Different sizes of sieves are used in the pharmaceutical industry during tablet manufacturing for the sizing of dry granules. The sieve selection is a critical process because it has a direct impact on many properties of the tablets during compression.
In this article, we will discuss how to select a proper sieve size for sieving of granules to avoid issues during and after tablet compression.
We are well aware that sieves are available in different sizes, e.g 12,14,16,18,20,24,30 numbers, etc, or 0.68 mm,0.8 mm, 1.2 mm, 1.7 mm.
For your understanding, 12 number sieve granules are larger in size than 16 number sieve granules & in the same manner, 0.68 granules are finer than 1.2 mm sieve.
For conventional sieves, the number of holes per linear inch is known as the sieve number.
The sieve selection may impact the following
- Product Flow
- Die Filling
- Disintegration/Dissolution
Product Flow
The flow of granules is greatly affected by the sieve used for the sizing of granules.
If a sieve with a higher sieve number (24,30) is used, it will result in the production of finer particles.
The flow of product in the hoppers is not smooth due to a higher ratio of fines.
To ensure a good flow of granules in the hopper, we must always use a sieve with wider holes, e.g 1.2 mm or 1.7mm instead of 0.68 or 0.8 mm or 12 number,16 number should be used in place of 20 or 24 number.
Die Filling
Die filling with granules is affected in two ways,
- Poor Flow from the hopper
- Over-sized granules
Poor Flow from the hopper
If the flow of granules is not proper from the hopper, it will result in improper die filling.
If dies are not properly filled, it will result in weight variation during tablet compression.
As we discussed previously, to improve flow, we must use sieves with wider holes to reduce the concentration of fines.
Over-sized granules
Die filling is also affected by the oversized granules. If tablet weight is less, i.e 100 mg, and we are using wider sieves like 1.7 mm, then the die filling is not smooth, as large-size granules result in improper die filling, which also results in weight variation and tablet capping issues.
For small-sized tablets, we must use narrow sieves, i.e 1.2 or 0.68, to ensure smooth die filling.
Disintegration
Disintegration and dissolution are greatly affected by sieve size so we must be careful during sieve selection, especially in cases where disintegration and dissolution are compromised.
Suppose we have used the sieve number 1.7 (large granule size), the disintegration time of the tablets will be prolonged, i.e 16 minutes, and the dissolution profile will be low.
To overcome this issue, we can change the sieve size from 1.7 mm to 1.2 mm, which will resolve the disintegration and dissolution problems.
Conclusion
Sieve selection during tablet manufacturing must be done based on many factors and the most important are the product flow, tablet weight variation and disintegration/dissolution issues.
A wider sieve results in large-size granules, which may result in prolonged disintegration whereas narrow sieves result in the production of fine granules to overcome disintegration-related issues, but narrow sieves may result in flow-related issues so, before sieve selectio,n always consider the above-mentioned factors to ensure a good-quality product.
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