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Importance of Punch Length and Cup Depth

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18 July 2016 No comments

Manufacturing tablets that have uniform shape, size, thickness and hardness requires that the tablet press punch dies used be of similar length. This article talks briefly about how one can determine the punch length, correct measurement as well as the effects of wear and tear. The article will also touch briefly on the importance of cup depth tolerance.

In using rotary tablet press punches of the same length, it is important to consider its consistency as it affects the uniformity of the tablet product in terms of weight, thickness and hardness. If one does not have any idea about its importance, it might lead to attributing the defects to the wrong cause. It is therefore important to conduct regular maintenance to ensure that all the punch s and cup depths are uniform.

Length

Tablet punches are manufactured based on a consistent length; the working length is measured by the distance from the head flat to the lowest measurable area of the punch cup (Figure 1). Figure 1 shows the cup depth as well as the overall length – from the head flat up to the punch tip.

Knowing the working length would lead to a consistent hardness, thickness as well as weight of the tablet. Should there be variation in the length; the attributes mentioned above would also vary. It is therefore essential that consistency in the working length is present. The standard length is 0.002. This means that, in a set of punches, the difference between the longest and the shortest working lengths is only 0.002 inch. It is recommended that one should regularly inspect the punches to check and ensure that the lengths do not exceed the tolerance or range that is specified. During inspection, it is important to inspect the upper punch and lower punch separately.

It is also important to measure the length correctly. It is wrong to measure the length by deducting the cup depth from the overall length, as this would lead to inconsistency of the result showing the tools are out of specification.

The tolerance of the overall length and cup depth is greater than that of the overall length and as such, one cannot use it to measure, as it would lead to a tighter tolerance. Measurement should be done from punch to punch rather than a calculated number. To measure, one can use a digital indicator that is mounted on a steel post and fixed to a granite base. The basic equipment costs less than $500 and provides the same accuracy as that of equipment that costs more.

Whatever equipment one uses, measuring the length of the punch with embossing or bisect at the lower area of the punch cup is complicated. To measure the working length, one should measure from the cup’s deepest area, which is accessible with the indicator’s point. After identifying the lowest area of the cup, it is important to ensure that one measures from that area consistently.

Some companies do provide a working length matching report when new punches are delivered. The reports pair the upper with the lower punch, longest to the shortest and are then numbered accordingly. Matched sets are ideal as they can provide consistency in terms of the hardness and thickness of the tablet and the report would be helpful during setup.

It is important to note that the lower punch length is more important than that of the upper punch. The reason behind this is that the length of the lower area determines the uniformity of the filling in the die. Changes in the amount would affect the weight and hardness of the tablet.

Cup Depth

In measuring the cup depth, one should start measuring from the tip edge of the punch to the lowest theoretical point of the cup. Some configurations have a different depth; examples are those tablets with scalloped edges. The cup determines the appearance and configuration of the tablet’s face. The tablet sidewall is the space between the two tablet faces that is created by the die. It is also known as the tablet band or tablet gate. Although the sidewall is not measured or inspected, it is still important in terms of the tablet’s appearance and during manufacturing process. Preferably, the sidewall width is proportioned to the tablet’s thickness, as a tablet that has an excessive sidewall would appear thicker creating the idea that it would be harder to swallow. An excessive thick wall would also make the press exert greater force in ejecting the tablet.

The sidewall width depends on the hardness, thickness and weight in relation to the cup depth. As the tip of the punch wears out, the cup depth decreases and the sidewall starts to thicken. Thus in comparing two tablets of equal weight and hardness, the 1st tablet is made with a shallow cup whereas the 2nd cup is made up with a deep cup. The 1st cup has a thicker wall but it is thinner than the deep cup tablet. Because of the sidewall’s thickness, it creates the illusion that it is thicker and thus visually undesirable. The shallow cup’s sidewall can also cause problems especially during film coating as erosion may occur on the sharp corners where the tablet’s radius and the sidewall meets.

Many press punches have a cup depth tolerance of 0.003 inch, which is widely accepted by the tablet compression industry and is followed by tooling manufacturers around the world. [Tableting Specification Method]. Although the 0.003 inch is widely accepted, it is not applicable with some applications especially if the diameter of the tablet is different from that which the company is currently manufacturing.

An example would be if the manufacturing company has to produce Schedule II drugs with a depth of 0.010 inch. The standard is 0.003 inch. If one complies with the standard, there would be a deviation of up to 60 percent of the desired cup depth. Another would be when one is manufacturing vitamin supplement tablets. The deepest cup would be 0.060 inch. In following the standard, there would be a deviation of 10 percent. There is a huge difference considering that the one we are using is a Schedule II or a small diameter size tablet.

In order to remove the deviation, it is important to specify the tolerance as the % of the desired cup depth. Example, in specifying a tolerance range of 20% of the desired cup depth for the 1st cup, changes the range from 0.007 – 0.013 inch to 0.009 – 0.011 inch. To inspect the cup depth, one can use the digital indicator mounted on a steel post to measure.

Overall Length

The overall length is least likely to consider in a punch. The overall length is the distance from the punch tip to the head flat. It is the reference dimension that is made up of two or more important dimensions – working length and cup depth. As both the working length and cup depth have their own tolerance, assigning tolerance to the overall length is not needed. As long as the tolerance for the other two is working, one no longer needs to inspect the overall length, as it will remain consistent. That said the overall length of the lower punch is important especially when it comes to setting the punch height to ensure uniformity of the tablet as well as reduce the probability of tablet damage. If it is standard procedure to check and inspect the overall length, then one can use the regular tools in inspecting the cup depth and working length to measure.

Wear and Tear

With regular use, punches would wear, especially in the tip area, which reduces the cup depth. This would then affect the overall length of the punch although it will not affect the working length. Wear in the head flat can also affect the overall length as well as the working length; however, it does not affect the cup depth. Regular maintenance of the tip, head and cup face can also affect the punch length. Using abrasive materials for cleaning, stiff brushes and bobs can also deepen the cup and alter the working length.

Wear can also occur at the narrow flat area located at the punch tip’s perimeter, in that when exposed to abrasives, the tip becomes thin and sometimes razor sharp causing a J-hook. J-hook usually causes tablet capping as well as lamination. To restore the land, use a soft cotton wheel and a polishing compound. Advantage would be that it prolongs the life of the punches; the disadvantage would be that it reduces the overall length and cup depth.

Summary

Uniformity is important to ensure consistency and smooth operation of the tablet presses. The most important part would be the working length followed by the cup depth and the overall length. In addition, to improve the uniformity of the tablets, it is important to obtain a matching report that will be useful as a guide for tablet consistency.