Twin Screw Extruder

Based on the design conditions and the specifications i prepared the 3D models of the parts and assembly

based on our requirements and resources we identified a set of objectives and a time bound gantt chart was made

OBJECTIVES

1.Design and development of a twin screw extruder for mixing of nanoclays with thermoplastics which can be used in future for the purpose of polymer research in IIT Delhi.
2.Test the microstructure of the product obtained by the extrusion from the above mentioned machine to check if the mixture is in homogenized form upto the nanoscale

DELIVERABLES :1st semester

Project proposal , work plan and gantt chart
The designed parameters for all the sub parts of the twin screw extruder: their 3 D models made in solidworks2004 , Engineering drawings
The process sheet for the fabrication of the screws
The screws
A progress report on the work accomplished as well as the deviations and their analysis

DELIVERABLES :2nd semester

Process sheets for the fabrication of the barrel , die , hopper
The final copy at the end of the semester of the project report
The twin screw extruder
Operation and maintenance manuals , users manuals
Extruded nanocomposite and the validation , calibration manuals
Statement of expenses
Project closure statement

LEARNING COMPONENT

Manufacturing processes and process planning
3D modeling using solidworks2004
Hands on experience of working on an CNC machine apart from the machining experience
A little about nanocomposites and their method of preparation as well as the advantages it holds in the polymer world
Most important , the experience of a research and free thought environment

WORK PLAN

Methodology
1.Design
Design of extruding screw(diameter, length , helix angle , pitch, clearance etc).
Design of die(Bore diameter)
Barrel(barrel diameter,length, clearence)
Frame
2.Fabrication
Fabrication of screws
Fabrication of Die
Barrel
Frame
3.Assembly
Specifications of gear box and motors, bearings, strainer plate etc.
4.Testing
Study of the microstructure achieved after extrusion.

the design details for the twin screw have been finalised

the sepecifications are
no of screws 2
l/d 20
dia 20
length 400
No of sectors 4
feed , knead , extruder and die

the other specifications have been given in the diagram attached iwth the next blog

the failure crieria that were identified during the design process are shear torsional stress failure and surface fatigue
the material required a steel with a high steel of 1000 MPa

Extrusion is a field which has naturally generated it's own terminology and this is an attempt to put together a small selection of the words used and what they mean.

The components of the extrusion line are relatively similar whatever type of extruder is used and a typical layout is shown diagrammatically below. The line consists of the basic extruder (drive, gearbox and screws), the extrusion die, the calibration units, the haul-off, the saw (or other cutting device) and finally the treatment devices for final finishing and handling. Major advances have taken place in all components of the extrusion line in the past decade but perhaps the most important have been in the output rates possible and in the command and control segments of the extruder. The advances in output rates have been driven by the demands of processors and the improved control systems have resulted from the availability of low cost computer processing power.
The basic components of an extrusion line are shown below:

• The extruder drive is electrical in operation and is geared via a thrust bearing to produce the rotational movement of the extruder screw.
• The polymer feed to the screw is from the feed hopper and the feed may be by gravity, metering screw or simple conveying spiral.
• The extruder barrel and screw are of high strength steels and are protected from wear and corrosion by a variety of hardening and coating treatments such as nitriding and hard chroming.
• The barrel and screw are zoned into between 3 and 7 sections which are individually heated and cooled depending on the material and process parameters. The multiple functions of the extruder screw are given in more detail below.
• The die channels the polymer melt from the front of the screw to form the basic shape of the desired product.
• The calibration units stabilise the form of the output to the detailed shape whilst the polymer is being cooled.
• The haul-off provides the dragging force to overcome the frictional forces in the calibrators and to pull the profile through the calibrators.
• The saw / cutter cuts the profile to the desired length.
• Additional operations may be performed in the line or at the end of the line depending on the operation.
The functions of the extruder screw
The extruder screw has the following basic functions:
• To bring the feedstock into the extruder and to move the material along the screw whilst at the same time compressing it and removing volatiles.
• To soften the melt by heating it (both from internally generated shear forces and externally applied heat).
• To mix the melt and produce a homogeneous melt without impurities.
• To apply the constant pressure (free of pulsation) required to force the material through the die.
These functions, at least for the single screw extruder, are generally achieved at different sections of the extrusion screw as the material progresses along the barrel and the functions are illustrated below.

