4202 :Compaction and compression

Definitions

• Compression

Compression means a reduction in the bulk volume of a material as a result of the removal of the gaseous phase (air) by applied pressure.

• Consolidation

Consolidation is an increase in the mechanical strength of a material resulting from particle-particle interactions.

• Compaction

Compaction of powders is the general term used to describe the situation in which these materials are subjected to some level of mechanical force.

The physics of compaction may be simply stated as "the compression and consolidation of a two-phase (particulate solid-gas) system due to the applied force."

Derived Properties of Powders or Granules:  
Some derived properties which help in quantification of important variables are
1.Volume
2. Density
3. Porosity
4. Flow properties. 

Volume: Measurement of volume of powder is not easy as the measurement of mass of powders, because in powders there will be inter and intra- particular voids. Hence three types of volume can be considered for a powdered mass, they are,

1. True volume
2. Granular volume    Interparticulate
                                                                                                                     Open
3. Bulk volume                                                                                           Closed

Volume is the quantity of three-dimensional space enclosed by a closed surface, for example, the space that a substance (solid, liquid, gas, or plasma) or shape occupies or contains. Volume is often quantified numerically using the SI derived unit, the cubic metre.

Mass-Volume relationships

1. Open intraparticulate voids-those within a single particle but open to the external environment.
2. Closed intraparticulate voids-those within a single particle but closed to the external environment.
3. Interparticulate voids-the air spaces between the individual particles.

Therefore, at least three interpretations of "powder volume" may be proposed:

True Volume of the powder (Vt): True volume is the   total volume of the solid particles. It is a volume of the particles excluding the inter and intra particulate spaces in a powder. Or it is volume of powder itself 

Granular Volume of the powder (Vg): It is the cumulative volume occupied by the particles, including all intraparticulate (but not interparticulate)   voids. Or it is the volume of powder itself and volume of intraparticulate spaces. 

Bulk Volume of the powder (Vb): It is the total volume occupied by the entire powder mass under the particular packing achieved during the measurement.

It comprises the true volume and inter and intra particulate voids.

Relative volume (VR): It is the ratio of the the volume, V of the sample under specific experimental conditions, to the true volume Vt.

VR = V / Vt

VR tends to become unity as all air is eliminated from the mass during the compression process.

Density
The ratio of mass to volume is known as the density (ρ) of the material
By considering the three types of volume of powders, we can define the respective densities as,

True density (ρt): Mass of the powder/ True volume of the powder.
True density, ρt = M / Vt  

Granular density (ρg): Mass of the powder/ granule volume of the powder. 
Granular density, ρg = M / Vg

Bulk density (ρb): It is the ratio of total mass of the powder to the bulk Volume of the powder. It is measured by pouring the weighed powder into a measuring cylinder and the volume is noted.

It is expressed in gm/ml and is given by: ρb = M / Vb  

Where, M is the total mass of the powder

             Vb is the Bulk Volume of the powder 

Porosity

The voids present in the powder mass may be more significant than the solid components in certain studies.   For  example,  a fine  capillary network of voids or pores has  been  shown to enhance  the  rate  of liquid  uptake  by tablets, which in turn increases the rate of their disintegration. For this reason, a second dimensionless quantity, the ratio of the total volume of void spaces (Vv) to the bulk volume of the material, is often selected to monitor the progress of compression. This is known as porosity.

The spaces between the particles in a powder are known to be voids. The volume occupied by such voids is known to be void volume. 

        Void volume (Vv) = Bulk volume –True volume 

The porosity of the powders is defined as ratio of the void volume to the bulk volume of the packing. Or, the ratios of the total volume of void spaces (Vv) to the bulk volume of the material.

      Porosity (E) = void volume /bulk volume.

                     =Vv/Vb

                     = [Vb - Vt / Vb] (As, Vv= Vb - Vt)

         = 1-(Vt/bt)

Porosity is frequently expressed in percent

                E = [1- Vt / Vb] x 100 

The relation between porosity and compression is important because porosity determines the rate of disintegration, dissolution and drug absorption.

Question: A cylindrical tablet of 10 mm diameter and 4 mm height weighed 480 mg and was  made  from  material  of true  density  1.6 g Cm-3 .  Clculate the bulk volume Vb .

