Extrusion process
Extrusion mechanism is very simple:
1: A screw in the cylinder rotates and the plastic to move forward. The screw is an inclined plane or ramp, is wound on the central layer. Its purpose is to increase the pressure in order to overcome the greater resistance. Extruder, there are three kinds of resistance need to be overcome: the solid particles (feed) and the friction on the wall of the cylinder and the screw rotation laps before (feed zone) the mutual friction between them; melt the adhesion of the tube wall; melt is pushed forward its internal logistics resistance.
Newton had explained that if an object is not a given direction of movement, the force on the object balanced in this direction. Screw is not axial movement, although in the circumferential near it may be laterally rapid rotation. Accordingly, the axial force on the screw is balanced, but if it is a large forward thrust is applied to the plastic melt so that it is also applied to an object a same backward thrust. Here, it is the thrust exerted role in the feed inlet behind the bearing - thrust bearing.
The majority of single screw is a right-handed thread, as used in woodworking and machine screw and bolt. If viewed from behind, they are reverse rotation, because they want to try to backward backed out of the cylinder. In a twin screw extruder, two screw reverse rotation in the two cylinder and intersect with each other, and therefore must be a right to another must be left to. In the the remaining occlusal twin-screw, the two screws rotating in the same direction and therefore must have the same orientation. However, in either case have to absorb the backwards force thrust bearing, Newtons principles still apply.
2 hot principles
The extruded plastic is a thermoplastic - which melts when heated and again during the cooling solidification. Melting the plastic heat come from? Into the preheating and barrel / mold heater may play a role in the start is very important, however, the resistance of the motor input energy - motor to overcome the viscous melt generated in the cylinder body when turning the screw friction heat - The plastic of the most important sources of heat, small systems, low-speed screw, high melt temperature of the plastic extrusion coating applications except.
For all other operations, it is important to recognize the main heat source in the tube heater is not operating and, therefore, the role of the extrusion may be less than we expected (see Principle 11). After the cylinder temperature is likely to remain important because it affects the teeth together or the solids in the feed conveying speed. The die and mold temperature should normally be desired melt temperature or close to this temperature, unless they are used for a specific purpose such as glazing, fluid distribution or pressure control.
(3) reduction principles
In the majority of the extruder screw speed changes are achieved by adjusting the motor speed. Motors are usually about 1750rpm at full speed rotation, but this is too fast for an extruder screw. If such a fast speed of rotation, it will generate too much heat due to friction and the plastic of the residence time is too short, can not prepare a uniform, well-stirred melt. The typical deceleration ratio between 10:1 to 20:1.The first stage can either gear pulley block, but the second stage gear, and the screw positioned at a large gear center.
Slow running machine (such as for UPVC twin-screw), there may be three deceleration phase and the maximum speed may be as low as 30rpm or less (ratio of 60:1). The other extreme is for stirring a long twin-screw at 600rpm or faster speed, therefore requires a very low rate of deceleration and a lot of deep cooling.
Sometimes deceleration rate with the task of matching wrong - there will be a lot of energy can not be used - and has the potential to increase in the first deceleration phase between the motor and change the maximum speed a pulley block. Either the screw speed is increased to more than the previous maximum or allow the system to reduce the maximum speed at maximum speed in a larger percentage of run. This will increase the availability of energy, to reduce the amperage and avoid motor problems. In both cases, according to the material and the cooling requirements, the output may increase.
4 play coolant feed
The extrusion is the energy of the motor - sometimes heater - transferred to the cold plastic, thereby converting it from the solid into the melt. Input feed ratio to the feed zone of the cylinder and the screw surface temperature is low. However, to the surface of the cylinder in the feed zone is almost always in the plastic above the melting range. It is cooled by contact with the feed particles, but the heat rearwardly from the front of the heat transfer heat and controlled heat and maintain. You even end heat by viscous friction to maintain and cylinder heat input is not required, you may need to open the heater. The most important exception is a slot into the barrel, almost exclusively in HDPE.
Screw root surface to feed the cooling by the plastic feed particles (and the air between the particles) from the cylinder wall insulation. If the screw is suddenly stopped, the feed is stopped and moved backward because the heat from the hotter the distal end surface of the screw in the feed zone becomes hotter. This may be caused by the adhesion of the particles at the root or bypass.
5 in the feeding area, and adhered to slide the screw on the cylinder
Solid particle transport in order to make a single-screw extruder feed zone of the smooth cylinder reaches the largest particles should be glued on the cylinder and slide the screw. If the particles stick to the screw root, nothing can pull them down; reduces the amount of channel volume and solid entrance. Another reason is the poor adhesion at the root plastic may soak here and produce gels and similar pollution particles, or adhesion with intermittent change of the output speed and interrupt.
Most plastics sliding naturally in the root, because it is cold, when they enter the roots and the frictional force is not heated to the tubular wall as hot. Some materials are more likely than others materials Adhesion: highly plasticized PVC, amorphous PET, and some end use desired adhesion properties of the polyolefin-based copolymer.
