Single Screw Extruder — Complete Technical Guide

Single Screw Extruder is the earliest widely used among various extruders. Due to simple structure and high efficiency, it has popular application in plastics processing industry. Similarly, in order to satisfy different requirement of processing, many explorations have been carried out on the structure of a variety of screw sand barrels by extruder producers. Single screw extruder has developed rapidly from the complete spiral structure to other kinds of structures, such as screw damper, venting and extruding structure, grooving barrel, pinning barrel, and block structure, etc. All the factors above have enabled single screw extruder to enjoy wider formation and application.

Thanks to space-saving, single screw extruder has been distinctive in compound processing and blown film. It is playing a more and more important role in extrusion processing.

Conventional single screw extruder is not suitable for WPC forming operation, because it processes the weak ability for material transmission and plasticization. The transmission affection of single screw extruder mainly depends on the role of friction. The wood structure is too fluffy to feed, and the filling of wood structure make the polymer melt viscosity increase which makes extruding more difficult, so that the material has a longer residence time in the cylinder, and at the same time it cannot operate to the plant fiber with high water percentage for its poor exhausting affection .Therefore, conventional single screw extruder will be greatly restricted in the extruding process of wood-plastic composite materials.

The single screw extruder, which can be applied to the WPC forming operation, should use the specially designed screw. This kind of screw should have strong ability for material transmission and mixing material plasticization, and it can often mix and pelletize materials before extruding.

Single screw extrusion is one of the core operations in polymer processing and is also a key component in many other processing operations. The foremost goal of a single screw extrusion process is to build pressure in a polymer melt so that it can be extruded through a die or injected into a mold. Most machines are plasticating: they bring in solids in pellet or powder form and melt them as well as building pressure.

The basic structure of single screw extruder mainly consists of the transmission system, feeding system, extruding system, heating system, and cooling system, as well as nose and mouth molds.

Transmission System The transmission system, whose purpose is to achieve the required torque and uniform RPM of the screw, is an important part for extruder to complete the extrusion process under specific conditions (such as die head pressure, screw rotation, extrusion, temperature, etc). The transmission system is composed of motor, reducing unit transmission and bearing system.

Feeding System
The feeding system of single screw extruder is composed of hopper and feeding devices. The shape of feed hopper can be conical, cylindrical or rectangular etc. On one side of hopper there is a hole used for material observation, and at the bottom there is an opening and closing door used for feeding control. The hopper cover can be closed keeping dust, moisture and other contaminants outside. The material of hopper is normally aluminum and stainless steel plate. And the volume of hopper should contain the extrusion capacity at least 1- 1. 5 hours. The shape of feeding port can be either an rectangle, which is the preference, or a circle. The feeding way can be manual or automatic. Automatic feeding device mainly consists of blast-furnace burden, spring feeding, vacuum suction and so on.

Extrusion System
Screw
The extrusion system of single screw extruder is composed of screw and barrel. Screw is a critical component. With screw’s rolling motion the material in the barrel moves ahead, then gets pressurized and creates some friction heat. At the end megatitude of constant pressure is conveyed to the head of the machine.

Heating and cooling system
In order to make plastics processing technology in the specific temperature,generallyextruders are equipped with heating and cooling system device, as well as measurement, temperature control instrumentation.
(1) Extruder heating
Currently, there are some heating methods of single screw extruder, containing hot body heating, resistance heating and electric induction heating.Due to a longer barrel extruder, extrusion process on the barrel temperature in the axis direction has certain requirements. According to the size of screw diameter and aspect ratio, barrel would be divided into several sections to heat. And the head of single screw extruder could set the heating section, depending on their type and size of the nose.
(2) the cooling extruder
Cooling process requirements is a significant part to ensure plastic out of single screw extruder stable, and it closely linked and inseparable with heating system.

Shunt Plate and Filters
In the barrel and the die junction of single screw extruder to set diversion board (also known as perforated plate), and filters, whose role is to make material flow from the rotary motion into linear movement, to prevent impurities and non plastics materials, to increase the back pressure, so that products more compact. And shunt plate acts as a supports for filters .But it is usually useless ,when the viscosity of extruded rigid PVC is large, while Unstable.

The screw of single screw extruder is generally divided into three sections, according to screw diameter, screw pitch, and screw depth to determine the effective length of the three sections, each by one-third of the general division.

The first section is called conveyor section: materials in the plastics can not be here, but the process of being warm-up, and squeezed is needed. The traditional theory holds that the material here is a loose body, but by proving the actual material is here solid plug, that is to say here, after the materials by extrusion of a solid as the same plug. Therefore, the transfer is its function.

