Method and apparatus for forming a striped extrusion
A polymer extrusion assembly for forming a striped extrusion. The assembly includes conventional components for conveying a primary and a secondary molten polymer of differing colors, arranged concentrically, toward a die sub-assembly, which may be rotatable. The die has an opening for conveying the primary polymer to form an extruded tube. The periphery of the die opening is modified to cause thinning or breaching of the secondary polymer skin layer as it passes through the die. The periphery may be provided with teeth which furrow the skin layer, exposing the primary polymer underneath, or the periphery may be comprise a plurality of facets meeting at sharp internal corners, wherein the skin layer is forced into the corners and is breached at the centers of the facets, again exposing the primary polymer underneath in longitudinal stripes. By causing the die sub-assembly to rotate during extrusion, a spirally striped extrusion is formed.
1 . An extrusion head assembly for continuous extrusion of an elongate element of a primary molten polymer and a secondary molten polymer, disposed in a layer coaxially around the primary molten polymer, wherein the secondary molten polymer layer is thinned or breached to visually expose a continuous stripe of the primary molten polymer underneath, the primary and secondary molten polymers differing visually and being supplied from first and second sources, comprising:
a) a fixed member for conveying said primary molten polymer and said secondary molten polymer in a coaxial annular stream axially of an axis of said head; and
b) a die sub-assembly rotatably disposed on said fixed member for receiving from said fixed member said annular stream of primary and secondary molten polymers and extruding said elongate element, said die sub-assembly including an extrusion die having an opening for extruding said elongate element, the periphery of said opening including means for varying the thickness of said secondary polymer layer at one or more selected angular locations of said die opening to form visual stripes in said elongate element resulting from said thickness varying.
2 . An extrusion head assembly in accordance with claim 1 wherein said means for varying the thickness of said secondary polymer layer include at least one protrusion extending inwards from said periphery for forming a furrow in said layer to expose a stripe of said primary polymer underneath.
3 . An extrusion head assembly in accordance with claim 2 comprising a plurality of said protrusions arranged angularly about said opening for forming a plurality of said furrows.
4 . An extrusion head assembly in accordance with claim 2 wherein said stripe is formed spirally of said extrusion by rotating of said die sub-assembly.
5 . An extrusion head assembly in accordance with claim 1 wherein said means for varying the thickness of said secondary polymer layer includes at least one facet for causing a thinning of said secondary polymer layer to expose a stripe of said primary polymer underneath.
6 . An extrusion head assembly in accordance with claim 5 comprising a plurality of said facets arranged angularly about said opening for causing a plurality of said thinnings to expose a plurality of stripes of said primary polymer underneath.
7 . An extrusion head assembly in accordance with claim 5 wherein said stripe is formed spirally of said extrusion by rotating of said die sub-assembly.
8 . An extrusion die for forming at least one visual stripe in an extrusion of dissimilar first and second molten polymers arranged concentrically, a second polymer layer surrounding the first polymer, wherein an extrusion opening of said die includes at least one protrusion extending inwards from said periphery for forming a furrow in said layer to expose a strip of said primary polymer underneath.
9 . An extrusion die for forming at least one visual stripe in an extrusion of dissimilar first and second molten polymers arranged concentrically, a second polymer layer surrounding the first polymer, wherein an extrusion opening of said die includes at least one facet for a thinning of said secondary polymer layer to expose a stripe of said primary polymer underneath.
10 . A method for forming a visual longitudinal stripe in an elongate element having a cylindrical shape formed of a molten primary polymer surrounded by a coaxial molten secondary polymer, comprising the steps of:
a) providing a fixed member for conveying said primary molten polymer and said secondary molten polymer in a coaxial annular stream axially of an axis of said head; and
b) providing a die sub-assembly rotatably disposed on said fixed member for receiving from said fixed member said annular stream of primary and secondary molten polymers and extruding said elongate element, said die sub-assembly including an extrusion die having an opening for extruding said elongate element, the periphery of said opening including means for varying the thickness of said secondary polymer layer at one or more selected angular locations of said die opening to form visual stripes in said elongate element resulting from said thickness varying;
c) introducing said primary polymer into said fixed member;
d) forming said primary polymer into a shape;
e) introducing said secondary polymer into said fixed member;
f) arranging said secondary polymer into a layer surrounding said primary polymer; and
g) passing said layered shape through said die opening and past said means for varying the thickness of said secondary polymer layer to form said elongate element having said visual stripe resulting from said thickness varying in said secondary polymer layer.
11 . A method in accordance with claim 10 comprising the further step of rotating said die during said passing step such that said elongate striped element is spirally striped.