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Is your tablet capping?

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
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46 Responses to “Is your tablet capping?”

  1. RYErnest
    November 30th, 2008

    Nice post u have here 😀 Added to my RSS reader

  2. Olechka-persik
    December 10th, 2008

    Thanks for post. Nice to see such good ideas.

  3. Zakir Hossain
    February 2nd, 2009

    Is there any process to get the information like article or discussion through my mail

  4. Mike
    February 2nd, 2009

    The article on Optimal Tablet Press Operation within our webite under Tech Articles should be helpful. You can also listen to a webinar in our archives on Capping go to recorded sessions and click on Capping. Let me know if this helps…we can always setup a webex meeting with your operators and techs to dicuss.

  5. Tony
    September 2nd, 2009

    Hey Mike,

    We have a product with a good dust control during compression. It is outsourced for packaging. What would be the recommended filling process (Cremer, slats,..) to minimize generation of dust during packaging and prevent dusting the sensors or camera ? Some dust has been observed from the bottom portion of drums, most probably generated by transportation and handling from the manufacturing site to the packaging site.


  6. magnesium
    October 29th, 2009

    This is really very good post and its very informative.You have given nice suggestions.Thank you very much for giving such a good information.You have done a good job.

  7. Pilule
    December 4th, 2009

    Dear Milke,

    Would you have a possible root cause for a product compressed on a 35-stn stokes (double-sided) on which we have a lot of variability in CU depending on the side – around 93% on the left and around 103% on the right. Weight variation is typically 105-113mg on the left -107-113mg on the right (limits 105-116mg) and the dosage is 1mg API. Any ideas are welcome. We are thinking of some wear on the tablet press.


  8. Mike
    December 18th, 2009

    Dear Pilule,

    If the segregation is really happening at the press the likely culprit is the difference in feed frame clearance settings. What is probably happening is that one side is segregating granules because it is slightly lower than the other side. This difference is not allowing larger granules to flow with all of the smaller granules; the larger ones are being segregated because they cannot sweep under the feed frame. A typical feed frame clearance between the die table and the feed frame base is .003″ – .005″ (as specified in the Stokes manual) which is the same as 200 mesh – 120 mesh. See if you can relate this segregation to your particle profile by doing a sieve analysis. A slight change in the feeder clearance setting up or down could net a completely different result. To make it more difficult the Stokes 35 station machines are some what temperamental when setting up the feed frames…it is flat out difficult if not impossible to get both sides the same…you don’t have to do this. What you need to do is to make certain that the side with the lower feed frame is increased slightly to allow the granules to not segregate.

    Remember no machine is perfect, even when they are new. A typical Stokes machine with several years of service can have a die table run out of .002″- .004″. This means that as the die table rotates there is a natural high spot. When setting up and checking the clearance of the feed frames the first step is to rotate the die table under the feed frame to the highest spot. To determine the high spot use a dial indicator and measure the die table run out. Then set the feed frame up at this location, then rotate the die table 180 degrees and set up the other feed frame on the high spot of the die table run out.

  9. Pilule
    December 23rd, 2009

    Thanks Mike for the helpful tips. For the feed frame clearance did you rather mean .03″-.05″ ?

  10. Mike
    December 23rd, 2009

    I’m glad you find our information helpful. The clearance between the bottom of the feed frame and the top of the die table is about the thinkess of a piece of notebook paper, which is equal to a 200 mesh particle size. The idea is to get the feed frame as close to the die table as possible without it touching, but high enough so it does not wedge or compact powder onto the die table….the .oo3″ – .005″ range is correct.

  11. Pilule
    December 24th, 2009

    Mike, maybe one more question. Some people mentioned to me the lower punch spring retainer as a possible cause, especially on these old BB2 presses but I can’t relate it to segregation but rather to a wider weight variation. Long life to your blog.

  12. Mike
    December 26th, 2009

    First off I had made an error in my first blog response to you regarding the feed frame clearance as it relates to the particle size distribution. The proper gap between the feed frame and die table is .003″-.005″ which is equal to particales in the range of 200 mesh – 120 mesh.

    My reasoning for potential segregation due to varation in feed frame clearance has a relatioship to particle size. If the feed frame clearance prevents larger particals from flowing with all the other particles it will cause segragation. I see this all the time. The quick fix is to change the clearance by decreasing or increasing the clearance.

    Now to answer your question regarding lower punch retainers; A retainer is meant to restrain the punch, preventing it from bouncing which should result in improved weight control. A loose or worn restainer does not relate to segregation but it does relate to weight variaion as you have pointed out.

    Segreation on the tablet press relates to flow, density variation, particle size distribution, cohesiveness, and feed frame setup.

    I hope have cleared this up, I did go back and correct the previous staements to make certain everyhting is now how I see it.

