Extrusion Profiles and Sheet
Along with Injection moulding, Pro-file Extrusion of plastics materials is a widely used method of forming plastics materials. The Plastic raw material is both melted and trav-ersed along by the action of heated rotary screws. It is a continuous process and is thus able to manu-facture long lengths of a product. This is ideal for such applications as pipes and gaskets. However very often the continuous extrusion is cut into application lengths
Unlike injection moulding where the process makes individual identical components, the extru-sion process makes a continuous length of plastic who’s cross section is constant. This cross section is called the profile. A couple of examples to illustrate this are hosepipes and curtain tracks.
An electric motor coupled to a hydraulic drive continuously turns a screw, which is contained in the machines barrel. Plastic granules are fed into the hopper of the extruder and are drawn down into the screw. The barrel and screw are heated by external heating elements. As the plastic granules move along the screw they melt and are forced through a die which is located at the end of the barrel. The die contains the cross section of the profile of the extrusion required.
|Photos of two extrusion dies along with the plastic section they produce
||A section of profile extrusion which is cored out to minimize the content of plastic
A section of co-extruded profile extrusion. Specialist extrusion machines are used in this application
Unlike the injection moulding process, profile extrusion depends more of ancillary equipment to shape and form the end product
When the plastic profile exits the die, it is still molten. It is channelled into a water bath which contains sizing formers and guides. During its passage through this cooling bath, the plastic solidifies and takes on its final shape. The bath is con-stantly cooled by either the introduction of mains water, or a process chillier, in line, extracting the excess heat.
Take Off Rollers
The still molten plastic coming out of the profile die has no strength. To ensure that the process has the correct process speed, Take Off Rollers, adjustable in speed, pull the solidified plastic through the cooling bath
Take Off Rollers control and coordinate the speed at which the plastic extrusion is drawn from the die
Certain hollow section extrusions and pipes need to be dimensionally accurate. A vacuum cali-bration device is therefore introduced into the cooling bath. The still soft plastic extrusion is passed through the centre of the calibration device, a vacuum is drawn on the outside of the device and small holes all round the central profile ensure that the soft plastic is drawn toward this profile. It’s a little like a continuous vacuum forming process.
Cutting to Size
The now formed profile extrusion needs to be cut to size to enable both transportation and to suit the application. Flexible extrusions are usually coiled by specialist coiling machines. Rigid extrusions are cut to length by either a saw or guillotine. If the extruded section of plastic to be cut is bulky, then a traversing saw cuts the extrusion. The length of both flexible and rigid extru-sion is measured by a counter activated by a roller, bearing on the extrusion as it passes.
This process combines two different materials or colours in a single profile. Special twin head extruders and purpose made dies are necessary. This technique is often used to replace two piece assemblies. The two materials are independently extruded and meet in the die, where be-ing molten, they alloy at the point of interface.
Post Extrusion Working
To enhance a profile extrusion, post extrusion work can be carried out. Punching, Drilling, and Slotting are typical process. Bending & Forming and the application of hot roll on adhesive tapes are carried out off-line. Inkjet printing enables batch and product marking
|Specialist Winders coil flexible extrusions in measured lengths ready for delivery to customers
||In line saws both measure the length of extrusion and traverse with the extrusion during the cutting process. Having completed its cut, the head of the saw traverses back to the left hand side of the machine ready for its next cut
Applications and Materials
Here are a few examples of applications and the materials used
Land Drainage Tubing HDPE resists corrosion underground
Animal Feeding Troughs Rigid PVC has integrity over lengths and is tough
Cable Harness PTFE Filled PVC Flexible and resists chaffing of cables
Door Seals Can be produced as a co-extrusion which provides dual properties in one profile.
Light Diffusers In acrylic provide a clear or diffused transmission of light.
Electrical Conduit and Cable Protector Rigid PVC for toughness and resis-tance to chemical action from plaster. Plus good electrical insulation.
Rainwater Pipes and Guttering Rigid PVC for good UV resistance and colour fastness. Also good impact resistance
Facia & Soffit Sections. Both compact and foamed PVC for good UV resistance and colour fastness.
Curtain Track Rigid PVC for good UV resistance and stiffness. Can be coloured.
Fridge Seals. Made from thermoplastic rubbers and PVC which have a metallic filler and subsequently magnetized.
Edge Trim produced in Rigid PVC occasionally with wood foil applied to match wood veneers. Lowers costs and enables consistency of colours.
Caravan Window Surrounds Semi Rigid PVC. Good UV resistance and ability to colour match.
Garden Fence Posts & Decking. Invariably extruded from recycled polysty-rene or polyethylene drinks bottles. Impervious to rotting and does not need painting.
Blood Drip Tubes & Catheter Tubes. A composite of plastics materials and reinforcement, high integrity materials post manufacture sterilized.
Dip Tubes in Aerosols and Spray Bottles Polyethylene both transparent and good chemical resistance.
Gas Pipes Polyethylene for resistance to corrosion from both the gas and the underground environment. Water Pipes Polyethylene for resistance to corrosion from the underground envi-ronment. Also assists in the purity of the water. Soil Pipes. Primarily in rigid PVC. Ability to be joined with fittings by solvent cements.
Window, Door & Conservatory Sections Rigid PVC for appearance, UV stability, ability to be cut and welded. Alternative sections have wood grain foils applied as to simulate wood.
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