Extrusion is a process used for creating a product (an extrudate) by forcing a material through a die or an orifice to form a shape, or alternatively an extruder is used to produce semi-finished or finished products.
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This article mainly focuses on the extrusion of thermoplastic products due to their importance in extrusion processes. Undoubtedly, thermoplastics are the largest group of plastic materials extruded; almost 65% of all plastics pass through an extruder.
As the single screw machine is the most popular type of extruder, this article focuses on processes based on this type of machine. As single screw machines are relatively cheap, simple and easily provide a continuous output, they are very popular.
In a thermoplastic extrusion, the material is first softened by heating so that it can be shaped. This process is performed by the extruder, or extrusion machine. This heat softening is referred to by different names, such as &#;thermal softening&#;, &#;plasticization&#;, or &#;plastication&#;.
Most extruders are single screw machines. The screw is what forces the material towards, and then through, the die. Shape is imparted by the die, and/or by post-extrusion forming, and the product is then set to shape by cooling while maintaining its shape. The equipment used to perform this process is known as the post extrusion equipment, while the entire system is known as an extrusion line.
Extruders may be categorized by three figures, for instance, 1-60-24. The first number specifies the amount of screws the machine has, the second number indicates the screw diameter in millimeters (mm), and the third number indicates the effective screw length as a multiple of the screw diameter. Therefore, in the given example, a single screw machine is being described that has a screw diameter of 60 mm and a length of 24 (i.e., a L/D ratio of 24/1).
Figure 1 shows a cut-away diagram of a simple single screw machine. This diagram shows the arrangement the different parts of the machine. The two units - screw and barrel - interact together to convey the plastic material, melt the material, and then push it through the die.
The screw is rotated at a predetermined speed with the electric motor drive unit and gearbox. Temperature controllers are connected to heating/cooling elements on the barrel to maintain the temperature at the set-point temperatures.
The capability of the screw and barrel assembly to extrude a given material is based on the characteristics or construction of the barrel and screw, the characteristics of the plastics material, and the circumstances under which the system is operated.
Figure 1. Single Screw Extruder (courtesy of Dr. Harold Giles)
When the extrudate leaves the die, it can either be set to the desired shape or its shape can be altered and then set to shape. The equipment that performs this process is known as the &#;post extrusion equipment&#; or the &#;haul off&#;, and in terms of size, it is generally much larger than that of the extruder. This is because plastics take a considerable amount of time to cool, and this cooling process determines the speed the line will operate.
The extrusion products include:
Co-extrusion is a process that allows two, or more, melt streams to combine in a die to make an extrudate from two or more materials. This process is now linked to thermoplastics materials, although it was first used with rubbery polymers to make an extrudate with layers of different colors. With thermoplastics, the layers of material are often combined in the die.
The simplest example combines only two layers, for instance a colored layer on a natural core, which saves on colorant costs. Films based on three or more layers are common in the packaging industry. Laminating two or more different polymer layers together can result in a product that has better barrier properties than those obtained when only a single polymer is used.
One of the polymers frequently used is a moisture or gas barrier layer based on polyvinyl alcohol (PVAL). Usually, two or three extruders are used to produce three-layered structures. In the blow-molding process, co-extrusion and bi-axial orientation can possibly be combined to produce lightweight yet strong bottles that extend the storage life of products.
Layered co-extrusion is not the only form of co-extrusion. A sequential co-extrudate can be produced by combining the materials one after the other. This may include a hard thermoplastic material combined with a soft thermoplastic material. Alternatively, two materials can be extruded side-by-side, for example, a clear material together with an opaque material, to produce a sheet for making thermoformed trays.
A vented extruder consists of a vent, which is used to extract volatiles from a plastic material during the extrusion process. Using a vented machine, the water (moisture) and volatiles content present in a plastic material can be reduced to satisfactory levels.
Part of the way along the extrusion barrel, the diameter of screw root is reduced to decompress the melt. At this point, a vent, through which the vapor escapes, is located in the barrel (when venting is not needed, the vent may be plugged and not used). Then, the vapor-free melt is sent towards the die and re-compressed by increasing the diameter of the screw root.
A dam or torpedo section is integrated on the screw just before the vent to make sure that melt does not escape through the vent. Due to the design and operation of the vented machines (a variable screw geometry and a high L/D ratio), it has been observed that these produce excellent mixing.
