For too long now the processing of plastic materials has been regarded as an art and not as the well developed technology that it really is. However, in an effort to establish a commercial advantage, more members of the plastics processing industry have come to realise that there is a need to maximise output rates and minimise scrap generation. To do this many have resorted to the fundamental principles of processing. One of the important topics to have been investigated is that of Rheology, i.e. "The study of the flow and deformation of matter". In order to understand and control any process involving the transfer of fluids it is necessary to know how that fluid behaves under different conditions of temperature and pressure etc. The behaviour of polymer melts under the influence of shear is very complex since they tend to be highly non-Newtonian, i.e. they do not obey Newton's Law of viscous flow. The viscosity of a polymer melt is not constant - it is highly dependent on the rate of strain, (see the figure below). This means that the viscosity of the melt is going to be influenced by the injection rate though the gate of an injection moulding machine or the screw speed of an extruder.
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Polymer melt rheology is a very important element of polymer technology since it enables an understanding of a large number of processing faults and defects, and it very often points the way to their correction. From an understanding of melt rheology it usually possible to select the most suitable polymer or polymer compound and link it to the most relevant processing method and set of processing conditions. Perhaps, most importantly, it leads to an understanding of the quantitative relationship between such factors as output rate, power consumption and machine geometry and operating variables such as screw configuration, screw speed, pressure and temperature. |
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The P7 (above) is a unique low cost rheometer that automatically measures and displays the rheological properties of polymer feed stock for production or compounding operations. The automatic multi-sample loading system, using a demountable cassette of containers, eliminates the need for tedious and time consuming manual testing. |
Here at PolyTech Consultants we have close ties with Porpoise Viscometers Ltd. We have access to the most modern and sophisticated rheometers, such as the P7 Sampling Rheometer (shown here on the left ) and software to analyse the mass of data that they generate. We also have access to their "P" series of viscometers; the P3 Online Rheometer, and the P5 Online Rheometer as well as the P7. We have a broad understanding of the flow properties of plastic materials and a wealth of practical experience in research and in industrial consultancy. We have worked successfully with many companies, large and small, both in the UK and internationally, to help solve processing problems associated with the flow properties of their materials. |
The P7 accepts samples in powder or
pellet form and, by using a unique method of conditioning and
re-melting, meters the polymers through a gear pump to a temperature
controlled die. The pressure drop is measured across the die
and the relevant rheological information is derived using Porpoise's
own software, developed in-house. The shear rate range is between 0.1
and 2.5 x 10 5 sec-1 and the corresponding viscosity range lies
between 0.1 mPa s and 1 x 10 6 Pa s, with a repeatability within
± 1%.
The Intelligent Rheometer (the P9) from Porpoise has arrived (see below), this is a very important addition to the Porpoise range of products.
