Liam Preston explains why tooling measurement, calibration and standardisation matters.
When it comes to tool measurement, not everyone gets it correct. Although it may appear to be a simple process, if it is not undertaken correctly, it can negatively influence the final tablet.
Measurement is crucial in the production of tablets. Similar to the weight control of the formulations being compressed, tooling dimensional controls are essential to ensure the tablets meet the required specifications and dosage.
In context to tablet compression tooling, measurement is the physical measurement of critical tooling dimensions using accurate and calibrated instruments. Before any work is undertaken it is important that that all equipment is calibrated.
Measuring and controlling the dimensions of punches and dies allows tablet manufacturers to monitor the tooling life. The tooling can be precisely inspected so that technicians can determine when it needs to be replaced and when it is approaching specification limits.
What to measure and when?
When measuring tablet tooling, there are two critical dimensions to include, working length and overall length. These have the most significant influence on the tablet.
The working length is the punch length from the bottom of the cup to the head flat. This is critical for tablet weight and thickness control. The overall length is the nominal reference length, or total punch length, from the head flat to the edge of the punch tip land.
Measurement should be performed on all new tooling. Although the tooling provider may have done so after the manufacturing process, in-house measurements should always take place to verify that the specification is correct, and the tools are ready for use.
It is also important to measure at regular intervals during production. During the manufacturing process, tools can wear which leads to inconsistencies in the overall and working punch lengths across the set.
Another crucial time to measure is if any repair work is undertaken, as this can also affect the tooling dimensions. Upper and lower punches can be measured and matched if weight control is hyper-critical to minimise dimensional variation.
The art of calibration
Another vital step to measuring tools is calibration, by definition, means correlating the readings of an instrument with those of a known standard to check its accuracy. This is critical when manufacturing solid dosage. If the tooling measurement is not precise, it can result in tablets being produced incorrectly impacting the weight, efficiency, and dosage of the medication.
By ensuring calibration is carried out correctly, manufacturers can be confident that any tool measurements taken are correct and accurate and that the equipment is compliant, within specification, and meets strict regulations, creating a standardised approach to tooling.
What Problems Does Measurement Solve?
Critical tooling dimensions will affect tablet weight if measurement is not routinely undertaken. Tablet weight is a universal quality control parameter to ensure the correct medicine dosage is contained within each tablet. This is achieved by accurately weighing a sample of tablets regularly during the production of a batch of product. A Standard Operating Procedure (SOP) will be specified for a particular product, or by the manufacturing centre, and should always be adhered to. Non-uniform or excessive variation in punch working lengths is one of the most common causes of tablet weight variation and should be monitored closely.
Tablet hardness can affect downstream processes such as coating, packaging, storage and transportation of the final product. If there is a variation in punch working length, it can alter the weight and hardness within tablets of the same batch. To avoid this issue, it is important to control the working length through effective maintenance procedures.
Altered Clearance and Increased Wear
Consistent measurement helps to monitor tool wear to make certain it is not excessive, as this can lead to problems such as tablet flashing, which occurs due to an increase in clearances. Flashing is when a thin layer of product forms on the outer edge between the punch tip and the die bore wall and will affect the tablet’s final appearance.
There are standard clearances for punches and dies within any tooling specification. Staying within this tolerance is critical when manufacturing tablets. Any granule that penetrates the gaps between the punch tip and die bore can cause friction. This can lead to abrasive wear and potentially cause tabletting issues like flashing, capping and die bore ringing.
Usually, increased clearance between the upper punch tip and die bore is used because the upper tip must continually re-enter the die bore during the compression cycle. The increased clearance ensures that the punch tip does not come into contact with the die bore. The lower tip has less clearance because it is always in the die bore during the compression cycle. This reduces the granule passing through the clearance, minimising the degree of lost granule, and stopping any machine contamination.
Punch Binding
Increased tool clearance can also result in punch binding. This is when the punch becomes tight in either the turret or die. There are two ways in which this takes place. The first is when the punch dimensions are too large, causing friction between the punch tip and the die bore wall. The second is when the punch dimensions are too small and the increase in clearance between the tip and die bore wall allows excessive product to escape. This can then build up, leading to punch binding or tightness increasing wear to the under-head angle or other areas of the punch.
Capping
Precise tool measurement can prevent capping, this is where the tablet upper cup is removed or partly removed. As a result, the tablet separates horizontally from the main body causing tablet ejection issues affecting tablet output.
Capping occurs when there is insufficient clearance between the punch tip and the die bore, so there is inadequate room for the air to escape when under compression. Air entrapped within the tablet can lead to problems with granule bonding, or capping if air is released at a later point.
A good option to reduce or eliminate capping is to use die tapers to increase the available clearance between the punch tip and the die bore. This will ease the passage for the air to escape. If dies do not have tapers, measuring can ensure clearances are kept within specification.
Liam Preston is with I Holland.
