EXTRUSION PIPE / PROFILE MACHINE

EXTRUSION

 Process

This is a widely used method of forming plastic sheet and profiles.  Unlike injection moulding which makes individual identical components, extrusion is a continuous process producing lengths of plastic with a constant section.  The cross section is called the profile.

The machine has a screw and barrel similar to an injection moulding machine.  The plastic pellets are fed into the barrel and drawn along the screw.  As with an injection moulding machine the barrel is heated and the plastic pellets are melted.  The molten plastic is forced through a die at the end of the barrel; this produces the required cross section of the profile.

The plastic profile is still molten when it exits the die.  It proceeds into a water bath containing sizing formers and guides.  During its passage through the cooling bath the plastic solidifies and takes on its final shape.

Certain hollow section extrusions and pipes need to be dimensionally accurate.  A vacuum calibration device is placed in the cooling bath in these cases.  The plastic extrusion is passed through the centre of this device whilst still soft.  A vacuum is drawn on the outside of the device and small holes around the central profile ensure that the soft plastic is drawn towards this profile.  This is similar to a continuous vacuum forming process.

The extrusion needs to be cut to size at the end of the bath.  Flexible extrusions are usually coiled whereas rigid extrusions are cut to size using either a guillotine or a saw.

Two different materials or colours can be co-extruded into a single profile by use of special twin head extruders.  The two materials are independently melted and meet in the die.

Materials

The main material used is rigid PVC.  High Density Polyethylene is also used for pipes, etc.  Although other materials can be extruded such as ABS, Styrene and Acrylic.

Extrusion is also used to produce plastic compounds where fillers and colour pigments are added to the base polymer which is then extruded into fine strands which are guillotined at the end of the cooling bath to form small pellets.

Engineering plastic

Engineering plastics are a group of plastic materials that exhibit superior mechanical and thermal properties in a wide range of conditions over and above more commonly used commodity plastics. The term usually refers to thermoplastic materials rather than thermosetting ones. Engineering plastics are used for parts rather than containers and packaging.

Examples of engineering plastics include:

• Acrylonitrile butadiene styrene (ABS)
• Polycarbonates (PC)
• Polyamides (PA)
• Polybutylene terephthalate (PBT)
• Polyethylene terephthalate (PET)
• Polyphenylene oxide (PPO)
• Polysulphone (PSU)
• Polyetherketone (PEK)
• Polyetheretherketone (PEEK)
• Polyimides
• Polyphenylene sulfide (PPS)

Commodity plastics

The more commonly used thermoplastic materials are known as commodity plastics as they are traded and used in great quantities. Examples are polystyrene (PS), polyvinyl chloride (PVC), polypropylene (PP) and polyethylene (PE).

Typical applications for commodity plastics are high production volume products such as ‘polythene’ bags (made from polyethylene), vacuum-formed food packaging (low density polyethylene), disposable drinking cups (high-impact polystyrene) and window frames/wire insulation (PVC).

Engineering plastics

Engineering thermoplastics are sold in much lower quantities and are thus more expensive per unit weight. Despite this, they are widely used in everyday products. For example ABS is used to manufacture car bumpers, dashboard trim and Lego bricks, polycarbonate is used in motorcycle helmets and polyamides (nylons) are used for skis and ski boots.

Typically, an engineering plastic is chosen for its range of enhanced physical properties e.g. polycarbonate is highly impact resistant and polyamides are highly resistant to abrasion. In these types of applications, designers are looking for plastics that can replace traditional engineering materials such as wood or metal. The advantage gained is the inherent ‘formability’ (ease of manufacture) of plastics as opposed to metal-working or fabrication.

Other properties exhibited by various grades of engineering plastics include high heat resistance, mechanical strength, rigidity, chemical stability and flame retardency.