Capping: What's your action plan when capping occurs?
Let’s start with the things we must know to evaluate the root cause(s) of capping.
What is capping? When the top (cap) of the tablet splits or fractures from the body of the tablet. Capping is caused by non-compressible fines particles that migrate when the air is pushed out during compression. The fines collect at the junction of the upper cup edge and the tablet band.
- Tooling root causes:
- Upper punch cup edges curl inward…called J-hooking.
- Is the cup polished to a nice finish?
- Die wear rings…also known as compression rings.
- Punch working length must be accurate, not more than .002” variation within the set.
- A tapered die will help the air escape and improve ejection.
- Make certain all die are installed in the same direction
- Are the head flats uniform within the set? A large head flat provides more dwell and a longer compression cycle than that of a head flat that is small.
- Press setup:
- Punch penetration should be set as high as possible without losing powders from the die. Typically 2.5mm-3mm punch entrance works best.
- Overload: make certain the overload settings are set to the maximum force of the tooling.
- Pre-compression should be set very lightly at first
- Press speed should be within pre-established ranges.
- Weight control:
- Weight variation nets hardness variation. Low weights mean lower compression forces which mean the tablet will be softer. As weights increase so does the hardness. When capping occurs weight variation can be the cause of capping whether there is too much force or not enough force. In other words if a tablet caps when the tablet weight is lower (this means compression force is lower) the ability to hold the powders together fails. When weights are high it is very possible to use too much force which can fracture; breaking the bond along the band and the upper cap.
- Powders characteristics:
- Moisture continent
- Particle size distribution. Small particle have lower compressibility
- Blend uniformity; a poor blend can be the root cause for capping.
OK, so you have capping: where do you begin?
- You need to have confidence the punches and dies are in good shape, polished and they have passed inspection. If the Dies are tapered you want to verify what the taper is…is it on both sides, and is it the same on both sides. If you don’t know the answers to these questions don’t tear the press down (yet). Let’s see what we can and should do next.
- The place we always start when solving defects: “Weight”. Always minimize weigh fluctuations. Any variation in weight can and will affect capping.
- Slow the press down just to see if the capping will improve. If it does this means that the slower speed provides ample dwell and air release time. This means we can make adjustments to punch penetration, and pre-compression which should allow us to increase the press speed. If slowing the speed down did not improve the capping, then the powders may need to be looked at; it probably isn’t the press. I understand that no one wants to say it’s the powder because they think formulation changes may lead to regulatory issues, and it is possible, but not always true.
- Compression force: some granulations need a lot of dwell time and small change can mean the difference between success and failure. A robust formula is what is needed to perform at high speeds otherwise they are very temperamental, meaning any slight change in particle size, and moisture content could mean failure. Start by making a soft tablet with low force, and then make small adjustments to increase hardness. This assures that over-pressing is not the issue.
- Pre-compression force: It is best to start off making a soft tablet with very little pre-compression force. The objective is to start to form a tablet without pushing the small particles outward toward the upper cap.