Compression is called the second section. The spiral groove volume gradually becomes smaller, and the temperature of plastic materials needs to achieve the degree of compression.Some single screw extruders also have changes in the completion of plastics material into the third.

Compression is called the second section. The spiral groove volume gradually becomes smaller, and the temperature of plastic materials needs to achieve the degree of compression. Some machines also have changes in the completion of plastics material into the third.

Measurement of the third section is here to keep plastic materials, temperature, metering pump as accurate as quantitative transport materials melt to supply the head of single screw extruder, when the temperature is not lower than the plasticizing temperature, in general, slightly higher point.

The effective length of the single screw is generally divided into three sections which are defined to the diameter, screw pitch and screw depth.

The last thread of feeder is called transportation section where the materials are required to be preheated and squeezed at pressure instead of plasticization. It is proved that the materials are actually the solid plugs rather than the loose bodies which are reported by the previous theory. As the result, the function the single screw extruder is to convey the materials.

When it comes to the second section, that is compression section, the volume of the spiral groove is gradually become smaller and smaller and the temperature reaches to the degree of the plastics. After that it has been compressed to the original statement according to the compression ratio— 3:1. It may changes sometimes.

The third section is called measurement section, where the temperature keeps the plastics degree or a little higher, just as precise and quantitative as the metering pump conveying to melt materials in order to provide to the top.

Among single screw extruders, the pin screw extruder is suitable for the extrusion screw of wood-plastic composite extrusion system, because the pin can be cut on the material flow and repeatedly to form a new interface and then repeatedly re-oriented so that the heat of the liquid can be passed to the solid phase in time. Thus the screw pin has a good ability of mixing, dispersion and plasticizing. The screw can be extruded with low-temperature under the conditions of the same formula and the critical of the screw can be increased at the same time, so that the production will be increased .Besides, the appearance and performance of the extrusion products are better than ordinary single screw extruders.

Actually, the stabilizator can be seen as a parallel single screw pumping device which has stabilization effect between extruder and its head. The part betwen pressure screw and extruder appears as the form of parallel connection, so there will be a pressure value just the same as extruder system.

A mathematical model of the parallel pressure stabilizator for a single screw extruder was proposed.The results of simulation by the proposed model indicated that by means of changing the speed of the subsidiaryscrew in the stabilizator and the self-adjusting length of the filled melt polymer , the stability of the pressure and production rate of the extruder can be effectively improved.

This stabilizator can heat and control pressure screw to make sure the polymer in pressure screw keep in molten state and drive motor connected with pressure screw though reducer. In the working process of extruder, extruder system will produce pressure in head of extruder; and polymer melt is pushed into parallel pressure ptabilizator and makes polymer melt keep in pressure screw within a certain length. On the other hand, pressure fluctuation will also change remaining length of polymer melt in pressure screw. The stabilizator can be adjusted its installation position and angle from from the longitudinal and transverse two directions and it is convenience for disassembly and cleaning.

Many single-screw extruders are used in industry to melt polymers and extrude them into a mold to make various pieces, mainly plastic or rubber. These devices, however, perform very poorly when asked to mix multiple substances together because of their shape and the high viscosity of liquid polymers. Over the years various attempts have been made to create an extruder that would produce a more evenly mixed outcome, mainly using spatially periodic flow techniques. Many of these changes increased the cost to produce as well as maintain the specialized extruders because of their more complicated design. Many of the specializations caused problems by creating pressure changes and other issues which inhibited steady throughput. This issue can be easily overcome by attaching a gear pump to the exit of the extruder. Much of the happenings inside an extruder are poorly understood, which is why Dr. Campbell and his team are studying chaotic mixing inside the free-helix extruder. The goal of this project is to develop a technique for visualization of the mixing process by using simulations of the process velocities and then using the velocities to predict the position of tracer “particles” in the system.

Project Specifics

Several graduate and undergraduate students in Dr. Greg Campbell’s laboratory decided to attempt using time period flow instead of spatially period flow to achieve chaotic mixing within the extruder. To do this, a third degree of freedom needed to be achieved, they made it possible to move the core of the screw separately from the helix. Using certain patterns of movement between the various extruder elements produces better mixing capabilities than with normal screw rotation and a more uniformly mixed output. The various combinations of movement give different mixing patterns.

Using normal screw extrusion gives a point stretched along the channel with no mixing (figure 1.a). Helical-motion only mixing provides stretching in the cross-channel section but not down the channel (figure 1.b) so the mixing is very poor again. The chaotic mixing (figure 1.c) forces the dye to mix in the cross-channel section and down the channel giving a better mix than a normal extruder.

