Processing of PVC
All the usual techniques used for the transformation of thermoplastic polymers are also used for the processing of PVC.
Extrusion of pipes and profiles
Extrusion is the most widely used processing technique for converting PVC compounds into finished products. The process consists of melting the compound, in the form of granules or dry blend, in the heated barrel of a single or multi screw extruder. The material is compressed, heated by friction, forced through a die of appropriate shape, cooled and calibrated in a vacuum before passing through a water section for complete cooling.
The two main application areas are:
Sheathing of cables
The technique remains the same as for pipe extrusion. However, the higher extrusion rates and insulation requirements necessitate the use of special formulations and resins with high plasticiser absorption and low gel count.
The extrusion may be made around a single or several cables.
Extrusion of flat sheet
Extrusion is made through a slit die. Special care must be taken to ensure an even melt flow as dead spots would lead to thermal degradation.
Furthermore excessive pressure must be avoided at the head, since this would produce unacceptable variations in thickness. The film is passed over a series of cold cylinders trimmed to width and wound up.
Calendering
Calendering is a process eminently suited to PVC. It produces rigid or flexible sheets of good surface finish and standardised thickness. The process demands a high level of technology, scale of production and capital investment.
PVC resin, plasticiser, stabiliser, lubricants, filler and colouring agents are mixed, homogenised in a rolling mill and then fed between the cylinders of the calender.
On leaving the calender, the PVC sheet is cooled, evened out and wound up.
Applications include flooring, sheeting for thermoformed packaging, soft sheet for upholstery and inflatables.
Blow moulding of hollow products
Blow moulding is used to produce bottles and containers of various shapes. The PVC compound is extruded into a tube shaped parison and is formed to the shape of the split mould by injection of air. The high output rates of the machines require the use of resins, which form easily and quickly.
For health and aesthetic reasons, PVC resins of high purity and clarity are used.
Blown film extrusion
A tubular film is extruded, either upwards or downwards, from a die head. Particular care must be taken in the design of the die head; an axial flow is necessary to avoid any dead spots in the direction of the melt flow. This process applies to the production of rigid or flexible films ranging in thickness from about 10 microns to 1 mm.
Injection moulding
In this process, the polymer is melted so that it can be introduced under pressure into the cavity of a mould.
An injection cycle can be broken down into several stages: In the first stage the correct quantity of polymer is heated to the required temperature to obtain the desired flow characteristics.
In the injection stage the molten polymer is injected into the mould. Finally, the item is cooled and solidifies to the shape of the mould.