Flow Properties: To get uniformity of the weight of the tablet, the powder should possess a good flow property. Flow properties of the powders depend on the-

• Particle size,
• Shape,
• Porosity and density,
• Moisture of the powder.

Particle size

The rate of flow of powder is directly proportional to the diameter to the particles.

• Beyond particular point, flow properties decreases as the particle size in increases. Because in small particle (10µ) the vanderwaal’s, electrostatic and surface tension forces causes cohesion of the particles resulting poor flow.
• As the particle size increases, influence of gravitational force on the diameter increases the flow property. But appropriate blends of fines & coarse improves flow characteristic, as the fines get absorbed and coarse particle reduce friction. 

Particle shape

Spherical, smooth particles improves flow properties, surface roughness leads to poor flow due to friction and cohesiveness , flat and elongated particles tend  to pack loosely, obstructing the flow

Density & porosity

Particles having high density and low internal porosity tend to posses good flow properties. 

Moisture
The higher moisture content, the flow property will be poor owing to cohesion and adhesion. 

Angle of repose

The flow characteristic is measured by angle of repose. Angle of repose is defined as the maximum angle possible between the surface of a pile of powder and the horizontal plane.

                    tan Ø= h/r

                         Ø = tan-1(h/r)

Where,    h  = height of pile

                r  =Radius of the base of pile.

                Ø = Angle of repose .

The angle of repose is calculated by measuring the height and radius of heap of powder formed.

The frictional forces in a loose powder can be measured by Angle of repose.

The lower the angle of repose better will be flow property.

The values of angle of repose are given below: 

Angle of repose (in     degrees)	Type of Flow
<25	Excellent
25-30	Good
30-40	Passable
40>	Very poor

Carr’s consolidation (Compressibility) Index (CI)

It indicates powder flow properties. It is expressed in percentage.

It is defined as:  

Consolidation Index = I = Tapped density-Poured density /Tapped density     

               Therefore = (Dt- Db/Dt) × 100 

Where, Dt is the tapped density of the powder

Db is the Poured density of the powder  

Determination of Tapped density & Poured density.

It is determined by passing a fixed quantity of powder into a measuring cylinder and the volume is noted .CI can be calculated by founding out by tapped density and Poured density of powder. 

Grading of the powder for their flow properties according to Carr’s index:

   Carr’s index (%)                                               Type of flow

       5-15                                                                       Excellent

        12-18                                                                      Good

       18-21                                                                     Fair to passable

         23-35                                                                      Poor

         33-38                                                                      Very poor

         >40                                                                         Very very poor

Compression properties

This involves compressibility and compactability.

Compressibility can be defined as the ability of a powder to decease in volume under pressure. Powders are normally compressed into tablets using a pressure of about 5.0kg/cm2.

Compactibility can be defined as ability of powder to be compressed in to a tablet of a certain strength or hardness.

These two relate directly to the tableting performance.

For proper compression to occur the tablet should be plastic i.e., capable of permanent deformation and it should also exhibit certain degree of brittleness. 

If the drug is plastic, then the excipients chosen should be brittle (lactose, calcium phosphate) and if the drug is brittle, then the excipients should be plastic (Microcrystalline cellulose).

In the absence of external force, molecules are in their equilibrium positions with lowest free energy as determined by the inter-atomic and intermolecular forces. In response to an applied stress, molecules or their parts will change their mutual positions from their original equilibrium positions to new positions with higher free energy. Thermodynamics then compels (চালিত করা) molecules to return to the original positions with the lowest free energy, resulting in a returning elastic force that counterbalances the applied stress. At the new equilibrium position, the internal elastic force equals to the applied external force.

Within a limited extent, the deformation of a solid body is reversible. This type of deformation is referred to as the elastic deformation. Elastic deformation obeys the Hook’s Law, which states that the deformation is proportional to the applied stress.

Plastic Deformation

Plastic deformation refers to irreversible changes in the internal molecular structure (or microstructure) of a material subject to applied stress.

Consolidation

An increase in the mechanical strength of the material resulting from particle or particle interaction. (Increasing in mechanical strength of the mass)

Consolidation Process

Cold welding:  When the surface of two particles approach each other closely enough, (e.g. at separation of less than 50nm) their free surface energies result in strong attractive force, this process known as cold welding.

