Extrusion and types of extruders

2. Introduction
•Extrusion is a thermo-mechanical process by which moistened,
expansible, starch and/or proteinaceous materials are
plasticized and cooked in a tube by a combination of moisture,
pressure, temperature and mechanical shear, and thus pre-
defined shaped through the die opening at extruder outlet
(Smith et al. 1976).
•Extrusion is simply the operation of shaping a plastic or dough-
like materials by forcing it through a restiction or die.

3. Contd…
• According to Rossen and Miller (1973) “Food extrusion is a
process in which food material is forced to flow under one or
more varieties of conditions of mixing, heating and shear
through a die which is designed to form and/or puff-dry the
ingredients.
• Extruders can be used to cook, form, mix, texturize and shape
food products under conditions that favour quality retention,
high productivity and low cost.

4. Principle
• A high-temperature short-time (HTST) processing.
• In extrusion processing, raw materials are processed in Baro-
thermal conditions (Pressure around 10-20 bar and
temperature around 160-190oC).
• Combines several unit operations including mixing, cooking,
kneading, shearing, shaping and forming.
• Main method of preservation is by the low water activity of
the product (0.1–0.4).
• After entering through barrel, food materials undergo through
very high pressure and temperature.
• During release, in extreme low pressure, the food material
expands & get the desired shape.

5. Equipment
Extruders are composed of five main parts (El-Dash, 1981) :
(I) Pre-conditioning system
(ii) Feeding system
(iii) Screw or worm
(iv) Barrel
(v) Die and cutting mechanism

6. Fig: Schematic diagram of an extruder

7. Pre-conditioning
• An assembly that adjusts moisture content and temperature of the
ingredients and may partially or completely cook them before
entering the extruder.
• Applied when moisture content is around 20 to 30% and long
residence time of the material is used.
• Main aim: Uniform hydration of the raw material in order to
eliminate any dry core (Strahm, 2000).
• Occurs with the addition of hot water (80-90°C) or steam.
• Pre-conditioners have 2 axis of different diameters and rotation
speeds.
• Residence time between 2 to 4 minutes.

8. Feeding system
• Normally composed of a holding bin where the material is loaded.
• Discharge of the material can occur through a vertical feeding
screw, a horizontal feeding screw, a horizontal vibrating trough
system, a disk feeder or a volumetric belt feeder.
• Necessary to guarantee a constant and non-interrupted feeding of
the raw materials into the extruder for an efficient and uniform
functioning of the extrusion process (El-Dash, 1981).

9. Screw
• Most important component of the extruder.
• Determine cooking degree, gelatinization and dextrinization
of starch and protein denaturation - ensure final product
quality.
• Can be mono-piece or multi-piece (El-Dash, 1981), vary in
number and shapes, each segment is designed for a
specific purpose.

10. Representation of extruder and its various components

11. Barrel or sleeves
• Surrounds the screw, made by solid material.
• Often jacketed to permit circulation of steam or superheated
oil for heating or cooling and for temperature adjustment.
• Equipped with pressure and temperature sensing and
temperature control mechanisms as well (El-Dash, 1981).
• In twin-screw extruders, the sleeves are usually smooth but
can be constructed with longitudinal or helical grooves.
• In single-screw extruders, the sleeves are usually fluted on
the inside, with either straight or spiral grooves.

12. Die & Cutting mechanism
• Two main functions:
 Give shape to the final product.
 Promote resistance to material flow within the extruder
permitting an increase in internal pressure.
• The die can present various designs and number of orifices.
• Cutting mechanism must permit obtaining final products with
uniform size.
• Product size is determined by the rotation speed of the cutting
blades.
• Blades can be horizontal or vertical.

13. Mechanism
• Raw materials (dry or pre-conditioned) are fed into the
extruder barrel through feeder.
• Extruder is divided into 3 zones
 Feeding zone: The screw convey & homogenise the food.
 Compression zone: Reduction in screw depth & pitch,
increase in shear rate, temperature & pressure.
 Metering zone: Also called as melt conveying zone. This
zone melts the last particles and mixes to uniform
temperature and composition.
• Conversion of solid material to liquid occurs inside the
extruder .
• Reduction in more screw depth & pitch, resulting in high
shear & more heat generation in the subsequent pressure
zone.

