During compression of tablets various problems are encounterd which need to be studied and rectified in order to get a quality product which must be free from any visual or functional defects. 
CLASSIFICATION OF DIFFERENT TYPES OF DEFECTS

PROBLEMS OCCURRING DUE TO PROCESS

• CAPPING: – It occurs when the upper or lower part of the tablet separates horizontally (partially or complete) from the tablet body and comes off as a cap, during ejection of tablet from the compression machine.

Causes– over drying of granules, lesser amount of binder, improper tooling, air entrapment.

• LAMINATION: – ‘Lamination’ is the separation of a tablet into two or more distinct horizontal layers.

Causes– over speed of turret, improper setting of lower punches, over dried granules.
PROBLEMS DUE TO EXCIPIENTS

• CHIPPING: – ‘Chipping’ is defined as the breaking of tablet edges during the release of tablet from the press or during handling and coating operations.

Cause – punch tips worn, over drying of granules, lower amount of lubricants.

• CRACKING: – When small and fine cracks are observed on the upper and lower central surface of tablets, or sometimes on the sidewalls of tablets, it is referred to as Cracking.

Cause– over dried granules, large size of granules, tablets expansion.

• STICKING: – when the tablet material adhere to the die wall, it is referred as sticking.
  Filming is a slow form of sticking and occur due to excessive moisture in the granules.

Cause– excessive binder, lesser drying of granules.

• PICKING:- when a small amount of material from a tablet is sticking and  removed off from the tablet-surface by a punch face, it is called picking.
  The problem is more common on the upper punch faces than on the lower ones.

Cause- lesser drying of granules, lesser addition of lubricants, binder addition was high embossing letters on punch tips.

• MOTTLING: – it is the  unequal distribution of color on a tablet, with light or dark spots present in an uniform surface.

Cause- migration of die to the surface of granules during drying improper mixing of die colored drug used with colorless excipients.
PROBLEM DUE TO MACHINE

• DOUBLE IMPRESSION: – ‘Double Impression’ occur due to free rotation of punches, which have a monogram or other engraving on their face.

Cause – occur due to free rotation of punches.
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Reference links
http://www.pharmainfo.net/rajapicta1023/blog/problems-associated-tablet-manufacturing
http://pharma-study4u.blogspot.in/2013/10/tablet-manufacturing-problems-causes.html?m=1