Case Study: How to increase tablet press capacity

Artur Jakubiak, international sales director at Adamus examines the different factors that can affect tablet press set-up.

Today in the race towards the maximum tablet press output, two different approaches are observed - that process efficiency can be influenced by the modification of the machine features or by applying compression tooling with special design.

Obviously, by buying a larger tablet press with a larger turret pitch diameter, the effect of the increasing capacity is easily addressed but is usually connected with high investment costs. Changing design parameters in the existing tablet press - such as the stations number or turret rotation speed - is usually connected with quite deep interference in the tablet press construction and/or control system.

The effect of the stations numbers in the turret can be seen in Table 1.

Increasing the number of stations can be connected with a decrease in the die outer diameter, as well as a  decrease in the maximum tablet diameter available to produce.

In the case of the tablet having a diameter (for round tablets) or a larger axis length (for shape tablets) of 11 mm or less, by changing the turret from D type to B/BBS type you can increase the production capacity of the tablet press by 185%.

Another way which has a huge impact on production capacity through increased output and reduced production time is through applying multi-tips tooling to existing machines. Additional benefits include less maintenance and reduction in press setup-time calculated per production batch. The fewer tablet presses needed to satisfy required output leads to less floor space, which gives more product volume processed per square meter. All these factors translate to reduction in overall plant running costs.

When using multi-tips, theoretically, the number of tablets per turret rotation could be multiplied by the number of tips. However, usually turret rotation speed has to be reduced due to increasing numbers of die bore to fill (right powder distribution) and the capacity increase factor can be smaller as a result. The influence of the tips number on capacity of Kilian Synthesis 500 can be seen in Table 2. These data should be treated as indicative only, as the real factor values strongly depend on the formulation to be compress.

Generally, there is no need for any special tablet press modification when applying multi-tips. Tablet presses must be in good working conditions with no significant wear in punch guides, keyways and die pockets. The upper punch table in the turret must have a keyway in the guide to ensure exact alignment with the tooling - as in the case of using any shape tooling. The lower punch guide does not require a keyway because the tips are guided by the die, but a keyway helps in the setting process, especially for tooling with higher tips number. Additionally, it should be considered that even if tips are guided by the die, during the contact of the punch head with the compression roller, the punch has a natural tendency to turn. Certainly, this rotation is restricted by the die and this leads to developing a stress between tips and die wall. As the stress is repeated many times during the compression cycle, finally it ends with the premature fatigue breaking of lower tips. Such stress can be eliminated by the keying of the lower punch or using a rotating head.

The formulation being compressed should be investigated to cope with the increased fill requirement and granulation flow. If product is not flowing well, a solution could be to modify the fill shoe paddles or reduce the machine speed to allow enough time for fill to take place.

Two different designs of multi-tip punches are generally available. One is the assembly design where individual tips are fastened into the punch shaft directly or by cup. The second is a monoblock design of punches machined out of a single piece of steel.

Choosing the type of configuration, the size of the tablet and the tool type (number of tips per punch size) has to be considered. Due to the more complicated machining operations, multi-tip monoblock tooling has been regarded as more expensive than an assembly design. However, current technological developments bring price equalisation to both constructions. Many companies prefer then to use monoblocks because they are easier to set up and can be cleaned much more easily than assembly construction tooling, that must be disassembled for cleaning. One of the most interesting benefits of an assembly construction is that the tips are removable, making the replacement of damaged punch-tips possible. Of course, replacing a damaged tip for a completely new one is risky, as the wearing of the remaining tips can lead to differences in the working lengths in assembly, which eventually may cause the rejection of all tablets produced in the station. That is why circulating the spare tips during the set lifetime to keep same wearing level is of great importance. With the solid monoblock design, broken tips in this case require the replacement of the complete multi-punch. The latter is much more expensive than replacing a single damaged punch tip.