Double Impression Tablet Defect

Various Tablet compression defects are observed during the compression process in the pharmaceutical industries.

Some of these tablet compression problems are related to the formulation & some are related to the compression machine or compression tooling like punches or dies.

In this article, we will discuss a common tablet compression defect known as Double Impression or double impressions.

What Is Double Impression Tablet Compression Defect?

As the name indicates, a double impression means repetition of the tablet punch impression on the surface of the core tablet.

In routine practice, when we compress tablets using tooling in which the surface of the punch has a bisect line or embossing of any digit, these are engraved or embossed on the tablet surface as it is on the punch surface.
But in the case of a Double Impression Defect, the surface of tablets gets one extra impression, the same as the original one but at different places.

Example

Suppose we are compressing a batch of tablets using tooling in which lower punches have a bisect line.

All the tablets have the same bisect line on the tablet surface but suddenly the operator noticed that an extra line is engraved on the tablet surface & this line looks as if it is crossing the original line.

This tablet compression defect is known as a double impression.

Reasons For Double Impression Defect?

Double Impression defect is directly related to the punches used for compression & is associated with the following

Punch Shape
Punch Position
Punch Embossing
Punch Locking

Punch Shape

One of the key factors for tablet double impression defects is the punch shape. The double impression defect is associated with the use of round shape punches.

Punch Position

Lower punches are mainly associated with the tablet double impression problem.
Upper punch free rotation may also result in double impression but in most practical observations it is caused by lower punches.

Punch Embossing

A double impression is caused by the presence of embossing of digits,monogram or bisect lines on the lower punches.

Punch Locking

Double impression problem is observed in those cases where lower punches have no keyways & are free to rotate under.

Principle Of Double Impression Defect?

Double impression defect is caused by the free rotation of lower punches.

The main mechanism involved in this defect is,

The original impression on the tablet surface is achieved during the compression process where the pressure is applied by the pressure rollers & tablet is formed.

After tablet compression, the lower punch is free to rotate for a short distance & if this punch is again compressed or comes under load during ejection, it gives an extra impression on the surface of the already compressed tablet so the double impression defect is observed over the tablet.

Solution Or How To Prevent Double Impression Problem?

Double impression effect can be prevented by following,

Use Of Keyed Punches
Use Of Anti-Turning Devices

Use Of Keyed Punches

Keyed punches are those punches that have sidelocks or keyways which prevent their free rotation.
The use of keyed lower punches is helpful to prevent double impression defect.
With the help of keyways, the lower punches are locked in the punch holders & their rotation is prevented so the extra impression is not achieved.

Use Of Anti-Turning Devices

Advanced compression machines have anti-turning systems which fix the punches in place & prevent their rotation.

With the use of anti-turning devices, the punches are free to move in an upward or downward direction but are not able to rotate.