I am acting as a Lynn surrogate and probably stating the obvious. But, since I am not Lynn, may be I can be excused as the novice and allowed to overstate your enquiry.

You say you used a folded A4 paper first and then a conical glass funnel. The rate of dischage or velocity of dicharge is dependent on: a) the surface finish of the flow container- paper vs funnel, b)paper corners and funnel shapes and c) orifice size wrt the powder size distribution. Having not seen the actual details I assume the following:

1. if powder leaves the conical glass funnel at a higher velocity it consolidates at a higher stress level in the receiving container from the higher kinetic energy of impact.

2. paper has a rougher surface finish and inhibits flow at the boundary between paper and powder

3. powder may flow in folded corners at a slower velocity due increased powder granular consolidation stress and doulble contacts in the folded corner regions.

4. combination of the rough paper surface and resistance to flow in corners yields slower velocity at exit and lower compaction velocity on impact.

However, there is a caveat . If the conical glass funnel convergence angle or included angle in the cone is too large, a restriction can form from the convergence and result in slower flow out of the glass funnel. There is more to this, but, I have already overstated the obvious. Since your observation appears not to fit the caveat, the above description in items 1-4 is probably appropriate.

More detail is needed and we await Lynn's more insightful response.

Regards,

Lawrence Nordell

Conveyor Dynamics, Inc.

www.conveyor-dynamics.com ■

Thinking about your question for a second.

Try using funnels of different convergence angles that alters the exit velocity.

Try roughing the funnel surface by gluing powder to the funnel neck at its convergence point.

Use a square shaped funnel and round conical funnel to observe the differences in exit velocity.

Try to find a way to measure the exit velocity and correlate with number of taps.

May be you get a beginning of a useful scientific observation and you can share the findings.

Lawrence Nordell ■

Lawrence Nordell has made a number of interesting and useful comments to your query. Another feature that may be relevant is whether there are any electrostatic differences between sliding the material on a glass surface compared with paper. It is also not clear what ‘getting a fixed volume’ after 500 and 900 taps means. These are large numbers for tapping and as powders tend to settle asymptotically to a stable value it may be difficult to determine the precise point at which further densification ceases. Does it mean that with an identical of powder the same fixed volume was achieved, or that no further change in volume took place at different densities?

Many factors bear on the consolidating characteristics of a particulate material. Particles are rarely isotropic and have shapes that are difficult to classify. The method of deposition can certainly affect the way in which irregularly shaped particles nest together in a bed and temperature influences the settling rate of fine powders due to the varying viscosity of the air in the voids. One reason why the phenomenon is not well documented is that some rigour and detail is essential to secure meaningful results and these are not readily transferable to other bulk materials. There is scope for some focussed research at a simple level but care would be necessary with generalised conclusions. ■

Two further observations regarding this interesting inquiry:

1. For repeatable filling of the cylinder I recommend instead using a small batching feeder, either screw or vibratory. This will eliminate the flowability variables created from the paper funnel surface, angle or corners. Many of the feeder manufacturers have lab-sized equipment, and a unit with a loadcell will give very repeatable sample weight and delivery time results.

2. You appear to be waiting for the material to fully deaerate by tapping before obtaining your reading. I believe the rate at which the material settles is as important as the final deaerated density, especially useful to know when the product is being packaged. “Tapping” may only represent storage in a hopper with external vibration.

Regards, Delmar Schmidt

Melfi Technologies Houston

www.melfitechnologies.com ■