Extruder key dimensions
In absolute terms probably the most referenced number for extruder specification is the L/D ratio (barrel length/barrel diameter) as this defines many of the operating characteristics of the extruder for all types of extruders The L/D ratio is a major factor in the effectiveness of the extruder and of the types of material that it can process. For most extruder types the L/D ratio has increased as technology has advanced. The limitation to high L/D ratios is the torque available from the motor (longer screws mean higher friction) and the capacity of the thrust bearings of the extruder. As advances have been made in these areas then the L/D ratios have steadily increased from L/D's of around 15 to up to 30 in 1998.

Basic extruder types
There are many different types of extruders and the illustration below attempts to divide these into a logical order.

The two main types - co-rotating and contra or counter rotating screw machines have different screw rotations in the barrels and these are shown below:

The problem:
The design and manufacture of a twin screw extruder for melt blending and compounding a mixture of clay and polymer substrate to give a nanocomposite. We have a specified L/D ratio, diameter of the screw and throughput. The other details like the helix angles total length other diameters have to be calculated.

Nanocomposites
This is basically a base polymer into which is added filler material just to achieve some properties. The property advantages that nanomaterial additives can provide in comparison to both their conventional filler counterparts and base polymer include:
•Mechanical properties e.g. strength, modulus and dimensional stability
•Decreased permeability to gases, water and hydrocarbons
•Thermal stability and heat distortion temperature
•Flame retardancy and reduced smoke emissions
•Chemical resistance
• Surface appearance
• Electrical conductivity
•Optical clarity in comparison to conventionally filled polymers
However it has been observed that the nanocomposites develop lesser toughness and weakened impact performance as compared to their components.
In our case the mixing of polymers with clay to form clay based nanocomposites is targeted. Clays are basically chains of alumino silicates with a sheet like layered structure, more specifically they are silica SiO4 tetrahedra bonded to alumina AlO6 octahedra in a variety of ways. The thickness of the layers (platelets) is of the order of 1 nm.

Synthetic Processing of Clay-Based Nanocomposites
The synthetic route of choice for making a nanocomposite depends on whether the final material is required in the form of an intercalated or exfoliated hybrid. In the case of an intercalate, the organic component is inserted between the layers of the clay such that the inter-layer spacing is expanded, but the layers still bear a well-defined spatial relationship to each other. In an exfoliated structure, the layers of the clay have been completely separated and the individual layers are distributed throughout the organic matrix. A third alternative is dispersion of complete clay particles (tactoids) within the polymer matrix, but this simply represents use of the clay as a conventional filler.

Polymers can be introduced either by melt blending, for example extrusion, or by solution blending. Melt blending (compounding) depends on shear to help delaminate the clay. Both thermosets and thermoplastics have been incorporated into nanocomposites, for e.g.
• Nylons
• Polyolefins, e.g. polypropylene
• Polystyrene
• Ethylene-vinyl acetate (EVA) copolymer

What is extrusion?
As has been already mentioned the melt blending can be done by extrusion of a mix of the clay and polymer. The extrusion can be performed by the either a single screw extruder or a twin screw extruder.

The single screw consists of a screw in a barrel .The single-screw extruder geometry relies on “drag flow.” Drag flow is the action of a rotating screw flight as it “drags” material that wants to stick or adhere to the barrel wall down the length of the extruder. If the material wants to stick to the screw more than the barrel wall, it will just remain at the same location on the screw. In this situation, there will be no material movement down the extruder

Twin screws can be :
1. Co-rotating and counter-rotating designs
2. Fully intermeshing and non-intermeshing designs
3. Parallel shaft and conical shaft designs.

The twin screw extruder is advantageous as it provides better mixing due to the fact there are 2 screws .Also in a single screw if the process material sticks to the screw and slips against the wall the objective is unfulfilled. This is a basic flaw of the drag flow mechanism that is employed in single screw design. This situation can be imagined as in the example of a nut and screw model shown in the diagram.

If both the nut and bolt are allowed to freely rotate, then the nut will remain at the same location on the bolt. However it the nut is fixed to a surface, it will move along the bolt as material would move down the extruder channel.

Another difference in the two can be due to the wiping effect.

The twin screw extrusion to be designed and fabricated will have the three basic sections:
1. drive section
2. process section
3. die section