Fusion bonding: Contacts of particles at multiple points upon application of load, produces heat which causes fusion or melting. If this heat is not dissipated, the local rise in temperature could be sufficient to cause melting of the contact area of the particles.

Upon removal of load it gets solidified giving rise to fusion bonding & increase the mechanical strength of mass.

Factors affecting consolidation

Both the “cold" and "fusion" welding, the process is influenced by several factors,   including:

1. The chemical nature of the materials

2. The extent of the available surface

3. The presence of surface contaminants

4. The inter-surface distances

Properties of tablet influenced by compression

1. Density and porosity:

The apparent density of a tablet is exponentially related to applied pressure
(or compressional force) until the limiting density of the material is achieved.

As compressional force increases the density of tablet also increases as a result of decrease in bulk volume.

                                                            
Fig.1.1: Influence of compressional force on apparent density

Fig.1.2: Effect of compressional force on porosity

As the porosity and apparent density are inversely proportional, the plot of porosity against log of compression force gives linear plot with a negative slope (Fig.1.2.).

Therefore the relation between density and porosity is

E=1-(1/Vr) [Where, E=Porosity and Vr= Relative volume)

2. Hardness and tensile strength

Tensile strength :( In the General Physics) a measure of the ability of a material to withstand a longitudinal stress, expressed as the greatest stress that the material can stand without breaking. Tensile strength measures the force required to pull something such as rope, wire, or a structural beam to the point where it breaks.

There is a linear relationship between tablet hardness & the logarithm of applied pressure except at high pressure.
               
Fig.2.1: Influence of compressional force on the tablet hardness.

The strength of tablet may be expressed as tensile strength. As shown in the following figure (Fig.2.2). The tensile strength of crystalline lactose is directly proportional to the compressional force .i.e. Increase in the compressional force increases the strength of the tablet.

Fig.2.2: Influence of compressional force on the tensile strength of a tablet.

3. Specific surface area:

Specific surface area is the surface area of 1 gm material. Specific surface area initially increases to a maximal value as the compressional force increases, indicating the formation of new surface due to fragmentation of granules.

Fig.3.1: Influence of compressional force on the Specific surface area of a tablet.

Further increase in force produce a progressive decrease in surface area due to bonding of particles.

4. Disintegration

Usually as the applied pressure used to prepare a tablet is increased, the disintegration time increases (lactose/aspirin alone).

Frequently, there is exponential relationship between disintegration time and pressure (aspirin-lactose).

Fig.4.1: Influence of compressional force on the Disintegration time of a tablet.

In some formulation there is minimum value when applied pressure is plotted against log of disintegration time (with 10% and 15% starch in sulfadiazine tablets)

For tablets compressed at low pressure, there is a large void, and the contact of starch grains in the inter-particular space is discontinuous. Thus there is a lag time before the starch grains, which are swelling due to imbibitions of water, contact and exert a force on surrounding tablet structure.

For tablets compressed at certain applied pressure, the contact of starch grains is continuous with the tablet structure, and the swelling of starch immediately exerts pressure, causing the most rapid disintegration.

For tablets compressed at pressures greater than that producing minimum disintegration time, the porosity is such that more time is required for the penetration of water into the tablet, hence increase in disintegration time.

5. Dissolution

The effect of applied pressure on dissolution rate may be considered from viewpoint of disintegrating and non-disintegrating tablets.
For a conventional tablet (uncoated) dissolution depends on

1. Temperature
2. The properties of the API
3. The properties of the excipients etc.
4. Properties of solvent

The effect of applied pressure on dissolution of disintegrating tablet is difficult to predict.

If fragmentation of granules occurs during compression, the dissolution is faster as the applied pressure is increased, because of increase in specific surface area.
If bonding of particle predominantly occurs during the compression, then it decreases the dissolution.

Pressure (N/cm2)	t50%   (min)
	Starch paste	Methylcellulose solution	Gelatin solution
200 400 600 800 1000 2000	54.0 42.0 35.0 10.0 7.0 3.3	0.5 0.8 1.1 1.2 1.4 1.8	10.0 4.5 3.0 4.6 4.9 6.5

Table: Effect of compressional force on dissolution of Sulfadimide tablets prepared with various granulating agents

Heckel Equation

The changes in the volume with the applied pressure ids defined by various equations among them the “The Heckel Theory” is the most important. Heckel considered that the reduction in the voids obey the first order kinetics relationship with the applied pressure. Where pores are reactant & densification is the product.

