Plastik malzemelerin ekstrüzyonunda kullanılan tek vidalı ekstrüderin incelenmesi

Abstract

Ekstrlizyon, belirli kesitli ürünlerin sürekli uzunî». İukta, ısı ile yumuşayan termoplastik malzemeler kullanıla rak elde edilmesinde yaygın olarak yararlanılan bir yönetim dir. Tek vidalı bir ekstrüder, bir besleme hunisi, bir vida ve silindir, bir matris ve ürünü uzaklaştırmak ve soğutarak katılaştırmak için kullanılan aletlerden oluşur. Besleme hunisinden beslenen malzemenin, ısıtılan silindir içinde vida yardımıyla eritilerek ve matristen basınç ile akması sağlanarak ürün elde edilir. Tek vidalı ekstrüder üç ana bölüme ayrılmıştır: besleme bölgesi, geçiş bölgesi ve boyutlandırma bölgesi. Tek vidalı ekstrüderde eriyik malzemenin taşınması üç akış me kanizması ile gerçekleşir: Sürüklenme akışı, basınçlı geri akış ve sızıntı akışı, ikış mekanizmalarının debi ve hızla-a. rı için geliştirilmiş yaklaşımlar, akışın sabit viskozite ve basınç gradyenine bağımlı olduğunu gösteren Mewtonien akış yaklaşımı ve viskozite ve basıncın kanal içinde sıcak lık ve konum ile değiştiği güç-kanunu yaklaşımıdır. Eriyik taşıma mekanizması kadar, katı taşıma mekaniz ması da önem taşımakta olup katı-blok, katı-yatak yaklaşım ları ayrı ayrı incelenmiştir. Ka£ı taşıma kapasitesi, doğru dan ekstrüder debisini etkilemese de, debiyi belirleyen bo yutlandırma bölgesi doluluk oranını belirlemektedir. Güç ve ısı enerjisi gereksinimleri birbirine bağım lı, olup, debi, ekstrüder boyutları ve polimer özellikleri tarafından belirlenir.