For the cylinder, plastic the necessary adhesion so that it can be scraped off and screw thread to move forward. Between the particles and the cylinder should have a high friction coefficient, friction coefficient, in turn, is also affected by the strong impact of the rear cylinder temperature. If the particle does not adhere to, they are only rotated in place without forward movement - which is why a smooth feed of the reason for the bad.
Surface friction is not the only factor affecting feed. Many particles never touch the cylinder or screw root inside the particles must have mechanical friction and viscosity chain.
With grooved drum body is a special case. The grooves in the feed zone, the feed zone and the rest of the tubular body portion is thermally insulated and the depth of water-cooled. Thread the particles are pushed into the tank and a high pressure is formed in a relatively short distance. This increases the nip of the lower screw speed grant the same output, so that the front end of the friction generated heat to reduce the melt temperature is lower. This may mean that the cooling limit blown film production line faster production.The slot particularly suitable for HDPE, it is the most slippery in addition to the perfluorinated plastic ordinary plastic.
Material cost
In some cases, material costs can account for 80% of the cost of production - more than all other factors - of particular importance in addition to a few quality and packaging products such as medical catheter. This principle naturally leads to two conclusions: the processors should as much as possible re-use of scrap and waste instead of raw materials, and, as far as possible, comply with strict tolerance so as not to deviate from the target thickness and problems.
Energy costs are relatively unimportant
Although a factory attractive and real problems and rising energy costs at the same level, the energy required to run an extruder is still a small part of the total production costs. Always the case extruder is an effective system, the introduction of too much energy then the plastic will quickly become very hot because the material cost is very high, so that can not be normal processing.
The pressure at the end of the screw is very important
This pressure reflects the resistance of the screw downstream objects: filters and pollution tie broken board, adapter pipelines, fixed stirrer (if any) and the mold itself. It is not only dependent on the geometry of these components is also dependent on the temperature in the system, which in turn affects the viscosity of the resin and speed. It does not depend on the screw design, it affects the temperature, viscosity and throughput, except when. For security reasons, it is very important to measure the temperature - if it is too high, the die and mold may explode and injure nearby personnel or machinery.
Pressure for mixing is advantageous, particularly in the final area of a single-screw system (the metering zone). However, the high pressure means that the motor output more energy - and thus a higher melt temperature - this can be specified pressure limit. In a twin screw two screws engaging with each other is a more efficient stirrer, and therefore no pressure when used for this purpose.
In the manufacture of hollow parts, such as the use of stents on the core positioning the spider mold manufacturing tube, high pressure in the mold to help separate logistics regroup. Otherwise, along the welding line of products may be weak and may use problems.
9 OUT = Last displacement of a threaded + / - pressure stream and leakage
Finally, a displacement of the thread is called positive flow depends only on the geometry of the screw, the screw speed and melt density. It is adjusted by the pressure stream actually included to reduce the amount of output resistance effect (represented by the maximum pressure) and any increase in the output of the feed over the occlusion effect. The thread leak may be an arbitrary direction in the two directions.
Calculated for each rpm (revolutions) of the output are also useful, because it means that any decrease in the ability of a screw pump out. Another related calculations are used per horsepower or kilowatt output. This means that the efficiency and be able to estimate the production capacity of a given motor and drive.
10 shear rate plays a major role in the viscosity
All ordinary plastic has a shear drop characteristics, meaning low viscosity plastic moving faster and faster. Plastic effect that was particularly evident. E.g., several PVCs thrust doubling the flow rate will increase by 10 times or more. Contrary, LLDPE shear drop was not too reasoning doubling its flow rate is increased by only 3 to 4 times. Reduced shear effect of reducing the mean high viscosity extrusion conditions, which in turn means more motor power. This could explain why the temperature than LDPE LLDPE run high. Shear rate, the flow in the screw channel is about 100s-1, in the majority of mold mouth is between 100 and 100 s-1 in the thread and the cylinder wall clearance and some small die clearance greater than 100s-1. Melt factor is the viscosity of a common measurement method, but is reversed (for example, flow / thrust / flow rate) thrust instead. Unfortunately, it is measured at a shear rate of 10s-1 or more hours and a melt flow rate in the extruder quickly may not be a true measured value.
11. Motor and cylinder antagonism, cylinder and motor opposition
Why is the control effect of the cylinder is not always the same and expectations, especially in the measuring area? If the cylinder is heated, the tubular wall at the viscosity of the material layer becomes smaller, less of the energy required by the motor in the cylinder body to run a smoother. Motor current (amperage) drop. Conversely, if the cylinder was cooled cylinder wall at the melt viscosity increases, the motor must be more strongly rotated, the amperage increases, some of the heat removed through the tubular body when the motor back.Typically, the regulator of the cylinder does have an effect on the melt, which is what we expect, but anywhere in the results that are not regional variables. The best melt temperature to truly understand what is happening.
Principle 11 does not apply to the die and mold, because there is no screw rotation. This is why the external temperature variation where more effective. However, these changes are uneven and thus from the inside to the outside, except in a fixed stirrer, stir for changes in melt temperature and stirring is an effective tool.
