The flowability of the filling material also refers to its smoothness. The types of filling materials often include powders and granules, and the flowability of the filling material will directly affect the control of capsule filling accuracy. The angle of repose, bulk density, particle size distribution, and moisture content of powders or granules filled with capsule filling machines directly affect their flowability.

Generally speaking, when the powder or granules filled by the filling machine are stacked, the angle of repose of the granules is smaller than that of the powder, so the flowability of the granules is good and the filling accuracy of the capsule filling machine is easy to control. And bulk density refers to the weight per unit volume of powder or granules, which is manifested as a larger weight per unit volume during the filling process, better flowability, and easy control of the filling accuracy of capsules; On the contrary, if the weight per unit volume is small, the fluidity will be poor, and it will be difficult to control the filling accuracy of the capsule. In the practice of capsule filling, when the bulk density of the filling drug is above 500mg/ml, it is easy to control the filling accuracy of the capsule, while when the bulk density of the filling drug is below 450mg/ml, it is difficult to control the filling accuracy of the capsule.

2、 Moisture absorption of filling materials

The moisture absorption of the filling material also affects the control of capsule filling accuracy. When the moisture content of the filled powder or granule is higher, the viscosity increases and the flowability is worse. If the viscosity is too high, the phenomenon of "powder blockage" will occur (powder blockage refers to the phenomenon that the powder column in the measuring hole of the measuring disk cannot be pressed out under a certain filling rod pressure). Generally, the moisture content of the filling drug is controlled to be less than 4% during the mixing or granulation process.

In addition, the moisture absorption of the filling material is related to its critical relative humidity. Generally speaking, when the purity of the filled powder or granule is not high, its critical relative humidity will decrease, and multi-component will also increase its hygroscopicity. Both of these factors will increase the viscosity of the filling process, leading to a larger filling volume and a higher risk of the aforementioned "powder blockage" phenomenon. Excessive moisture absorption can also cause drug deliquescence and failure. In this way, it is necessary to control the environmental humidity at its critical relative humidity during the filling process, and the relative humidity corresponding to different drugs also varies. For example, penicillin ≤ 47% is better, while aminoglycoside ≤ 58% is better. However, the capsule body also has certain requirements for humidity. If the humidity is too low, the capsule body is prone to rupture. Therefore, the humidity of the capsule filling environment is generally controlled at 40-60%.