To more easily analyze the flow of fluid down the channel, the channel can be ‘unwrapped’ from the core giving rise to a rectangular channel in which the top and bottom can be moved at specific velocities. This technique provides a good approximation of what really occurs and eliminates the need for the complicated math involved in working with helical coordinates. The points a, b, c, d in figure 2.a become a straight line down the channel (as transposed into figure 2.b) while the points e, f, g, and h define a square on the lid. Moving the barrel of the extruder equates to moving the lid of the channel, while moving the core is like moving the bottom.

As well as collecting data, a set FORTRAN computer programs were written modeling the data and theory. My task this summer has been to learn the programming language of FORTRAN, learn the theory behind the experiments and code, and begin to figure out what the existing code does. Not having had any of the classes that cover the basics of the theory has made understanding the theory difficult. Most of the theory appears to deal with fluid mechanics and transport phenomena in regards to highly viscous polymers. In turn, not grasping the theory well has made understanding the existing code more complicated. The code is largely based off the equations that dictate the fluid flow, so not grasping what the different variables mean and what they represent makes matching up the code with the theory rather difficult. I, however, am beginning to grasp the basics of the theory and the experiment as well as understand the existing code even though much still eludes me at this point in time. Continued work on this project would allow me to expand my understanding of the project, the theory of fluid mechanics and chaotic mixing, especially within an extruder.

Continuation of Project

Eventually our goal will be to create an interactive, digital simulation of the mixing within the extruder. A single still image, like the one in figure 3, could previously be created. These stills could not be easily combined to make a video clip and they were not completely accurate with the experimental data collected. The code was then revised so more accurate numbers were given but not in such a way to allow visual representation. The existing code needs to be polished and expanded so the digital animation is possible. The program must be able to accurately determine the paths and speeds of the various points of dye, efficiently calculate the path and speed, and effectively display the path and speed of the particles for any given set of inputs. Such a digital clip would provide a way to watch the happenings inside the extruder and demonstrate the mixing capabilities of the various patterns of movement.

1. In the traditional system of screw extruder, screw is driven by DC motors. The screw is directly driven by the gear box in direcrt drive; in the case of indirect transmission, it is pushed by the belt and disk drives. There are some shortcomings of traditional DC motor itself, for example, the brush of DC motor must be replaced once a month, and it is necessary to clean DC motor regularly. Sometimes even cooling air is needed to pass DC into the cleaner from outside the workshop.

2. The drawback of indirect drive screw extruder is: the existence of slip on the belt, a belt can cause a certain degree of energy loss, and additional mechanical devices have increased the possibility of failure. Noise is the biggest flaw of DC motor, due to brand brush, rotor pollution, motor temperature too high, inadequate exhaust and electrical shock. Therefore, it must be at high expense for screw extruder to use DC motor, such as higher maintenance cost.

Screw extrusion is widely used but little is known about the design and operation of this type of equipment save in the polymer industry. Particularly with the advent of high-performance, high-value ceramic materials, it has become important to extend knowledge. Here a mathematical model for a flooded screw brings together recent paste rheology and a model of screw extrusion developed previously for frictional solids flow. The model suggests that helix angles in current use are significantly below the optimum. Furthermore it appears that the deepest channel screws will produce the highest pressure gradients, a fact already established in practice which is in contrast with screws for polymers. Experiments both with and without flow on a 20 mm diameter laboratory extruder used screws of varying angle and channel depth with pastes of various rheologies. Pressure measurements from experiments without flow could show a marked offset from predictions with screws of low helix angles, this apparently being the result of an increased surface shear stress at the barrel-paste interface. The effects of changing the screw speed were reliably predicted. Experiments with flow carried out on a 76 mm diameter industrial machine showed general consistency with predictions.

When mixing in single screw extruders, one must have a practical guide to finish the single-screw extrusion. This authoritative handbook empowers the reader to achieve good results with their plastic mixing. This is very practical, and user-friendly not a mathematical, confusing theory. The single screw extruders will be a good choice to all involved in the art of plastic extrusion.

◎High speed and high capacity extrusion with high quality guaranteed;
◎Plasticization at a lower temperature is conducive to higher quality;
◎Integral double-stage model enhances the plasticization effect and ensures high-performance extrusion;
◎Special barrier and comprehensive BM design ensure good compounding;
◎High torque output and super thrust bearing;
◎Gears and shafts are made of high-strength alloy steel after carbonization and teeth-grinding treatment;
◎High hardness, high cleanness, ultra low noise;
◎Intelligent PLC control makes it possible to interlock main extruder and its downstream equipments;
◎HMI makes it easy to monitor the process condition and machine status;
◎It’s possible to change the control method (temperature controller) if needed;
◎The material is 38CrMoAlA nitride with high anti-wearing performance;
◎Strict control on temperature accuracy combined cooling by air and water;
◎Single screw extruder is equipped with a unique feeding port with perfect water cooling system.

Single screw extruders have many advantages, which include the ability to process a wide variety of materials in many different applications. They are an industry leader, and have maximum flexibility to process the toughest materials. It is also available for a variety of different L/D’s. Their flame-hardened screw is abrasion and corrosion resistant, screw based coatings are also available.

There are three kinds of single screw extruders products in our company.

Standard single screw extruder is designed and developed on the traditional extrusion theory of sing screw. It is widely applied in extrusion processes from a great variety of materials and acts as a main machine for different kinds of plastics products, such as pipes, sheets and films.

Venting single screw extruder is designed and developed on the extrusion theory of venting single screw. It majors in processing materials that need degassing during their extrusion. These materials include:

1. PET/ABS/PA for extrusion
2. PET/ABS/PP/PE for recycling process
3. PE/PP/ABS for modification processes like coloring.

Grooved single screw extruder is the enhanced type of single screw extruder based on new extrusion progress. As per the theoretical analysis, it can greatly increase the conveying efficiency of screw and its capacity to make some grooves in axial direction in the barrel of feeding section. This kind of extruders are mainly applied for polyolefin, and act as the main extruder for plastic tube, sheet and film.

The invention refers to a single screw extruder made up with a barrier screw and a barrel in which the barrier screw is held in a way that allows rotation and combining at least one feed zone longitudinal section and at least one melting zone longitudinal section. The extruder is characterized in that the barrel, on its inner wall in the area of the melting zone longitudinal section , has at least one groove which runs in a longitudinal direction. The invention also includes a method for extruding plastic material using a single screw extruder comprising a barrier screw which is held inside a cylinder in an extent that grants it to rotate, depending on the extruder contains a melting zone and a feed zone , and the barrier screw has at least one solid matter channel and a melting channel. The method is described in that solid plastic material (solid matter) is conveyed in the area of the melting zone in a defined quantity into the melt channel and out of the solid matter channel.

Thermoplastics resins are typically formed by extruding the material through a through a die which shapes the material. The raw material is placed in a hopper where it is moved into and through a chamber by a screw. The mechanical action of the feeder screw heats and feeds the material through chamber which is typically in the shape of a barrel. The heated and compressed material is forced through the die at the discharge end of the extruder. The extruded part assumes the cross-sectional shape of a die.

In a single screw extruder, the screw typically has an external diameter corresponding to outer edges of the helical threads or flights. The flights may be under sized in relation to the barrel in the feed or hopper area so as to reduce feedthroat friction. Typically, a single keyway in the shaft area of the screw matches a keyway in a sleeve portion of a drive shaft so that a key positioned in the matching keyways is utilized to transmit rotational movement from the drive shaft to the screw. The key is a separate piece of rectangular metal which fits into the matching keyways. A disadvantage attendant with the use of keys is that they may be lost or misplaced. Also, worn keys can contribute to screw wobble.

As the viscosity of the thermoplastic material increases, it has been observed, that in single screws of the above described type, radial screw movement or wobble may develop. Clearance in the feed area between the outside diameter of the screw and the inside of the barrel permits movement of screw. As the screw wears, metal may be removed between the exterior of the screw and the inside of the barrel. This creates additional clearance which may cause even more wobble or radial movement.

Hence, it is desirable, to reduce radial movement without reducing clearance in the feed screw area which clearance is desirable for extruding higher viscosity materials. Reduction in radial movement beneficially reduces screw wear and contamination of the finished product with residual metal particles from the screw or barrel. Reduction in radial movement also helps maintain tighter tolerances between the screw and the barrel and can result in higher feed rates. Any reduction in radial movement can result in a more efficient powder feed and less wear on single screw extruders.

The mainframe of a screw extruder consists of three systems: extruding system, wheel system and heating and cooling system.

Extruding system, the most critical part of an extruder, is made up of a screw and a machine barrel.

The main function of wheel system is screw driver which can ensure the torque and speed of screw working.

The heating and cooling system of the screw extruder is mainly used to ensure temperature control in the forming of materials and extrusion system.

