Analysis of plastic processing technology - on the mechanical principle of extrusion molding

Mechanical principle

The basic mechanism of extrusion is simple – a screw rotates in the barrel and pushes the plastic forward. The screw is actually a bevel or slope that is wrapped around the center layer. Its purpose is to increase the pressure in order to overcome the large resistance. In the case of an extruder, there are three types of resistance that need to be overcome: the friction of the solid particles (feed) against the wall of the cylinder and the mutual friction between the coils before the rotation of the screw (feeding zone); Adhesion on the wall of the barrel; the internal flow resistance of the melt as it is pushed forward.

If an object does not move in a given direction, the force on the object is balanced in this direction. The screw does not move in the axial direction, although it may rotate laterally rapidly near the circumference. Therefore, the axial force on the screw is balanced, and if it applies a large forward thrust to the plastic melt, it also applies an identical backward thrust to the object. Here, the thrust applied is the bearing acting on the thrust bearing behind the feed port.

Most single screws are right-handed threads, like screws and bolts used in woodworking and machinery. If they look from the back, they are rotating in the opposite direction because they try to spin out the barrel as far as possible. In some twin-screw extruders, the two screws rotate in opposite directions in two cylinders and cross each other, so one must be right-handed and the other must be left-handed. In other occlusal 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

The extrudable plastics are thermoplastics - they melt when heated and solidify again upon cooling. Where does the heat of molten plastic come from? Feed preheating and barrel/die heaters may work and are important at start-up, however, the motor input energy - the friction of the motor against the viscous melt - the frictional heat generated in the barrel when turning the screw - is all The most important heat source for plastics, except for small systems, low speed screws, high melt temperature plastics and extrusion coating applications.

For all other operations, it is important to recognize that the barrel heater is not the primary source of heat in operation, and therefore the effect on extrusion is less than we expected (see Principle 11). The post-cylinder temperature may still be important because it affects the rate of solids transport in the teeth or in the feed. The die and mold temperatures should generally be the desired melt temperature or close to this temperature unless they are used for a specific purpose like glazing, fluid distribution or pressure control.

3. Deceleration principle

In most extruders, the change in screw speed is achieved by adjusting the speed of the motor. The motor typically rotates at full speed of approximately 1750 rpm, but this is too fast for an extruder screw. If it is rotated at such a fast speed, too much frictional heat is generated and the residence time of the plastic is too short to prepare a uniform, well-stirred melt. Typical deceleration ratios range from 10:1 to 20:1. The first stage can be either gear or pulley, but the second stage uses gears and the screw is positioned at the center of the last large gear.

In some slow-running machines (such as twin-screws for UPVC), there may be 3 deceleration stages and the maximum speed may be as low as 30 rpm or lower (up to 60:1 ratio). At the other extreme, some very long twin-screws for agitation can run at 600 rpm or faster, thus requiring a very low rate of deceleration and a lot of deep cooling.

Sometimes the deceleration rate is mismatched with the task - there will be 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 above the previous limit or decreases the maximum speed to allow the system to operate at a greater percentage of maximum speed. This will increase the available energy, reduce the amperage and avoid motor problems. In both cases, the output may increase depending on the material and its cooling needs.

4. Feeding as a coolant

Extrusion transfers the energy of the motor, sometimes the heater, to the cold plastic, converting it from solid to melt. The input feed is cooler than the barrel and screw surface temperatures in the feed zone. However, the surface of the barrel in the feed zone is almost always above the melting range of the plastic. It is cooled by contact with the feed particles, but the heat is retained by the heat transferred back to the hot front end and controlled heating. Even after the current end heat is held by the viscous friction and no barrel heat input is required, the post heater may be required. The most important exception is the slotted feed cartridge, which is almost exclusively for HDPE.

The screw root surface is also cooled by the feed and is insulated from the barrel wall by the plastic feed particles (and the air between the particles). If the screw suddenly stops, the feed also stops, and as the heat moves back from the hotter front end, the screw surface becomes hotter in the feed zone. This can cause adhesion or bridging of the particles at the roots.

5. In the feeding area, stick to the cylinder and slide onto the screw

In order to maximize the amount of solids transported in the smooth barrel feed zone of a single screw extruder, the particles should stick to the barrel and slide onto the screw. If the particles stick to the root of the screw, nothing pulls them down; the volume of the passage and the amount of solids are reduced. Another reason for poor adhesion to the roots is that the plastic may heat up here and produce gels and similar contaminating particles, or intermittently adhere and break with changes in output speed.

Most plastics slide naturally at the roots because they are cold when they enter, and the friction does not heat the roots as hot as the walls. Some materials are more likely to adhere than others: highly plasticized PVC, amorphous PET, and some polyolefin-based copolymers with adhesive properties desired for end use.

