GMP News. H- e, m, ?$ d ]2 \, s& k
04/03/2015
Causes for Weight Variations during Tableting
压片时重量差异的原因
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.
片剂重量差异是一个压片工艺中普遍的问题。每个压片工艺的目的都是制出重量恒定的片剂。这个问题的实际原因是缺乏称重系统，不能很快地称出每个片子的所需重量。在压片过程中，每个片子所用的粉料被放入模具内，然后由上下冲片挤压成片。这就是说，实际放入的量是按体积来计算的，而质量要求的却是重量。因此，在一定限度内存在重量差异是很正常的，因为粉料的比重会有差异，并且在模具填充时可能会不完全。
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.
药典指定了重量差异的可接受程度。如果重量差异太高，则每片中的活性成分可能太高或太低，这样片子就不再符合质量标准了。因此，必须要让粉料填充又快又恒定。为此，粉料具有非常好的流动特性是非常重要的。这就是为什么压片前通常会进行制粒，这是为了使得填充的粉料比起物理混合物具有更好的流动特性。粉料中各不同成分的粒径分布不能太宽，因为在将粉料填充至容器的过程中可能会由于震动而引起不均匀。否则，在填充过程中较大的颗粒会因为其更容易滑下去，而较小的颗粒则停滞在后面。这种情况下，可能需要给进料仓配制机械式偶合推动。还有一个原则是最大的颗粒不应超过模具的直径的20%。上游的制粒不应生产出很大的颗粒，必要时，应在其后进一步磨粉和筛分。
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.
有时这种情形也可以通过强制填充来改善。通常下位模在模具到达填充单元区域前已放置到位，这就是说，粉料进入模具仅是靠重力。这时，如果采用强制填充，则下位模被拉至低于填充单元的目标位置，这时粉料填入模具，这样即使粉料流动性不够理想，也可以采用高冲压速度来压片。
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