Determining whether a powder will form a good tablet is a complex process because for a material to make a good tablet it requires a number of different attributes which in some ways conflict with each other.
For example it needs to be well lubricated but also to be easily penetrated by fluid in the gut. It also needs to be physically robust but also break down rapidly in the body.
The parameters we have identified which determine manufacturability are set out below.
The most important requirements are for it to be readily compressible, well lubricated, and to exhibit acceptable recovery (as excessive elastic recovery has been associated with manufacturing defects such as capping.
|What is it?||Why does it matter?|
|Tabletability||Tabletability is the relationship between tablet mechanical strength and the pressure used to make the tablet.||Tabletability is maximised to ensure that the porosity of the tablet is as high as possible for any given compaction pressure. This maximises liquid penetration into the tablet, and therefore tablet dissolution rate.
|Ejection stress||Ejection stress is the stress generated during the tablet ejection process. It is a measure of how well lubricated the tablet is; reducing ejection stress is the purpose of adding lubricant to the formulation.
|Poor lubrication results in picking and sticking problems during production. These are one of the most common causes of tablet defects which result in slow running of the tablet press and reduced output. High ejection stress (>5 MPa) is strongly correlated with manufacturing problems.|
|Detachment stress||A force is needed to detach a tablet from the punch tip during manufacture. Detachment and ejection stresses are related for some products, but unrelated for others.||If detachment stress is too high, defects such as picking and sticking are seen. These can be reduced by punch material selection. We are also concerned about formulations in which detachment stress is substantially higher than ejection stress as this shows unusual interaction between the material and the punch tip.
|Elastic recovery||The tablet thickness of some materials changes between the tablet being compressed and it being ejected. In some cases, contraction of the tablet is seen while in others, and more commonly, expansion occurs.
|Elastic recovery is widely held to be one of the causes of capping, a major tableting problem which requires slowing of the tablet press for resolution, sometimes with a massive impact on productivity. Checking for elastic recovery during development and manufacture is therefore important.|
|Risk of over-compaction||Solid fraction is a measure of the porosity remaining in the tablet. It is calculated from the weight and dimensions of the tablet, and the absolute density of the components.||The recommended maximum solid fraction for a tablet is 0.85 although many products exceed this. Products with very high solid fraction are prone to capping and dissolution problems. Over-compaction is therefore a serious tableting problem.
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