This article originally published in Tablets and Capsules’ “Solid Dose Digest” sent July 25, 2016.
Industry experts have long debated the best tooling for tablet manufacturing. To understand why their opinions differ, you must first understand the history of the punch.
In this edition of the column, I show that the standard head flat has a smaller dwell-flat diameter than the domed head flat as defined in the Tableting Specification Manual (TSM), and I discuss why it matters. The information is relevant, in principle, to both B and D punches.
Two early manufacturers of rotary tablet presses—one in the USA and the other in the UK—are generally regarded as the originators of two common, yet different, punch designs. The primary feature that differentiates the designs and causes the most debate is the head, but they have at least four other differences, as shown in the table below. It compares the British punch, the predecessor of the current European Union (EU) punch, to the IPT punch, which is the predecessor of the TSM punch.
|Characteristic||British Punch||IPT Punch|
|Radius of domed head||0.625 inch||0.3125 inch|
|Back angle of punch head||30 degrees||37 degrees|
|Barrel diameter||0.748 inch||0.745 inch|
|Head thickness||0.360 inch||0.406 inch|
Additionally, the British domed head stayed in contact with the compression roll longer, which reduced stress and unwanted heat, resulting in lower rates of punch failure. Its biggest drawback was the smaller (0.25 inch) head-flat area, which required you to operate the press more slowly to satisfy the longer dwell time requirements of many tablets.
More dwell time, faster failure
While the design differences have caused people to argue about which punch type is better, let’s acknowledge that the original IPT/TSM punch did at least one thing very well. It offered more dwell time. It could do so because its dwell-flat diameter was double that of the British punch, i.e., 0.50 inch versus 0.25 inch. Figure 1 shows the differences.
The drawbacks in the original IPT punch design include its sharp angle. While that feature was mitigated by blending the head radius, the unwanted heat and stress and high failure rates persisted. The photo shows how the IPT design can lead to problems at the punch head’s point of impact with the compression roller. The force of that impact can cause the steel of the punch to become work-hardened. As the roller repeatedly impacts the punch, a hardened skin can form on top of the base steel. As it hardens and becomes more rigid, the movement of the metal underneath can cause serious scarring and pitting, which eventually render the punch unusable.
Semi-domed head versus standard head
Today, manufacturers of the standard TSM head often claim that their punches provide a dwell flat of 0.50 inch. When they manufacture the punch, however, they blend the 0.3125-inch radius into the 0.50-inch head, resulting in a smoother transition but causing a loss of dwell-flat diameter. That loss shrinks the actual diameter to only 0.367 inch.
That’s why the TSM domed-head profile is sometimes preferred. By using a slightly thinner head flat and the 0.625-inch radius of the original British punches, it provides a larger dwell-flat diameter. That, in turn, provides a head-flat diameter of 0.375 inch, increasing dwell time and allowing for greater tableting speed. Figure 2 shows the differences and the change in dwell-flat distance.
The TSM domed head was introduced just as tablet presses began to operate at higher speeds, but the British design didn’t change to accommodate those speeds until later. They now have the EU configuration, which uses the 0.375-inch dwell-flat diameter.
Proof in the numbers
Evidence of the additional dwell time is easy to find. A standard TSM head provides a travel distance—measured from the point of contact between the compression roller and head to the opposite side of the dwell-flat area—of 0.4375 inch. The TSM domed head, however, has a travel distance of 0.4475 inch. That difference may seem small, but the additional compaction time allows you to increase turret speeds and thus output, while achieving the same dwell time offered by the smaller standard TSM head. Conversely, you could reduce the turret speed with the semi-domed head and reduce the potential wear on the tablet press and tooling.
Additional benefits of the semi-domed head include:
- No premature failure or potting
- Less tablet capping
- Smoother running of the tablet press through the cycle of compression
- Cleaner lubrication and longer service life
- Better consistency in the tablet properties
To improve performance even more, Natoli Engineering has developed a modified, semi-domed TSM head that includes an extended head flat. With a dwell-flat diameter of 0.470 inch, it prolongs the dwell time even more to meet the needs of today’s tablet manufacturers.