Swept leg spider for an extrusion apparatus

A spider for an extrusion apparatus located upstream from a die, through which a profile is extruded. The spider can have an outer housing with a central flow passage therethrough. An inner hub can be positioned within the central passage for supporting an inner portion of the die. At least one spider leg can be secured to the outer housing and the inner hub, and support the inner hub within the central passage. The at least one spider leg can recess radially rearwardly outwardly in the downstream direction such that molten polymer flowing through the central passage flows around the at least one spider leg, separating and rejoining together earlier in inner radial regions close to the inner hub than in outer radial regions close to the outer housing for reducing spider lines on inside surfaces of extruded profiles.

BACKGROUND

In the prior art, when extruding plastic pipe with an extrusion die having inner and outer die portions, the inner die portion is typically secured to a central or inner hub of a spider pipe. The central hub is supported within the spider pipe by straight spider webs or legs. Polymer flowing through the spider pipe towards the dies must flow around the spider legs, separating at the upstream side of the spider legs and then rejoining together at the downstream side. In some situations, the polymer rejoins together in an incomplete manner, resulting in weld or spider lines on the inside of the pipe when extruded, and making the wall thickness of the pipe uneven. The wall thickness of the pipe is thinner along the weld or spider lines, and thicker in the regions between the spider lines.

SUMMARY

The present invention provides a spider or spider pipe for an extrusion apparatus that can be located upstream from a die for extruding profiles such as pipe, and can have a configuration that can reduce spider lines within extruded profiles or pipe.

The spider can have an outer housing with a central flow passage therethrough. An inner hub can be positioned within the central passage for supporting an inner portion of the die. At least one spider leg can be secured to the outer housing and the inner hub, and support the inner hub within the central passage. The at least one spider leg can recess radially rearwardly outwardly in the downstream direction such that molten polymer flowing through the central passage flows around the at least one spider leg, separating and rejoining together earlier in inner radial regions close to the inner hub than in outer radial regions close to the outer housing for reducing spider lines on inside surfaces of extruded profiles.

In particular embodiments, the at least one spider leg can recess radially rearwardly outwardly in a configuration to separate and rejoin molten polymer starting earlier in the inner radial regions and subsequently progressively continuing moving radially outward to the outer radial regions. The at least one spider leg can have upstream and downstream edges which are angled radially rearwardly outwardly in the downstream direction at the same angle relative to a flow direction axis. The at least one spider leg can have a longitudinal flow direction length between the angled upstream and downstream edges that is generally constant at any radial location of the at least one spider leg. The upstream and downstream edges can be pointed for reduced flow resistance. The spider can have three equally spaced spider legs for supporting the inner hub. The profile in some embodiments can be a pipe.

The present invention can also provide a spider for an extrusion apparatus upstream from a die for extruding pipe including an outer housing with a central flow passage therethrough. An inner hub can be positioned within the central passage for supporting an inner portion of the die. A plurality of spaced spider legs can be secured to the outer housing and the inner hub, and support the inner hub within the central passage. The spider legs can be angled radially rearwardly outwardly in the downstream direction such that molten polymer flowing through the central passage flows around the spider legs, separating and rejoining together earlier in inner radial regions close to the inner hub and subsequently progressively continuing moving radially outward to outer radial regions close to the outer housing for reducing spider lines on inside surfaces of extruded pipe. The spider legs can have upstream and downstream edges which are angled radially rearwardly outwardly in the downstream direction at the same angle relative to a flow direction axis.

The present invention can also provide a method of reducing spider lines in a profile extruded through a die having an inner die portion supported by a spider positioned upstream from the die. The spider can have an outer housing with a central flow passage therethrough. An inner hub can be positioned within the central passage for supporting the inner die portion. The inner hub can be supported within the central passage with at least one spider leg secured to the outer housing and the inner hub. The at least one spider leg can recess radially rearwardly outwardly in the downstream direction such that molten polymer flowing through the central passage flows around the at least one spider leg, separating and rejoining together earlier in inner radial regions close to the inner hub than in outer radial regions close to the outer housing for reducing spider lines on inside surfaces of the profile.

In particular embodiments, the at least one spider leg can recess radially rearwardly outwardly in a configuration to separate and rejoin molten polymer starting earlier in the inner radial regions and subsequently progressively continuing moving radially outward to the outer radial regions. Upstream and downstream edges of the at least one spider leg can be angled radially rearwardly outwardly in the downstream direction at the same angle relative to a flow direction axis. The at least one spider leg can have a longitudinal flow direction length between the angled upstream and downstream edges that is generally constant at any radial location of the at least one spider leg. The upstream and downstream edges can be pointed for reducing flow resistance. The inner hub can be supported with three equally spaced spider legs. The spider lines can be reduced in an extruded pipe.

