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Causes and Remedies for Tablet Weight Variation

Causes for Weight Variations during Compression
  • 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.
  • 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 afterward. In this case, 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.

  • 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.
How to Avoid Tablet Weight Variation?
  • The issue that manufacturing a batch of tablets without an occasional defect would be unusual. Tablet weight variation is one of the most common problems we met that usually creates tablet defects. Consistent tablet weight is so essential to make a good tablet. If not carefully handled, solving other tablet defects will be difficult.

  • A Compression machine could not weigh powders, it fills volumetrically. In other words, the volume created by the filling position that will determine the final tablet weight. If the power density has varied then the final tablet weight will change though the volume is the same. 
  • Controlling tablet weight within a tight range will contribute to better tablet hardness and friability. How to avoid tablet weight variation and how to eliminate it has become our priority concerning problem.

1. Machine parts are aging and wearing
  • Lifting poker moves abnormally that the machine could not measure tablet weight precisely, which varies the tablet weight. Check whether the mini turbine and filling guide have been wore and, timely change the wearing part with a new one.

2. Inappropriate installment of feeder
  • Inappropriate installment of feeder will possibly bring about the problem that the powder filled in the feeder is not sufficient and continuously stable. The best way is to increase powder in the feeder and control filling powder within an appropriate limit. Adjust the screws beside the feeder in order to fix feeder in the right position. Further more, adjust the rotating speed and feeding speed to the same frequency.

3. Mismatching between upper and lower punch
  • Due to mismatching between upper and lower punch, the filling powder in the middle die varies from time to time. It is difficult to control the powder weight at an exact level. Timely clean and maintain the punch and die so as to extend the punch die life. Change a new set of punches and die if necessary.

4. Defect Punch Tooling
  • One set of punch and die tooling includes upper punch, lower punch and middle die. Any part of the punch and die tooling will definitely cause tablet defects. Capping, piking and sticking, chipping, lamination and double pressing are some of the tablet pressing problems we usually encountered. Please clean the punch die to remove some sticking powders as well as apply some lubricant before storing. As for some rupture and wearing tooling, it is better to change it.

5. Filling Powder
  • The filling powder is the most direct factor to influence the tablet weight. If the powder is too hard, moist, too rough, or too fine, the tablet weight will also vary. The best way is to regrind the powder in order to make it with uniform size, not too rough and not too fine. If possible, improving the way of manufacturing.
  • However, the importance of weight control cannot be over-emphasized. Weight must be uniform in order to troubleshoot most other tablet defects. Use the machine in a correct way, regular check and change unqualified machine parts, and reasonably maintain, you will manufacture qualified tablets.

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