Also the temperature distribution, and hence viscosity distribution, is poor. However, it is appropriate for short product lengths, eg strip for tyre tread. The cylinder is heated to maintain the softened state.Ī hydraulically actuated ram (plunger) forces the material through a die clamped to the end of the cylinder. The polymer is fed to the cylinder (barrel) in a plasticised state. In its simplest form an extruder is analogous to icing a cake. Sheet, cast film, tubular film, tape, oriented film Rod, edging strip, window frame, curtain rail This is achieved by softening (plasticising) the material using heat and/or pressure, forcing the softened material through and orifice (die) and maintaining the desired cross-section by cooling or by chemical reaction. In injection moulding, where a hot melt flows into cooler channels, a “frozen” skin layer is established at the wall with the melt flowing inside the skin in an unfolding melt front (“fountain flow”).Įxtrusion is a process in which polymeric materials, in the form of powder, granules, trip or melt, are converted into products of controlled cross-section in a continuous fashion. The highest velocity will be at the centre of the channel and reducing to zero at the wall. In an isothermal channel, such as an extrusion die where the wall is at the same temperature as the melt, the flow front will be parabolic. In most situations it can be assumed that flow is laminar. Polymer dispersions (latex, plastisols) can exhibit both shear thinning (pseudolplasticity) and shear thickening (dilatancy).īecause of their high viscosities, thermoplastic melts rarely show turbulent flow. Increases with increasing molecular size (MW)ĭecreases with increasing lubricant content In the melt state thermoplastics show varying resistance (viscosity) to applied flow stress.ĭecreases with increasing shear strain rate (shear thinning, pseudoplastic) ![]() Semi-crystalline thermoplastics have enthalpies almost double that of their amorphous counterparts.īecause of their susceptibility to thermal decomposition it is advisable to heat polymers up quickly, form quickly and cool down as soon as possible and avoid long residence times at elevated temperatures. The energy required to raise unit mass of polymer from room temperature to processing temperature is defined as the enthalpy. In conduction heating, the poor thermal conductivity results in undesirable temperature gradients and shear (frictional) heating is faster and also provides more uniform temperatures. Consequently, the best form of feed is as finely divided granules or powders. Therefore, this makes them unsuitable for heating by conduction in thick sections. Polymeric materials are characterised by high specific heat and also low thermal conductivity. Polymer manufacturing processes – The common features of polymer processing are: extrusion, there is more efficient use of energy and it is easier to maintain a consistent quality.įor batch or cyclic processes, such as moulding processes, there is a higher probability of batch-to-batch variation and lower efficiency due to unproductive parts of the cycle (down time). In continuous process where raw material is fed in continuously and the product flow appears continuously e.g. ![]() Polymer manufacturing processes can be divided into continuous processes and batch processes. Setting the shape is achieved by either cooling or carrying out a chemical process (crosslinking) to achieve the necessary dimensional stability. Usually the softened state is achieved by heating the polymer. ![]() Manufacture of a particular product may require more than one forming process.Ī crucial feature of most polymer processes is the preparation of the polymeric material in a appropriately softened state to suit the forming stage. Thermoforming Moulding: compression, transfer, Injection, blow rotational The advantage of polymer processing over manufacturing with more traditional materials is that there are opportunities for cost savings through minimising finishing processes. ![]() In most manufacturing there will be a number of finishing steps. Inherent in the forming stage is the requirement to set or maintain that shape.įorming can be conveniently divided into two-dimensional forming, where products have a relatively simple geometry, and also three-dimensional forming with complex geometry. The second stage is to create the desired shape. The first step consists of mixing additives into the polymer to achieve the required modification to the properties of the raw polymer. The conversion of raw polymers into finished products involves a series of polymer manufacturing processes.
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