Dale Natoli explains how improved tablet quality can be achieved by checking a number of vital inspection points
Inspecting tooling and press components for wear could be the most important step in improving tablet quality and process efficiency. Certain inspection points are easy to overlook yet can cause considerable tablet quality issues and limit tool life. Implementing the following inspection points will assure physical tablet quality and mitigate possible damage to the tooling and tablet press.
Working length is the most critical punch dimension when using a rotary press and is responsible for the consistency of tablet parameters related to weight, hardness and thickness. The working length dimension is the value from the head flat to the deepest area of the punch tip cavity and measured directly and not from a value derived from subtracting the cup depth from the punch overall length. Most reputable tooling vendors can provide an inspection report verifying conformance of the tool lengths for new tooling as well as other critical inspection data used as a basis to realise punch wear. It is recommended to establish inspection protocols to re-confirm the adherence to the engineered working length tolerance.
During the life of the tools, it is uncommon for the working length to deviate from the required tolerance, hence deviation is typically due to product flow, tablet press set-up and tablet press wear. When purchasing tablet compression tooling it is common to order spare punches in the case a single punch becomes damaged. Adding a spare punch in a set of in process tools can cause an inconsistency with tablet specifications due to the new spare punch being full length and absent of tool wear hence being longer than the in-process tools. Before adding the spare punch in the set of in-process tools it is important to inspect the spare punch length to confirm its length conforms to the working length of the in-process tools.
Die Bore Wear
Inspecting for die bore wear is often overlooked during in-process tool inspection and until recently was often a visual inspection due to the lack of effective and affordable equipment to accurately measure this dimension. New technology has allowed this inspection to be performed effortlessly. Equipment such as the Wi-gauge quickly measures the die cavity for round dies and the minor dimension on shaped dies. The Wi-gauge is an excellent tool to measure die wear and when used in Wi-Fi mode will display measured values on the hand-held unit and send the data to a computer to track, monitor and store die and segment measurements.
Once a die bore becomes ringed it allows formulation to migrate between the punch tip and die resulting in excessive friction causing heat which can be related to tool binding, sticking and accelerated punch tip wear. Die bore wear can also cause capping, laminating and excessive tablet flashing, which can often be the culprit for weak and soft tablet edges causing havoc during tablet coating and excessive dust in packaging for uncoated tablets.
Punch Tip and Cup Wear
Punch tips should be inspected for excessive clearance to mitigate the same tablet quality conditions that can be experienced with die cavity wear. Tip wear is generally pronounced at the very edge of the punch tip, making it virtually impossible to measure with traditional inspection equipment. Once the punch tip edge wears the punch cup should be inspected for a wear condition referred to as “J-hook.” The condition is characterised by the edge of the punch tip curling inward toward the punch face. Tip wear can cause capping during tablet production and lamination as the tablet de-compresses after the compression event. The optical comparator is one of the best and most affordable inspection tools to measure punch tip edge wear.
The lower punch retainers can play an important role with tablet quality. Worn retainers should be replaced immediately to mitigate variances in tablet weight. Punch retainers are properly adjusted when the retainers just hold the lower punch in position against its own weight. Worn or improperly adjusted punch retainers can cause excessive tool and cam wear if adjusted too tight and excessive tablet weight deviation if they are worn.
Another inspection point on the tablet press is the pressure rolls and these should be visually inspected for surface pitting, which can transfer to the punch head. The pressure rolls should be free spinning and checked for “run-out” or concentricity. An out of round pressure roll will cause inconsistencies in tablet hardness and weight. Check by using a dial indicator affixed to a magnetic base. Manually spin the roller while the indicator is slightly pressed on the roller. The measurement is taken and averaged to determine whether pressure roll wear is within the tolerance range.
Fill Cam Selection
Proper fill cam selection is imperative to consistent tablet weight and clean operation of a tablet press. A fill cam that does not have a sufficient depth of fill will result in inconsistent tablet weight. Rotary tablet presses operate on an overfill principle where an approximate 20% overfill is required to achieve consistent tablet weight. The weight/dosing cam will raise the lower punch to scrape off the excess formulation resulting in consistent tablet weight. If the fill is too deep, excess formulation, due to centrifugal force of the turret rotation may be thrown into the die cavity, again causing inconsistent tablet weight. The excess formulation will also result in contamination of tooling lubricants.
Ejection cam height should be checked with a straight edge across the die table above the die cavity. The edge should just meet the lower punch tip to ensure there is adequate clearance during tablet take-off. Setting ejection cams to a proper height will reduce tablet chipping and damage to the punch tip and feeder.
Checking the suggested inspection areas and correcting any wear issues or defects is a step that is well worth its time, as it will improve product quality and reduce tooling cost due to wear and breakage.
It’s also important for written inspection procedures to be developed to ensure that best practices are followed. It is crucial for staff to thoroughly check that press parts and tooling are within their respective tolerances and in good condition.
Dale Natoli is president of Natoli