Difference between single screw extruder and twin screw extruder

Polymer Extrusion - Single Screw Extruder vs. Twin Screw Extruder

Hi friends, today we are discuss about difference between Polymer Extrusion - Single Screw Extruder vs. Twin Screw Extruder. If you like my article, please share in your groups and follow my site www.automationwale.in

Polymers are amazing material, but for majority of its application Polymers are mixed with other fillers additives and colorant for better performance and appearance. Performance of these mixed polymers depend on the quality of mixing in plastic industry.

Single screw extruder and twin screw extruders are the most common machine used to mix polymer and additives. Here we know about basic characteristics of single screw and twin screw extruders and then we discuss about how do they differ from each other?

Both single and twin screw extruders are consists of some basic elements like screw, feeder, barrel, heating element, motor and gearbox. Single screw extruder contains one screw where as twin screw extruder contains two screws presence of one or two screw significantly changes the flow and Amount of shear that can be applied on the polymer. Extruders mostly contain more than one feeder to achieve desired loading of additives in the polymer.

Difference between single screw and twin screw extruder ppt

In case of single feeder polymer and additives are pre-mixed. Barrel is a long cylinder which hosts screw and keep molten polymers confined inside the extruder the inner layer of barrel is specifically treated to eliminate any reaction with polymer at high temperature.

Heaters are mounted on the barrel to melt the polymer and keep it at desired temperature during extrusion. Several heaters are used to separately control the temperature at different zone. Mostly extrusion is performed at 50 ‘C higher temperature than the melting point of polymer. Which has a melting temperature of 165 ‘C And Finally all extruders contain motor and gearbox.

To generate motion and transfer motion from motor to screw every component of the extruder influences the mixing quality and efficiency single screw extruders contain one screw the screw mostly is single metallic shaft except Containing flights to guide the polymer inside the barrel most screw contains three distinct sections,

Twin screw extruder working principle

1) feed section

In this part flight depth is high so that more amount of plastic blades can fit in As plastic is fed through the hopper it melts and move forward due to back pressure. Back pressure is the most important factor which controls flow of polymer inside single screw extruder.

2) Compression Section

In this section the flight depth gradually decreases It allows polymer melt to get compressed and release any volatile material which can be harmful for the polymer.

3) Metering Section

In this section flight depth is very narrow Which creates high shear and thus help additive to disperse in the polymer matrix.

It is important to note that the relative length of each zone Depends on the polymers melting behavior and the viscosity of polymer melt. If polymer are difficult to melt it needs longer feed zone but if polymer possess very low melt viscosity, Then we need to use small feed zone and long compression zone.

Therefore various types of screw design is available in the market. Pressure inside a single screw extruder is gradually increases from feed zone to metering zone, the increase in pressure is related to decrease in flight depth the pressure also reflects the amount of shear applied in each zone.

what kind of screw elements normally used in screw

As polymers are always under pressure inside the barrel. We cannot put vent in single screw extruder Because polymer melt will shoot out from the vent recent development in a screw design Have allowed putting vent in the single screw extruder, but it is still not widely adapted.

Now coming to twins Screw extruder Twin-screw is much more versatile than single screw extruders presence of two screws allows several variations in terms of rotation, position, and time, Therefore there are several types of twin screw exist in the market Based on rotation Twin-screw can be classified into Co-rotating and counter rotating.

In case of co-rotating both screw moves in the same direction Where as in counter rotating the screw rotates in opposite direction. For plastic compounding co-rotating twin screw extruders are mostly used. In terms of position Twin screw can be classified into inter Mixing and non intermixing,

In inter mixing two screws are positioned Very close to each other so that flights of one screw enters in the flight depth of other screw. Arrangement of screw in this way allows self wiping of screws and better transfer of polymer melt from one screw to another screw, In non inter mixing screws are positioned far apart, So that flights do not enters in each other space These types of twin screw are used for high throughput compounding.

Twin-screw can also be classified by the screw types like single shaft or modular, Single shaft screws are very similar to single screw extruder. Single shaft screws provide option to achieve very high shear. But to process different types of polymer and additives whole screw needs to be changed.

Twin screw extruder working principle

The modular screw consists of individual elements arranged on a shaft. As elements can be arranged in a wide number of ways, the screw design can be extremely versatile.

Most common elements in a twin screw extruders are conveying element, needing block and flow restrictor. Number and location of these elements can be easily changed thus provides huge amount of flexibility during compounding. Needing blocks provides high amount of shear, thus help to achieve excellent dispersion of additives.

However, if the polymer is sensitive to extensional flow Having needing block can be a problem. Flow restrictors reverses, the flow of polymer milt thus allows additives to mix properly in the polymer matrix. Depending upon the difficulty of dispersion a typical screw obtain, screw extruder will consist of one or more section of kneading and flow restrictor blocks.

Twin screw extruders a basic understanding

Presents of various types of elements in the screw also leads to different pressure level inside the barrel. Feeding pressure increases as polymer melt moves forward. Highest pressure is developed at kneading block. The pressure again drops as we move to conveying element from the kneading element. Depending upon the offset angle of kneading block different amount of pressure is developed at different kneading block section and almost no pressure is developed at the conveying section.

Presence of sections with zero pressure allows manufacturers to put extra feeder and vent a different part of the machine. Between screw extruders ability to provide opportunity to have multiple feeders and vents significantly enhances its mixing ability and efficiency, Actually the presence of two screws significantly changes.

How polymer is processed in twin-screw compared to single screw extruder. The most important differences is in single screw extruder polymer melt flows passively due to back pressure. On the other hand in twin screw extruder there is active flow of polymer melt since polymer melts are actively transferred from one screw to another screw. The active flow of polymer melts provides two very important benefits.

Application of twin screw extruder

First twin screw extruder can run in a starved mode which means it is not necessary to completely fill twin screw to move material forward But in case of single screw extruder, it is important to flood fed otherwise polymer melt will not move forward.

As twin screws are starved fed there is almost no pressure in conveying zone. Another benefit of active melt transfer is that we can control how materials can flow inside the barrel and how long it could stay in a specific segment of the screw.

There by mixing efficiency can be optimized Overall, it can be said that extremely good mixing efficiency of twin screw extruder can be attributed to presence of high shear elements, vents and active material flows,

Advantages and disadvantages of twin screw extruder

So, to summarize the difference between the single screw and twin screw extruders We can say that,

The first difference is number of screws, Single screw extruders contains one screw, where as twin screw extruder contains two screws, The screw of twin screw extruders are mostly modular therefore provides opportunity to design in an infinite number of ways where as screws of most.

Single screw extruders contents will define three sections so provides little flexibility in screw design as High shear elements are present into in screw extruder It can apply significantly high amount of shear then single screw extruder Modular design and starve feeding in twin screw extruder also allows putting vents and additional feeders at different parts of the machine.

Overall twin screw extruder provides significantly better mixing efficiency then single screw extruder,

However initial investment in single screw extruder is significantly low compared to twin screw extruder and single screw extruder is significantly better in processing shear sensitive polymers and additives,

So, if Additives is compatible and easy to mix single screw extruder is preferred compared to twin screw extruder In summary, we can say that both single and twin screw extruders offer some benefits and both of them have some limitations. Therefore the choice of extruder will depend upon type of polymer and additives.

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