  13. Pilule
    January 5th, 2010

    Thanks again Mike and Happy New Year 2010.
    I really appreciate this interactive technical forum, the first one on the Web, as far as I know.
    This is making Techceuticals a pioneer !
    Best regards

  14. Mike
    January 5th, 2010

    We get calls everyday for help with a formula, equipment, tooling and a host of challanges people run into. For the most part if one person is experiencing it is most likely someone else is having a similar issue. At least this way we can all share in the solution. I’m happy to know this is helpful…thanks for the kudos!

  15. Denis Delaney
    February 10th, 2010

    Nice to see these helpfull suggestions. They are always what I try when having issues! However, this time I have tried all these with a trial batch of tablets but capping still occurs which is quite a pain. Any1 any suggestions

  16. Mike Tousey
    February 20th, 2010

    Tell me more, what is the shape of the tablet, machine type, punch penetration depth, speed. Is the product granulated for direct compression? Is the tooling new or old, what kind of conditon is it in. Have you inspected the dies? Doe the product cap all all speeds? Does pre-compression help or hurt the issue. Please send me photos of everything; tooling, press, powders, and capped tablets.

  17. Packaging Equipment
    July 8th, 2010

    Capping is the term used, when the upper or lower segment of the tablet separates horizontally, either partially or completely from the main body of a tablet and comes off as a cap, during ejection from the tablet press, or during subsequent handling.

  18. Mike
    July 8th, 2010

    Capping is when the upper cap seperates from the rest of the tablet. If the lower cap seperates it is not capping, it is splitting or lamination. This point is very important because to the actions to correct capping are not the same actions that would be taken for lamination or splitting. When a tablet caps it is directly related to how the air moves fine particles to the line between the cap and the rest of the tablet. This line of fine particles that were moved along the cap edge creates a weak, non-compressible collection of fines which causes the top of the tablet to cap.

  19. B Doddamani
    July 14th, 2010

    We are observing inconsitant batch problems of Sticking and Capping, means in a compagin of 15 batches we observe 3-4 batches of sticking and capping how to avoid even though we are following wet granulation method for hygroscopic active.

  20. Mike
    July 14th, 2010

    Powders that stick are generally over wet and powders that cap are dry, non-granulated, and fine. Focus to make certain the solution delivery is uniform. Do not rely on the mixing step to evenly distribute unevenly wetted powders. Why is it happening on 3-4 batches of 15? There is some inconsistency and usually it is related to the solution addition, batch volume variations, solution prepartation, or different operators.

  21. B Doddamani
    July 26th, 2010

    Dear Mr Mike,

    we are studied the Particle size and moisture content of those batches compared with validation batch, the results of those batches were good in case of particle size and moisture content, and one more thing the operators were expirenced one so please suggest me the possible root cause other than particle size and moisture content leading for capping. please mail me if possible.

  22. Mike
    July 26th, 2010

    I have seen many formulas that have passed moisture content testing but continue to cap. I do not believe that current instrumentation can adequately measure moisture levels in particles for compression. So telling me it matches a validation batch confirms once again I can have good testing information but bad results, what does that tell you?
    Capping is a compressibility issue; the particles along the top cap are not locking together. Most of the time the dryer lighter particles are aligned along the cap by the evacuating air during compression. So saying your particles are not a problem does not begin to address the issue. We are looking for a small percentage of fine particles that move during air evacuation.
    Slow the machine down; if capping decreases with longer dwell time and slower air release than you do have a particle /moisture issue. If you slow the machine down and capping gets worse than you have a compressibility issue. Meaning the locking mechanism between particles is broken by too much force. The problem becomes that reducing compression may stop the capping but then the tablet hardness is usually not acceptable. This comes back to fine tuning the press, and if that doesn’t work you must look back up stream at the poor quality of granulation or the components within the formulation itself.
    This problem can be solved, and I’ll bet that it is related to moisture and particle size….provided that you have tablet press operators that know how to run a press and make adjustments for this type of problem. Tell me what press you are using, the tooling design, the press setup parameters, the formulation, granulation process, and send me some photos of the capped tablet.

    The bottom line is that there are many Band-Aids to fix the problem at the tablet press, but the problem is within the formula and the granulating process.

    I cannot begin to tell you how many formulations are poorly granulated or poorly processed and blamed on the press operation. I get calls all the time with this very same issue “It worked fine in validation and now it does not work the same”. If material tests are the same and the nothing else changed than it tells us the tests are not worthy.

