Twin Screw Extruders (TSE)

TWIN SCREW EXTRUDERS (TSE):

TSE’s are classified according to the following three categories

Screw Rotation

1. Co-rotating

2. Counter-rotating

Intermeshing

A. Separated

B. Tangential

C. Intermeshed

Confinement

a. Lengthwise closed or open (L-closed or open)

b. Crosswise closed or open (C-closed or open)

(a) Co - Rotating

(b) Counter - Rotating

Counter – Rotating Non-Interesting (CRNI)

Originally developed for natural rubbers, foodstuffs, and elastomers, at present these machines are used as a stirred tank reactor or a twin-rotor continuous mixer. Their bearing is exposed mainly to radial, not axial, forces. By using one screw longer than the other, once can separate the mixing and pumping functions. 

In CRNI the material flow is based on a drag, not positive pumping. There is a low shear stress field, responsible for the absence of dispersive mixing. However, the interchange of material between the screws provides good distributive mixing. The chemical reaction proceeds on the continuously renewed surfaces, related to reorientation of the laminar flow patterns and the total strain. CRNI is well suited for the polymerisation of miscible, low viscosity systems, viz. polymerisation of monomers grafting, halogenations, and PP-visbreaking (degradation by addition of organic peroxides). The main advantage of CRNI is long enough residence time, sufficient to complete slow reactions. The molecular weight of the generated polymer was found to be virtually independent of Q and N. Mild mixing during compounding prevents the mechanical degradation of elastomers as well as crushing glass microbaloons.

CRNI can offer larger outputs, and more interchange of material between the two screws than the other type TSE’s. These machines are frequently used for preparation of composites with fibrous fillers. In general, the closer the clearances and intermeshing the more rapid the build-up of pressure. The narrower the lands and the larger the clearances between the screws, the greater the longitudinal mixing. 

INTERMESHING COUNTER-ROTATING (ICRR)

ICRR was developed as a positive displacement screw pump for viscous, difficult fluids (e.g., Coal-Oil, Ceramic or Rubber compounds). This is the only TSE type that is fully, axially and radially, closed. The extrusion speed depends on the intermeshing geometry and the screw speed. The fully intermeshing ICRR has narrower distribution of residence times and better precision in controlling rapid reactions between liquid reagent and molten polymer that a CORI. The low speed ICRR’s have been used for PVC compounding and forming. At higher screw separation and speeds, the machines can be used for incorporation of high viscosity toughening elastomer. For example, ICRR’s have been used for the reactive extrusion of ABS.

CO-ROTATING INTERMESHING (CORI)

These are the most popular and commercially important Twin Screw Extruders. The machines were originally developed in the 1850’s for pumping stone past. Their advantages arises from the movement of the intermeshing surfaces in opposites directions, thus the melt free surface is continuously renewed and the screws clean each other. In addition, since at the intermeshing the material passes from one screw to the other (Change of the drag direction) there is low probability that the material would go through the gap. Thus, there is no calendering pressure that may cause the screws to bend (a possibility for the ICRR machines). This inturn permits to use higher screw speeds and longer barrels. 

During the 1950’s and 1960’s the design criteria for these extruders were:

1. Incorporation without degradation

2. High shear stress for dispersing

3. Homogenisation of materials with different viscosity

4. Uniform stress and heat history

5. Precise control of mixing process.

Screws are available with 1, 2 or 3 screw flights; the first has the highest solid conveying capacity (used mainly for feeding powders). Two flight screw have large free volume per unit length and low average shear rate – they are mainly used for processing the temperature sensitive materials. The Three-flight screws are used when high shear stress is required. CORI’s are efficient in alternating the direction of applied stresses, providing distributive mixing by lamellae folding, as well as controllable shearing (dispersing mixing). They are operated in starve-fed mode what demands high performance feeders. The material transport mainly depends on drag flow, with only local contribution from the screw pumping in the so-called pressure zones. The maximum conveying is achieved when the pitch is equal to screw diameter, CORI’s are used for compounding, polymerisation, and devolatilization.

Fundamentals of Extrusion: 

Principles of Extrusion:

1. Although the solid-state extrusion is known, in the everyday language extrusion is a melt deformation process in which the following operations sequentially take place.

2. The required materials (Polymers, additives, reinforcements, etc.) are fed through a feed hopper into a barrel.

3. The polymeric material softens by the heat from the external heaters and generated by frictional forces.

4. The energy needed to convert the cold particle into a hot viscous melt depends on the specific heat, heat of melting (of crystalline polymers), the maximum temperature of the extrusion.

5. The transfer of heat into individual pellets by thermal conductivity.

6. Thermal conduction is important in removing the heat from the melt in order to solidify it.

7. The pressure required for the continuous extrusion can be generated by the extruder screw alone, or by attached pumps, viz., gear or screw type.

8. The pressure required for economic production depends on the material properties (especially the melt rheology), type of restriction (filter pack, die, post-die equipment), the screw geometry, and the required throughput.

9. The forward flow of the material is achieved due to the drag flow in the screw channel.

Materials

1.Extrusion techniques can be used to process most thermoplastics and some thermoset plastics. 

2. The resins most commonly extruded for medical applications include polyethylene, polypropylene, polyurethane, polystyrene, fluoropolymers, polyamide, polyester, and flexible polyvinyl chloride. 

3. Combinations of various resins can be used to gain special physical, biological, or chemical properties. Many additives can be used during the extrusion process to enhance processing characteristics of the polymer or to alter product properties. 

4. Such additives include lubricants, thermal stabilizers, antioxidants, radiopacifying agents, and colorants. 

Advantages of Extrusion

1. Low cost tooling and short lead times 

2. Low Cost parts. 

3. Use of multiple materials in a variety of durometers. 

4. Reinforcement via fiber wrapping. 

5. Colour matching.