In order to ensure accurate measurement, the Chinese Pharmacopoeia stipulates the standard for tablet weight difference and its inspection method. That is, 20 tablets are precisely weighed to determine the total weight and the average tablet weight is obtained. Then, the weight of each tablet is weighed and compared with the average tablet weight. The weight difference limit for tablets below 0.3g and above is ± 7.5%, and for tablets above 0.3g, it is ± 5%.
tablet pressWhen compressing tablets, the dosage is integrated by volume, so the main reasons for the difference in tablet weight are the structural quality and particle factors of the tablet press.
（1） The influence of the structure and die quality of tablet press
(1) There are three types of feed methods for tablet presses, namely: ① After the down punch position is fixed in the appropriate position, the powder feeder moves to the mold hole and swings, and the particles flow into the mold hole and fill it up. The excess particles are scraped off by the powder feeder, and most single punch tablet presses use this feeding method; ② When filling, the particles are first stacked on top of the mold hole, and then washed down to the specified position. The particles fill the mold hole and scrape it flat; ③ When filling, first lower the punch to a slightly lower position than the specified height, fill the mold hole with particles, then raise the punch to the specified height and push the excess particles out of the mold hole and scrape it flat. The order of uniformity for the above three filling methods is ③>②>① method. The rotary tablet press adopts the third method. For materials with poor liquidity, mandatory feed devices should be added.
(2) The quality of the stamping die on the rotary tablet press is equipped with multiple sets of punches and dies, so the precision requirements for the punches and dies are high. For example, the length of the punch and the diameter of the die hole should be consistent, otherwise the difference in weight between the large pieces will increase.
（2） The influence of particle quality on sheet weight
(1) The flowability of particles should be good, otherwise it is difficult to fill the mold holes due to the short feeding time.
(2) The density of particles here refers to their bulk density. The bulk density is related to the particle density and particle morphology. When the prescription is the same, the process has an impact on the density. Long term kneading of soft materials can increase the particle density; When the particle size distribution is wide, small particles are prone to move to the lower part through the gaps between large particles due to vibration, causing changes in bulk density; When using a swing type particle machine for granulation, the density of the resulting particles varies due to differences in screen material, tightness, and other factors.