14. Contd...
• As it moves further, the screw kneads the material into a semi-
solid, plasticized mass.
• Mass reaches maximum temperature and pressure and a
reduction in viscosity immediately before exiting the extruder
(Fellows, 2000; Riaz, 2000).
• Finally, it is forced through one or more restricted openings,
called dies.
• The food expands to the final shape as it emerges under
pressure from the die.
• In extrusion cooking (in case of hot extrusion) the food is
heated above 100ºC.
• Cools rapidly as moisture is flashed off as steam.

15. Classification
Extruders
on the basis of number of screws on the basis of heat generation
Twin-screw extruder
Single-screw extruder
Polytropic extruder
Isothermal extruder
Adiabatic extruder

17. Single-screw extruders
• Most common extruders applied in the food industry.
• Composed of single axis.
• Screws may be made up of only one piece or screws of multiple
pieces.
• Single element screws may present different configurations:
 Screw with constant depth and flight (straight)
 Screw with constant flight and variable depth (tapered)
 Screw with a reduction in depth just after feeding (tapered-
straight)
 Screw with flight openings (interrupted flight)
• Screws of multiple elements can be built up to desired
configuration.

18. Different configurations of single screws

19. Twin-screw extruders
• Composed of two axis that rotate inside a single barrel.
• Internal surface of the barrel is smooth.
• Depending on the position of the screws and their direction
of rotation, four different types are
Co-rotating intermeshing screws
Co-rotating non-intermeshing screws
Counter-rotating intermeshing screws
Counter-rotating non-intermeshing screws
• Little used in the food industry, even though they present
more efficient displacement properties (El-Dash, 1981).

20. Different configurations of twin screws

21. Adiabatic(autogenous) extruder
• These extruders develop essentially all heat by friction and
little heat is removed from the barrel e.g, dry extruders and
low cost extruders.
• Some extruders need to be heated by supplementary
sources initially but then will operate autogenously.
• Adiabatic extruders operate at low moisture levels (8-14%).

22. Isothermal extruders
• These extruders operate at an essentially constant
temperature throughout the entire length of the barrel.
• Water-cooled jackets are sometimes used to maintain the
temperature.

23. Polytropic Extruders
• These extruders have provisions for alternatively adding or
removing heat as required by the specific process.
• Examples include most cooking extruders with external
cooling and heating sections which generate heat by friction.

24. Quality evaluation
The quality of the extruded products
depends on:
 Temperature inside the extruder
 Water content of the raw material
 Rotation speed of the screw
 Time duration for which the material is kept inside the
extruder for processing

25. References
• El-Dash, A.A., 1981. Application and control of thermoplastic extrusion of
cereals for food and industrial uses. POMERANZ, Y.; MUNCH, L. Cereals: a
renewable resource, theory and practice. Saint Paul: American Association
of Cereal Chemists, pp.165-216.
• Kirby, A.R., Ollett, A.L., Parker, R. and Smith, A.C., 1988. An experimental
study of screw configuration effects in the twin-screw extrusion-cooking of
maize grits. Journal of Food Engineering, 8(4), pp.247-272.
• Riaz, M.N. ed., 2000. Extruders in food applications. CRC press.
• Smith, O.B., 1976. Extrusion cooking. New protein foods, 2, p.86.
• Steel, C.J., Leoro, M.G.V., Schmiele, M., Ferreira, R.E. and Chang, Y.K.,
2012. Thermoplastic extrusion in food processing. In Thermoplastic
elastomers. InTech
• Huber, G.R., Strahm, B.L., Sevatson, E.S. and Kesler, D.L., Wenger
Manufacturing Inc, 2001. Method and apparatus for the production of high
viscosity paste products with added components. U.S. Patent 6,294,212.