This document discusses size reduction and the relationship between stress and strain forces during size reduction. It explains that applying stress can cause elastic or permanent deformation depending on whether the applied strain exceeds the material's elastic limit. Various size reduction equipment are also described, including slicing, dicing, shredding, and pulping equipment as well as instruments like ball mills, disc mills, hammer mills, and roller mills. Sieving is discussed as a method for determining particle size using screens of decreasing size to separate particles into fractions.

1. Size reduction
Dr. Chirag

2. Relationship between stress and strain force in size reduction
When stress (force) is applied the resulting
internal strains are first absorbed, to cause deform
ation.
 If the strain does not exceed a certain critical level
named the elastic stress limit (E), the particles retu
rn to their original shape when the stress is remove
d, and the stored energy is released as heat (elastic
region(O–E))

3. Relationship between stress and strain force in size reduction
 If the strain area exceeds the elastic stress limit,
the material gets permanently deformed. If the
stress is continued, the strain reaches a yield point
(Y).
 Above the yield point the material begins to flow
(Y–B) Finally, the breaking stress is exceeded at t
he breaking point (B) and the
material fractures along a line of weakness.
 Part of the stored energy is then released as
sound and heat.

4. Relationship between stress and strain force in size reduction
The size of the piece is reduced, there are fewer lines
of weakness available, and the breaking stress that m
ust be exceeded increases.
When no lines of weakness remain, new fissures
must be created to reduce the particle size

5. Size-reduction Equipment

6. SLICING EQUIPMENT
The blade advances with the product on
the conveyor to ensure a square cut edge re
gardless of the conveyor speed or cut lengt
h which can be adjusted.

8. Dicing equipment
The products are first sliced and then cut
into strips by rotating blades.
The strips are fed to a second set of
rotating knives which operate at right angle
s to the first set and cut the strips into cubes

10. Shredding equipment
is a modified hammer mill in which knives
are used instead of hammers to produce a cutti
ng action.

11. Pulping equipment
This uses a combination of compression and
shearing forces for extraction from crude
drugs,

13. INSTRUMENTS USED IN SIZE REDUCTION
Ball mills
Disc mills
Hammer mills
Roller mills

14. Ball mills
These have a slowly rotating, horizontal steel cylinder
which is half filled with steel
balls 2.5–15 cm in diameter.
 At low speeds, the small balls are used.
 At higher speeds, the larger balls are used.
They are used to produce fine powders, such as
pharmaceutical colourants.

15. Disc mills
• Single-disc mills in which drug passes through an
adjustable gap between a stationary casing and a grooved
disc, which rotates at high speed.
• Double-disc mills which have two discs that rotate in
opposite directions to produce greater shearing forces.

16. Pin-and-disc mills
which have intermeshing pins fixed either
to the single disc and casing or to double
discs. These improve the effectiveness of
milling by creating additional impact and
shearing forces.

18. Hammer mills
These have a horizontal cylindrical
chamber, lined with a toughened steel bre
aker plate.
A high-speed rotor inside the chamber is
fitted with swinging hammers along its le
ngth.
Using for crystalline and fibrous
materials.

19. Hammer mill

20. Roller mills
Using to mill wheat.
Two or more steel rollers revolve towards
each other and pull particles of food through t
he ‘nip’ (the space between the rollers).
The size of the nip is adjustable for different
foods and overload springs protect against acc
idental damage from metal or stones.

22. Applications of size reduction equipment

23. Size Determination

24. Sieving
Sieving is a mechanical size
Separation process.
Widely used in the food
Industry for
* separating fine from
larger particle
* removing large solid
Particle from liquid stream

25. Sieve Analysis
Involves :
- Passing the material being sized through
openings of a particular standard size in a screen.
- The particle-size distribution is then reported
as the weight percentage retained on each of a seri
es of standard sieves of decreasing size and the per
centage passed of the finest size.

26. Sieving is a gravity-driven process. usually a stack of
sieves are used when fraction of various sizes are
to be produce from a mixture of particle size

27. The shaker may be in the form of an eccentric
drive
which a screens a gyratory or oscillating motion or
vibrator which gives the screens small-amplitude,
high frequency, up and down motion

28. When the sieve are inclined, the
particles retained on the screen fall
off at the lower end and are
collected by a conveyor. Screening
and particle size separation can
thus be carried out automatically

29. Standard sieve size
Sieves may be designated by the opening size,
US sieve mesh or Tyler sieve mesh
The Tyler mesh designation refer to the number of
opening per inch.
The US-sieve mesh designation is the metrication
The two mesh designations have equivalent opening
size although the sieve number designations are not
exactly the same.

30. Standard US-sieve size