Clearances and tolerances
Unlike typical mechanical processes, powder compaction inevitably results in granule coming between the lower punch tip and
die bore, resulting in resistance. Consideration should be given to the clearance between such surfaces.
Figure 3: Clearances and tolerances.
|
Lower punch tip to die bore clearance.
Clearances and tolerances (see Figure 3) are critical to ensure accurate functionality of the tooling, and reduce wear of
tooling and press components. The clearances between the punch tips and die bores are important to ensure good, problemfree
tablets and to eliminate problems such as capping (see Figure 4). The ISO standard stipulates the use of DIN Norm for tolerances,
adopting the same size and clearance for both upper and lower punches irrespective of tablet size. This also creates a wider
variation in clearance between the lower punch tip and die bore regardless of granule size, which can cause granule leakage
past the lower tip, resulting in potential wear and damage to tooling and press parts.
Figure 4: A typical example of capping.
|
Upper punch tip to die bore clearance.
ISO adoption of DIN Norm on upper punches also results in insufficient clearance leading to rejected tablets through capping.
To counter this, all dies have to be tapered at both ends to enable air to escape from the tablet. This requirement increases
tooling costs for the tablet manufacturer.
The Eurostandard and the TSM standard avoid these issues by adopting customized clearances, which offer advantages over ISO
standards, although there are differences between the two. The Eurostandard adopts a tighter clearance range to help improve
general operation and product yield.
Conclusion
Although there are other small variations between the standards, such as seal groove configuration, these variations do not
prevent interchangeability and are not detrimental to the running of the tablet press. All of these differences are surmountable.
In recognition of the need to eliminate these variables and specifically take into account the sensible and efficient provisions
for cross-platform/press compatibility laid out in ISO 18084, the Eurostandard has been updated to make it compatible with
the ISO standard.
A single global tooling standard should be highly desirable for the industry; however, even if tablet press manufacturers
could be convinced that it is in their commercial interest to adopt it, it will take time for these changes to evolve. Leading
tablet-press manufacturers are always seeking to innovate and move the industry forward so tooling standards will always require
revision.
In turn, this means that highquality tooling suppliers will always need to retain the ability to solve complex problems and
preserve the skill set within their workforce to manufacture tooling that is compatible with any press worldwide.
In conclusion, I believe that calls for the standards laid out in ISO 18084:2005(E) to be adopted globally should be fully
endorsed. However, I also believe that the Eurostandard already represents a comprehensive global standard because it is the
most widely adopted tabletting tooling standard today and is compatible with ISO.
Steve Deakin is business development director at I Holland, steve.deakin@iholland.co.uk
.
|
