Extrusion is a process that converts raw materials into a desired shape and form by forcing it through a die under pressure. There are four main types of extrusion systems - cold extrusion, extrusion cooking, single screw extruder, and twin screw extruder. A single screw extruder uses one rotating screw to move material through the barrel, while a twin screw extruder uses two intermeshing or counter-rotating screws. Extrusion is used to manufacture many food products like cereals, pastas, and snacks. Power requirements and rheological properties of the extrudate impact extruder design and operation.

1. Extrusion
Unit Four

3. • Extrusion is a process that converts raw
material into a product with desired shape
and form by forcing the material through a
small opening using pressure.
• The process involves a series of unit
operations such as mixing, kneading,
shearing, heating, cooling, shaping and
forming.
• .

4. • Many food products are manufactured by
extrusion cooking—a process that uses both
thermal energy and pressure to convert raw
food ingredients into popular products such as
breakfast cereals, pastas, pet foods, snacks
and meat products

5. Introduction and background
• The origins of the extrusion process are closely associated
with polymer science and technology.
• In the mid-1850s, extrusion was used to produce the first
seamless lead pipe.
• The manufacturing of Bakelite in 1907, and the protective
coating resin, glyptal, in 1912, was dependent on extrusion
processing.

6. • Formal applications of extrusion processes to
foods began in the 1930s and evolved over
the following 50 years, as equipment for
extrusion processing increased in capabilities
and complexity.

8. Five components of Extrusion System
1) Primary feed system consisting of a container
and delivery system for the primary
ingredients involved in the process
2) Pump to move all ingredients through the
steps associated with the extrusion process
3) Reaction vessel where key actions such as
mixing, kneading, shearing, heating, and
cooling occur

9. 4) Secondary feed system for adding secondary
ingredients or energy as needed to achieve
the desired product characteristics
5) Exit assembly designed to restrict flow and
contribute to the shaping and forming of the fi
nal product (usually referred to as the die)

10. • These components appear in different ways
and may be identified differently in various
extrusion systems, depending on the specific
equipment used and the product being
manufactured.

11. Examples of extrusion processing
• Pasta products (macaroni, spaghetti, etc.)
• Pellets for conversion into ready-to-eat cereals.
• Cereal based products (corn flakes, puffed rice,
crisp breads, snacks),
• Fruitbased products (fruit gums, licorices, hard
candies),
• Protein-based products (textured vegetable
proteins),
• Animal feeds (pet foods),
• Spice-based products (flavors).

13. • Food ingredients of various types may be
processed by extrusion and are referred to as
extrudates.

14. Basic Principles of Extrusion
• Transport Processes :
• Flow of materials within the system,
• Thermal energy transfer to and within the material,
• Mass transfer to and within the material during
extrusion.

15. What we are going to study
• Flow and deformation behavior within
extruder ie Rheology
• Power Consumption of extruder

16. • All ingredients involved in the extrusion
process flow through a channel with a
defined geometry.
• The power requirements for the process are
directly dependent on the flow characteristics
through the channel
and
on properties of the fluid used.

17. Equations/relations involved in
extrudate rheology
• The primary property of the extrudate in this
relationship is viscosity (μ).
• Since most food extrudates are highly non-
Newtonian, the apparent viscosity decreases
with increasing rate of shear.
• These materials are described by the
Herschel-Bulkley model

18. Herschel Bulkley Model

19. • where the consistency coeffi cient ( K) and the
flow behavior index ( n ) are parameters
describing the rheological properties of the
extrudate.
• Based on investigations of the flow behavior
of food extrudates, the flow behavior index is
normally less than 1.0.

