The extrusion process is a very common manufacturing process of different engineering materials such as metals, ceramics, and polymers. In polymer production, plastic extrusion processes and extrusion machines are very common systems in the industry. Here, you can find general information about;
- Working principles of plastic extrusion machines,
- Also, tooling and equipment for plastic extrusion,
- Furthermore, plastic extrusion process parameters,
- Information about die configurations in extrusion processes.
How Do the Plastic Extrusion Machines Work?
You can see the basic expression of a plastic extrusion machine. And also, in a typical machine, there is a hopper the operator adds plastic pellets which are the starting form of the used polymer inside the machine. So, these plastic pellets enter inside the barrel section which includes a rotating screw inside it.
As you see, there is a rotating screw inside the plastic extruding machine. This rotating screw pushes the plastic pellets to the die section. There is also a specific mechanism inside the screw.
In general, we divide the screw section of the plastic extrusion machine into three sections; feed section, compression section, and metering section.
Also, in the feed section of the plastic extrusion machine, plastic pellets are fed inside the barrel. Here, the machine melts plastic and is not in plastic melt form. There are plastic pellets inside the feed section.
Compression and Extrusion
In the compression section, the machine melts plastic pellets with the application of compression and heaters. Moreover, heaters give the required heat to melt plastic pellets. The rotating action of the rotating screw which is generally 60 RPM, also heats the plastic inside it. Sometimes, we require external chillers for excessive heat generation because of the rotating action, instead of heaters.
Also, barrel diameters can change from a few centimeters to 1.5 meters. And the barrel length to barrel diameter ratios can be 5 to 30. According to the used material, these values can change.
As you see above illustration, the thickness of the rotating screw body increases, so the space between the screw and barrel decreases. This is because we need to increase the pressure on polymer melt to force it to die orifice.
Furthermore, in the metering section of the extrusion machine screw, pressure on polymer melt increases up to the required values.
So, there is a special section after the metering section that we force polymer melt into. Also, we call this section of the polymer extrusion machine a breaker plate. In this breaker plate, they set the flow form of the polymer alright, and they give the required molecular form with a bunch of holes.
Without the breaker plates, we can obtain uneven extrudate geometry. Or die swell at unwanted levels can occur.
The length of barrel sections inside polymer extrusion machines can change according to the melting characteristics of the used polymer. For example, crystalline polymers melt abruptly. So, the metering section of machines for crystalline polymers can be shorter. So, we require different barrel and screw geometries for different plastic materials.
Flow Rate Calculation in Plastic Extrusion Machine
And also, the flow rate is a very important parameter in plastic extrusion processes that the mean prediction of it can lead to the estimation of the required working parameters of plastic extrusion machines.
In an ordinary plastic injection machine, flow is take place as ‘drag flow’. But there is also ‘backpressure flow’ which is because the backpressure that is the increase in pressure from the feed section to the metering section. You can use the calculator below;
Flow Rate Of Polymer Melt Calculator
The use of the calculator above to calculate the flow rate inside a plastic extrusion machine is very basic. You just need to enter the required parameters inside the brackets, then click on ‘Calculate!’ the see ‘Flow Rate’. If you want to do another calculation, you just need to click on the ‘Reset’ button and then re-enter the values.
The explanations of parameters inside the calculator;
- D: It’s the inside diameter of the barrel which is also the diameter of the screw. The unit must be a meter in SI or US Customary.
- dc: The distance between the barrel wall and the screw body, as you can see in the illustration on the calculator. Also, the unit must be a meter or in.
- N: It’s the rotational speed of the screw that has the unit of rev/s.
- θ: The angle of screws which we imply as ‘A’ in the above illustration. Enter an angle value in degrees.
- p: Backpressure inside the barrel. And also, the unit is MPa in SI and lb/in2 in US Customary.
- L: Length of the screw of the screw section. The unit is the same as others.
- η: It’s the viscosity of polymer melt inside the barrel. The unit of viscosity in SI units N-s/m2 and US Customary lb-sec/in2.
If you enter the parameters in the unit set described above into the calculator, you will have the flow rate in the SI units m3/s and US Customary in3/sec.
As you see that there are a bunch of parameters to obtain flow rate inside a plastic extrusion machine. For a typical machine, we do not change some of these parameters but we can change and optimize some parameters. So, backpressure, rotational speed, and melt viscosity are the optimizable parameters, that you can make optimizations upon them to obtain the required flow rate inside a plastic injection machine.
Maximum Back Pressure Calculator
Also, if there is no backpressure inside the barrel, the flow rate will be the maximum value. If the back pressure is high enough to make the flow rate of melt flow 0 inside the barrel, this is the maximum backpressure inside the barrel. Furthermore, knowing the maximum pressure is very important at which pressure the flow stop inside the plastic injection machine. By using the same parameters in the above calculator, you can calculate the back pressure;
Maximum Back Pressure Calculator
The pressure value that you obtained is in the SI unit as MPa and US Customary lb/in2.
The maximum backpressure and the maximum flow rates define the extreme points of a typical plastic injection machine. So, we calculate the required flow rate and the backpressure for a die cross-section by using die characteristics. If you select a flow rate that must be below the maximum flow rate for a specific die cross-section, you can obtain the backpressure value of this cross-section by multiplying the flow rate with the die characteristic value. Also, die characteristics change according to the shape of die cross-sections.
Type of Products in Plastic Extrusion Processes?
There are three main categories for conventional plastic extrusion processes available in the industry; Wire and cable coating, production of colid cross-sections from polymers, and hollow products such as tubes or pipes.