  23. sudhakar
    January 3rd, 2011

    I have severe capping problem and having particle size distribution data indicates around 70 % of granules are 120 mesh passsed
    Iam using concave capsules shaped punches and iam getting maximum hardness 11Kp beyond that capping starts, at 11 kp also friability is 0.4 and below 9Kp fraibility fails
    so please advice me to avoid capping

  24. Mike T
    January 3rd, 2011

    You say the tablet is good until 11kp, more pressure and the tablet breaks apart and looks like capping. You have reached the point of maximum compressibility, any more pressure and the tablet breaks apart. Further evidence is the friability issue. Though the particle size is good the moisture content and cohesiveness of the powders is weak and limited. Slow the machine down and the hardness will go up only slightly before you have a failure again.

    In the old days this is when they would add some steric acid, which would increase hardness and reduce friability.

    So tell me how close are you to success? What is the condition of your tooling? What kind of accuracy are you having with tablet weights? Are the dies tapered, do youhave pre-compression. What percentage of the tablet are capping? I’d like to see a photo of the defect.

    Thnaks again for your contribution to our blog!

  25. Navneet
    February 25th, 2011

    Thank you for very nice and useful information…but if
    What if punch is deep concave,shallow concave or standard concave? Which should prefferable and why?

  26. Mike T
    March 9th, 2011

    Hi Navneet, Capping is the inability for the top cap of the tablet to compress to the rest of the tablet. The deeper the concavity the greater the issue becomes with removing air and forming a good bond without moving the fine particle along the top cap which causes capping. The shallower the better when it comes to capping. The deeper the cup the more likely capping will occur.

  27. S Narayana Reddy
    April 11th, 2011

    Good information to find out the rout cause for this problem, and also required more information.

  28. shahebaz
    August 15th, 2011

    i have problem of capping for my last 4 batch

  29. PONTY
    September 30th, 2011

    I found lots of infrmations on ur blog…i want to know that we have gliclazide tablet formulation . and at every batch we have got capping probs and hv to mix moisture each time…can u suggest

  30. Dhananjay Pai
    November 1st, 2011


    Its quite well established that capping is not due to entrapped air. Entrapped air is a old theory discarded already. Capping happens due to residual wall stresses. It can be reduced by increasing press dwell time.

  31. Shoaib
    November 8th, 2011

    Hi Mike,

    Our 3rd validation batch failed at compression stage due to capping.

    The first 2 validation batches were perfect. And even the first 80 kg of the 350 kg 3rd validation batch was ok. But after 80 kg, the 3rd batch has capping problem such that its about 1 or 2 tabs per 20 tabs. We have changed all tools even replaced with virgin tools (never used before) but the problem exists.

    Any comments for root cause would be appreciated.


  32. yogesh
    December 21st, 2011

    dear mike.

    article is really nice to read. but i have an issue regarding capping. see when i start compression initial capping observed. i slow down the compression machine, increase the thickness, LOD of the lubricated blend was 2.7%, fines are just 25% in blend. Now suggest me what else we can do to remove capping. one more thing do tell me what is “Add a taper into the die”.
    yogesh. bansal

  33. Mike
    January 9th, 2012

    Capping issues?? The process of making a tablet is to first attain uniform weight (volume of fill), then compress the powder into a tablet, and then eject the tablet. My question is…do you really have capping or do you have a form of de-lamination. Are the particles locking together to begin with? Capping is a collection of fines along the air escape vent created between the upper punch tip and the die wall. During compression air is released at this point and as the air leaves it often pulls the fines light dust particles with it…forming a line of non-compressible particles which results in capping. If you slow the press down and you can never eliminate capping even at very slow speeds you have a compressibility issue: if all other things are equal. If punch tips have a “j-hook”, the dies have a “die wear ring”, or the blend is not uniform, then these issues must be addressed first. Some products need a bigger air vent: this can be gotten by adding a taper into the die. A taper is commonly added to the top of the die bore to allow more air to escape during compression and it reduces the amount of force needed to eject the tablet…another possible reason you are seeing capping.

  34. sunny
    January 11th, 2012

    i am sunny .i am chemist in pharma compney. but i face more problems of capping in compression.more capping in diclo & serra granules.plz give me solution ,how to solve this problems.

  35. Prasad Kanitkar
    February 29th, 2012

    Dear Mike,
    I am an ardent reader of your articles on art of tabletting. They are really informative, and provide practical advice on trouble shooting different aspects of tabletting.
    I have a doubt about ‘land on tablet punch.
    Will you please elaborate role of ‘land’ on the punch?
    Does it help in reducing friability of tablets as compared to the tablets made with punches that do not have ‘land’?
    Warm regards,

  36. Mike
    February 29th, 2012

    Hello Prasad, Thank you very much for your kind words! The “land” on a punch adds strength, which in turn allows for higher compression forces and less likely hood of the cup edge curling inward and making the tablet look like there is a capping issue. Allow me to explain: when a cup is formed the punch tip cup edge is the weakest area of the punch. If we then grind the edge down and polish it, it becomes a “land” or a “blended land”. A land is a flat spot along the cup edge. The bigger the land the stronger the punch tip becomes. The 2nd half of your question is regarding friability. A blended land allows air to escape during compression, thus improving friability because air is not trapped between particles, when air is removed from between particles then the particles can lock together reducing the likely hood of friability issues. However, friability may also be a formulation issue and changing tool designs, adding a larger land, may not correct the problem. I hope this helps…stay in touch….Mike T

  37. thahera
    April 16th, 2012

    hey Mike,
    im thahera, im facing problem of capping for mefenamic acid, will u please suggest me how can i will try to avoid it.

