High thickness and large-diameter pipes: how to avoid sag in pipe extrusion

The increased usage of large- diameter pipes, from 630mm to 1,200mm, across various applications has encouraged the development of PE100 materials suitable for large-diameter pipes to prevent problems during extrusion, like sagging.
Maintaining dimensions within specifications is problematic for the extrusion of large-diameter thick-wall HDPE pipes (> 75mm wall) due to sag caused by insufficient resin melt strength.

As the diameter of HDPE pipe increases during extrusion:

1. The thickness increases;
2. The pipe does not effectively cool from inside and within the core;
3. The linear speed decreases.

​Large-diameter pipes take typically 3.3 hours to produce and may have various segments of:

• Different crystallinity;
• Different thickness;
• Different moisture content, etc.

Development of crystallinity:

In most HDPE extrusion processes, 60% to 80% of the crystallisation takes place during the cooling phase of processing and as much as 90% occurs within a week of processing. Remaining crystallisation can take months to complete, depending on the ambient temperature. However, crystallisation continues until a stable crystal structure is achieved.

The problem of sag in pipe extrusion:

For thick-walled pipe, the inside of the wall remains molten for a long time, causing downward melt flow called sag.
Sag in pipe extrusion can cause serious non-uniformity in pipe wall thickness, increases ovality and offsets concentricity of the pipe and creates waste of material at the bottom of the pipe, adding extra production costs and causing non-optimal final product quality.
Sag always happens with large-diameter thick-walled pipe production and is the flow of the material from the top to the bottom of the pipe before it has been frozen off by the cooling water.

There are two ways to aid the elimination of sag in pipe extrusion:

a) By offsetting the die gap – but this takes time and always leads to the use of additional material and variation in thickness. Offsetting the die also helps to prevent high wall thickness at the bottom.
b) By using low-sag HDPE material and optimising the cooling process. It is believed that a bimodal polyethylene composition with a high viscosity at a low shear stress improves the sagging behaviour of the polymeric melt. The pipe is extruded through a ring formed die and cooled on both inner and outer surfaces.

Offsetting the die gap:

The conventional way to reduce sag in pipe extrusion processes is by manually adjusting the die eccentricity, until an acceptable wall thickness profile is achieved. This tedious trial-and-error procedure can take up to several attempts to get the right profile. To minimise efforts and compensate the effect of sag, the die gap is adjusted before starting the extrusion in such a way that the die gap is more at the top and less at the bottom of the die.
We can use an ultrasonic inline thickness measuring instrument, with four locations at 90° to each other, and display thickness variation on the screen. Alternatively, portable equipment may be used to measure inline thickness at various places of the pipe.Once we have knowledge of thickness variation, we can fine-tune it by altering the temperature of the segmented heater adequately, to control thickness and save wastage, as well as improve quality.

What is low-sag HDPE?

The modern “low-sag” resins make it possible to produce pipes with larger diameters and thicker walls than before. There is a need for special polyethylene compositions, which show an improved balance of low sagging behaviour and processability, to support large-diameter pressure pipes (up to 1,200mm) with wall thicknesses of 100mm, which can be extruded with existing lines and standard adjustments of die heads. The composition should also show a good balance of mechanical properties and pressure resistance to meet PE100 requirements. (Backman, M & Lind, C. 2001).
Due to the high wall thickness and the slow cooling process governed by the thermal conductivity of PE, it is of utmost importance that the HDPE in molten state possesses sufficient melt strength to prevent the material from sagging to the bottom of the pipe.
Attempts have been made to achieve this by a molecular design of HDPE that balances high melt strength with good processability and throughput.

The use of hexene as comonomer in PE100 resin specifically developed for very large diameter pipes is known to provide the following benefits:

• Better slow crack growth resistance;
• Better resistance against rapid crack propagation;
• Superior melt strength (low sag).

BorSafe HE3490-ELS-H, PE100, is a material in which the molecular weight distribution has been adjusted to increase the viscosity at low shear rates, which reduces the sag in pipe extrusion processes, while allowing the same material to be used for smaller diameter pipes. It is a bimodal, high-density polyethylene MRS 10 material specially designed to simplify the production of thick-walled, large-diameter HDPE pipes (above 80mm thickness) through its exceptional resistance to sagging and superior melt strength. Numerous trials have demonstrated an average of up to 7% material savings and better dimensional control compared to standard PE100 when producing pipes with wall thickness in excess of 80mm, independent of the pipe outer diameter. For example, trials were conducted for 1,200mm x SDR 11 pipe with standard low-sag material and extra-low-sag material. The trial clearly showed much better wall thickness distribution achieved with the extra-low-sag material. (Abdullah Saber & Hussein Basha,2021).

Moreover, by using the correct tooling and low-sag material, the overweight value can be kept down, which leads to a reduction in raw material and consequently a reduction in production costs. Normally, all tube manufacturers should try to work to 30% of the thickness tolerance. This is for two reasons: to have a high level of quality but, above all, to reduce production costs. The target is to have an overweight of 3–3.5%.