External and internal calibration systems to size the extruded plastic pipes such as pvc pipes.

1. CALIBRATION SYSTEMS
Presented by,
Vishal K P.
Dept. of PS&RT
CUSAT

2. Introduction
• Extrudate emerges out from die as semi-molten
substance and need to be cooled to harden.
• Calibration systems or Extrusion sizing chills and
sizes the extrudate without much defects.
• For tubular films, cold air impingement may be
sufficient, but for rigid tubes more positive
methods must be employed.
• They are also known as forming boxes, sizing
formers, sizing die, etc.

3. Two types of calibrators for rigid &
semi-rigid tubes:
• External calibrators
• Internal calibrators

4. External calibrators
• Employs a water-cooled forming tube in
conjunction with internal air pressure.
• It consist of a water jacketed tube, the bore of
which is made to the required outer diameter
of the pipe + any allowances.
• This assembly is mounted in exact alignment
at the front of the extruder die.

5. Working:
• The thermoplastics tube is continuously
inflated from inside such that it comes in
sliding contact with the chilled interior wall of
the calibrator.
• The outer layer become frozen and become
supporting.
• The final cooling is done by passing through a
water bath.

6. Length of the calibrator tube:
• Length must be chosen such that the outer
layer of the pipe gets frozen to a sufficient
depth for self supporting during cooling.
• The shortest length must be chosen so as to
avoid friction, strain and inconvenience.

7. Problems:
• Reason: The tubes are inflated by internal
pressure, so must be sealed at both ends to
prevent air loss.
• Commonly used system to prevent air loss:
Floating plug attached to the die mandrel by
means of a chain or other flexible members.
Air loss through ends of the tube

8. • Vacuum is used to minimize the pressure loss.
• Two methods of vacuum sizing are used:
*Single unit, alternative cooling and vacuum
sections control the sizing of the tube.
*Longitudinally adjustable vacuum sizing units
within a cooling tank.

9. Internal calibrators
• Employs a cooled mandrel and secondary
external cooling.
• It consist of a tapered water cooled extension
to the mandrel which fixes the diameter of
pipe as in external calibration.
• It cools the tube both from inside and outside.
• It has higher rate of cooling hence sizing of the
tube.

10. • The extrudate is drawn over the extended
mandrel-It tends to shrinks on cooling.
• The shrinkage provides the calibration pressure
to size the product.
• The usual tapering dimensions are 0.004mm per
mm of extended mandrel length.
• Mandrel temperature for medium density P.E-
55°C, for rigid PVC pipe-lower ranges.

11. Drawbacks of Internal and External
calibrators
• In external system, the outside of the tube is
in sliding contact with smooth cooled metal
surface and gives polished surface, but interior
remains rough.
• The internal system gives better product and it
give a smooth product both from inside and
outside.
A combined external-internal system can be
used to overcome the drawbacks.

12. Combined External and Internal
calibration
• Here, a short water cooled external calibrator
is combined with an extended mandrel which
is heated at its upstream end.
• A high finish is given to the tube exterior by
contact with external calibrator, and by means
of carefully designed internal flow channels.

13. References

14. THANK YOU