  38. Prasad Kanitkar
    June 26th, 2012

    Hello Mike,
    I would like to have your view on validation of lubrication stage in tablets.
    More often than not, samples from lubricated blend are analyzed for content of ‘Acitve’.
    But then, I guess when uniform distribution of active is achieved at dry mixing stage in the granulator (it could be high or low shear, or mass mixer). Once that uniform dry mass is subjected to wet granulation, uniform distribution of acitve is only reassured and consolidated due to formation of granules.
    Hence, I do not see any value addition happening by testing content of ‘active’ during lubrication stage.
    What really matters, I feel, is that the granules after mixing with lubricants should be tested for their effective ‘run’ on compression machine, producing tablets that meet the specifications of hardness, friability, weight variation, and flow of granules from hopper. And these tests must be conducted at various time points of blending with lubricants. It will be only then, that we can ‘Validate’ lubrication as a unit operation in tabletting.
    may I know your views please !

    Prasad Kanitkar

  39. aidy
    May 2nd, 2013

    we run comprima 300,s have done for year,s now we have problems capping on are last 4 batches nothing has changed with m/c settings or blends i have been told

  40. Mike
    May 3rd, 2013

    Hello Aidy, Those words “nothing has changed” are the hardest part of the challenge. There are many reasons that a tablet will cap. If nothing has changed with the blend or machine settings then it is possible that the dies have wear rings in them. I would first slow the machine down until capping goes completely away or improves significantly…only as a test. If the capping goes away it is probably not the dies. Now we are back to something has changed! The next thing I would do is to reduce punch penetration. If you are already higher than 2mm then I doubt that any further reduction in this setting will work without product loss. Next I would look at adjusting pre-compression force. Often times too much pre-compression force or not enough force can be the cause. I prefer to reduce pre-compression force as a first step, and then I would go the other way. There is a relationship between rpm and pre-compression force. If the problem still continues I’m now more inclined to think I have a tooling issue or too many “fines” (dusty particles). If it was a “fines” issue then you should have seen improvement with longer dwell time when you slowed the machine down. However if the blends is considerable dryer and dustier than other batches this could be the reason. Finally, I’m always concerned with segregation. When you use the words multiple batches this tells me that you potentially have an older blend. All products have an optimum blend hold time. When a blend is fresh it usually flows really good and when it gets old (usually after 4-6 days) it has settle and de-aerated and the air moves the fines to the top as the air releases from the blend. This change effects the lubrication which in turn creates capping. Run a few tests and let me know you findings.

  41. Shruts
    December 20th, 2013

    Thanking for your Blog ! It is very informative. I am facing capping for a Direct compression of water soluble drug (67% of the formulation) using magnesium sterate as lubricant. I add MgS towards the end of blending for 5 min. Can insufficient blending with powder blend and lubricant be one of the reasons for the capping?

  42. Mike
    January 30th, 2015

    1. MgS should always be added very last, by itself!
    2. Mgs can be over-blended or under-blended.
    3. MgS will lower hardness.
    4. Variation in uniformity can resulting in capping

  43. Mike
    February 1st, 2014

    Capping of one tablet when performing friability test would be one tip on a punch correct?

  44. Mike
    January 30th, 2015

    Maybe! Not always! Capping occurs for many reasons. When a tablet caps in the friability tester are you sure it’s the top of the tablet or the bottom. Capping only occurs on the top. It is Delamination when it occurs on the bottom cap. I will assume you know it is the top because of an embossing or some other indication. In which case I go back to looking for all root causes. I’d refer to my articles on capping…there is a lot to discuss and consider before we blame a punch tip.

  45. sumit
    February 23rd, 2015

    I want to know capping test not to capping so if you know then please comment

  46. Mike
    June 27th, 2015

    I’m not sure I understand your question. If you are asking how do you know if your tablet is capping? There are a few methods used; the most common indicator of capping is to see how the tablet breaks a during the hardness test. Years ago and still to this day I place a few tablets in a bottle and shake it hard, tablets that might cap often appear after a good hard shake.

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