FILM RHEOLOGHY - STILL HALF WAY TO GO
By Andrey R.Smyshlyaev, PhD
The technology of polymer extrusion was always a tricky thing. Half science, half magic. Film extrusion is much worse then usual. Just imagine, how sophisticated has to be the machinery to transform polymer pallets into thin film.
Film extruders have undergone the long way within past decades. First machines barely produced dozens of pounds of low quality film per hour. The material, mostly polyethylene and PVC, was very unstable, melting ability and homogenization of screw presses was uneven, design of extrusion dies was primitive. In the given conditions stable extrusion process was difficult to maintain and greatly depended on human factor. For this reasons skills of designers and operators of the extrusion machines were valued extremely high.
Main advance in extrusion technology was achieved within 70-s and 80-s. Know how started to spread around gurus. As a result extrusion screws got longer, from initial 22.25 to 28..30 L/D. Polyethylene and other raw materials suitable for film extrusion undergone long ways in stability of melt index and molecular mass distribution that effected strength, rigidity, stability and other key features of the end product. Usual film tolerances went down from 15 to 10 per cent, output of single machine achieved 500 kg per hour for film blowing extrusion. But there were still many blank fields within the technology despite tons of scientific reports.
Research of polymer melt behavior is much more complicated then those of other liquids because of it’s anomaly behavior, known as Power law. In simple words viscosity of the material greatly depends on speed of flow and on type of polymer. The polymer also dissipates mechanical energy into heat due to shear of layers. As the result polymer melt may be overheated in some areas or stop moving in other. Both worsen extrudate temperature distribution and leads to degradation of polymer. Specialists know that even slight change of processing temperatures may greatly affect quality of the film.
There were many attempts to predict result of extrusion process by numerical simulation on computers in 80-s and 90-s, but due to the luck of appropriate mathematic models and low computer power practical result was achieved only in straightforward cases like calculation of pressure drop within simple shaped channels.
By the end of 90-s variation of film gauges was reduced down to 5..7 per cent. This was a result of simultaneous development of polymer materials, extrusion presses and extrusion dies. New simulation programs were able to utilize increased power of PCs to compute not only pressure drops, but velocity, pressure distribution and temperature fields within and at the nozzles of extrusion dies. Programs develop and will most likely result into better dies geometry with even distributions of materials.
We may see very good results in mathematical optimization of hot part even now. New computing methods provides even distribution of material within each layer in co-extrusion dies. The layers interference after merging is lesser now. Output of dies continues to increase along with increase of specific and total output of the entire line. Mathematics helps to reduce consumption of electricity, to shorten residual time and thermal-mechanical destruction of polymer material due to improved design of hot part zones. All this provides wider “technological window” for operation.
Another real breakthrough was achieved by development and commercialization of automatic profile gauging that may improve gauge distribution of film down to 2..5 per cent. First systems of this type appeared 30 years ago but remain quite expensive.
Why every percent of tolerance is so important? Because one per cent saved on tolerances means about the same saving of manufacturing expenses. Profit usually constitutes small fraction of the expenses, so the savings of one per cent means several percents of profit increase. But this is not the only benefit. Gauge tolerance correlates with product quality. Modern lamination, bag making and printing machines may only achieve good quality and high speed with film of low gauge variation. Uneven distribution of thickness also affects mechanical and barrier properties of film and final products.
Industry needs further development of computer modeling of polymer processing, especially within complicated geometry of dies such as spiral Mandrell dies. The programs do not work automatically, practical results of their use greatly depend on special knowledge and patience of researchers. The lau6614547_test finite element programs may fairly good simulate flow of polymer at given temperatures within pre-designed shape of spirals. Variation of M given parameters of die (geometry, temperature, pressure, etc.) within n increments gives Mn possible combinations of parameters of the extrusion die. To find a global optimum computer has to calculate every variant. Modern PCs spend about 5 seconds for each calculation, so it may take several thousand years to find the best solution with needed precision. Supercomputers might have reduced this term to several months. So at the moment the practical solution is to combine power of calculation of existing PCs with human brain supremacy. This is not a simple task because there are many ways to go. An experienced designer has to scan multi-parametric space using heuristics. There are analytical methods of optimization which may replace heuristic approach, but at the moment none of the existing programs develops in this direction. So far we may speak about computer aided simulation rather then about automated design of the extrusion dies. This means that for the near future only the companies with experienced designers who are qualified to use lau6614547_test simulation programs may design dies with best distribution of tolerances.
We may also expect appearance on the market of better and less expensive sensors of film thickness. The lau6614547_test developments in infra-red semiconductor sensors which are specific to different polymer show the possibility to define thickness of each layer of coextruded film, if the layers are made of materials of different nature. The modern safe and easy to operate IR sensors finally became as fast and precise as gamma-backscatter, X-ray and beta-ray sensors.
Other breakthrough to come is appearance of affordable methods of gauge variation control. Every part of the gauge control system such as sensors, actuators and software are not expensive, but the complete system still costs around hundred thousand US Dollars. As soon as competition grows we may expect reduction of the prices within next years, so it would be economical to equip with such devices even small film extruders.
Providing all this developments are in place we may expect the average gauge variation to be reduced from today’s best 4..6 per cent down to 2..3 per cent of absolute thickness.
Scientists spent more then 50 years to make a technology that may win human in Chess. Nobody knows for sure how long t may take to get the complete control over extrusion technology. But “caravan goes”.
Film Extrusion on Plastic Technology online
Computer analysis of film extrusion
British Plastics & Rubber
Plastics News magazine
Modern Plastics magazine