Polymer Melt Viscosity in an Extrusion Based Process - Essay Example

Polymer Melt Viscosity In An extrusion Based Process The worldwide production of polymer, according to an article published by AIM Magazine en d “Polymers in Europe, Quo Vadis?” in 2001, has increased from 27 million tons in the year 1975 to about 200 million tons per year in 2000, and the figures are still rising (H.U. Schenck). In the USA, as reported by the Washington DC USA, Soc. Plast. Ind., Inc. (SPI), shipments of plastic products amounted to $330 billion, with the supplying industries having $90 billion of sales, amounting to an annual total of $420 billion. This brought an estimated 2.4 million employment that is about 2% of the U.S. workforce. Indeed, the booming growth of the polymer industry and its impact to the world is very much visible and evident in everyday lives. The growth of polymer industry is brought by the unique properties of plastic products (De Laney). Plastics are characterized by easy shaping and fabrication, low densities, resistance to corrosion, electrical and thermal insulation, and often favorable rigidity and toughness per unit weight. Extrusion is one of the most important operations in polymer processing, injection molding being the other one. Both these processes involve the following steps: (a) heating and melting the polymer, (b) pumping the polymer to the shaping unit, (c) forming the melt into the required shape and dimensions, (d) cooling and solidification. Other processing methods include calendering, blow molding, thermoforming, compression molding and rotational molding. More than 30000 grades of polymers are being processed by these methods (De Laney). However, nowadays, the plastics compounding and extrusion industries are being affected by high resin and labor costs and the demand and need for superior product performance and consistent quality. Therefore, it is important that suppliers and producers of many polymer products such as film, sheet, profile, tubing, and pipe avoid inconsistencies and flaws in the products they produce. This is the only way by which they could ensure their survival in the growing market of polymer industry. Unfortunately, even when all of the operational parameters in an extrusion process such as pressure, temperature, screw rotation rate, and other factors are taken to utmost control and monitoring, there are still possibilities that the product would fail the set specifications and may exhibit inconsistencies. An extruder is a machine aimed to produce more or less continuous lengths of plastic sections. It is composed of a tubular barrel, usually electrically heated; a revolving screw, ram or plunger within the barrel; a hopper at one end from which the material to be extruded is fed to the screw, ram or plunger; and a die at the opposite end for shaping the extruded mass. The main function of an extruder is to deliver a homogeneous plastic melt to a die at a specified uniform temperature and pressure. To achieve such objective, the well -designed extruder is generally equipped with an efficient drive and feeder system, and a screw designed to melt and convey the product. Also, thermocouples and melt pressure transducers are normally installed to monitor the system for safety and process control. However, this machine simply a plastic melting and melt conveying machine and sometimes designed to be capable for mixing. Therefore, there is no chance that the extrusion process could improve the quality of the resin fed into the machine, because what goes in one end of the machine is what comes out in the other end. . The extruder, however, is mainly a plastic melting and melt conveying machine, which is sometimes provided with some capacity for mixing. In general, what goes in one end of the extruder is what comes out the other end. And even if the feed resin properties are adequate for an application, the resin may be degraded by the presence of contaminants or by exposure to excessive temperatures, pressures, and residence time within the extruder. However, once material has been melted and mixed within the extruder, the quality and process stability of the resulting melt stream can be precisely controlled and even improved before it is delivered to the die and downstream equipment (De Laney). There are useful components of an extruder which can have a significant impact to the quality of the melt stream. This includes a melt filtration system, a melt pump and static mixer, and an online rheometer. The melt filtration system removes any contaminants and ensures a uniformly clean product. A melt pump and static mixer control the rate, pressure, temperature, and stability of flow of the molten resin to the sheet, film, or pipe die. The online rheometer monitors the melt flow or viscosity of the resin which is one critical parameter that determines the quality of a polymer product. This parameter determines whether it is possible to extrude a certain material or how well it can be extruded. It also provides information on the basic molecular structure of the resin introduced in the machine. Thus, the basic tests for the quality control of plastic compounds and resins include the measurement of viscosity of the material. It is also helpful to monitor and observe changes in a resin or compounds melt viscosity because these observations could aid in the prediction of any deviation in the compounds composition or degradation which may cause failure of the extruded product. One factor that needs to be controlled in assuring the required or needed