Tablet capping problem and solution

Tablet capping is a common problem which is observed during the compression of tablets in pharmaceutical industries.

Definition of capping

Removal of the crown edges of the tablet is called capping.

Removal of the edges of the tablet from the main part of the tablet is called capping.

Reasons for Capping

Tablet capping is a defect and there are many reasons for this defect. Capping may occur because of the following main reasons,

Formulation Related Problems
Machine Related Problems

Formulation Related Problems

The following formulation, related problems may cause tablet Capping,

Amount of Binder
Moisture Content
Grain Size
%age of Fine
Lubricant

1.Amount of Binder

Binder is added in a tablet formulation to hold the granules and give strength to granules. Binder gives strength to tablets and prevents breakage. Binder amount may also result in tablet capping.

Reason

Low level of binder addition during granulation.

Solution

Add more binder to the formulation.

2.Moisture content

The optimum level of moisture should be present in granules for strong bounding.

Low moisture content results in less binding of granules during compression and may result in capping.

Reason

Over drying of granules results in low moisture contents.

Solutions

Moisten the granules.

3.Granules size

If granules size is large and % age of granules is very high and fine is very low capping may occur.

Reason

Lower sieve number(#10,#12)used for sieving of granules after drying results in large size granules.
More granules and less fine is produced if the tray dryer is used.
If granules size is large and %age of the fine is very low then void spaces are not filled during die filling, granules are not compressed well and may result in capping.

Solution

Pass the granules through higher sieve number (#16,#20)to generate some fine.

4. %age of fine

Capping of the tablet may occur if the %age of the fine is high and granules percentage is low because fine powders are not compressed well.

Reason of fine

Over-drying in FBD may produce more fine.
Lower sieve number used for screening of granules after drying.
Less binder addition so granules are a week.

Solution

To generate less fine and more granules during granulation use a higher number of the sieve. (Depending on formulation)
Add more binder to strengthen the granules.

5.Lubricant

Capping is also related to the lubricant type and its concentration.

Reason

Higher concentrations of lubricant.
Over mixing after addition of lubricant.
Lubricants form a layer around granules to prevent sticking so if higher concentration of lubricant is added or mixing time after lubricant addition is increased it may cause slipping of granules under pressure and reduction of hardness and may cause capping.

Solution

Change the lubricant type or concentration.
Optimize the mixing time after lubrication, it should be 3- 5 minutes.

Machine Related Reasons For Tablet Capping

Apart from formulation related problems, tablet capping also occurs due to the compression machine and its tooling. Following are some machine and tooling related reasons which may cause capping.

Machine Speed
Air Entrapment
Upper punch penetration
Punches and Dies Condition

1.Machine Speed

During the compression of tablets, the speed of the compression machine may cause capping.

Reason

The highly plastic powders are not compressed properly on high speed due to short dwell time.
When compression machine speed is high, dwell time is short and it may result in capping because granules do not have sufficient time to relax after the formation of the tablet.

Solution

Slow down the compression machine speed.
By reducing the speed of compression machine, dwell time is increased so it provides more time to compress the tablets in die cavities and after compression proper time is given for relaxation and capping is prevented.
Increased dwell time by reducing the machine speed also allows air to escape easily and give more time to particles to bind with each other.

The time which is spent by the punch head under compression roller is called the dwell time.

2.Entrapment of air

Tablet Capping may occur due to entrapment of air in granules during compression.

Reason

When tablets are compressed, the air in granules in pushed out allowing particles to stick with each other. When air comes outward it also pulls fine powder or fine granules along it and these fine powders are compressed on the corner of tablet or tablet band so prevent strong bounding of granules at corners and result in tablet capping.
Entrapped air in granules reduces inter particular bonding.
Entrapped air reduces tensile strength which results in capping.

Solution

To remove entrapped air following techniques may be used,

Pre-compression
Tapered dies

Precompression

Precompression in modern machines is provided to eliminate the capping. Precompression is the process in which tablets are slightly compressed with low pressure before the main compression to form soft tablets.
Precompression compresses the fine powders and granules and prevents their migration to upward during the main compression and removes entrapped air to prevent capping.
Precompression should not be too high.

Tapered Dies

When upper punches enter into dies and compress the powders, the entrapped air comes outward.
In normal dies, there is no space for air to escape from dies, so it may break the tablet edges.
In tapered dies, space is available for air to escape from dies.
Entrapped air escapes through taper patterns on dies and it prevents capping.

3.Upper Punch Penetration

Punch Penetration is an indication that at which height tablet will be formed inside the die cavity.

The degree of upper punch penetration in the dies is used to prevent capping.

Reason

When puch penetration in the die cavity is increased than the tablet is formed at the lower side of the die and air escape is difficult because air has to travel more distance and it results in capping.

Solution

The tablet should be compressed in the upper portion of the die cavity by less punch penetration.
Due to less upper punch penetration in the die cavity air has to travel less distance to escape from die cavity and capping is prevented.

4.Punches & Dies condition

Condition of compression machine tooling like punches, dies and scrapers or swipe off blades may also cause tablet capping.

Reason

Not properly maintained punches may develop j hocks or have scratches on cup surface which result in capping.
J hock is like j shape.J hock is produced when the edge of the punch tip is curled inward in the shape of j.
Lower lunch length is small.
Dye wear rings also result in capping.
The wrong Position of swipe off blade may also break the tablet edges.
Deep Concave shape punches promote capping.

Solution

Properly maintain punches and dies.
Develop a procedure for routine.
inspection.
J hock can be checked by rubbing a finger on the inner side of the punch cup, if it feels irregular surface, repair it or replace the punch.
Punch length should be accurate because if the length of the lower punch is short it may not properly eject the tablet and can result in capping.
Swipe off blade should be adjusted properly
Change punch dimensions like deep concave to the standard concave or flat face.

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