11 important principles of plastic extrusion
11 important principles of plastic extrusion
1. Mechanical principle WeChat public no
The basic mechanism of extrusion is simple - a screw turns in the barrel and pushes the plastic forward. The screw is actually an inclined plane or slope that wraps around the center layer. The aim is to increase pressure to overcome greater resistance. In the case of an extruder, there are three kinds of resistance to overcome: the friction between the solid particles (feed) against the cylinder wall and the first few laps of the screw (feeding area); The adhesion of the melt on the cylinder wall; When the melt is pushed forward, its internal logistics resistance.
Newton had explained that if an object does not move in a given direction, the force on the object is balanced in this direction. The screw is not an axial motion, although it can rotate rapidly around the circumference. So, the axial force on the screw is balanced, and if it's putting a big forward thrust on the plastic, then it also exerts a similar backward push on something. In this case, the thrust is applied to the bearing on the back of the feed - thrust bearing.
Most single screws are right-handed threads, such as bolts and bolts used in carpentry and machinery. And if you look at it from the back, they're going backwards, because they're going to try to spin out the cylinder back. In some twin screw extruders, the two screws rotate in opposite directions and cross each other in the two cylinders, so one must be right and the other must be left. In other interlocking twin screws, the two screws rotate in the same direction and must have the same orientation. However, in either case there is a thrust bearing that absorbs the backward force, and Newton's principle still applies.
2. Thermal principle WeChat public no
The extrudable plastic is thermoplastics - they melt when heated and solidify again when they cool. Where does the heat from molten plastic come from? Feed preheating and barrel/mould heater may play a role and at boot time is very important, however, motor energy input, overcome the resistance of the viscous melt the generated when the screw on cylinder body friction heat - is the most important of all plastic heat source, small system, low speed screw, high melt temperature except plastic and coating application.
For all other operations, it is important to recognize that the tubular heater is not the main source of heat in the operation, and thus is less effective than we might expect (see rule 11). The temperature of the posterior cylinder may still be important because it affects the speed of solid material delivery in the teeth or feed. Molds and mold temperatures should usually be the desired melt temperature or proximity to this temperature unless they are used for a specific purpose such as glazing, fluid distribution or pressure control.
3. The deceleration principle WeChat public no
In most extruders, the change of screw speed is realized by adjusting the motor speed. The motor usually turns at full speed of about 1750rpm, but this is too fast for an extruder. If you rotate at such a fast speed, you will have too much friction and the plastic is too short to produce a uniform, well stirred melt. The typical rate of deceleration is between 10:1 and 20:1. The first stage can be either a gear or a pulley block, but the second stage is geared and the screw is positioned at the last big gear center.
In some slow-moving machines (such as twin screws for UPVC), there may be three deceleration phases and the maximum speed may be as low as 30rpm or lower (ratio of 60:1). The other extreme is that some of the long twin screws used for stirring can be run at 600 RPM or faster, so you need a very low cut rate and a lot of deep cooling.
Sometimes the reduction rate is mismatched with the task - there is too much energy to use - and it is possible to add a pulley block between the motor and the first deceleration phase that changes the maximum speed. This either increases the screw speed to more than the previous limit or decreases maximum speed allowing the system to run at a greater percentage of the maximum speed. This will increase the availability of energy, reduce amperes and avoid motor problems. In both cases, the output may increase depending on the material and its cooling needs.
4. The feed ACTS as coolant WeChat and the public no
The extrusion is to transfer the energy of the motor - sometimes the heater - to the cold plastic to convert it from a solid to a melt. Input feed is lower than the cylinder and screw in the feeding area. However, the surface of the tube in the feeding area is almost always above the plastic melting range. It is cooled by contact with incoming particles, but heat is kept back by heat from the hot front end and can be controlled by heating. Even if the current end heat is maintained by viscous friction and does not require a cylinder heat input, it may be necessary to open the heater. The most important exception is the slot feeding tube, which is almost exclusively for HDPE. The screw root surface is also cooled and is insulated from the cylinder wall by the plastic feeding particles (and the air between the particles). If the screw stops suddenly, the feed stops, and because the heat moves back from the hotter front, the surface of the screw gets hotter in the feeding area. This may cause the particles to adhere or bypass the root.
5. In the feeding area, slide to the cylinder body and slide on the screw to WeChat public
In order to make a single screw extruder the solid particles in the feeding area of the smooth barrel reach the maximum, the particles should stick to the cylinder and slide onto the screw. If the particles stick to the root of the screw, nothing can pull them down; The volume of the channel and the volume of the solid is reduced. Another reason for the adhesion of the roots is that the plastic may be heated and produced in this area to produce gels and similar particles of pollution, or intermittent adhesion and interruption as the output speed changes.
Most plastic slides naturally in the roots, as they are cold when they enter, and the friction does not heat the root to the heat of the wall. Some materials are more likely than others to adhere to: highly plasticized PVC, non-crystalline PET, and some of the eventual use of the adhesive properties of polyolefin copolymer.
