FINER POINTS IN CONTROLLING THICKNESS VARIATION IN UPVC PIPES

Introduction:

UPVC pipes are sold on length basis as per customer or applicable standard requirements.

Therefore, ideally allowing any thickness increase more than the minimum thickness required will amount to additional PVC compound going to customer free of cost. It is a non value adding activity.

In reality one cannot produce pipes with exact thickness along the circumference.

Minimum thickness for UPVC pipe mentioned in IS 4985:2000 is the actual minimum thickness, rounded off to next higher digit. Thus, 90 mm 0.4 MPa the thickness of 2.04 mm as per calculation is rounded off to 2.1 mm.

Controlling the thickness variation, while maintaining minimum thickness during processing, is the most appropriate way to save losses. Higher the diameter and thicknesses of the pipe, more are the losses, especially in higher output machines.

Thickness variation is the sum total of various cause as below:

1.      Variation in the compound:

·        Over lubrication of the compound

·        Variation in the consistency of compound due to imperfect mixing procedure.

2.      Variation in process parameters:

·        Variation in dozing from the hopper if the dry blend is not free flowing.

·        Variation in the RPM of the screw, haul off etc,

·        Variation in the process variables like load, temperature etc.

3.      Variation in the tools:

·        Variation in the mandrel and bush diameter

·        Variation in chrome plating

4.      Variation in the procedure:

·        Variation in the tightening of die assembly

·        Non alignment of die, sizer, cooling tank, haul off etc.

5.      Variation in the temperatures

·        Variation in temperature of compound in the hopper (should be 40 – 45° C)

·        Variation in temperature of extrudate along the circumference ( should be 180-190° C)

·        Variation in cooling of the extrudate due to breeze from the nearby window etc.

6.      Variation in operator skill

·        To fix the die or adjustment in eccentricity

Let us consider tools alignment related aspects not normally addressed –

Centering bolts:

Not all the industries can afford software that displays thickness variation on the monitoring screen so that operator can adjust it with ease; leave apart the close loop system that alters the temperatures in the segmented bush to correct the thickness.

In such places, operator keeps on adjusting centering in order to minimize thickness variation. But the thickness is not adjusted to his satisfaction.

It is advisable to use shorter length of allen bolts for centering. Also, if allen bolts having more number of threads per cm are used, the finer adjustment becomes easier.

Ovality in the mandrel and bush:

It is observed especially with older dies that the pipe has higher thickness on opposite sides and lower thickness on the perpendicular sides. An operator is unable to adjust the eccentricity.

On inspection, it was found that the mandrel as well as bush has become slightly oval over a period of time. This aspect is neglected by the processor.

Fitting of the die assembly:

Fitting of the die assembly is done by the operator, or die fitter’s team.

He uses Allen keys for tightening the flange with the use of G.I. pipe inserted in to the allen key.

He is assisted on the other side by the helper.

Variation in tightening forces comes from –

1.      The difference between physical strength of the operator and the helper,

2.      The difference in the length of pipe inserted into the allen key by them,

3.      Method of tightening etc.

 After the heating period is over, there is slackness due to expansion of the die parts and bolts. It is mandatory to ensure the tightness once again by retightening, before starting the production. It is advisable to apply high temperature grease to the threads of the bolts.

Over tightening of the flanges results in inability to adjust the eccentricity and sometimes, the allen bolt breaks either due to over tightening or weaker bolt used and has to be removed by welding process.

The correct way is to use bolts of appropriate size and torque level (given by the manufacturer of allen bolts) and use torque wrench of appropriate size for tightening to prefixed level. Deploying a separate well trained team for die fitting as per work instructions is advantageous.

Alignment:

Basic principle of engineering is that – the extruder, die, sizer, cooling tank, haul off and cutter must be levelled and aligned in all the three axes.

Let us assume that the die gap is uniform before starting production.

And the sizer is 0.5 mm off centre (in any direction).

Therefore during production, the extrudate will touch the sizer on one side and will undergo friction.

The diagonally opposite side of the extrudate will be slightly loose and offer very less friction.

This will cause differential stretching resulting in bad surface, differential thickness, stresses and reversion variation.

The root cause is misalignment, but the operator will often adjust the eccentricity to correct the thickness, introducing more faults in the quality of pipe.