Causes for Weight Variations during Tableting
壓片時重量差異的原因
Weight variations of the tablets produced is a relatively common problem of the tableting process. But each tableting process aims at producing tablets with a constant weight. The actual cause for this problem is the lack of weighing systems that are sufficiently fast to weigh or dose the required weight for each single tablet. Since this is not possible, each tablet press doses a certain amount of powder into the die and this powder is then pressed into tablets by the upper and lower punch. This means that a volume is dosed, but the quality requirement is the weight. Hence, weight variations in a limited extent are quite normal due to variations in the density of the powder material and to a partially incomplete filling of the dies.
片劑重量差異是一個壓片工藝中普遍的問題。每個壓片工藝的目的都是制出重量恆定的片劑。這個問題的實際原因是缺乏稱重系統，不能很快地稱出每個片子的所需重量。在壓片過程中，每個片子所用的粉料被放入模具內，然後由上下衝片擠壓成片。這就是說，實際放入的量是按體積來計算的，而質量要求的卻是重量。因此，在一定限度內存在重量差異是很正常的，因為粉料的比重會有差異，並且在模具填充時可能會不完全。
The pharmacopoeias specify the acceptable level of weight variations. If the weight variations are too high the level of active ingredient in each tablet might be too high or too low and then the tablets don't comply with the specifications any more. It must therefore be aimed at tableting a powder that can be dosed quickly and consistently in the dies. For this reason very good flow characteristics are extremely important. That is why tableting often is preceded by a granulating process in order to give the feed material better flow characteristics than the physical mixture has. Is the particle size distribution of the powder rather broad a separation of the components of the powder must be prevented that might occur for example because of vibrations in the feed container. Hence it has to be prevented that the bigger particles are dosed first because they slipped downwards and the smaller particles afterwards. In this case a mechanical decoupling of the press from the feed container might be helpful. A further rule of thumb is: the biggest particles should not exceed 20 % of the diameter of the die. The upstream granulation should produce no bigger particles; if necessary a further grinding/sieving process must be carried out.
藥典指定了重量差異的可接受程度。如果重量差異太高，則每片中的活性成分可能太高或太低，這樣片子就不再符合質量標準了。因此，必須要讓粉料填充又快又恆定。為此，粉料具有非常好的流動特性是非常重要的。這就是為什麼壓片前通常會進行制粒，這是為了使得填充的粉料比起物理混合物具有更好的流動特性。粉料中各不同成分的粒徑分布不能太寬，因為在將粉料填充至容器的過程中可能會由于震動而引起不均勻。否則，在填充過程中較大的顆粒會因為其更容易滑下去，而較小的顆粒則停滯在後面。這種情況下，可能需要給進料倉配製機械式偶合推動。還有一個原則是最大的顆粒不應超過模具的直徑的20%。上遊的制粒不應生產出很大的顆粒，必要時，應在其後進一步磨粉和篩分。
But sometimes the situation can also be improved by means of a forced filling. Usually the lower punch already is pulled down before the die reaches the area of the filling unit. This means that the material falls into the die only due to gravitation. In the case of a forced filling however, the lower punch is flush with the die table. The lower punch is pulled down into its target position only below the filling unit. The material is sucked into the die. This often makes it possible to work with a higher pressing speed even if the material doesn't flow optimally.
有時這種情形也可以通過強制填充來改善。通常下位模在模具到達填充單元區域前已放置到位，這就是說，粉料進入模具僅是靠重力。這時，如果採用強制填充，則下位模被拉至低於填充單元的目標位置，這時粉料填入模具，這樣即使粉料流動性不夠理想，也可以採用高衝壓速度來壓片。
本文來源：洛施德諮詢Julia/Candy。