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Physical attributes of tablets and ink printing
Q: Do the physical attributes of tablets impact the quality of their ink-printed logos?
imageAs drug-development technology advances, solid oral dosage forms (SODFs) become available in an increasingly wide variety of shapes, sizes, colors, and material compositions. Given the various options for tablet coatings to enhance the look, feel, and taste of drug products, the possibilities seem endless.
This vast array of physical variations exists for several reasons, from specific drug-delivery mechanisms to marketing and general appeal. The inherent need to label and otherwise mark SODFs requires marking machine vendors to develop technology that can adapt to these variations.
With the drug development process being lengthy and expensive, having a clear understanding of the impact a tablet's physical attributes can have on the final product can save a significant amount of time and money in the long run.
Changes to tablet shape can affect print quality. Tablet designers must consider the factors that drive acceptable criteria for printed logos, including the logo's position, legibility, and completeness. These print criteria are determined by tablet shape, dimension, identity, and print area and are susceptible to geometric changes in the product to be marked.
Tablet shape and dimension
A critical step in achieving a highly successful printing operation is ensuring that you thoughtfully design the tablet shape and dimensions. As a general rule, the more geometric variation a product has, the more difficult it becomes to manipulate during printing.
A product's physical appearance is usually described as being dimensional. If a sphere has one dimension, a flattened sphere becomes a tablet with two dimensions, and an elongated tablet becomes a caplet with three dimensions. To perform printing and packaging tasks, the more dimensions a product has, the more manipulation you must perform to orient or position it.
imageWhile almost any tablet shape can receive a quality print, you can feed certain shapes through a printing machine at a higher rate and with a higher probability of a good print than other shapes. A round shape provides the best output, and the probability of a good print decreases progressively for simple caplets, bi-convex caplets, and special shapes (photo).
Tablet identity
The ability to feed an SODF into a printing machine and precisely position it is key to optimal print quality and consistency. A number of tablet-and-capsule ink printers are available that can take blank, bulk tablets or capsules and position them for precision rotogravure printing.
When you can't limit your SODF's dimensions to a sphere, which would allow printing in any position or direction to create a complete and legible logo, you must identify and isolate its target face for printing relative to its other sides or orientations. The ability to distinguish a tablet's target face and orientation from the remainder of the tablet is known as its identity. A tablet's identity is good if it has a face that is easily distinguishable from any other side.
Tablet identity is key to ensuring proper positioning during tablet manufacturing. You use it in much the same way as you would position people to take driver's license photos. You would define their identities by properly lining them up for the photo, determining if you have positioned them so that they are roughly in the photo's center and facing the camera to entirely capture their faces.
These criteria align with the goals of a good print on tablets as well, even without measuring equipment. In the same way that people identify each other by matching expected and observed geometrical patterns on their faces, such as eye distance or nose curvature, printing machines recognize tablet features to position the tablet in the center of the printing equipment.
A well-designed pocket for positioning a tablet is machined to match expected geometric patterns, such as curves, so that the printer can position the tablet in one way only as it's fed. This method works best when your tablet's side is distinctly different from its face. Recognizable features, such as crowning curves or tapers instead of the straight lines in a square, increase the probability that the printing equipment will position your tablet correctly.
A chief example of dangerous indistinction in tablet design for high-volume production is overexposure of a compressed tablet's belly bands. Belly bands are the straight sides on compressed tablets that tooling and manufacturing processes require. When the ratio of the tablet face width to its belly band approaches 1:1, the belly band is overexposed. This causes feedrate issues, which can, in turn, affect print quality.
Tablet designers must scale a tablet's belly band to the thickness and shape of its top and bottom faces for the tablet to remain manufacturable, and they also must consider how overexposure of the belly band affects the process after compression. Because belly bands are a product of tooling and are flat to aid in manufacturing, they inherently have lower identity than the curves on the tablet's front or back faces. Even if the belly band follows a curve around a tablet's perimeter, it can only be curved in one axis.
If we define identity as the number of possible alignments available based on a tablet's geometry, a belly band curved on one axis will have lower identity than a tablet's face curved on two axes. If we compare two tablets with the same surface area, with one having a larger belly band, the tablet with the smaller belly band will have a higher identity. Its surfaces with the capacity to be curved in two axes will take up a higher percentage of the area. The closer the band's width is to the width of a tablet's face, the less identifiable the tablet is and the less likely it is to receive a good print.
A marking machine vendor must design tooling to minimize the possibility that the tablet will stand on its edge, causing the marking machine to print on the belly band, which results in an incomplete and/or illegible logo print. The difference between the width and thickness of a compressed tablet should be at least 0.100 inches (2.54 millimeters). The greater the width-to-thickness ratio and the smaller the belly band, the greater success you will have in positioning the product properly and in creating high-quality print output.
Print area
For a quality printed logo, the tablet must not only be positioned consistently and correctly from random entry into the machine, but also the logo's recognizable features must be present and legible after printing. You must determine the last print criterion for a good print, the print area, at the beginning of the process because it ultimately dictates your choice of machine type.
Single-lane printing machines are available for production of small batches, but logos printed on a single-lane printer can realistically be expected to cover no more than half of a tablet's face. In addition to allowing a higher production volume, flatbed printers and drum multilane printers offer an increased marking area. This is due to their precision manufacturing and the addition of vacuum inside the pockets in the case of the drum printer. These improvements allow the tablet faces to be more exposed with the same amount of security.
As markings become larger and more intricate to distinguish products from competitors, include more trackable details such as bar codes, or add function to product dissolution, tablet manufacturers often choose to change to flatbed or drum printers because these printers can hold the tablets more securely and allow larger print areas.

Jonathan Bry is a mechanical engineer at Ackley Machine, Moorestown, NJ. The company designs, engineers, and manufactures machinery for confectionery and pharmaceutical tablet printing, laser marking, laser drilling, and vision inspection.
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July 26, 2021
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