For the cylinder, the plastic is necessary to adhere to it so that it is blown off and pushed forward by the screw thread. There should be a high friction coefficient between the granule and the tube, and the friction coefficient is also influenced by the temperature of the back tube. If the particles don't stick, they just rotate in place without moving forward - which is why smooth incoming materials are not good.
Surface friction is not the only factor affecting feed. Many particles will never touch the cylinder or screw root, so there must be friction and mechanical and viscosity interlocking within the particulate matter.
A slot tube is a special case. In the feeding area, the feeding area and the rest of the barrel are heat-insulated and deeply water-cooled. The thread pushes the particles into the slot and creates a very high pressure within a fairly short distance. This increases the occlusion of the same output with low screw rotation speed, resulting in less friction heat in the front end and lower melt temperature. This could mean cooling down the production of faster production in the blown film production line. The groove is especially suitable for HDPE, which is the most slippery common plastic except for fluorinated plastics.
6. The maximum cost of materials is WeChat, the public number of the public
In some cases, material costs can account for 80 per cent of the cost of production - more than the sum of all the other factors - except for a few products that are particularly important in quality and packaging, such as medical catheters. This principle nature leads to two conclusions: processors should be as much as possible instead of reuse wastes and waste of raw materials, and strictly comply with the tolerance as much as possible so as not to deviate from the target thickness and product problems.
7. The cost of energy is relatively unimportant
Despite the attractiveness and real problems of a factory and rising energy costs on the same level, the energy needed to run a extruder remains a fraction of the total cost of production. Always the same because the material cost is very high, extruder is an effective system, if the introduction of the plastic will soon become very hot too much energy that cannot be normal processing.
8. The pressure at the end of the screw is very important
This pressure reflects the resistance of all objects downstream of the screw: the filter screen and the pollution puller board, the adapter conveyor tube, the fixed agitator (if any), and the mold itself. It depends not only on the geometry of these components but also on the temperature of the system, which in turn affects the viscosity and speed of the resin. It does not depend on the screw design, which affects temperature, viscosity and passing. Measuring temperature is important for safety reasons - if it is too high, die and die may explode and injure nearby personnel or machinery.
Pressure is favorable for stirring, especially in the final area of the single screw system (metering area). However, high pressure also means that the motor has to output more energy - and therefore the melt temperature is higher - which can specify the stress limit. In a twin screw, two snails are a more efficient blender, so no pressure is needed for this purpose.
When making the hollow parts, such as the pipe used for the core positioning of spider molds, there must be high pressure in the mold to help the separated logistics reassemble. Otherwise, products along the welding line may be weak and may be problematic when used.
9. Output = last thread displacement +/- pressure logistics and leakage WeChat public no
The displacement of the last thread is called a positive flow, which depends on the geometry of the screw, the speed of the screw and the density of the melt. It is regulated by pressure logistics, which actually includes any overbite effects of reducing the drag effect of output (expressed by the highest pressure) and increasing the output. The leakage on the thread can be either direction in either direction.
It is also useful to calculate the output of each RPM, since this represents any drop in the pump capacity of a time screw. Another related calculation is the amount of output per horsepower or kilowatt. This represents efficiency and can estimate the production capacity of a given motor and drive.
10. The shear rate plays a major role in the viscosity of WeChat public
All ordinary plastics have a drop in shear, meaning that the viscosity gets low when the plastic is moving faster and faster. The effect of some plastics is particularly pronounced. For example, some PVCs can increase the flow rate by 10 times or more when the thrust doubles. On the contrary, LLDPE shear has not fallen too much, and its flow rate increased by only three to four times when the reasoning doubled. Reduced shear reduction results in high viscosity under extrusion, which in turn means more motor power. This can explain why LLDPE runs at a higher temperature than LDPE. The flow rate is indicated by the shear rate, which is about 100s-1 in the screw passage, between 100 and 100s-1 in most mold mouths, and greater than 100s-1 in the gap between the thread and the cylinder wall and some small die. The melt coefficient is a commonly used measure of viscosity but is reversed (for example, flow/thrust rather than thrust/flow). Unfortunately, the measurement is in the shear rate at 10s-1 or more and may not be a real measurement in the extruder with a fast melt flow.
11. The motor and barrel are opposite each other WeChat public number of the wind polyvinyl plastic
Why is the control effect of the tube not always the same as expected, especially in the measuring area? If you heat the tube, the viscosity of the material at the wall is smaller, and the motor will need less energy in this smoother cylinder. Motor current (amperes) decreased. Conversely, if the tube is cooled, the melt viscosity increases in the cylinder wall, the motor must rotate more vigorously, the number of amperes increases, and some of the heat removed from the tube is returned by the motor. Normally, the tube regulator does have an effect on the melt, which is what we expect, but nowhere is the effect greater than in the region. It's best to measure the melt temperature to really understand what's going on.
Rule 11 does not apply to dies and molds because there is no screw rotation. That's why external temperature changes are more effective there. However, these changes are not uniform from the inside out, unless stirred in a fixed agitator, which is an effective tool for melting temperature changes and stirring.