For the compressional process Heckel has proposed the following equation known as the “Heckel Equation”

lnVV-Vα=KP+VₒVₒ-Vα-----------------------(1)

Where, V= Volume at the applied pressure “P”
Vₒ=Original volume of the powder including the voids
Vα=Volume of the Solid Powder excluding the voids
K=A constant related to the “yield pressure” of the powder
P=Applied pressure

We know that, porosity “E” is the ratio of the total volume of the void space to the bulk volume of the powdered material.

i.e.  E=V-VαV
Or, 1E=VV-Vα--------------------------(2)
From the equation 1 and 2 we get

ln1E=KP+VₒVₒ-Vα--------------------------(3)
This is the rearranged or moderate form of Heckel Equation.

Heckel Plot

When  ln1E  or   lnVₒVₒ-Vα  is plotted against the applied pressure P, we get a plot known as Heckel Plot. The nature of the plot depends on the characteristics of the material to be compressed. The Heckel plot explains the mechanism of bonding.

Materials that are comparatively soft and that readily undergo plastic deformation retain different degrees of porosity, depending upon the initial packing in the die. This in tum is influenced by the size distribution, shape, etc. of the original particles. Heckel plots for such materials are shown by type a in Figure 4-17; sodium chloride is a typical example.

Conversely, harder materials with higher yield pressure values usually undergo compression by fragmentation first, to provide a denser packing. Label b in Figure 4-17 shows Heckel plots for different size fractions of the same material that are typical of this behaviour. Lactose is one such material.

                                       
Fig. 4-17:  Examples of  Heckel plots. Curves i, ii, and iii represent decreasing particle size fractions of the same material.  Type a curves are typical of plastically deforming materials, while those in which fragmentation occurs initially tend to show type b behaviour.

Application of Heckel Plot: The plot is used

i. To check lubricant efficacy.
ii. For interpretation of consolidation mechanisms.
iii. To distinguish between plastic and elastic deformation characteristics of a material.

Limitations

1. The plot is linear only at high pressure.
2. The plot can be influenced by time of compression and degree of lubrication.

Processes of Compression

In pharmaceutical tableting an appropriate volume of granules in a die cavity is compressed between an upper & lower punch to consolidate the material in to a single solid matrix, which is subsequently ejected from the die cavity as an intact tablet. The subsequent events that occur in the process are:

     1. Transitional repacking or Particle rearrangement.

     2. Deformation at the point of contact.

     3. Fragmentation.

     4. Bonding.

     5. Deformation of the solid body.

     6. Decompression.

     7. Ejection.

 

Transitional repacking or Particle rearrangement

• The particle size distribution and shape of granule determines initial packing.
  In the initial stages of compression, the punch and particle movement occur at low pressure.
• During this, particle move with respect to each other & smaller particles enter the voids between the larger particles. As a result the volume decreases and bulk density of granulation increases.
• Spherical particles undergo less rearrangement than irregular particles as spherical particle tend to assume a close packing arrangement initially.
• To achieve a fast flow rate required for high speed presses the granulation is generally processed to produce spherical or oval particles; thus, particle rearrangement and energy expended in rearrangement are minor consideration in the total process of compression.

Deformation at the point of contact

When a stress is applied to a material, deformation (change of form) occurs. If the deformation disappears completely (returns to original state) upon the release of stress, then it is called an elastic deformation. If the deformation that does not completely recover after release of stress is known as plastic deformation.

The force required to initiate plastic deformation is known as yield stress. When the particles of the granulation are so closely packed so that no further filling of the void can occur, a further increase of compressional force causes deformation at the point of contact. 

Both plastic and elastic deformation may occur although one type predominates for a given material.

            
Fragmentation

As the compressional load increases the deformed particle starts undergoing fragmentation. Because of the high load, the particle breaks into smaller fragments leading to the formation of new bonding areas. The fragments undergo densification with infiltration of small fragments into voids.

In some materials where the shear stress is greater than the tensile strength, the particles undergo structural break down. This is called brittle fracture.

            Example: sucrose – shear strength is greater than the tensile strength. 

With some materials fragmentation does not occur because the stresses are relieved by plastic deformation. Plastic deformation may be thought of as a change in particle shape and as the sliding group of particles in an attempt to relieve stress (viscoelastic flow). Such deformation produces new, clean surface that are potential bonding areas.