20. This relationship contains two additional parameters: an
activation energy constant ( A) to account for the influence of
temperature(T), and a similar exponential constant ( B) to
account for the influence
of dry basis moisture content ( M).
• Two additional parameters with signifi cant influence on flow
of food extrudates are moisture content and temperature. In
order to account for the additional factors, the following
relationship has been proposed:

22. Type of Flow geometries In
Extruder
• Tube
• Channel
• Usually described as rectangular cross-section
within the barrel of the extruder

23. Assumptions in Deriving Flow
Behaviour
Flow is incompressible
 Steady
 Laminar and fully developed
The rectangular cross-section channel formed
between the screw flights and the barrel
surface is assumed as an infinite plate sliding
across the channel

24. • Based on these assumptions, for an axial flow in rectangular
Cartesian coordinates, the differential equation for the
momentum flux is
where L represents the length of the channel (distance in
the downstream direction), and y is the element
distance from the barrel surface to the surface of the
screw (distance in the vertical direction).

25. • Based on these assumptions, for an axial flow
in rectangular Cartesian coordinates, the
differential equation for the momentum fl ux
is
• where C is a constant of integration. It must be
noted that the preceding equation holds for both
Newtonian and non-Newtonian fl uids.

26. • In the case of a Newtonian fl uid this expression is
combined with Equation (2.28) to obtain the
following expression

27. • The fluid velocity distribution within the channel can be
obtained by integration of the previous expression in
the following manner,

31. • As indicated earlier, most food materials involved in extrusion
are non-Newtonian and require appropriate relationships to
describe the flow characteristics.
• In an extrusion system, the flow would most likely occur in a
rectangular cross-section channel (generally assumed to be a
plane narrow slit) and the expression for shear stress is given
by Equation (14.4).

38. Extrusion Systems
• Extrusion systems can be divided into four
different categories.
• These four categories include two different
methods of operations—cold extrusion or
extrusion cooking—and two different barrel
configurations—single or twin screw.
• Both barrel confi gurations may be used for
either method of operation.

39. System 1: Cold Extrusion

41. • As illustrated, one component of the final
product is pumped through an opening of
defined shape to create a continuous tube of
the first component.
• At the same time, the second component of
the final product is introduced just before the
die and becomes a filler for the interior space
within the outer tube.
• During a final step in the process, the tubular
product is cut into appropriate lengths

42. • In general, cold extrusion is used to mix,
knead, disperse, texturize, dissolve, and form
a food product or product ingredient.
• Typical food products include pastry dough,
individual pieces of candy or confections,
pasta pieces, hot dogs, and selected pet
foods.
• These types of extruders would be considered
low-shear systems and would create relatively
low pressures upstream from the die.

43. Cold Extrusion Process

44. Filled Products Using coExtrusion

45. Extrusion Cooking
• When thermal energy becomes a part of the
extrusion process, the process is referred to as
extrusion cooking.
• Thermal energy may be added to the extrudate
during the process from an external source or may
be generated by friction at internal surfaces of the
extruder in contact with the extrudate.
• As illustrated in Figure 14.3 , the addition of thermal
energy occurs at the surface of the barrel of the
extrusion system.

46. • Thermal energy may be transferred through
the walls and surfaces of the barrel to
ingredients used to create the extrudate.
• In addition, mechanical energy created by
friction between surfaces and ingredients
within the barrel is dissipated as thermal
energy in the extrudate.

48. • A portion of the cooking process occurs downstream from the
die due to the rapid change in pressure.
• The pressure change results in a rapid reduction in
temperature and a release of moisture from the extrudate.
These changes are illustrated in Figure 14.4 .
• The combinations of temperatures, pressures, and moisture
contents may be used to create an unlimited range of product
characteristics.
• For example, the density of individual pieces of extrudate will
be a function of the pressure difference across the die.

50. Single Screw Extruder
• In a single-screw extrusion system, the barrel of
the extruder contains a single screw or auger that
moves the extrudate through the barrel.
• As shown in the photograph in Figure , the
extrudate is carried through the barrel in the
space between the core of the screw and the
barrel.
• The flow rate of the extrudate through the
system will be proportional to the rotation speedrotation speed
(rpm) of the screw.(rpm) of the screw.

51. 3 components of single screw extruder
• Feed section, where the various ingredients are
introduced and initial mixing occurs.
• The rotating action of the screw moves the
ingredients to the transition or compression section.
• Compression or transition section, where the
ingredients begin the transition to the extrudate as
pressure and temperature begin to increase.
• As the dimensions of the flow channel decrease, the
material is compressed and mechanical energy is
dissipated as temperature increases.