In this study, extrusion process in which thermoplas tic materials are molten by the movement of the screw and by heat and they are passed through a die that gives a shape to the material under pressure is examined. In the extrusion process, flow is devided into two parts as solid transport and viscous flow. By taking base the flow theories, desing of an extruder and a die is examined sample processes are defined. The results that is obtained show that in the extrusion process, design can be made for a private ther moplastic material. Extrusion is a molding process for producing continuous lengths of thermoplastics with consistent cross- sectional shapes such as rod, tube, sheet and film. It is similar, to injection molding in some ways and probably processes a greater volume of material in a given time than any method of molding. The earliest study to formulate the theory of single- screw extruders are due to Rowell and Finlayson, who in 1922 and in 192S", published expressions for the output, the power requierements and the efficiency. There are four phases to the process of extrusion. First, the thermoplastic in powder or pellet form is placed in a hopper. Second, the plastic drops into a heating cylinder which has a revolving screw inside. The plastie is liquified by the heat of the cylinder and the friction created between the screw, the plastic and cylinder walls. Heating cylinders are commonly kept at 150 to 260 C. Third, the liquified plastic is forced through a die opening at the and of the heating cylinder with pressures of 10 MPa to 45 MPa. The opening in the die determines the cross-sectional shape of the piece, called the extrudate, Fourth, the extruded plastic is cooled and thus hardened as it leaves the die on a conveyor belt. It is cut into convenient lengths or rolled up. A plastics extruder consists of a hopper, a heating cylinder with ii feed "screw inside, a die with a properly shaped opening, some means of cooling the plastic, and a take off device. viii The heating cylinder must be very strong, capable of withstanding pressures as hing as 70 MPs. Heating is most commonly accomplished with electricity,, but steam and oil are also used. The die must be hardened steel to resist the wear of the plsstic as it is forced through the orifice. The opening in the die maynotbe the exact size and shape of the desired extrudate because dimensional changes occur as the material cools. The feed screw is an extremenl important part of the equipment. Different types and sizes of threads are employed for the extrusion of different materials. They are usually made from heat-treated alloy steel to withstand tensile stresses of 700 MPa. As plastic leaves the die, it usually passes through a water bath for cooling. Some sort of conveyor belt or take-up rool removes the extruded plastic. A single-screw extruder can be devide into three zones. - Feed zone - Transition zone - Metering zone The purpose of feed zone is to pick up the cold material from hopper and to feed it to the transition zone. Peed materials differ widely in their physical form and may be supplied as free-runnijig fine powders, regular cubes, random cut chips or small spheres. Channel depth in the feed section is greater than the others. The second zone is the transition zone. Transition zone follows immediately after the feed zone and can be formed by the gradual increase of the root diameter of screw thread until the diameter of the metering section is reached. Solid material is compacted until melting occurs. The metering zone is the final part of the screw and acts rather as a metering pomp from which the molten plastics material is delivered to tK;e die system at constant volume and pressure. The ratio of the volume of one screw flight in the feed zone to the volume of a flight in meterring zone is termed the compression ratio of a screw. ix First studies on the single-screw extrusion theory- are about the pomping mechanism of molten material that occurs in metering zone There are two approximations on the process of molten material conveying. - Newt onien- flow approximation - Power-law model fluid approximation The movement of viscous material in theflights of a single-screw machine is envisaged as being composed of three distinct types of flow. They are: - Drag flow - Pressure back flow - Leak flow In Newtonien flow, velocities of fluid and the output is the function of pressure gradient which differs by zdirection and constant viscousity. Another approximation about solid conveying mechanism is solid-bed approximation. Here, material in various forms nove in screw channel, granular materials are deformable and there are no relative movements. In solid-bed approximation, a linear stress-strain equation is used. As material heated, firstly molten films, secondly molten pools occur. Extrusion is ordinarily used with thermoplastics. Commonly extruded materials are acrylice, cellulosics, fluorocarbons, nylon, styrenes, polyethylene and vinyls. The only shapes which can be extruded are those with consistent cross sections such as rods, tubes, sheets, moldings, or other architectural shapes. Extruded shapes, espepially sheets, can be identified by the stress lines on the surface. Extrusion is a very rapid production method because it is continuous. Several times the amount of material is provessed this way if contrasted with injection molding of course, the two processes are not used for the same purpose. Special adaptations of extrusion are often used, Wire may be coated by passing it through the center of an extrusion die which produces a tube around.it. Some film is made by extruding a tube and blowing air inside whicd expands and thins it to be desired size. The thin tube is then slit and flattened out for use as film. Many polyethylene "bags are mode by cutting thin extruded tubing and sealing the ends. Monofilament fishing line is extruded nylon. Sheet extruders may be placed in line with thermoforming equipment so that the.sheet is fed directly into the forming machine before tt is cooled. Resin Screw > Thermocouples, Screen pack Breaker plate Hopper / cooling jacket ^UM^LUiy/ZQ Fig: Standart Single-Screw Extruder CT 3 Screws are characterized by their length/diameter (1/D) ratio. The ratios most commonly used for single-screw extruders range form 15/1 to 30/1. Ratios of 20/1 and 24/1 are common for thermoplastics. A long barrel yields a more homogeneous extrudate, which is particularly desirable when pigmented materials are used. Screws are also characterized by their compression ratios, or the ratio of the flingt depth of the screw at the hopper end to the flight depth at tke die end. Compression ratios of single-screw extruders are usually 2/1 to 3.5/1. The -screw is usually divided into feed, compression, and metering sections, each of which operates at its own r^e The feed section pick up the powder, pellets, or beads form under the hopper mouth and conveys them in the solid to the compression section. This section is deep flighted to supply enough material to the forward sections. The gradually diminishing flight depth in the compe ression of the melting granules. This results in the trapped air being forced back through the feed section instead of being carried forward with the resin, 10 ensure a porosity- free extrudate. An increase in the shearing action on the melt, caused by the motion of the screw surfaces in relation XI to the barrel wall, results in good mixing and generates fractional heat, which increases melt fluidity and leads to more uniform temperature distribution in the molten ex- trudate. The resin should be fully melted into a fairly uniform melt as it enters the final metering section of the screw. This section forces the molten polymer through the die at a steady rate and eradicates pulsations. Many screw designs use a compression ratio of 3/1 to 5/1» that is, the flight depth in the metering section is roughly :one- third to one-fifth that in the feed section. The motor drive-, for the screw should possess more power than it actually needs for normal use and, ideally, should be capable of varying its speed. Either variable- speed motors or constant-speed motors with variable-speed equipment should be used. Because of the very high back pressure generated in an extruder and the trend toward higher screw speeds, thrust bearings must be strong. Overload protection should be provided. A simple continuous-flight screw with constant pitch is common. More sophisticated screww designs include flow disrupters or mixing sections, which act to break up and rearrange the laminar flow of the melt within the flight channel. These actions result in more thoroigh mixing and more uniform heat distribution in the metering section of the screw. Mixer screws can be used to mix dissimilar materials and to improve extrudate uniformity at screw speeds greater than 100 rpm. The fluted mixing section barrier-type desing is very useful when extruding polyolefins. Rings, mixing pins and parallel interrupted mixing flights with wide-pitch angles can be used to solve mixing problems.