WHY HDPE PIPE EXTRUSION NEEDS MULTIPLE COOLING TANKS WITH GAPS?

Introduction:

HDPE is a semicrystalline polymer having 60-80% crystallinity. Its Tg is (-) 125°C whereas its Tm is 133-137°C.

At room temperature, HDPE pipe is softer owing to its Tg of (–) 125 °C, and stronger to withstand hoops stresses owing to its crystallinity.

HDPE is processed at temperature well above its melting point at which, the original crystalline structure is destroyed completely and entire HDPE is converted into a disordered amorphous state.

During cooling, the crystalline structure has to be effectively recovered to regain its original mechanical strength throughout the pipe wall.

Standardizing cooling process is therefore necessary to bring consistency in quality.

It is worth noting that HDPE pipe have almost 2.5 times thickness than UPVC pipe for the same pressure rating and hence the recrystallization should be uniform across the wall thickness of pipe.

Rate of crystallization:

The longer the semicrystalline polymer remains midway between the Tg & Tm, the greater is the amount of crystallization.

Tg for HDPE is (-) 125 deg C, while Tm is + 135 deg C. Therefore the ideal recrystallization temperature would be + 5°C, midway between Tg and Tm. At shop floor temperature of around 40 °C, HDPE will continue to crystallize slowly, until equilibrium is reached.

It is reported that 85% of shrinkage will take place in 24 hrs, 98% will take place within a week and remaining shrinkage will be completed within 3 months. Crystallization continues until a stable crystal structure is achieved.

This raises a question:

After how many Hrs of manufacturing, we should test the mechanical properties of HDPE pipes and what should be the conditioning parameters? This has to be standardized.

Entry into quench tank:

As the extrudate is drawn through the air gap, extrudate entry into the quench tank (vacuum tank) is an apparently motionless area, in a well aligned die and production line.

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

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

The problem:

Since it is usually necessary for the extrudate to quickly freeze the outer dimensions to conform to a required round shape, the initial cooling usually needs to be adequate to set the shape (usually below 71 °C for HDPE pipe). Here, an amorphous structure can be temporarily “frozen in” with rapid cooling (quenching).

But since HDPE is poor conductor of heat, the outer surface can be cooled and solidified while the inner surface of the extruded pipe may remain hot, even as it exits the production line, especially for higher diameter and higher thickness pipe.

This results in a largely amorphous structure on outer side of the pipe and a partially crystalline structure on the inner surface.

In pipe this effect will cause a high stress in the wall that will reduce its physical properties, particularly impact and stress-crack resistance.

Also, since the crystalline portion will have a higher density and more shrinkage than the amorphous portion, there is a great deal of internal stress developed within the part, as one side shrinks more than the other.

The solution lies in controlling the cooling rate:                                            Control of the crystallization or shrinkage and resultant stress depends on control of the cooling rate through the entire pipe part in three ways -

1.      By reducing the overall cooling rate,

2.      By ensuring that the pipe is cooled uniformly over entire outer surface, and

3.      By interrupting the cooling by using more cooling tanks, so that the outer surface of the extruded part cools slower and allows heat from the other side to diffuse to the cooled surface.

Need for more cooling tanks with spaces between them:

Since it is usually necessary for the extruded parts to quickly freeze the outer dimensions to conform to a desired shape, the initial quenching usually needs to be in the optimum range of 4° to 10°C, to set the shape.

However, with continued rapid cooling, there is a potential for developing internal stress in the wall.

Residual stresses generated by the cooling process within the pipe wall are minimized by providing “annealing zones”.

These zones are spaces between the cooling baths which allows the heat contained within the inner pipe wall to radiate outward and anneal the entire pipe wall. Long-term pipe performance is improved when the internal pipe wall stresses are minimized.

Multiple cooling tanks and appropriate spacing between them is important in controlling pipe wall stresses.

If number of cooling tanks are curtailed or annealing is denied due to any reason (length of the shed constraint, lack of understanding about cooling or comparing it with UPVC pipe process), then different layers in the thickness will have differential crystallinity, shrinkage and weaker pockets.