Calibration mandrel

Each profile model needs its die, mandrel, calibrators and cooling fixtures whose cost is charged on each run. Consequently, the costs of the profiles depend on ... 

Calibration is a technique for maintaining the shape and dimensions of the product from the time the melt exits from the die until it can be cooled enough to solidify and stabilize it. Depending on the size and shape of the product, calibrators can use water-cooled plates, internal mandrel, internal air pressure, external vacuum, or a post-extrusion die to change the shape of the molten extrudate before it solidifies. 

The extruder and die, as well as down-stream devices for the outside and inside calibration of the pipes cross sectional area, if required, use air pressure and/or vacuum to contain the pipe shape. Wall thickness measuring device, mandrel designs (such as while water cools outside ... 

Inward expansion (Celuka process) where the extrudate is fed on to the calibrator walls via a die mandrel, the torpedo creating a space into which the extrudate expands. The die and calibrator are... 

Figure 5.50 Calibration methods for pipe extrusion (a) extended water-cooled mandrel, (b) vacuum forming, and (c) pressure forming.

While barrel, die, and water-bath temperatures and extrusion rate can be varied in pipe extrusion, the primary controls are the die gap, calibration rings or extended mandrel, and the take-off speed. Die gap and centering of the mandrel provide the initial wall thickness and thickness uniformity, whereas take-off speed determines the final thickness. While extended mandrels fix the inner diameter, calibration rings establish the outer diameter in vacuum and pressure cahbration systems. The other diameter varies with the take-off rate. Water-bath temperatures and temperature profiles control the cooling rate for the pipe or tubing. This alters the shrinkage, crystallinity, and retained stresses in the products. 

The air pressure is maintained by a sealing mandrel located inside the extrudate downstream of the calibrator. The mandrel is attached to the tip by a cable to fix its position. This type of calibration is used in larger diameter pipe for PVC (D > 350 mm) and PE (D > 100 mm). An example of an external vacuum calibrator is shown in Fig. 9.47. 

Figure 7 Internal calibration using a tapered mandrel.

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