DETAILED DESCRIPTION

Referring toFIGS. 1-8, spider or spider pipe16can be employed in a plastic extrusion apparatus10for extruding profiles such as plastic pipe, for example, PVC pipe. The spider16can be positioned inline with passage11of extrusion apparatus10between upstream and downstream members or portions10aand10balong longitudinal flow axis A. Upstream member10acan have an upstream passage portion11aconnected to the upstream end16aof spider16, and downstream member10bcan have a downstream passage portion11bconnected to the downstream end16bof spider16. The spider16can be upstream of an extrusion die14having inner14aand outer14bdie portions, and secured in place by bolts34through holes32positioned along a circumferential bolt circle. The inner14aand outer14bdie portions can form an extrusion gap14ctherebetween, through which molten polymer12is extruded to form a desired profile, such as a pipe. The extrusion gap14cis shown to be circular, but can be any suitable shape. The gap14ccan be adjusted with an adjustment mechanism, device, apparatus or arrangement13.

The spider16can have a cylindrical outer wall, ring or housing17surrounding a central flow cavity or passage20for receiving molten polymer12, and can support the inner die portion14aof the extrusion die14along the flow direction and longitudinal axis A. The central flow passage20can be generally circular or round. A central or inner hub15can be positioned in the center or along the longitudinal axis A of the central flow passage20for supporting the inner die portion14aalong longitudinal axis A, and can be generally rod or cylindrical shaped, and elongate. Consequently, the positioning of the inner hub15within the central flow passage20can form a generally annular flow path or passage20athrough the spider16. The inner hub15can have an upstream female threaded hole15afor securing a contoured, pointed or generally cone shaped flow member19by engaging a male threaded stem or member22. The inner hub15can also have a downstream female threaded hole15bfor securing the inner die portion14aby engaging a male threaded stem or member24at the base26of inner die portion14a. The flow member19and the base26of the inner die portion14acan be shaped to provide a smooth flow transition with the inner hub15once tightened thereto.

The inner hub15can be supported by a plurality of fin shaped spider webs struts, supports, members or legs18, for example, three equally spaced spider legs18(120° apart). The spider legs18can be integrally formed, connected, secured or extended between the outer ring17and the inner hub15. The longitudinal flow direction length L of the spider legs18along or parallel to the longitudinal axis A can be greater than the thickness or width W. The spider legs18can have a midsection18cand have pointed or reduced thickness contoured upstream18aand downstream18bedges for reduced flow resistance as the polymer12separates to flow around the spider legs18while flowing through central flow passage20. The polymer12flowing around the sides21of the spider legs18recombines before flowing through the die14.

FIG. 9shows a profile40such as a pipe having an inner surface44with spider lines42, and a round theoretical inner surface46for comparison. In order to prevent or reduce spider lines42on the inner surface44in an extruded profile40, such as a PVC pipe, the upstream edge18aof the spider legs18can be progressively inclined, recessed, directed, extended or angled backwardly or radially rearwardly outwardly in the downstream direction. This radial outward direction, is radially outward relative to the longitudinal axis A. The backwardly directed angled upstream edge18aof the spider legs18can progressively radially outwardly cut, split or separate the flow of the molten polymer12flowing around the spider legs18through the central flow passage20along longitudinal axis A. Such progressive cutting by the backwardly outwardly angled upstream edge18acan in some cases more easily cut through the molten polymer12than a straight or vertical upstream edge. In addition, the downstream edge18bcan be directed or angled backwardly or radially rearwardly outwardly in the downstream direction by the same amount or angle θ (FIG. 10). The backwardly directed or angled spider legs18allow the molten polymer12flowing around the spider legs18to separate at the upstream edge18aand rejoin or recombine on the downstream edge18bfirst or earlier in the generally central or inner radial regions of the central flow passage20at, close to, near, or around the inner hub15. The molten polymer12flowing past the backwardly directed or angled spider legs18at radially outward locations of the central flow passage20, moving towards, at or closer to the outer ring17, separates and rejoins progressively, subsequently and sequentially later, as the molten polymer12flows longitudinally past the spider legs18. This can provide a smoother transition of flow around the spider legs18to reduce spider lines and obtain an inner surface approaching or similar to theoretical surface46.

Referring toFIG. 10, when the upstream18aand downstream18bedges are angled at the same angle θ relative to longitudinal axis A, the distance between the upstream18aand downstream18bedges along or parallel to the longitudinal axis A, forms a longitudinal flow direction length L which is generally constant over the radial height H or at any radial location on the spider legs18. As a result, the molten polymer12flows over an equal amount of longitudinal flow direction length L of the spider legs18along longitudinal axis A, regardless of the radial location or height on the spider legs18or position within central flow passage20. This can provide a consistent flow speed, friction and temperature along different radial locations on the spider legs18which can aid in providing a consistent extruded profile with reduced spider lines. Spider legs that have different longitudinal flow lengths at different radial locations can result in inconsistent flow rate, friction and temperature of the molten polymer. Furthermore, as can be seen inFIG. 7, the opposing sides21of the spider legs18can have surfaces that have the same shape so that the molten polymer12will have the same or similar flow characteristics on both sides21.