viscosity of a polymer in an extruder-based process is pressure. When there is sufficient inlet pressure for melt to fill the gear teeth, the discharge pressure remains virtually constant although the feed pressure of the molten polymer from the extruder to the melt pump may fluctuate. To ensure sufficient melt pressure that will feed the pump, a pressure transducer is usually placed at the inlet. This is very important because insufficient pressure could lead to inadequate lubrication, resulting in seizure of the bearings. Generally, a second pressure transducer needs to be inserted at the pump exit to control the outlet pressure. Melt pressure transducer traces are measured at the screw tip, after the screen pack, and at the melt pump discharge. The stability of this process on an extruded product such as film or sheet can give way reduction in product thickness variation, which in turn will result to an improved quality and cost reduction. The melt pump, aside from being capable of controlling the flow, also allows the extruder to run at a lower head pressure. This reduction in head pressure provides an increase in the specific output rate and a substantial reduction in melt temperature. And lower melt temperatures reduce the chance of thermal degradation during the extrusion process. Thus the melt pump also has an impact on the quality of the molten resin. Since the melt pump cannot improve thermal uniformity, a static mixer can be placed in the line after the melt pump and before the die. The static mixer consists of a series of stationary mixing elements inserted in the melt stream to disperse the polymer flow. This dispersion of the flow produces a well -mixed melt, with a uniform distribution of temperature, for delivery to the die. For direct process control, test of material flow are impractical to be done on a laboratory scale. Thus, rheometers are now interfaced directly with the extrusion process. This provides real-time melt flow or viscosity values, providing efficient monitoring for the quality of the material as well as valuable data for process control. De Laney describes how a return-stream online rheometer, interfaced to an extruder works: “Molten polymer flows into the instrument through the central tube of the annular melt transfer line that protrudes into the stream. The molten polymer is pumped through a capillary die by the first of a pair of stacked melt pumps, which accurately control the rate of polymer flow through the capillary die. After passing through the capillary die, a second stacked melt pump returns the molten polymer to the process through the outer tube of the transfer line. The temperatures of all of these components are precisely controlled. Very sensitive, melt - pressure transducers are used to measure the pressure drop that occurs as the polymer flows through the capillary. High precision and accuracy are required of the melt - pressure transducers and melt pumps in order to obtain reliable measurements of melt flow or viscosity.” In the extrusion process, if the melt temperature is constant, it is assumed that the viscosity will also be constant. This avoids degradation of the polymer (McAfee and McNally). For example, in the extrusion process of polyethylene, any irregularity in a recycle feeder could cause viscosity change. This change as a result of process deviation could be possibly detected by an interfaced rheometer. The operator will be able then to make necessary adjustments to avoid the extrusion of large amount of off-specification products. The presence of moisture lowers the viscosity of the polymer. Absorbing moisture by the compound causes a degradation of the resin and leads to the failure of the extruded product. If this is the case, the operator could utilize a different source of resin which is sufficiently dried and the production process could resume. The changes in such variables as the molecular weight or composition of the feed can either increase or decrease the viscosity of the compound. Even if these changes occur very gradually, they can be observed as a trend in the viscosity value. Collection of these data over time permits the establishment of critical limits that ensure extrusion of a consistent product. Interfacing of the rheometer output to a computer that contains the established control parameters can then provide automated responses to deviations and provide process control (De Laney). It is a need that in polymer processing, that the delivery of a homogeneous melt be given with utmost importance in order to prevent product failure. Differences or fluctuations in the melt viscosity of the product will cause fluctuating die stresses, which may induce viscoelastic instabilities. Aside from this, non-homogeneous melt affects the stability of other extrusion processes such as cooling, stretching, forming, and others. These existing control schemes for melt flow or viscosity in an extrusion – based process, controls and monitors significant parameters such as pressure, temperature and moisture (McAfee and McNally). Works Cited De Laney, Donald E. Plastics Auxiliaries & Machinery, July/August 2002. http://www.dynisco.com/products/pt/literature/PMarticle.pdf H.U. SCHENCK: AIM Magazine (Special Issue: ‘Polymers in Europe, Quo Vadis?’), 15-18; 2001, Eur. Polym. Fed. McAfee, M. and G. McNally. “Real-time measurement of melt viscosity in single screw extrusion”. Transactions of the Institute of Measurement and Control 2006; 28; 481http://tim.sagepub.com/cgi/content/abstract/28/5/481. Accessed: December 7, 2009. Read More