WHAT HAPPENS IN THE AIR GAP DURING HDPE PIPE EXTRUSION?

Introduction:

HDPE used in pipe is a predominantly crystalline polymer. It has 60-80% crystallinity.

Processing is carried out around 200° C, at temperature well above the Tm of HDPE. At this temperature all the crystalline structures are converted into an amorphous structure as the material exits the die.

Extruded plastics often have a higher melt viscosity, which allows the extrudate to retain the shape imparted to it by the die.

What is an Air gap?

The portion of extrudate between the extrusion die and the entry to the sizer is a critical area, known as “Air gap”.

This is the space in which the extruded resin is first seen by the operator.

There are many micro processes going on in this area. Even though correct extrusion processing is taking place, it will be difficult to tell exactly what is happening in the air gap.

The air gap should appear to the human eye as a completely undisturbed area. If there is any disturbance, there is some processing problem.

One of the reasons could be the alignment in the extrusion line elements from extruder through die – sizer – tanks – and haul off.

What happens in the Air gap?

After the material comes out of die, the HDPE extrudate undergoes sequence of micro processes –

1.      Die Swelling,

2.      Shrinking,

3.      Increase in density,

4.      Melt fracture, (under certain conditions)

5.      Air contact and surface oxidation,

6.      Stretching while the extrudate is hot (melt orientation),

7.      Draw resonance, (under certain conditions)

8.      Annealing while the material is hot before entering the sizer.

9.      Sizing ( vacuum sizing or pressure sizing)

10.  Cooling well below the solidification temperature.

These entire “micro processes” take place simultaneously, within the air gap.

Extrudate condition:

Swollen extrudate is the output of die. The shape of the HDPE extrudate in the air gap will be conical.

This die swell and its variation for HDPE depends upon –

1.      HDPE used – More the MFI, lesser the die swell.

2.      Die gap – Lesser the gap, more will be die swell

3.      Die land – longer the die land, lesser the die swell

4.      Eccentricity in the pipe wall will cause variation in die swell.

5.      Temperature of extrudate – more the temperature lesser is the die swell.

Ideally, the extrudate should have –

1.      Same thickness along the circumference, and

2.      Same temperature along the circumference.

It is beneficial to keep any random air currents from impinging on the “air gap” area, as these air currents would adversely affect the uniform cooling of the extrudate.

The air gap distance should also be held constant.

Usually, the air gap distance will be kept as short as possible to minimize the area where the extrudate can be negatively affected.

The air gap distance determines the rate of elongation that the melt cone or extrudate experiences in the air gap.

This is the area where hot HDPE comes in contact with oxygen from the air causing oxidative degradation at the surface. Antioxidants are not very effective in preventing oxidation in the air gap, because most of the aldehydes or acids are formed here.

There are two types of instabilities in the air gap:

1.      Melt fracture

2.      Draw resonance

Both melt fracture and draw resonance produce a decrease in extrudate quality which will often limit productivity.

The occurrences of these two types of instability are produced by different processing conditions.

Melt fracture starts to occur at some critical melt throughput rate, even without extrudate stretching or drawing.

On the other hand, draw resonance occurs only during extrudate stretching or drawing.

The effects of melt fracture and draw resonance:

The severity of surface deterioration ranges from a simple roughness, (sharkskin) to random helical configurations or gross distortions (melt fracture).

Sharkskin/melt fracture may result in -

·        Tensile failure,

·        Buckling of the extrudate,

·        Stick-slip phenomena of the polymer on the die surfaces.

Draw resonance is most interesting, and distinct from melt fracture.

·        “Draw resonance” is more cyclic in nature, having pulsations in the extrudate diameter.

·        It occurs only when the draw down ratio (DDR) and/or draw down rate reach a critical value at a fixed throughput rate.

·        The severity of draw resonance increases with increase in the draw down ratio and/or rate until the extrudate breaks.

Entry into a vacuum tank:

As the extrudate is drawn through the air gap, another motionless area is the extrudate entry into the quench tank.

This is quite challenging as the quench tank entry is occupied with running water.

The water is usually under strict control by the use of dams, diaphragms, and weirs to keep the water flow as constant as possible to prevent unstable or unsteady water contact with the melt cone.

This area is quite important, where the extrudate first makes contact with water from the quench tank creates an irreversible condition in the product.

Here, the outer surface of the extruded product is altered from a high-temperature viscous liquid state to a solid state.