article to provide a layer of foamed thermoplastic composition thereon, comprising:

• an extruder housing having an inner surface defining an extruder barrel, an input end at which a composition is fed, an output end at which the composition is extruded, and means comprising a feed opening located in a vicinity of said input end and communicating with said extruder barrel for feeding of the composition therein;
• a rotatable extruder screw axially positioned within said extruder barrel having a ratio of length to diameter of at least 16:1;
• means for heating at least a portion of said extruder barrel and thereby the composition within said extruder barrel;
• gas input means extending into a portion of said extruder barrel;
• and a die head means attached to the output end of said extruder housing having a longitudinally extending aperture communicating with said extruder barrel and through which the elongated article may be passed for extrusion thereon of the composition;
• the improvement comprising:
• said extruder screw having a compression zone and ratio of length to diameter of at most 28:1 and comprising, within said extruder barrel, a shaft portion from whose circumference extend only
• a first helical conveyor part;
• means for providing and substantially maintaining a constant high pressure relative to that in other portions of the extruder barrel in a pressure retention chamber comprising only two axially spaced apart shearing members constituting a first shearing member having an outer diameter at portions substantially equal to a corresponding diameter of said inner surface of said extruder barrel positioned from said input end of said housing a distance of 1/2 to 2/3 the length of said extruder screw, but no further distant from said input end than said gas input means and a second shearing member having an outer diameter at portions substantially equal to the corresponding diameter of said inner surface of said extruder barrel positioned from said input end of said housing a distance of at least 4/5 the length of said cylindrical extruder screw;
• said shearing members each includes a plurality of obliquely extending ribs evenly disposed about the periphery thereof and said oblique ribs having an outermost surface, said outermost surface constituting said portions, said portions having a diameter substantially equal to a corresponding diameter of said inner surface of said extruder barrel and rotatably tightly slidably engaging said inner surface of said extruder housing and a protrusion between adjacent said ribs, said protrusions defining a minor distance spaced from said inner surface and said periphery of said shearing members undulating between said adjacent ribs defining a larger distance spaced from said inner surface than said minor distance and defining restricted volumes for transverse passage of the composition;
• and
• a second helical conveyor part disposed between said first and second shearing members, said shearing members each have an axial length several times greater than the axial thickness of said helical conveyor parts;
• and
• said compression zone being in said extruder barrel between said input end and said first shearing member,
• said gas input means positioned opposite to a longitudinal central portion of said first shearing member for injecting an inert gas at elevated pressure into said extruder barrel between said two shearing members, whereby, as said extruder screw is rotated, there is continuously provided (i) a flow path for the composition from said input end to said outlet end of said extruder housing and (ii) said pressure retention chamber defined by said first and second shearing members having therein said high pressure relative to the other portions of said extruder barrel with said high pressure being maintained substantially without change throughout said pressure retention chamber and defined in said extruder barrel delineated by said first and second shearing members, the inert gas being injected into said pressure retention chamber to increase the pressure applied to the composition passing through said pressure retention chamber to obtain an increased quantity of the inert gas dissolved in the composition.