Cable and Wire Coating
This cable and wire coating is one of the most important applications of plastic extrusion machines. The working principle of these machines is very simple. There are two feeding sources at the die section. And also, one of them is plastic melt feeding which provides coating material, and bare wire or cable feed. Plastic melt comes from the screen pack section which directs the streamlines to obtain laminar flow at the dying opening. At die opening, the machine coats the bare wire feed with the plastic melt, with the laminar opening and exiting of coating and bare wire at the same time. They apply vacuum also for adhesion between bare wire and coating.
Production of Solid Plastic Profiles
In this application of plastic extrusion, the polymer melt is drawn into the die opening with the application of pressure. There are a screen pack and breaker plate just like in the wire coating process. The size of the dire opening is constant, but obtained extrudate has a bigger cross-section because of the die swell. We must consider the die swell and we need to calculate the design required to die for a specific product. The extrudate will be soft after the die opening. So, to harden the extrudate, we can apply water sprays or air blow. We must design different cross-sections of dies to obtain different solid extrudate shapes from polymers. The most important product types are; door and window frames, automobile trimmings, etc.
Hollow Tube and Pipe Production
For the production of tubes or hollow shapes, they use mandrel structures. With these mandrels, we obtain an internal hollow shape. And also, to obtain tubes, we use spider legs structures in the die opening section. Inside the mandrel structure, we blow the air to ensure the hollow shape after the die opening. We use screen pack and breaker plate mechanisms also like other plastic extrusion techniques.
What are the Most Encountered Defects in Plastic Extrusion Processes?
The most encountered defects in polymer extrusion processes are; sharkskin, bambooing, and melt fracture.
Sharkskin occurs because of the occurrence of the velocity profile at the polymer melt inside the die hole. Velocity profile occurs because of the high friction between the die and flowing polymer melt. If the friction increases further, the bambooing problem occurs on extrudate. So, the solution is the minimize the friction between the die opening and extrudate.
Melt fracture is another problem that results from the uneven flow profile inside between the die opening and screw. So, an uneven profile means turbulent flow, which leads to uneven geometries of the extrudate. To compensate for it, we recommend converging entrance geometry for the die and the use of breaker plate and screen pack geometries.
Before starting to explain general extrusion processes applied to polymers to produce sheets and films, we need to know what is the film and the is sheet.
To say ‘film’ to a polymer product, the thickness of the products must be below 0.5 millimeters. From 0.5 millimeters to 12.5 millimeters, we call it a sheet. In film production, we generally use low-density polyethylene.
The three sheet and film extrusion processes are; the slit die extrusion process, blown film extrusion process, and calendaring.
What is Slit Die Extrusion Process?
The slit-die extrusion process is a plastic extrusion process that has a very specific die configuration. Because of the shape of the die opening, this process is called a ‘slit’ die. So, slit refers to the very thin and wide shape of the die opening to obtain the required sheet and film sections. The lengths of the slit die can be up to a few meters, and the narrowness of the slit opening can be below 0.4 millimeters.
Uniform cooling must be established in the silt die extrusion process. And also, if there is non-uniform cooling of the extruded sheet from the slot die, the temperature gradient and pressure gradient around the slit die can cause non-uniformity in the shape of the extruded sheet.
Uniform cooling is established by the use of quenching of the sheet directly after the slit dies. This quenching solidifies the sheet abruptly. Another process is the cooling of the sheet on rollers. In this cooling system, rollers are much cooler than usual to abruptly solidify the extruded sheet, right after slit die opening.
The calendaring process to produce thin layers of plastics is the most efficient way by the most expensive way. The production rates of calendering of plastics such as general rubbers or rubbery thermoplastics reach up to 3 meters per second.
In the calendering process, there are a set of rollers in which the starting form of material is rubbery. When this rubbery form enters into sets of rollers, it takes the shape of a film or sheet after each step by decreasing the thickness.
The most important products that are produced with the calendering method are inflatable boats, PVC curtains, vinyl tablecloths, etc.
Air Blown Sheet Extrusion Process
This process to produce plastic films and sheets is more complex than other processes. These are feeder that feeds the polymer melt onto guide rolls. Air is blown inside this polymer melt to obtain a bubble. The flow of polymer melt is in the upward direction and after a point, solidification of the bubble occurs. This point is called as ‘frost line. This bubble was then quenched by collapsing rollers. Then it’s wound to windup reel.
With air blown sheet extrusion process, many more strong sheets and films can be obtained to use in the packaging industry. But the control over the dimensional tolerances is harder than other processes.
Conclusion for Plastic Extrusion
This article can be a very good assistant for designers or engineers to design a plastic extrusion process for a plastic extruded product.
Also, Mechanicalland does not accept any responsibility for the calculations done by its calculators. A good engineer must be about the results of the calculations and whether they are sensible or not in real applications.
Finally, do not forget to leave your comments and questions about the plastic extrusion machines and their features below.
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FAQs About Plastic Extrusion
The plastic extrusion process is the manufacturing process where the molten plastic material is forced through a nozzle to form a long cross-section of parts. They are very common in the production of long parts with the same cross-sections.
The main purpose of the plastic extrusion process is the production of plastic parts that have the same cross-section and long parts. In the production of window frames, plastic tubes and long plastic rods are produced with this process.
The first advantage of plastic extrusion, it makes it very easy to produce long cross-sections of plastic parts. And also, there is nearly zero scrap material. And energy is saved with plastic extrusion processes if we compare it with other kinds of processes.
The plastic pellets are put from the hopper. And also, the machine melts the plastic material. And plunger forces this molten plastic to the nozzle of the machine. Molten plastic takes the shape of the nozzle.