A feed bushing for single screw extruder for the plasticizing extrusion of plastics comprises an outer casing, a grooved bushing enclosed in the outer casing, and a cooling system. The groove bushing is assembled from a tubular grooved inset and a tubular reinforcement surrounding this inset, which is shrink-fitted into the reinforcement. The grooved inset of the grooved bushing is made of cemented carbide, a metal oxide which is an extremely hard and wear-resistant material.

The competition of single screw extruder is pretty strong in international plastics market. In order to ensure the expansion of its market share of extruder, it is necessary for extruder manufacturers to improve their equipment reliability, reduce costs and ensure that the manufacturing process of products doesn’t be interfered.

It is essential to choose machines and components with high reliability. Extruder manufacturers begin to involve tubes and the membrane system in the huge market of single screw extruder.

There are a variety of requirements for the speed box of single screw extruder:

◆ the axial force in the process of absorbing and extruder
◆ to provide high torque
◆ to push the screw out from the drive side
◆ to resist overloading
◆ gear without limit life
◆ high efficiency
◆ low Noise
◆ high reliability
◆ excellent price / performance ratio

Feed zone of a screw extruder, which uses forward transmission components with larger pitch, can gain a larger feed capability. Besides, feed zone will compact the powder filler by following measures in order to promote melting and plasticization of materials:

a. Part the modular screw (such as co-rotating twin screw and screw of disc extruders) in order to descend pitch change.

b.Reduce the pitch of non-modular screw gradually.

It is the fluctuation of feed that make the distribution and mixture of materials become particularly important. It is directly related to the consistency of the product.

As for parallel counter-rotating twin screw, the mixture capability of screw can be improved by several measures, such as enlarging the space between screw diameter and barrel wall on the solid transmission part and mixture part, setting reaction thread on pre-plasticization part of screw, setting the kneading disc and so on.

A single screw kneading extruder is a type of single screw extruder, which includes a first-stage kneading portion and a second-stage kneading portion alignment. The first kneading portion disperses a polymer filler by a powerful shearing force and has kneading blades on its rotor. The polymer which is incorporated into the filler is uniformly blended by the first portion and dispersed by the second kneading portion. After extruding, there are cavities in its rotor and stator respectively.

Single screw extruders usually work on high velocities. When the speed of the screws is increased, it causes higher friction and subsequently, the temperature of the polymer increases due to friction in screw length. Sometimes, due to the heat caused from friction, it may cause burning and chemical disintegration of the polymer. One way to prevent this problem, is to use a cooling system. A single-screw extruder is not suitable for polymers with thermal-mechanical sensibilities (such as PVC), because of debility in thermal transmission of polymers, there are also problems with the consistency of melt, especially in the metering zone.

A gaseous phase disengaging apparatus for the liquid phase process material contaminated with a substance of lower boiling point consists of a twin screw extruder. The raw feed is passed through the twin screw extruder and the feed is repetitively spread over the apparatus inside surfaces. The apparatus is provided with paired raw material supply inlets and gaseous phase vent ports. These are maintained in continuous communication during screw rotation.

There are impurities in most of the plastic materials, but they can be cleaned effectively by a screen changer, which is installed on the back of an extruder, and it is a kind of honey net, sharing the same diameter of the barrel for extrusion machine.

Generally speaking,a screen changer is composed of different nets. The net inside can filter the smallest solid particles from the stream of melt; while the outside coarse mesh can prevent larger particles from entering the dense inner net.

In order to resist the force due to melt flow, there is a round steel plate, which is called fender, imbedded in the groove of the end of an extruder‘ s barrel.

Please remove the down stream equipment to change the blocked mesh with a new one, and reinstall the new mesh onto the production line.

Screen Changer

It is unnecessary to stop working with the way above; at the same time it can dispense much labor. As per the design, there are two main types:interchangeable type and continuous type.

An interchangeable screen changer owns two identical mesh groups, one of which is installed over the path of melt flow. Another mesh is used when the first group is ineffective; while the used group is changed outside. These two groups of mesh are put on the same steel plate, revolving round the same shaft, or sliding along the guide. The former works based on a lever by manual control; while the latter moves its mesh group by hydraulic pressure from one side to another. A valve is needed in another method to change mesh, so that the melt can flow into a new mesh group from the old one.

Continuous screen changer: it removes the new mesh into melt flow, from which the old one is moved away. The speed of this process can be adjusted according to the amount of impurities automatically. A continuous screen changer enables to get rid of the pressure and temperature change of melt due to the discontinuous operation.

A continuous screen changer is designed based on swivel slide.There are numerous mesh groups lining along a turntable, driven by a wheel machine. When the turntable is moved, the used mesh is removed from melt flow.