Overview
The extrusion processes offer the advantages of complete versatile plastic processing techniques unsurpassed in economic importance by any other process. Worldwide, extruder lines are the largest converters of plastics and can be considered the most important production machinery in the plastic industry.
EXTRUDER TYPE AND CONSTRUCTION
There are many different extruder types that have been designed to produce the wide variety of plastics and products. The widely used are the single-screw extruders and twin-screw extruders.
1.1 Extruder with tapered twin-screws
1.2 Extruder with single-screw
In choosing the size of an extruder and its driving mechanism, considerations to be evaluated include:
The range of screw speed screw erations to be evaluated include:
(1) the range of screw speed screw needed;
(2) the requirement that screw speed should be infinitely variable,and may require certain speed levels or several ranges;
(3)maximum power required from the drive based on the plastic to be processed;
(4)relationship required between the screw speed and the torque on the screw shaft;
(5) whether the machine to be used for a single or multiple products. The drives consist of motor or belt drives, which are linked to the screws through systems such as double reduction gearboxes.Belt drives are used for the smaller machines. The drives simplify the transformation of high motor speeds into lower speeds and high torque required to operate and control the screws.
EXTRUDER OPERATION
Machine operation can take place in three stages. The first stage requires operating the extruder for warm-up with operational settings of up-stream and down-stream equipment. The next stage involves setting the required processing conditions to meet product requirements at the lowest cost. The final stage is devoted to fine-tuning and problem solving the complete line. A successful operation requires close attention to many details, such as the melt quality, temperature profile adequate to melt but which does not degrade the plastic, production of a minimum of scrap,and procedures for start-up and shut-down that will not (or minimize)
degrade the plastic. Figure 1.8 shows examples of temperature
profiles for different plastics.
PROCESSING PLASTIC
An extruder is a combination of screw conveyor and compressor. As a conveyor, it mechanical acts as 'screwing' itself backward. Since the screw itself stays in one place, the plastic does the moving and is forced out through the die. Since the plastic is viscous and develops frictional resistance to flow, power is required to turn the screw. As a compressor, the screw moves solid plastic (pellets, etc.) initially and gradually the plastic melts. In the solid stage, it is denser, especially the crystalline plastic;with this plastic air spaces exist with the result that they have a low bulk density (weight per volume). The screw compresses the solid plastic together. With this pressure action, the air usually takes the least path of resistance and is pushed back out of the feed opening. If any air remains in the melt, it could be released when the melt exits the die with no damage to the product or it may remain in the product as a contamination.
EXTRUDER HEATING AND COOLING PROFILE
Examples of melt temperatures in an extruder
Heat to soften the plastic is supplied in two ways: by external barrel heating and internal frictional forces brought about on the plastic due to the action of the metal screw in the metal barrel. The amount of such frictional heat supplied is appreciable. In many extrusion operations it represents most of the total heat supplied to the plastic. Electricity, fluid such as steam, or hot oil can be used. Electrical heating is generally preferred because it is the most convenient, responds rapidly, easiest to adjust, easy to clean, requires a minimum of maintenance, covers a much larger temperature range, and is generally the least expensive in terms of initial investments.
AUXILIARY EQUIPMENT
Down-stream of the die different types of takeoff equipment are used to meet different product requirements. They include sizing fixtures, cooling tanks, coiling devices, line speed controls (mechanical, electrical, and/or sonic), tension controls (capstans, etc.), pulling devices (caterpillar,pinch rolls, opposite moving belts, etc.), cutters in-line, collecting equipment (troughs, tanks, stackers, etc.), postforming, fast takeoff rolls, and so on.
ENERGY CONSERVATION
Figure 1.3 Examples of energy losses from an extruder
When examining energy consumed or lost, the equipment used in the complete production line as well as the plastic is involved.There is a continuing effort by our POLYTECH planstic machinaery , to reduce energy consumption as well as energy loss. Figure 1.3 is
an example of where energy/heat losses from an extruder occur.
In comparison with other materials, plastics have the lowest specific energy requirement for their manufacture (Fig. 1.4),fabricating products,and lowest recycling energy consumption. Energy conservation can be considered in the extrusion as well as the overall plastic industry from several viewpoints that go from producing plastic to processing to recycling products. In practically all evaluations, utilization of plastics saves or reduces energy requirements. In automobiles, aircraft, and other means of transportation, the increasing use of plastics reduces weight and fuel consumption; an example is replacement of the small whiskey
Bottles aboard aircraft with extruded blow molded plastic bottles rather than glass, resulting in lots of fuel energy saved. Using plastics in construction saves energy via insulation gains. Also, just shipping the lighter weight plastic products save energy.
Figure 1.4 Energy requirements for different materials.
ADVANCES IN EXTRUSION PROCESS
From this beginning, advances in plastics have been followed by advances in extrusion equipment . With all this action, we Polytech plastic machinaery co.,Ltd has been extensive work in rheology and thermodynamics in better understanding the process of extruding plastics and other materials. The results continue to increase production rates, increasing amount of control over the extrusion variables, and/or reducing costs. Major developments continue to be in the refinement of existing extrusion methods.