2. CONSOLIDATION
Due to particle-particle interaction, the
mechanical strength of the material is
increased, which consolidates the material
into a single solid matrix. This phenomenon
is known as CONSOLIDATION.
3. CONSOLIDATION PROCESS
1. COLD WELDING -
When the surfaces of two particles approach each other closely
enough, their free energies results in strong attractive force, this
process is called COLD WELDING.
2. FUSION BONDING -
Multiple points of contacts of particle upon application of load
produces heat which causes fusion / melting. Upon removal of load
it gets solidified and increase the mechanical strength of the mass,
this process is known as FUSION BONDING.
4. MECHANISMS INVOLVED IN CONSOLIDATION
1. MECHANICAL THEORY- As the particle undergo deformation,
the particle boundaries that the edges of the particle intermesh,
hence forming a mechanical bond.
2. INTERMOLECULAR FORCES THEORY- Under pressure the
molecules at the point of true contact between new, clean surface
of the granules are close enough so that the van der waals forces
interact to consolidate the particles, for eg. MCC is believed to
undergo hydrogen bonding during tablet compression.
3. LIQUID-SURFACE FILM THEORY- Thin liquid films form which
bond the particles together at the particle surface. The energy of
compression produces melting of solution at the particle interface
followed by subsequent solidification or crystallisation thus forms
bonded surfaces.
5. FACTORS AFFECTING CONSOLIDATION
1. THE CHEMICAL NATURE OF THE MATERIAL.
2. THE EXTENT OF AVAILABLE SURFACE.
3. THE PRESENCE OF SURFACE CONTAMINANTS.
4. THE INTERFACE DISTANCE.
6. COMPRESSION AND CONSOLIDATION UNDER HIGH
LOADS
1. EFFECT OF FRICTION-
a) INTER PARTICULATE FRICTION- This arises at paricle-
particle contacts and can be expressed as
interparticulate friction coefficient µi. It is more significant
at low applied loads. Glidants reduce this friction.
b) DIE- WALL FRICTION- This results from material being
pressed against the die-wall and moved down it, and it
can be expressed as die-wall friction coefficient µw.
7. 2. FORCE DISTRIBUTION-
• Most investigations are carried out at sinlge station presses or even
on isolated punches and die sets in conjugation with hydraulic
press, there must be an axial balance of forces ,
FA = FL + FD
FA = Applied force to the upper punch.
FL = Applied force to the lower punch.
FD= Reaction at die-wall due to friction at surface.
8. 3. DEVELOPMENT OF RADIAL FORCE-
As the compressional force is increased and the repacking of the tabletting
mass is completed the material may be regarded as a single solid body. Hence,
the compressive force applied in vertical direction results in decrease in
height,ΔH. In case of unconfined solid body this would be accompanied by an
expansion in the horizontal direction, ΔD. The ratio of these two dimensional
changes is known as poisson ratio(λ) of the material. λ is characteristic constant
for each solid.
λ = ΔD/ΔH
Consequently a radial die-wall force FR develops perpendicular to the die-wall
surface. Materials with larger values of λ shows larger values of FR , hence the
relationship between FD and FR will be given by expression,
FD = µW . FR
where µW is coefficient of die-wall friction.
9. 4. DIE-WALL LUBRICATION- Die-wall lubricants function
by interposing a film of low shear strength at the interface
between tabletting mass and die-wall, there is some
chemical bonding between boundary lubricants and the
surface of die-wall as well as the edge of the tablet.
The best lubricants are those of low shear strength but have
strong cohesive tendency in direction at the right angle to
the plane of shear.
10. 5. EJECTION FORCES- It is the force necessary to eject a
finished tablet. It consist of three stages-
• Stage 1- peak force, required to initiate ejection by
breaking of tablet / die-wall adhesions.
• Stage 2- small force, that is required to push the tablet up
the die wall.
• Stage 3- declining force of ejection , as the tablet
emerges from the die.
