All Aspects Of Plastic Extrusion Process; Equipments, Dies, Design Calculations

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Extrusion process is a very common manufacturing process of different engineering materials such as metals, ceramics and polymers. In polymer production, plastic extrusion processses and extrusion machines are very common systems in industry. In here, you can find general information about;

Working principles of plastic extrusion machines,
Tooling and equipments of plastic extrusion,
Plastic extrusion process parameters,
Informations about die configurations in extrusion processes.

How The Plastic Extrusion Machines Work?

Illustration of a typical plastic extrusion machine(Source Of Image: Fundamentals Of Modern Manufacturing).

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 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 plastic extrude machine. This rotating screw pushes the plastic pellets to the die section. There is an also specific mechanism inside the screw.

In general, the screw section of plastic extrusion machine divided into three sections; feed section, compression section and metering section.

In feed section of plastic extrusion machine, plastic pellets are fed inside the barrel. In here, plastic to be melt is not in plastic melt form. There are plastic pellets inside feed section.

In 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 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 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 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 metering section of extrusion machine screw, pressure on polymer melt increases up to required values.

There is a special section after the metering section that polymer melt is forced into. This section of polymer extrusion machine is called as breaker plate. In this breaker plate, the flow form of polymer is set alright, and required molecular form is given with the 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 used polymer. For example, crystalline polymers melt abruptly. So, 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 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 an also ‘back pressure flow’ which is because the back pressure that is the increase in pressure from feed section to metering section. You can use the calculator below;

Flow Rate Of Polymer Melt Calculator

The use of calculator above to calculate the flow rate inside plastic extrusion machine is very basic. You just need to enter 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 ‘Reset’ button then re-enter the values.

The explanations of parameters inside calculator;

D: Its the inside diameter of barrel which is an also the diameter of screw. The unit must ne meter in SI or in. in US Customary.
d_c: The distance between the barrel wall and the screw body, as you can see at the illustration at calculator. The unit must be meter or in.
N: Its the rotational speed of screw that has the unit of rev/s.
θ: The angle of screws which is implied as ‘A’ at above illustration. Enter a angle value in degrees.
p: Back pressure inside the barrel. The unit is MPa in SI and lb/in2 in US Customary.
L: Length of the screw of screw section. Unit is same with others.
η: Its the viscosity of polymer melt inside the barrel. The unit os viscosity in SI units N-s/m2 and in US Customary lb-sec/in2.

If you enter the parameters in the unit set that described above into calculator, you will have the flow rate in the SI units m3/s and in US Customary in3/sec.

As you see that there are bunch of parameters to obtain flow rate inside plastic extrusion machine. For a typical machine, some of these parameters are not changed but some of parameters can be changed or optimized. Back pressure, rotational speed and melt viscosity are the optimizable parameters, that you can make optimizations upon them to obtain required flow rate inside plastic injection machine.

If there is no back pressure inside the barrel, the flow rate will 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 back pressure inside barrel. Knowing the maximum pressure is very important at which pressure the flow stop inside plastic injection machine . By using same parameters at 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 in US Customary lb/in2.

The maximum back pressure and the maximum flow rates define the extreme points of a typical plastic injection machine. The required flow rate and the back pressure for a die cross section is calculated by using die characteristics. If you select a flow rate which must be below the maximum flow rate for a specific die cross section, you can obtain the back pressure value of this cross section by multiplying the flow rate with die characteristic value. Die characteristics change according to the shape of die cross sections.

Conlusion For This Article

This article can be a very good assistant for designer or engineers to design a plastic extrusion process for a plastic extruded product.

Mechanical Base does not accept any responsibilities of the calculations done 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 MB-Unit Converter tool to convert your units between different sets of unit systems.

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