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How to Evaluate Powder Flow

In the July/August 2018 issue of EPM magazine, we wrote that most challenges in the tableting process begin with, or can be traced back to, powder flow. We mentioned in our article. In our article, we mentioned that conducting studies on a formulation in the R&D stage, and later in scale-up, can help identify and solve potential powder flow issues before moving into commercial production.

The EPM article generated some good discussions with customers and readers. We’d like to discuss here two questions that warrant further discussion.

Q: “Can rheology studies conducted on an FT4 replace powder flow studies conducted on a Flodex?”

A: We would not recommend replacing the Flodex studies with an FT4. We use these two pieces of equipment in a sequence. At Natoli Scientific, we approach Flodex as an inexpensive and preliminary screening experiment. We recommend using the FT4 as a final analysis due to time and cost considerations. In sequence, we’ll test the intrinsic flowability of formulations on a Flodex. Once we find formulations that have intrinsic flow and are acceptable for a product specification, we further our knowledge of the formulations on an FT4.

An example is a round tablet with a 10 mm diameter. The specification on a Flodex would be a target of 10 mm orifice diameter. If the intrinsic flow is equivalent to the diameter of the die (the smallest cross-section in which our powder must flow or the constraint of our process), we have a good probability of experiencing good powder flow flow during tableting.

Once we have candidate formulations, we conduct further tests using an FT4 to understand the potential for segregation, agglomeration, and properties such as:

  • electrostatic nature
  • caking
  • angle of wall friction (for example, which steel type to use for a hopper)
  • and powder flowability while it is being forced to flow (e.g. feeder screws and forced feed frames).

These can help provide the best chances for success during pilot, scale-up, and eventually, full-scale production.

If you are interested in an alternative for an FT4, we suggest a PFT powder flow tester from Brookfield Ametek. This is budget conscious alternative to an FT4, and thus it will not have as much functionality.

Q: “Is measuring angle of repose, Carr index, and Hausner ratio enough to evaluate flowability?”

A: In our experience, angle of repose, Carr index, and Hausner ratio are the most basic measurements and do not give you enough data. We would not use only these measurements. Therefore, we highly recommend using a Flodex in combination with this method. Visual observations from a Flodex will always give you a better chance for success. If you don’t have a powder flow tester, FT4 or otherwise, pay attention to the visual observations on a Flodex. Here are some questions you should be able to answer through your observations:

  • What does the angle of the powder bed left in the cup of the Flodex look like?
  • Does the powder left in the cup have an angle?
  • Did a cylinder shape form as the powder flowed through the orifice?

In conclusion, when measuring powder flow, keep in mind as a general rule that the greater the angle, the less adhesive the powder.

Still have questions?

If you have any further questions about powder flow, Please contact us or visit our website.

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