Transmission Efficiency of plastic Films Part 1

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2.1 EXTRUSION OF FILM: TRANSMISSION EFFICIENCY OF PLASTIC FILMS FILM FORMING PROCESS There are fundamentally two different methods of extruding film, namely, blow extrusion and slit die extrusion former method produces tubular film, which may be gusseted or lay-flat while the latter results in flat film. If desired, lay-slit film can be produced. The equipment for film extrusion consists of an extruder, fitted with a suitable die, equipment to cool the molten film, haul-off machinery and a wind-up unit. Extrusion and slit die extrusion vary in the design of die used and in the type of cooling. The haul-off and wind-up equipment is also different. The design and operation of the extrusion up to the die is the same for both methods however, and’ will be briefly described here before considering the different types of film manufactured. ‘The basic extrusion process is designed to convert continuously a thermo plastics material into a particular form. The basic sequence of events is as follows. A typical extruder is shown in Figure. The plastics granules are fed through a hopper mounted at one end of the barrel and carried forward along the barrel by the action of the screw. As the granules move along the screw, they are melted by contact with the heated walls of the barrel and by the heat generated by friction. The screw then forces the molten plastic through the die which determines its final form the most important component of the extruder is the screw and different designs of screw are used for extruding different polymers. Extruder screws are characterized by their length to diameter ratio (commonly written as L/D ratio) and their compression ratios. The compression ratio is the ratio of the 6
TRANSMISSION EFFICIENCY OF PLASTIC FILMS volume of one flight of the screw at the hopper end to the volume of one flight at the die end. L/D ratios is most commonly used for single screw extruders are between about 15:1 to 30:1 while compression ratios can vary from 2:1 to 4:1. An extruder screw is usually divided into three sectors, namely, feed, compression and metering. The feed section transports the material from under the hopper mouth to the hotter portion of the barrel. The compression section is that section where the diminishing depth of thread causes a volume compression of the melting granules. The main effect of this is an increase in the shearing action of the molten polymer due to the relative motion of the screw surface with respect to the barrel wall. This improves the mixing and also leads to an increase in frictional heat and a more uniform heat distribution throughout the melt. The function of the final section of the screw is to homogenize the melt further, meter it uniformly through the die and smooth out pulsations. Multi-screw extruders, particularly twin-screw extruders also have their own advantages and limitations. In general, multiple-screw machines are more expensive and because of their more complicated construction are likely to be less sturdied. A more positive transport of the molten polymer is possible, and better mixing is obtained. By virtue of their positive pumping action, multiple-screw extruders produce less shear heat and this makes them very suitable for materials that are heat sensitive, have low coefficients of friction or must leave the die at low extrusion temperatures. 2.2 COEXTRUSION: This method of producing composite films differs in one important detail from all those already described, As it can be adapted to produce a composite film in either lay-flat tubing form or as a flat film. The process for making tubular composite films is often referred to as coaxial extrusion. Basically the process consists in coupling two or more extruders to a single die head. It is essential that precise control is available over screw speed, melt pressures and temperatures and power capacity of the extruder drives. Size of extruders used will depend on the ratio of the components in the laminate. Extruders of similar capacity are usually employed when a wide range of composite film thicknesses is required, but for composites of widely different polymer content, different sizes of extruders are usually combined. So minimizing capital cost. However, the success of the technique lies chiefly in the design of the die and the way in which the individually extruded melts are brought together, prior to being extruded as a multi-layer film. There are, essentially, two different methods of carrying out this operation. In one system, each separate melt is extruded into a separate manifold and then brought together at a common point inside the die. The channels within the manifold are normally calculated to match the flow properties of 7
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Butyl, GRS, polyurethane, neoprene
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Transmission Efficiency of plastic Films Part 1

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