Irrespective of behaviour of large particles, small particles may deform plastically, a process known as microsquashing, and the proportion of fine powder in a sample may therefore be significant.

Fragmentation do not occur when applied stress-

• Is balanced by a plastic deformation.
• Change in shape.
• Sliding of groups of particle (viscoelastic flow).

Bonding and Consolidation

After fragmentation of the particles, as the pressure increases, formation of new bonds between the particles at the contact area occurs. The hypothesis favouring for the increasing mechanical strength of a bed of powder when subjected to rising compressive forces can be explained by the following theory.

Bonding/Consolidation Mechanism

There are three theories about the bonding of p[articles in the tablet by compression

1. Mechanical theory
2. Intermolecular force theory
3. Liquid-Film surface theory 

The mechanical theory

It occurs between irregularly shaped particles.

The mechanical theory proposes that, under pressure the individual particles undergo Elastic / Plastic deformation and the particle boundaries that the edges of the particle intermesh forming a mechanical Bond.

Mechanical interlocking is not a major mechanism of bonding in pharmaceutical tableting.

Intermolecular force theory

The Molecules at the surface of solids have unsatisfied forces which interact with the other particle in true contact.

According to this theory, under compressional pressure the molecules at the points of true contact between new clean surfaces of the granules are close enough so that vanderwaals forces interact to consolidate the particles.

Material containing plenty OH group may also create hydrogen bond between molecules. E.g. microcrystalline cellulose is believed to undergo significant hydrogen bonding during tablet compression

The intermolecular forces theory and the liquid-surface film theory are believed to be the major bonding mechanisms in tablet compression

 

Liquid-surface film theory

Due to the applied pressure, the particles may melt (due to lowering of M.pt.)
or dissolve (due to increased solubility).

Many pharmaceutical formulations require a certain level of residual moisture to produce high quality tablets. The role of moisture in the tableting process is supported by the liquid-surface film theory. Thin liquid films form, which bond the particles together at the particle surface.

The energy of compression produces melting or liquefaction of the particles at the contact areas. As the pressure is withdrawn the melted ingredients solidifies causing fusing of the particles.

In addition the solubility of the solution at the particle interface under pressure is increased and as the pressure is released it gets super saturated and followed by subsequent solidification or crystallization thus resulting in the formation of bonded surfaces.

Deformation of the Solid Body

On further increases of the pressure, the non- bonded solid is consolidated towards a limiting density by plastic or elastic deformation.

Decompression

As the applied force is removed, a set of stresses within the tablet gets generated as a result of elastic recovery. The tablet must be mechanically strong enough to accommodate these stress, otherwise the tablet structure failure may occur.

If the degree and rate of elastic recovery are high, the tablet may cap or laminate. If the tablet undergoes brittle fracture during decompression, the compact may form failure planes as a result of fracturing of surfaces. Tablets that do not cap or laminate are able to relieve the stresses by plastic deformation.

*The tablet failure is affected by rate of decompression (machine speed).

Ejection

Finally as the lower punch rises and pushes the tablet upward, there is continued residual wall pressure and considerable energy May be expanded due to the die wall  Friction.

Strength of the Tablets

The tablet should be sufficiently strong to withstand the mechanical shocks during the subsequent handling and transport. The mechanical strength of tablet is described by the following parameters.

1. Crushing Strength
2. Friability
3. Hardness
4. Bonding Strength
5. Fracture resistance.

Crushing Strength

The most popular estimate of tablet strength has been crushing strength,  Sc,  which may be defined as "that compressional force (Fc) which, when applied diametrically to a tablet,  just fractures  it.”

It may then be described by the equation=

Where ST is the tensile strength, Fc is the compressional force and D & H are the diameter and thickness of the tablet, respectively.

Friability

The crushing strength test may not be the best measure of potential tablet behaviour during handling and packaging. The resistance to surface abrasion may be a more relevant parameter. For example by the friability test.

These test measure the weight loss on subjecting the tablets to a standardized agitation procedure. The most popular (commercially available) version is the Roche Friabilator, in which approximately 6 g (Wₒ)     of de-dusted tablets are subjected to 100 free falls of 6 inches in a rotating drum and are then reweighed (w).  

