All Aspects Of Plastic Extrusion Process; Equipments, Dies, Design Calculations
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,
- Tooling and equipment of plastic extrusion,
- Plastic extrusion process parameters,
- Information about die configurations in extrusion processes.
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How Do The Plastic Extrusion Machines work?
You can see the basic expression of a plastic extrusion machine. In a typical machine, there is a hopper that operator adds plastic pellets which are the starting form of used polymer inside the machine. These plastic pellets enter inside the barrel section which includes a rotating screw inside it.
As you see that, 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, the screw section of the plastic extrusion machine is divided into three sections; feed section, compression section, and metering section.
In the feed section of the plastic extrusion machine, plastic pellets are fed inside the barrel. In here, plastic to be melted is not in plastic melt form. There are plastic pellets inside the feed section.
In the compression section, plastic pellets are melted with the application of compression and the heaters. 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, external chillers are also required for excessive heat generation because of the rotating action, instead of heaters.
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 screw and barrel decreases. This is because the pressure on polymer melt must be increased to force it to die orifice.
In the metering section of the extrusion machine screw, pressure on polymer melt increases up to the required values.
There is a special section after the metering section that polymer melt is forced into. This section of the polymer extrusion machine is called a breaker plate. In this breaker plate, the flow form of the polymer is set alright, and the required molecular form is given with a bunch of holes.
Without the breaker plates, there can be uneven extrudate geometry will be obtained. 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, different barrel and screw geometries are required for different plastic materials.
Flow Rate Calculation In Plastic Extrusion Machine
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 machine.
In an ordinary plastic injection machine, flow is take place as ‘drag flow’. But there is also ‘backpressure flow’ which is because of 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 then re-enter the values.
The explanations of parameters inside 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 at calculator. 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 is implied as ‘A’ in the above illustration. Enter an angle value in degrees.
- p: Backpressure inside the barrel. The unit is MPa in SI and lb/in2 in US Customary.
- L: Length of the screw of screw section. The unit is the same as others.
- η: It’s the viscosity of polymer melt inside the barrel. The unit os viscosity in SI units N-s/m2 and US Customary lb-sec/in2.
If you enter the parameters in the unit set that 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, some of these parameters are not changed but some parameters can be changed or optimized. 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.
If there is no backpressure inside the barrel, the flow rate will be the maximum value. If the backpressure is high enough to make the flow rate of melt flow 0 inside the barrel, this is the maximum backpressure inside the barrel. Knowing the maximum pressure is very important at which pressure the flow stop inside the plastic injection machine. By using the same parameters at 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. The required flow rate and the backpressure for a die cross-section are calculated 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. Die characteristics change according to the shape of die cross-sections.
Conclusion For This Article
This article can be a very good assistant for designers or engineers to design a plastic extrusion process for a plastic extruded product.
Mechanical Base does not accept any responsibilities for the calculations done by its calculators. A good engineer must be about the results of the calculations if they are sensible or not in real applications.
You can use the MB-Unit Converter tool to convert your units between different sets of unit systems.
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