Central Institute of Plastics Engineering and Technology, INDIA

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PIPE EXTRUSION

Central Institute of Plastics Engineering and Technology, INDIA

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Pipe plant elements

• Extruder

• Die

• Sizing device

• Cooling bath

• Puller

• Cutter or Winder

Central Institute of Plastics Engineering and Technology, INDIA

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Pipe Extrusion Plant

Central Institute of Plastics Engineering and Technology, INDIA

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MATERIALS

• PVC

• HDPE

• PS

• ABS

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PVC Pipes are available in two forms:

• Rigid PVC or Unplasticised PVC

• Soft PVC or Plasticised PVC

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PVC can’t be processed alone. It needs to be

compounded by adding different ingredients.

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Ingredients added in PVC compounding

• Plasticizers• Fillers• Heat stabilizers• Lubricants

• Processing aids• Flame retardants• Colorants• Impact modifiers

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Plasticisers are used to make the resin softer, reduce the viscosity and thus make processing of PVC easier. Di-allyl phthalate (DAP) and di‑2‑ethyl hexyl phthalate (DOP) are examples of common PVC plasticisers.

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Heat stabilizers are required to prevent the thermal degradation of PVC at the processing temperatures. The degradation involves resin discoloration and evolution of HCl. In the past lead stabilisers have been used (viz. lead stearates). Owing to toxicity of Pb, today the most common stabilisers are based on

tin, viz. di-butyl tin maleate.

Central Institute of Plastics Engineering and Technology, INDIA

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RPVC-pipe formulation

PVC Resin (K = 67.68) 100 parts

Tin stabiliser 0.4

Calcium stearate 0.8

Wax 1.2

TiO2 (colorant) 1.0

CaCO3 20 to 40

Central Institute of Plastics Engineering and Technology, INDIA

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Lubricants are used to reduce friction during processing that may lead to degradation. Two types of lubricants are used: external and internal. The former (viz. calcium stearate and wax) is used to reduce friction between metal surfaces and the polymer, the latter (viz. stearic acid) the friction arising out of polymer chain slippage.

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Fillers and reinforcements are added for cost reduction (fillers) and improvement of the mechanical properties (reinforcements). Both increase rigidity, hardness, density and fire retardancy. A typical filler is calcium carbonate; typical reinforcements are chopped glass fibre and mica flakes, while such additives as clay, talc, and silica are in-between.

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Flame retarding agents are needed for plasticised PVC. Chlorinated paraffin oil or wax, oxides of Sb, Mo, P, or B, as well as aluminium hydroxide have been used.

Central Institute of Plastics Engineering and Technology, INDIA

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Processing aids are added to rigid PVC to improve flow through the barrel and die. To this group of additives belong polymers and copolymers miscible with PVC, e.g., PMMA, ABS, SAN.

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Impact modifiers are added to improve the impact strength. Impact modifiers are composed of at least two parts: one that ascertains miscibility with PVC, the other elastomeric in nature that provides the toughening effect. Examples of these include MBS, EVAc, CSR, and NBR.

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PVC compounding is usually carried out in

two stages: • Dry‑blending PVC resin with additives in

an intensive dry mixer, equipped with a high speed impeller.

• Melt blending in a suitable compounder, e.g., single-, twin-, or multi-screw extruders, single or twin shaft compounders, Kinetic energy mixer, etc.

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Sizing equipments

The extrudate emerging from the pipe die usually shows variation of diameter, caused by variability of the rheological melt properties. Thus, the pipe has to be calibrated or “sized” while hot.

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Four methods are in use for pipe sizing

• Vacuum trough

• Sizing sleeve

• Extended mandrel

• Sizing plates

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Vacuum trough

The pipe, coming out of the die passes through a long closed trough filled with cooling water. Inside the trough, the pipe passes through fixed metal rings of predetermined size. Since, vacuum is drawn over the water, the internal pressure in the soft pipe causes it to expand against the rings. Thus the pipe gets the outside diameter.

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Sizing sleeve

This method fixes the outside pipe diameter as it hardens by contact with a water‑cooled metal sleeve. The contact is achieved by air pressure inside the pipe or by drawing vacuum through the perforated inside surface of the sleeve. The method produces glossy, smooth, closely controlled outer surface.

Central Institute of Plastics Engineering and Technology, INDIA

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Extended mandrel

This method uses a water-cooled metal mandrel that extends beyond the die. Shrinkage of pipe as it cools helps to keep a light contact with the mandrel. Thus this method provides additional internal cooling and internal support.

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Sizing plates

This method involves pulling the pipe through a series of brass or stainless steel plates. Air pressure in the pipe forces it against the plates, thus controlling the outside pipe diameter. This method is mainly used for smaller diameter pipes.

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Cooling water troughs

• During the sizing also the pipe gets cooled.

• Cooling is done by spraying water over the surface of the pipe.

• The length of the cooling troughs for PE is longer than for PVC.

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Pulling Machines

• Pulling machines maintain uniform pipe size.

• Pullers are either the caterpillar type or the gripping wheel type.

• The puller speed is controlled by electrical, electronic or magnetic speed variators.

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Controls

• They provide automatic measurements and control of pipe size.

• In one system the capacitance of the pipe is measured and is related to the thickness.

• The sensing head moves around the pipe, showing when the pipe is out of round and when it is generally too heavy or too light.

• All these systems can be made to feed back to control the puller or the extruder drive.

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Cutting devices

• Circular saw cutters, with blades specially designed for cutting plastics, are used to cut the pipe into specified lengths.

• Automatic cutting devices are also used. They travel with the pipe to cut, then after cutting go back to pick up the next pipe length.

• Larger sizes are sawed to length.

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Coiling devices

• Smaller diameter Soft PVC and HDPE pipes are coiled on a large centre‑wood reel.

• In simple methods pipe is coiled by a band rank.