The friability, f, is given by:

Factors Affecting Strength of the Tablets

The following factors affect the strength of tablet

1. Particle size

2. Particle shape & surface roughness

3. Compaction pressure

4. Binders

5. Lubricants

6. Entrapped air

7. Moisture content

8. Porosity

Particle size

Smaller particles have larger surface area & when these are exposed to atmosphere may be prone to oxidation and moisture absorption takes place which affects the strength of tablet.

Extensive fragmentation during compaction of a brittle material may result in a large number of interparticulate contact points, which in turn provide a large number of possible bonding zones. The tablets made of these materials can have a high mechanical strength.

The increase in mechanical strength is attributed to an increase in the surface area available for interparticulate attractions, as the particles become smaller.

Particle shape & surface roughness

The mechanical strength of tablets of materials with a high fragmentation tendency are less affected by particle shape and surface texture. Particle shape affects the inter particulate friction & flow properties of the powder. Spherical particles are considered to be ideal.

General particle shapes and their effect on powder flow are as follows:

Spherical particles - Good

Oblong shaped particles - Poor

Cubical shaped particles - Poor

Irregular shaped particles - Medium

Compaction pressure

The compaction pressure and speed affects the strength of the resulting tablet.
A fragmenting material has been shown to be less affected by variations in compression speed. The behaviour of granules during compaction, the extent to which they bond together & the strength of the inter granule bonds relative to the strength of the granules determine tablet hardness.

Binders

A binder is a material that is added to a formulation in order to improve the mechanical strength of a tablet. In direct compression, it is generally considered that a binder should have a high compactibility to ensure the mechanical strength of the tablet mixture. Addition of a binder which increases elasticity can decrease tablet strength because of the breakage of bonds as the compaction pressure is released.

Lubricants

Lubricants are used to improve granule flow, minimize die wall friction & prevent adhesion of the granules to the punch faces. Lubricant decreases the strength of the tablets. When lubricants are added as dry powder to granules, they adhere & form a coat or a film around the host particles during the mixing process. The Lubricant film interferes with the bonding properties of host particle by acting as a physical barrier. When the tablet is blended lightly, the lubricant is present as a free fraction. Prolonged mixing time will produce a surface film of lubricants over the drug particles due to which inter particulate bonding is reduced.

Entrapped air

When the air does not freely escape from the granules in the die cavity, the force created by the expansion of the entrapped air may be sufficient to disrupt the bonds.

The presence of entrapped air will produce a tablet which can be broken easily & it lowers the tablet strength.

Moisture content

A small proportion of moisture content is desirable for the formation of a coherent tablet. At low moisture content there will be increase in die wall friction due to increased stress, hence the tablet hardness will be poor. At high moisture level the die wall friction is reduced owing to lubricating effect of moisture. At further increase in moisture content there will be decrease in compact strength due to reduction in inter particulate bond.

Optimum moisture content is in the range of 0.5 – 4%.

Porosity

When particles of large size are subjected to light compression the tablet will be highly porous–low tablet strength. Reduction in porosity is due to granule fragmentation giving smaller particles which may be more closely packed & plastic deformation which allows the granules to flow into the void spaces.

Parts of a Tablet Press

Tablet presses are designed with following basic components:

1. Hopper for holding and feeding granulation

2. Dies that define the size and shape of the tablet.

3. Punches for compressing the granulation within the dies.

4. Cam tracks for guiding the movement of the punches.

5. A feeding mechanism for moving granulation from hopper into the dies.

Auxiliary Equipment’s

1. Granulation Feeding Device: In many cases, speed of die table is such that the time of die under feed frame is too short to allow adequate or consistent gravity filling of die with granules, resulting in weight variation and content uniformity. These also seen with poorly flowing granules. To avoid these problems, mechanized feeder can employ to force granules into die cavity.

2. Tablet weight monitoring devices:- High rate of tablet output with modern press requires continuous tablet weight monitoring with electronic monitoring devices.

3. Tablet Deduster: In almost all cases, tablets coming out of a tablet machine bear excess powder on its surface and are run through the tablet de-duster to remove that excess powder.

4. Fette machine: Fette machine is device that chills the compression components to allow the compression of low melting point substance such as waxes and thereby making it possible to compress product with low meting points.

