Adjustable sizing tube

There is provided a plastic extrusion sizing tube having a cylinder with a split along its length to allow for radial expansion and reduction of the sizing tube. The cylinder has holes to allow an extrusion within the cylinder to be drawn to the cylinder by a vacuum outside the cylinder. The cylinder also has extensions on opposite sides of the split with an adjustment screw connecting the extensions. The diameter of the cylinder can be expanded or reduced when the adjustment screw is turned in respective directions. A set screw can be connected to one of the extensions. The diameter of the cylinder can be expanded or reduced when the set screw is turned in respective directions. The diameter of the cylinder can be locked in a desired position when the adjustment screw and the set screw are turned in the same direction. The adjustment screw and the set screw can be located in a flange at one end of the cylinder. The holes in the cylinder can be slots.

BACKGROUND OF THE INVENTION

When extruding polyvinyl chloride (PVC) pipe, an adjustable sizing tube is used to guide, create and maintain the final outside dimensional configuration of the extruded pipe material before the pipe material has completely hardened. The sizing tube is located within a vacuum-calibration tank (VCT) which is located downstream from an extruder and respective extrusion die. The sizing tube can be either fixed or adjustable. However, the fixed sizing tube is made from a rigid-walled tube and cannot be adjusted. Both types of sizing tubes are characterized by circumferential slotted channel configurations.

As shown inFIGS. 1A and 1B, a prior art adjustable sizing tube10has various slotted channel14configurations cut circumferentially throughout the length of the sizing tube10. The slotted channels14allow a negative vacuum created in the VCT's chamber to “pull” the extruded pipe material against the inner wall of the sizing tube10to maintain the required outer diameter dimensional characteristics of the extruded pipe.

As shown inFIG. 1B, the prior art adjustable sizing tube10has a single slot18cut the entire length of the tube10which allows for minor reductions to the diameter of the sizing tube10.

SUMMARY OF THE INVENTION

The prior art adjustable sizing tube10as shown inFIGS. 1A and 1Ballow only for compression/reduction of the diameter of the sizing tube10. To reduce the diameter of the pipe, strapping devices22are placed over the sizing tube10after it is installed in the vacuum-calibration tank (VCT). The strapping devices22are tightened or loosened, respectively, to achieve the desired extruded pipe diameter. Since the strapping devices22can only make the diameter of the sizing tube10smaller, the tolerances needed to machine the inner diameter of the tube10should error to a few thousandths of an inch in a larger diameter. The larger diameter is desirable because raw steel expresses an internal memory when machined. As layers of metal are removed from the sizing tube10, the engineered-to-specification wall thickness may cause the adjustable sizing tube10to spring-in or spring-out by a few thousands of an inch. The worst condition is the spring-in effect because the diameter of the sizing tube10can only be reduced.

There is provided a plastic extrusion sizing tube having a cylinder with a split along its length to allow for radial expansion and reduction of the sizing tube. The cylinder has holes to allow an extrusion within the cylinder to be drawn to the cylinder by a vacuum outside the cylinder. The cylinder also has extensions on opposite sides of the split with an adjustment screw connecting the extensions. The diameter of the cylinder can be expanded or reduced when the adjustment screw is turned in respective directions.

A set screw can be connected to one of the extensions. The diameter of the cylinder can be expanded or reduced when the set screw is turned in respective directions. The diameter of the cylinder can be locked in a desired position when the adjustment screw and the set screw are turned in the same direction. The adjustment screw and the set screw can be located in a flange at one end of the cylinder. The holes in the cylinder can be slots.

DETAILED DESCRIPTION OF THE INVENTION

In an extrusion system26as shown inFIGS. 2-3B, an extrusion such as extruded PVC pipe is formed by feeding plastic pellets into an extruder30where it is subjected to high temperatures to create a molten substrate. The substrate or melt proceeds through an extrusion head, at the end of which the melt passes through an extrusion die34. The die34contains the circular cross-sectional profile shape to be extruded. The extruded pipe38exits the die34and flows in direction40into a vacuum calibration tank (VCT)42. The VCT42cools the walls of the extruded pipe38to maintain a fixed shape while it passes through VCT42. An adjustable sizing tube50is located within the VCT42to maintain the outer diameter of the extruded pipe38. The VCT42draws a negative vacuum which maintains the walls of the extruded pipe38against the inside diameter of the sizing tube50. The extruded pipe38proceeds from the VCT42to a spray tank54for further cooling of the extruded pipe38. The now hardened pipe38proceeds through a pulling machine66which “pulls” the extruded pipe38from the die34through the system26. The hardened pipe38exits the pulling machine66and enters a cutting machine70which automatically cuts the pipe38to a desired length.

As shown inFIGS. 4A-5D, the adjustable sizing tube50has slotted channels14cut circumferentially throughout the sizing tube50. The adjustable sizing tube50has a single slot18cut the entire length of the tube50to allow for both reduction and expansion of the tube50. An adjustment flange74(an example of extensions) located at the end of tube50allows for the reduction and expansion of the slot18to maintain a desired outer diameter of the extruded pipe38.

The adjustment flange74contains at least two adjustment/set screws78,82which allow for both a controlled reduction and expansion of the sizing tube's50diameter. While the extruded pipe38flows through the adjustable sizing tube50the adjustment screws78,82are either tightened or loosened to reduce or expand the slot18. Contraction of the slot18reduces the diameter of the sizing tube50and expansion of the slot18expands the diameter of the tube50.

The flange74contains a lip75and seating surface76which are located on opposite sides of the split18. The lip75contains two holes for accepting the adjustment screws78,82. Hole80is threaded for connecting adjustment screw82. Hole80allows adjustment screw78to slide through and connect to threaded hole79in the seating surface76.

To reduce the diameter of the sizing tube50, adjustment screw82must be loosened to create a gap77(FIG. 5B) between the end of the screw and the seating surface76. Adjustment screw78is then tightened until the desired diameter is achieved or the end of adjustment screw82reaches the seating surface76. Further loosening of adjustment screw82may be required to obtain the desired diameter. Once the desired diameter is achieved, adjustment screw82is tightened until it reaches the seating surface76thereby “locking” the diameter in place.

To expand the diameter of the sizing tube50, adjustment screw78must be loosened to create a gap83(FIG. 5C) between the head of the screw and the lip75. Adjustment screw82is then tightened until the desired diameter is achieved or the head of adjustment screw78reaches the lip75. Further loosening of adjustment screw78may be required to obtain the desired diameter. Once the desired diameter is achieved, adjustment screw78is tightened until it reaches the lip75thereby “locking” the diameter in place.

The adjustment screws78,82allow for adjustments measurable by a few thousands of an inch. The “locking” prevents the diameter of the sizing tube50from wandering. It should be understood that any type of device capable of expanding or reducing the diameter of the sizing tube50can be used. One device may be a single screw that can expand and reduce the diameter of the sizing tube50and a set screw used to prevent the diameter from wandering. The process can also be automated by using a hydraulic piston or other type device.