Sizing comprises the tooling and techniques that are used to manage the controlled shrinkage of an extruded tube or pipe from the time it exits the die as hot extrudate to the time that it has cooled to the dimensions of the finished product. A typical sizing process involves three key elements:
- Tooling used to calibrate the exterior of the tube, either by direct contact or by using a stream of water.
- Differential air pressure to shape the interior of the tube.
- Continuous heat transfer from the extruded product through a cooling medium.
Sizing tooling is almost always used in conjunction with a vacuum cooling tank. Vacuum cooling tanks do two vital things:
- They expose the external surface of the extruded pipe or tube to vacuum, so that the differential air pressure inside the tube or pipe maintains outward pressure on the sizing tooling. The outward pressure ensures that the finished product has a proper ID and OD.
- They cool the tube or pipe by immersing it in water or by continuously spraying it with water, enabling it to cool to its final dimensions.
Three approaches to sizing
There are three basic approaches to sizing, which dictate the selection of tooling based on the material that is being extruded. These approaches can be applied to everything from microbore medical catheters to flexible tubing to residential and commercial pipe.
Contact sizing is typically used for materials like polyethylene and polypropylene which have good surface lubricity. Contact sizing involves passing the hot extrudate through a fixture called a calibrator, which makes direct contact with the hot extrudate to control and reduce its size as it passes through. The tooling used for contact sizing typically consists of a wafer-disk calibrator or a sleeve calibrator, positioned at the entrance to a vacuum cooling tank.
|Wafer-disk calibrator||Sleeve calibrator|
Although these two calibration tools differ in appearance, their functions in contact sizing are essentially similar. The metal in successive wafer disks, or along the interior of the sleeve, pulls heat out of the extrudate and transfers it to the surrounding cooling water. This contact cooling process begins to “harden” the tubing and stabilize its dimensions.
The selection of a wafer-disk calibrator versus a sleeve design is dependent on several factors:
- Smaller diameter, thin-wall tubing made of polyolefin materials is normally run with a wafer-disk design and lower vacuum levels
- Larger diameter, heavier-wall tubing in stiffer materials like ABS and RPVC typically requires the use of a sleeve calibrator and higher vacuum levels. (Understandably, using a wafer-disk calibrator with high vacuum levels could cause the extrudate to expand upward between individual disks, resulting in “chatter.”)
- Line speeds also play a role in calibrator selection. For higher line speeds, wafer-style calibrators have less surface contact and thus reduce the levels of frictional drag on the extruded product.
Non-contact sizing is typically used for softer, stickier materials including flexible PVC, polyurethanes, and thermoplastic elastomers (TPEs). Extrudate passes out of the die and through a glass-filled PTFE disk positioned at the entrance to a vacuum cooling tank. However this non-contact calibrator is slightly oversized relative to the tube so it never makes contact. Instead, the extra space surrounding the tube is filled by a stream of cooling water, which surrounds and shapes the tube as both pass through the calibrator and are pulled into the vacuum cooling tank. The non-contact calibrator shown below is equipped with an adjustable iris. Positioned at the entrance to the vacuum cooling tank, it surrounds the incoming extrudate with a film of water that shapes, cools, and lubricates it as it enters the tank and shrinks down to size.
|Non-contact calibrator||Non-contact with entrance water well|
Hybrid sizing is a less common, but still important approach. It uses specially built tooling that combines various elements of non-contact and contact sizing for materials and applications that don’t fit neatly with either approach.
The drawing below shows a hybrid contact calibrator that combines a sleeve with wafer disks. This type of calibrator might be used for a sticky material like semi-rigid PVC that requires a high level of vacuum in the cooling tank. The sleeve calibrator contains the soft material under high vacuum while providing intensive initial cooling. Then, once the product is pre-skinned, the wafer disks allow for additional sizing while minimizing surface friction so that line speed can be maintained.
|Hybrid contact calibrator (sleeve plus disks)|
Sizing tooling is attached to the front of the cooling tank and can easily be changed, making your tank a versatile piece of equipment for multiple processes. Conair has helped thousands of processors identify the best sizing tooling for applications of all sizes and shapes. Always willing to help you determine the best tooling and equipment for your process, Conair is a great resource. An expert can walk you through options and even run testing in a fully-operational lab, simulating your process environment, eliminating days, weeks, or months of unnecessary “trial and error” learning time (and untold pounds of scrap) in your plant. Contact Conair for more information.