Last week we had some really good participation for our Tablet Capping webinar…thanks to all that attended. This is one of those defects that can be really frustrating because once you feel you have a product figured out, and have it fine tuned, suddenly capping occurs. Getting the air out from between particles without moving the “fines” to the “air evacuation point is the key to success”. During compaction as the upper punch tip enters the die, air is being pushed out of the die. The design of the upper punch tip being smaller than the lower creates an avenue for air to escape. I call the point between the upper punch cup edge and the die wall the point of “air evacuation”. As the air moves and evacuates it takes with it many of the dusty, fine, dry, light, non-compressible particles. These particles can form a line along the point of air evacuation between the top of the tablet (aka the “Cap”) and the “Band” of the tablet, forming a line of non-compressible particles, which results in the “cap” not adhering to the rest of the tablet. The use of pre-compression is to lightly tamp or lock the small particles in place so they cannot migrate during final compression. Upper punch penetration is another tool most modern machines have; adjust it as high as possible and the air will have a shorter distance to travel allowing for more complete air evacuation. Machine speed can also play a roll to prevent capping because the time to evacuate the air is related to the clearance between the upper punch tip and the die wall and the depth of punch penetration…basically slowing the press gives the air more time to leave without pushing the fines to the air evacuation line which will result in capping. If you want to fix capping…1.Optimize weight control, 2. Reduce Punch Penetration (2-3mm), 3. Use pre-compression to lightly tamp, 4. Slow the press down until capping stops, 5. Add a taper into the die, 6. Change the radius of the upper punch tip to a compound radii, 7. Use a machine with 2 pre-compression stations (like a Manesty Nova), 8. Reduce the percentage of fines within the formula. Never change more than one item at a time and test to see the result. Call or write to me if you need me to clarify any of this information.
Posted by Mike on November 10th, 2008 :: Filed under On line seminars,Tableting
Tags :: band, cap, capping, fines, tablet
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