52. • This section may be referred to as a kneading
section, with significant changes in the physical and
chemical characteristics of the ingredients occurring.
• Metering (or cooking) section, where additional
compression of the extrudate occurs as a result of
additional reductions in the dimensions of the flow
channel and increased shearing action.
• In some designs, the overall dimensions of the barrel
are reduced as well.

53. Sections within the barrel of single screw extruder

54. Photo of single screw extruder

55. • Single-screw extruders may be classifi ed
according to shearing action as well.
• Generally, low shear extruders will include
smooth barrel surfaces, relatively large fl ow
channels, and low screw speeds (3–4 rpm).
• Characteristics of moderate shear extruders
include grooved barrel surfaces, reduced fl ow
channel cross-sections and moderate screw
speeds (10–25 rpm).

56. • High shear extrusion systems operate at high
screw speeds (30–45 rpm), variable pitch and
flight depth screws, and grooved barrel
surfaces.
• Each of these types of extrusion systems will
create extruded products with different
properties and characteristics

57. • An additional dimension of operation within
the single-screw extrusion system is
associated with flow characteristics within the
barrel and around the flights of the screw.
• Although the forward flow is caused by action
of the screw, backward flow occurs between
the flights and the barrel surface.

58. • The backward flow is the result of increased
pressure as the extrudate moves from one
section of the extruder to the next.
• The second component of backward flow is
the leakage between the flights and the
barrel; this type of flow may be reduced by
grooves in the barrel surface.

60. Twin Screw Extruder
• Twin-screw extrusion systems incorporate
two parallel screws into the extruder barrel.
• The screws may be co-rotating or counter-
rotating, as illustrated in Figure .

61. • Various configurations of the screw or auger
have been developed, including the fully
intermeshing, self-wiping, co-rotating twin-
screw system as shown in figure on next page
• This particular system has been used in many
food applications due to the self-cleaning,
better mixing, moderate shear force and
higher capacity characteristics.

62. • Twin-screw extrusion systems have numerous
advantages.
• The throughput of these systems can be
independent of feed rate and screw speed.
• Process variables include degree of fill,
temperature-shear history, and heat transfer,
all of which may infl uence the properties of
the extruded product.

63. • Twin-screw systems provide increased
flexibility in terms of higher moisture content
extrudates, as well as higher concentrations of
ingredients (lipids, carbohydrates, etc.)
• These systems usually have less wear due to
shorter sections of the barrel being exposed
to the high pressures required for product
extrusion.
• Finally, twin-screw extruders will
accommodate a wider range of particle sizes
in the ingredients.

64. • Other configurations of twin-screw extrusion
systems include counterrotating with non-
intermeshing systems, counter-rotating with
intermeshing systems, and co-rotating with
non-intermeshing systems.
• Each of these configurations may be used in
specific applications and require unique and
complex analysis for design and scale-up.

65. Screw rotation in a twinscrew
extruder.

66. Confi guration of screws in a
twin-screw extruder.

70. Extrusion System Design
• The power requirement for operating an extrusion system is a
key design factor.
• Power consumption is a complex function of properties of the
material being extruded, extruder design, extruder motor
type and extrusion conditions.
• Although there are unique aspects associated with estimating
power consumption for single- versus twin-screw systems, a
general approach for estimating total power consumption (pt)
is:

71. • where ps is the portion of the power consumption
for viscous dissipation associated with shear of the
feed ingredients, and the second term of the
expression is the power needed to maintain flow
through the barrel and die of the extrusion system.

72. • The power needed for viscous dissipation has been
expressed in terms of the Screw Power number (
Np ), as follows:
• where the screw speed ( N ), screw diameter ( D ), screw
length ( L ), as well as density of extrudate ( ρ) are
considered. The magnitude of the Screw Power number is
dependent on the Screw Rotational Reynolds number ( N Res
), as illustrated by Figure 14.9 .

73. • For extrudate with rheological properties described
by the power-law model, the Screw Rotational
Reynolds number is defined as follows:

74. Dimensionless correlation
for extruder power consumption