However, it must be remembered that it would be more appropriate to pre-dry the polymer, because heating certain plastics in contact with water may degrade or decompose them.
This information has been sourced, reviewed and adapted from materials provided by Dynisco.
For more information on this source, please visit Dynisco.
A single screw extruder will generally be used for melting continuously and then it will be compressed, extruded, and solid material will be formed within a heated barrel.
The single screw extruder actually has been developed from the basic spiral structure, e.g., damping screw block, slotted screw barrel, exhaust screw, building block structure, pin barrel, and many other different types of structures.
Since the single screw extruder will generally occupy a small space, it has turned into the main equipment that is used in the area of compound processing as well as plastic blown film.
Extrusion is a kind of process that is used to create a product by forcing a certain material through an orifice or die to form a certain shape. Alternatively, an extruder will be used for producing semi-finished/finished products.
StructureBasically, a single screw extruder consists of a screw, drive mechanism, barrel, resin feed arrangement, and various controls. Through a heated barrel, a constantly turning screw will move the resin where it will be heated to a suitable temperature and then blended into a certain homogeneous melt.
Turbulent back pressure will be build-up which is going to push the melt out of this extruder in the form of the die. Sometimes the resin may not be completely melted in your basic extrusion screw. There will be a barrier screw designed for countering this problem. Often, additional flights are also attached to its transition section to separate both molten and solid plastic to certain different channels.
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As this solid pellet will move forward it will melt because of shear against the wall. Thus, it will melt and flow into the liquid channel. In this way, the solid channel will gradually narrow and the liquid channel will widen.
Over the years, the design of extrusion screws has been improving, with various new ideas and innovations. Nowadays, single screws will be available that have got secondary flights that can improve speed by allowing faster melting.
Single screw extruders are available with many different types of accessories such as
Also, they are available with a wide range of geometrics of screws for different products and materials.
PrincipleHere the material is not needed to be plasticized, rather it should be preheated and compacted. Earlier, on the old theory of extrusion, it was believed that all the material here was loose. However, later, it was finally proved that the material is actually a solid plug, and this means the material will be solid like any plug after being squeezed, hence its function will be to complete the total delivery task.
The volume of the screw groove, at this time will gradually become smaller, and the temperature should reach the degree of material plasticization. The compression that is produced here will be from the third conveying section.
Here the compression will be one that is called the screw compression ratio of 3:1. Also, there are a few changes for other machines. Then the plasticized material will enter the third stage.
Here the material will be maintained at the plasticizing temperature, almost like any metering pump for accurately and quantitatively transporting the melt material for supplying the machine head. During this time, all the temperatures cannot be lesser than the temperature needed for plasticizing, generally slightly higher.
Mainly these single screw extruder machines are used for extruding hard and soft polyethylene, polyvinyl chloride, and other thermoplastic plastics. Also, it can process many different plastic products, like tubes, films, and plates, in combination with the corresponding auxiliary and it also can process the granulation.
How It WorksSingle screw extrusion generally uses one screw that is within a certain cylindrical barrel for continuously pushing plastic through a certain constant profile die. Typically, the production rate is measured in mass/hour that is controlled by the machine&#;s screw speed.
Advantage And DisadvantageThe advantages of these single screw extruders are that they have an advanced design, good plasticization, high quality, low energy consumption, low noise, large bearing capacity, stable operation, and long life.
The single screw extruder can adopt a 2-stage overall design for strengthening the plasticization function for ensuring high-speed, stable extrusion, and high performance.
The special barrier complete mixing design will ensure the material mixing effect and the high shear, as well as low melting temperature will be guaranteed.
In addition, to that, a single screw plastic extruder will be quite cheap in design and offer high-performance and low-temperature low-pressure metering material extrusion so the single-screw extruder is very widely used.
There are many single screw extruder manufacturers available in China who can offer you this kind of extrusion machine.
The disadvantages are:
As the transmission of any plastic materials in a single screw plastic extruder will be through friction, hence there is a certain limitation when we consider the feeding performance.
A few materials like powder or paste possess difficulties during mixing. This will make using the machine unsuitable in some processes.
ApplicationsThe following are a few applications of single screw extrusion machine to manufacture the following products: