The ideal inspection

John Norman, technical service representative for the Tooling and Tablets division at Natoli examines the importance of both dimensional and visual tool inspections and their direct relationship to final tablet quality.

Why Inspect Punches and Dies?

Tooling inspection procedures generally encompass a check of all critical dimensions, but frequently a visual inspection of the punches and dies may be overlooked. Even though a dimensional inspectional of the tooling may find the punches and dies to be within specification, a thorough visual inspection may find punch tip and die bore wear that could be responsible for tableting defects observed during production. An inspection of the critical punch dimensions should be performed periodically, but a visual inspection of the punches should be performed each time a tool set is removed from the tablet press.

We believe there are three dimensions that should be considered in a dimensional inspection of an upper or lower punch. Checking other dimensions is either unnecessary because it rarely changes and therefore isn’t worthy of the time and expense of measuring, or they cannot be properly measured with typical equipment and is better served by a visual inspection.

The three dimensions include the following:

1. Working Length is considered the most critical punch dimension as it largely determines tablet weight, thickness, and hardness. Tolerances should be determined by comparing the working length to the length of the other punches within the set, rather than comparing it to a specific value. Uniformity of punch working lengths is the key. As long as the punches are the same length (within the specified tolerance of each other) it doesn’t really matter how they compare to a specific predefined length.
2. Cup Depth should be calculated by subtracting the measured working length from the measured overall length. Cup depth is not considered a critical dimension with regards to controlling tablet weight.
3. Overall Length is not critical, except that it is necessary to accurately calculate Cup Depth. Therefore, do not assume a standard European (EU) overall length of 133.60mm for every punch. However, it is important to understand that the overall length of the lower punches is responsible for tablet ejection and smooth take-off.

Understanding that the consistency of working length between punches in a tool set is primarily responsible for the resulting weight, thickness, and hardness of the tablets, solidifies it as the most important dimension in your tool inspection programme. When inspecting working lengths, the upper and lower punches should be separated and inspected for variances independently of one another. The majority of all new upper and lower punches are engineered and manufactured to a standard tolerance of 0.05mm T.I.R. (Total Indicator Reading). This means that, within a set of upper or lower punches, the difference between the working lengths of the longest and shortest punch would be no more than 0.05mm T.I.R. For in-process (used) tooling, it is recommended that you periodically inspect the punches to ensure working lengths do not exceed this tolerance. The lengths of the lower punches are a bit more critical than the upper punches because they largely determine how uniformly product fills the dies.  Deviations in the amount of product in the dies may affect tablet weight and hardness.

The working length of the punches should be measured for deviation from punch-to-punch rather than to a calculated number.  You should measure working lengths using a digital indicator mounted on a sturdy steel post fixed to a granite base. This relatively inexpensive basic measuring equipment provides the same accuracy as an elaborate inspection system that can cost $50,000 USD or more.

Visual Inspection of Punches

Tooling inspection procedures often overlook wear to the edges of punch tips. Many factors can cause tip wear, including poor design of tablets and tooling, abrasive formulation characteristics, improper steel selection for an application, and improper press setup. While inspection technicians typically pay attention to the punch cup's face, typical surface degradation is generally not responsible for tablet defects such as capping, lamination, and flashing. These tablet defects are more often related to wear on the punch tip's outer edge and land, the narrow flat surface around the perimeter of the punch cup.

While proper inspection can help to prevent many common tableting defects, punch-tip inspection often only checks the size of the tip by measuring it with a micrometer or calipers. Since the measuring anvils, i.e., contact surfaces of these instruments are three to six millimetres wide, they actually can only check for the largest dimension. Wear to the very edge of the punch tip is relatively small and is undetectable using traditional measuring equipment and techniques, and unassisted visual inspection may not easily observe it either. Some form of magnification, such as a horizontal optical comparator or a stereo microscope, is necessary to properly and thoroughly inspect the punch tips for edge wear.

If excessive tip wear is discovered on punches, then also checking for the presence of J-hook is recommended. This type of tooling defect can be detected by checking the inner periphery of the punch cup with a fingernail. J-hook will cause the fingernail to catch the inward curled metal at the cup edge. Without repair, this condition can lead to capping and lamination of tablets. To remove J-hook from a punch, a quick buffing of the punch cup can be performed to remove this, but will not repair any of the damage on the outer perimeter of the punch tip. Excessive wear to the outer edge of the punch tip can cause flashing and possibly cause binding of the upper punch tip in the die.

For lower punches, checking the back edge of the tip for sharpness is another visual inspection that will have to be done so the relief is functioning as designed. If the back edge has become worn or rounded, it will not clean out the die bore on the downstroke. To check the sharpness of this area, you can drag this against your fingernail, and it should shave some of the nail. If this area isn’t sharp, this could lead to lower punch tip binding in the die bore due to build-up of product. This condition can cause excessive friction, heat generation, sticking and picking, and excessive wear to the back angle of the lower punch head.

Die Inspection (Both Visual and Dimensional)

Inspecting the dies of a tool set begins with a visual check for wear rings in the bores. Simply holding a die up with a bright light on the opposite side of the bore and reflecting the light throughout the entire bore will highlight any wear rings that may be present or beginning to form. Depending on the abrasiveness of the formulation being compressed, the definition of this wear ring can vary from fairly faint to clearly visible during this visual inspection. A premium wear resistant steel such as PM15V or utilising a carbide/ceramic lined die would be a viable option to address this type of wear and extend the service life a die set before replacement would be necessary.

To determine the extent of any die wear seen visually, the use of a split-ball bore gauge connected to an indicator or a digital handheld gauge could be employed to measure the wear ring(s) within the die bore. If the dies have visible wear and are still producing acceptable tablets, you may deem the measured wear as acceptable. Allowable wear within a die bore depends on the characteristics of the powder, which varies product to product. With each compressed tablet, the wear will continue to increase and eventually affect final tablet quality and hinder smooth operation of the punches and press. Flipping the die over to compress tablets in the other half of the die is an option if the die only has wear in one end of the bore.

Conclusion 

Regular and frequent inspections of punches and dies, for dimensional consistency within the set and visually for wear, can prevent issues from arising during tablet manufacturing. Inspecting working lengths within a punch set to confirm they are within the specified tolerance range for your company, will help ensure consistency in resulting tablet weight, thickness, and hardness.  Visual inspection of the punch tip’s outer edges and die bores can prevent common tableting defects, such as capping, lamination, and flashing. For the best results, include a detailed inspection in your Standard Operating Procedures (SOP’s) so each tooling technician inspects the punches and dies in the same way.