For the barrel, it is necessary for the plastic to adhere here so that it is scraped off and pushed forward by the screw thread. There should be a high coefficient of friction between the granules and the barrel, and the coefficient of friction is in turn strongly influenced by the temperature of the rear barrel. If the particles do not stick, they simply rotate in place without moving forward - which is why smooth feeding is not good.

Surface friction is not the only factor affecting the feed. Many particles never touch the barrel or the root of the screw, so there must be friction and mechanical and viscosity linkages inside the particles.

A grooved cylinder is a special case. The trough is in the feed zone and the feed zone is thermally insulated from the remainder of the barrel and is deeply water cooled. The thread pushes the particles into the groove and creates a very high pressure over a relatively short distance. This increases the bite tolerance of the lower output of the same screw at the same output, so that the frictional heat generated at the front end is reduced and the melt temperature is lower. This may mean faster production in cooling-limited blown film lines. The tank is particularly suitable for HDPE, which is the smoothest common plastic except for fluorinated plastics.

6. The most expensive material

In some cases, material costs can account for 80% of the cost of production—more than all other factors—except for products that are particularly important in quality and packaging, such as medical catheters. This principle naturally leads to two conclusions: processors should reuse scrap and scrap as much as possible in place of raw materials, and strictly adhere to tolerances as much as possible to avoid deviations from target thickness and product problems.

7. Energy costs are relatively unimportant

Although the attractiveness and real problems of a factory are at the same level as rising energy costs, the energy required to run an extruder is still a small fraction of the total production cost. This is always the case because the material costs are very high and the extruder is an effective system. If too much energy is introduced, the plastic will quickly become so hot that it cannot be processed properly.

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 contaminated shredder plate, the adapter transfer tube, the fixed stirrer (if any), and the mold itself. It depends not only on the geometry of these components but also on the temperature in the system, which in turn affects resin viscosity and throughput. It does not depend on the screw design, except when it affects temperature, viscosity and throughput. For safety reasons, measuring temperature is important – if it is too high, the die and mold can explode and harm nearby people or machines.

The pressure is advantageous for agitation, especially in the last zone of the single screw system (metering zone). However, high pressure also means that the motor has to output more energy - and thus the melt temperature is higher - which can dictate the pressure limit. In a twin screw, the engagement of the two screws with each other is a more efficient agitator, so no pressure is required for this purpose.

In the manufacture of hollow parts, such as tubes made from spider-centered spider molds using brackets, high pressure must be created within the mold to aid in the recombination of separate streams. Otherwise, the product along the weld line may be weak and problems may occur during use.

9. Output = displacement of the last thread / - pressure flow and leakage

The displacement of the last thread is called positive flow and depends only on the geometry of the screw, the screw speed and the melt density. It is regulated by the pressure stream and actually includes a drag effect that reduces the output (indicated by the highest pressure) and any overbiting effect in the feed that increases the output. The leak on the thread may be in either of two directions.

It is also useful to calculate the output per rpm (rotation) as this represents any drop in the pumping capacity of the screw at a time. Another related calculation is the output per horsepower or kilowatt used. This represents efficiency and is capable of estimating the production capacity of a given motor and drive.

10. Shear rate plays a major role in viscosity

All common plastics have shear-reducing properties, meaning that the viscosity becomes lower as the plastic moves faster and faster. This effect of some plastics is particularly noticeable. For example, some PVCs increase the flow rate by a factor of 10 or more when the thrust is doubled. On the contrary, the LLDPE shear force is not reduced too much, and the flow rate is only increased by 3 to 4 times when the reasoning is doubled. The reduced shear reduction effect means high viscosity under extrusion conditions, which in turn means more motor power is required. This can explain why LLDPE operates at a higher temperature than LDPE. The flow rate is expressed in shear rate, approximately 100 s-1 in the screw channel, between 100 and 100 s-1 in most die profiles, and greater than 100 s-1 in the gap between the threads and the wall and some small die gaps. Melt coefficient is a commonly used measure of viscosity but is reversed (eg flow/thrust rather than thrust/flow). Unfortunately, the measurement is not a true measurement in an extruder with a shear rate of 10 s-1 or less and a very fast melt flow rate.

11. The motor is opposite to the cylinder, and the cylinder is opposite to the motor.

Why is the control effect of the cylinder not always the same as expected, especially in the measurement area? If the cylinder is heated, the cylinder

Product Name:	Water Decoloring Agent
Solid Content:	50%~55%
Component:	Dicyandiamide-formaldehyde Resin
Other Names:	Decolorant; Color Removal Agent; Dicyandiamide-formaldehyde Resin Flocculant
Certificates:	ISO, SGS,BV	Service:	Trade Assurance protection for quality, shipment, payment

Decolorizing Agent

Decolorizing Agent,Effective Decolorizing Agent,Waste Water Decoloring Agent,Water Decolorizing Agent

Shandong Tiancheng Chemical Co., Ltd. , https://www.akdchemical.nl