In some embodiments, the molten polymer12flowing around the sides21of the spider legs18can rejoin together in the central or radially inward regions of the central flow passage20, at or close to the inner hub15, between about 1 or 2 inches earlier or further upstream, or about 0.5 to 0.1 or 0.5 to 2 seconds faster, than the molten polymer12rejoining at the radially outward regions, at or close to the outer ring17. This can be illustrated inFIG. 10, where the upstream18aand downstream18bedges of a spider leg18are shown angled at the same angle θ. The longitudinal flow direction length L is the same at different radial locations, positions or heights on the spider leg18, such as at h1and h2. Radial height h1is shown located close to the inner hub15at a radially inward region, and the radial height h2is shown close to the outer ring17at a radially outward region. Molten polymer12flowing through the central flow passage20can separate at radial height h1at point50aon upstream edge18aand rejoin at point50bon downstream edge18b, and can separate at radial height h2at point52aon upstream edge18a, and rejoin at point52bon downstream edge18b. As can be seen, the molten polymer12flowing in the direction of longitudinal axis A separates at the upstream edge18a, first or earlier at radial height h1or point50a, by a distance d before separating at radial height h2or point52a, and also rejoins at the downstream edge18bfirst or earlier at radial height h1or point50b, by a distance d before rejoining at radial height h2or point52b. It is understood that molten polymer12will separate and rejoin later at any radially outward location or region relative to any radially inward location or region, or conversely, separate and rejoin first or earlier at any radially inward location or region relative to any radially outward location or region. The separating and rejoining of the molten polymer12starts first or earlier at the inner radial regions, and subsequently progressively continues moving downstream and radially outward relative to longitudinal axis A to the outer radial regions on spider leg18. The extra length, distance or time that molten polymer12can mix, before or earlier, at or near the radially inward regions of central flow passage20to recombine or mix together, allows the molten polymer12to recombine or mix together more completely at those regions than at or near the radially outward regions, that mix subsequently or later. Also, the backwardly outwardly angled upstream18aand/or downstream18bedges of the spider legs18can in some cases, direct some molten polymer12radially outwardly, and cause another mixing action, which starts earlier at inner radial regions close to the inner hub15. As a result, when the molten polymer12passes through the die14, the molten polymer12in the radially inward locations of central flow passage20when extruded through the die14, is on the inner surface of the extruded pipe and is more completely recombined or mixed, resulting in reduced weld or spider lines. Since the wall thickness of extruded pipe is measured at the thinnest location, which is at the spider lines, a pipe with spider lines contains more polymer than in a pipe with reduced spider lines. Therefore, a pipe with reduced spider lines has a more consistent wall thickness and uses less material, resulting in cost savings.

In one embodiment of the spider16, the outer ring17can have a diameter of about 5 inches. The central flow passage20can be about 4.3 inches long and about 3.3 inches in diameter. The radial height H of the spider legs18can be about ⅞ inches and the upstream18aand downstream edges18bof the spider legs18can be at about a 60° angle relative to longitudinal axis A. The upstream18aand downstream18bedges can also be pointed about 35° on the side surfaces, as seen inFIG. 7. It is understood that other suitable angles other than 60° and 35° can be employed. The longitudinal flow direction length L of the spider legs18along or parallel to longitudinal axis A, can be about 2.9 or 3 inches long. The upstream edge18aof the spider legs18at the location joining the inner hub15can be recessed within the central flow passage20, and the downstream edge18bat the location joining the inner hub15can be recessed a greater distance due to the angle of the downstream edge18b. The spider legs18can join the inner hub15with a radius37, and can join the outer ring17with a radius38(FIG. 6). Radius37and Radius38can be about ¼ inch. The inner hub15can have a diameter of about 1.5 inches and can be the same length as the central passage20. The female threaded holes15aand15bin the inner hub15can be M30×3.5-6H threads about 1½ inches deep. A counterbore about 1¼ inches in diameter and about 0.2 inches deep can be located at the entrance of the threaded holes15aand15bfor engaging with locating diameters22aand26aof the flow member19and base26of the inner die portion14a. The upstream16aand downstream16bends of the spider16can have a shoulder or lip which can be about 3.6 inches in diameter, can protrude about 0.2 inches, and can aid in aligning and sealing the spider16. One or more bore holes28can extend through the outer ring17into the inner hub15through one or more of the spider legs18. The bore holes28can include a threaded hole28ain the outer ring17into which a plug30can be secured. The bore holes28can be used for supply lines, such as power and/or control lines to devices or instruments, for example, for temperature probes, heating elements, pressure sensors, etc. The bores28can be about 0.3 inches in diameter and the threaded hole28acan be a ⅜ pipe thread.

Although the upstream18aand downstream18bedges of the spider legs18have been shown angled, in some embodiments, the edges18aand18bcan recess with other suitable configurations, such as with curves. The amount of rearward recession, formed by angling and/or curving or other configurations, does not have to be constant. In addition, more or less than three spider legs18can be employed (at least one), for example, one, two, four, etc. Furthermore, depending upon the situation at hand, it is understood that a range of different dimensions are contemplated for different sized spiders16, or for different features. Also, the spider16, central flow passage20and inner hub15do not have to be round in cross section but can have other suitable shapes.