John Norman, Technical Service Representative for Natoli’s Tooling and Shelves Division, discusses the importance of both dimensional and visual tool inspections and their direct relationship to the final shelf quality.
Why inspect punches and dies?
Tooling inspection procedures generally include a check of all critical dimensions, but frequently a visual inspection of punches and dies can be overlooked. Although a dimensional inspection of tooling may find that the punches and dies are within specification, a close visual inspection may find wear to the punch tip and die bore that could be responsible for the defects in the die. tablet manufacturing observed during production. An inspection of critical dimensions of the punches 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 a top or bottom punch. Checking other dimensions is either unnecessary as they rarely change and therefore not worth the time and money for measurement, or they cannot be properly measured with typical equipment and are best served by visual inspection.
The three dimensions are:
- The working length is considered the most critical punch dimension as it largely determines the weight, thickness and hardness of the tablets. Tolerances should be determined by comparing the working length to the length of other punches in the set, rather than comparing it to a specific value. Uniformity of the working lengths of the punches is key. As long as the punches are the same length (within the specified tolerance of each other), it doesn’t matter how they compare to a specific preset length.
- Bucket depth should be calculated by subtracting the measured working length from the total measured length. Cup depth is not considered a critical dimension for tablet weight control.
- The overall length is not critical, except that it is necessary to accurately calculate the depth of the cup. Therefore, do not assume a European Standard (EU) total length of 133.60mm for each punch. However, it is important to understand that the total length of the lower punches is responsible for the tablet ejection and smooth release.
Understanding that the consistency of the working length between the punches of a tool set is primarily responsible for the resulting weight, thickness and hardness of the tablets, consolidates it as the most important dimension of your program. tool inspection. When inspecting working lengths, the upper and lower punches should be separated and inspected for deviations independently of each other. The majority of all new upper and lower punches are designed and manufactured with a standard tolerance of 0.05mm TIR (Total Indicator Reading). This means that in a set of upper or lower punches, the difference between the working lengths of the longest and the shortest punch would not be more than 0.05mm TIR For current tooling (used) it It is recommended to periodically inspect the punches to ensure that the working lengths do not exceed this tolerance. The lengths of the bottom punches are a bit more critical than the top punches because they largely determine the consistency with which the product fills the dies. Deviations in the amount of product in the dies can affect the weight and hardness of the tablets.
The working length of punches should be measured for the punch-to-punch deviation rather than a calculated number. You should measure the working lengths using a digital indicator mounted on a sturdy steel pole attached to a granite base. This relatively inexpensive basic measuring equipment offers the same accuracy as an elaborate inspection system that can cost $ 50,000 or more.
Visual inspection of punches
Tooling inspection procedures often overlook edge wear on the punch tips. Many factors can cause tip wear, including poor tablet and tooling design, abrasive formulation characteristics, inappropriate steel selection for an application, and improper press setup. While inspection technicians generally pay attention to the face of the puncture cup, typical surface degradation is generally not responsible for tablet defects such as plugging, lamination, and flashing. These tablet defects are more often related to wear of the outer edge of the punch tip and the narrow flat surface around the perimeter of the punch cup.
While proper inspection can help prevent many common tableting faults, inspecting the punch tip often only verifies the tip size by measuring it with a micrometer or caliper. Since the measuring anvils, i.e. the contact surfaces of these instruments, are three to six millimeters wide, they can in fact only verify the largest dimension. Wear to the edge of the punch tip is relatively low and undetectable using traditional measuring equipment and techniques, and unattended visual inspection may not easily be observed either. Some form of magnification, such as a horizontal optical comparator or stereomicroscope, is required to properly and thoroughly inspect the punch tips for edge wear.
If excessive tip wear is found on the punches, it is also recommended to check for the presence of a J-hook. This type of tooling fault can be detected by checking the inner periphery of the punch cup with a nail. The J-hook will cause the nail to catch the metal coiled inward at the edge of the cup. Without repair, this condition can lead to styling and rolling of the tablets. To remove the J-hook from a punch, a quick polish of the punch cup can be done to remove it, but will not repair any damage to the outer perimeter of the punch tip. Excessive wear on the outer edge of the punch tip can cause burrs and possibly seizure of the upper punch tip in the die.
For lower punches, checking the sharpness of the back edge of the point is another visual inspection that will need to be done in order for the relief to work as intended. If the trailing edge is worn or rounded, it will not clean the die bore on the downstroke. To check the sharpness of this area, you can slide it against your fingernail, and it should shave off part of the nail. If this area is not sharp, it could result in less seizure of the punch tip in the die bore due to product build-up. This condition can cause excessive friction, heat generation, galling, and excessive wear to the rear angle of the lower punch head.
Matrix inspection (both visual and dimensional)
Inspection of the dies of a tool set begins with a visual inspection of the wear rings in the bores. Simply holding a die with a bright light on the opposite side of the bore and reflecting the light throughout the bore will highlight any wear rings that may be present or start to form. Depending on the abrasiveness of the formulation to be compressed, the definition of this wear ring may vary from fairly low to clearly visible during this visual inspection. A high quality wear resistant steel such as PM15V or using a carbide / ceramic coated die would be a viable option to deal with this type of wear and extend the life of a die set before a die set. replacement is necessary.
To determine the extent of visually visible die wear, the use of a slotted ball bore gauge connected to an indicator or portable digital gauge could be used to measure the wear ring (s) in the die. the die bore. If the dies show visible wear and still produce acceptable tablets, you can consider the measured wear to be acceptable. The allowable wear in a die bore depends on the characteristics of the powder, which vary from product to product. With each tablet compressed, the wear will continue to increase and will eventually affect the final quality of the tablet and hamper the proper functioning of the punches and press. Turning the die over to compress the tablets in the other half of the die is an option if the die only has wear in one end of the bore.
Regular and frequent inspections of punches and dies, for dimensional consistency within the assembly and visually for wear, can prevent problems from occurring in tablet manufacturing. Inspecting the working lengths in a punch set to confirm that they are within the tolerance range specified for your business will help ensure consistency in the weight, thickness and hardness of the resulting tablets. Visual inspection of the outer edges of the punch tip and die bores can prevent common compression defects, such as plugging, rolling and flashing. For best results, include a detailed inspection in your standard operating procedures (SOP) so that every tool technician inspects punches and dies in the same way.