Instrumented tablet machines and tooling

The tablet press is a high-speed mechanical device. It compresses the ingredients into the required tablet shape with extreme precision. It can make the tablet in many shapes, although they are usually round or oval. Also, it can press the name of the manufacturer or the product into the top of the tablet.

Tablet punching machines work on the principle of compression.A tablet is formed by the combined pressing action of two punches and a die.

Punches & Dies

Tooling Station: - The upper punch, the lower punch and the die which accommodate one station in a tablet press.

Tooling Set: A complete set of punches and dies to accommodate all stations in a tablet press.Tooling

Tablet compression machines are made in keeping in view the type of dies and punches will be used on them, the dies and punches and their setup on compression machine is called tooling, it is classified as B and D mainly.

The B tooling dies and punch can be further have specifications as BB and D tooling can also be dies and punches can be utilised on B tooling machine which is called as DB

Mainly there are two standards, D and B.

Difference between B and D tooling

Different Shapes of Dies and Punches

1. Round shape punch die set
2. Oval shape punch die set
3. Capsule shape punch die set
4. Geometric shape punch die set
5. Irregular shape punch die set
6. Core rod tooling punch die set

Round shape Punch Die Set

Used by pharmaceutical and veterinary industry.                 Fig: Round Shape

Can manufacture following type of tablets:                         

• Shallow Concave Ball Shape
• Deep Concave Flat Faced
• Concave with Edges Flat with Bevel Edges
• Normal Concave

Oval Shape Punch Die Set           Fig: Oval Shape

Applicable to pharmaceutical and ayurvedic industries.

Can manufacture following types of tablets:

• Flat Faced Flat with bevel edges
• Concave/Deep/Deep Concave with bevel edges.

Capsule shape punch die set Fig: Capsule Shape

Applicable to pharmaceutical and ayurvedic industries.

Can manufacture following types of tablets:

• Concave with Edges
• Deep Concave Flat Faced
• Normal concave Flat with Bevel Edges.

Geometric Shape Punch Die Set
Applicable to pharmaceutical, confectionery, chemical, industrial powder metallurgy industries. Can manufacture following types of tablet:

• Triangular
• Benzene
• Rhombus
• Rectangular Square

Irregular Shape Punch Die Set

Are applicable to confectionery industries.  

Available with different size, concavity, and flat in plain or engraved break line.

Problems associated with large scale manufacturing of tablets.

In olden days tablets were initially punched on small scale with hand operated machines, which suffered the problem of varied strength and integrity.

But now the tablet punching machines are all mechanized, the mechanical feeding of feed from the hopper into the die, electronic monitoring of the press, but tablet process problem still persist.

The Imperfections known as: ‘VISUAL DEFECTS’ are either related to Imperfections in any one or more of the following factors.

1. Formulation design,
2. Tableting process,
3. Machine.

The defects related to Tableting Process.

• CAPPING: Partial or complete separation of the top or bottom of tablet due to air-entrapment in the granular material.
• LAMINATION: Separation of tablet into two or more layers due to air-entrapment in the granular material.
• CRACKING: Small, fine cracks observed on the upper and lower central surface of tablets, or very rarely on the sidewall.
• CHIPPING: Breaking of tablet edges.

The defects related to Formulation.

• STICKING: The adhesion of granulation material to the die wall.
• PICKING: The removal of material from the surface of tablet and its adherence to the face of punch.
• BINDING: Sticking of the tablet to the die and does not eject properly out of the die.

The defect related to Machine.

• DOUBLE IMPRESSION: Due to free rotation of the punches, having some engraving on their faces.

The defect related to more than one factor.

• MOTTLING: Unequal distribution of colour on a tablet with light or dark areas.

PDF:https://lookaside.fbsbx.com/file/merged_Compaction%20and%20Compression%20MANIK%20wm.pdf?token=AWzVjctgTz-9GyjgrvP0jsrfwr7xSKbplXJdsldGzeI4_RfZ1tRPW42z_Pxqmk5CHxvgA9lVB_CwuZo21KeL2q8NMZuIA9jWUiHE5Bqa0XgbChlZGQAZU3PtZKqa48RUdBsz7FsRXNQKTStyIX61dgXsmUHK3Am-q3vilv-IxKiZZwzQ5Cg2y1pylATz-ClV0qw

Slide Share:http://www.slideshare.net/ImranNurManik/compaction-and-compressionmanik