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Guidance on tablet design

13th March 2017


Evenly broken tablet
Unevenly broken tablet
Stress and fatigue analysis of a punch tip using Finite Element Analysis, or FEA. FEA is a software based numerical technique for calculating the strength and behaviour of engineering structures. It is used to calculate deflection, stress and strain to determine fatigue limits of both material and design
The application of a blended land to a punch tip. The correct method of applying the land is to ensure that the flat area on the tip edge is maintained, whilst blending the intersection between the profile and the flat. This is achieved by applying a radius to the finished punch tip
A good example of the importance of spacing on logos. The top example clearly shows the embossing within the safe zone for this particular tablet shape. The bottom example shows embossing that goes beyond the safe zone and on the side view you can see how the embossing protrudes
Stcking has occurred to the punch tip on the left. This is because the soft part of the tablet, at the top of the convex, is sticking to the punch tip face. There are several remedies for this

Why is good tablet design so important and how to achieve it? By Steve Deakin and Steve Osborn

Good tablet design is extremely important; it has an impact upon anti-counterfeiting, tooling strength, tablet coating, durability and functionality. It also helps to avoid downstream manufacturing problems such as tablet sticking, picking, lamination, capping and premature tooling failures. Several elements need to be considered when designing a tablet.

Tablet shape and profile

The tablet shape plays a decisive role in the final quality of the tablet. There are two basic tablet shapes, round and non-round, however the complexity of non-round shapes can be very varied and require specialised tool manufacturing capability. Once the base shape has been decided, tablet size must be determined, consideration should be given to the type of press available for tablet manufacture as this can limit the size of the tablet. 

Next follows selection of the tablet profile, which is influenced by several factors; the granule, embossing requirements, coating process, packaging and the company’s branding. Thought should also be given to the volume of the tablet and if it will be coated. Successful coating is dependent on tablet profile. Coated tablets, present challenges for the tablet designer. Typically, the core of a tablet, is lower in hardness, so during the coating process core erosion may take place. This is when the tablet comes into contact with the coating pan and other tablets causing wear. This vulnerability can be reduced by avoiding very deep concaves and ensuring a robust design. 

For shallow tablets with hard, sharp edges, the coating process will damage the exposed edge of the tablet resulting in chipped edges and sometimes cracks. Therefore, flat and shallow tablet profiles should be avoided as these will not roll effectively in the coating pan. Double radius designs ensure a strong tablet edge and a balanced profile.

Poor marking and branding design on the tablet can lead to bridging (where the coating collects in the detail on the face of the tablet) and infilling (when too much coating material has filled the detail) during film coating. Possible causes for this can include:

1)    Inadequate adhesion of the film coating.

2)    Inappropriate marking design where the angle may be too acute or too deep (bridging) or the stroke or section of the embossing being too wide or too shallow (in-filling). 

3)    Inappropriate coating procedure i.e. spray rate, drying time etc.

‘Twinning’ can also occur where tablets stick together during coating. This is normally caused by the flat surfaces of the tablets coming into contact and adhering to each other. To avoid this a slightly curved surface can be applied, which reduces the contact area.

Tablet breakability

Breaklines that are used to divide a tablet must be both functional and effective. Uneven breaking of a tablet may result in significant fluctuations in the administered dose. The degree of inaccuracy may be associated with breakline design, tablet hardness, and/or formulation. 

The following factors should be considered when selecting a breakline:
•    Accuracy of breakage, which is important for equal dosage
•    Holding of the tablet and ease of breakage. This relates to tablet size and hardness.
•    Inclusion of other detail such as a logo and its influence on the breakline. Product identification must be maintained to ensure brand integrity when the tablet is divided.
•    Robustness of the tablet during compression, coating and packing, because the tablet’s physical qualities are changed by adding a breakline, it may become weaker.

Tooling performance

Tablets are becoming more complex in both shape and profile for brand identity and marketing. This demands for improved tooling strength, durability and overall performance. 

When assessing the results of the FEA analysis, it is important to identify areas of high stress concentration. When reviewing the image and colour banding, the red areas are those with higher stress concentrations. If the fatigue limit is exceeded in these areas during cyclic loading, eventually a breakage will occur. A good design should ensure that the stresses are equally dispersed across the shape.

An expert designer will be able to add appropriate strengthening features to the design such as blended lands. Often, tablet manufacturers elect not to apply a land as it may not be visually acceptable on the finished tablet. Lands that are applied incorrectly, either unevenly or made too large can present a range of issues including: flashing or lamination during compression; chipping of the land during take‐off or, build‐up of coating on the edge of the tablet which eventually will chip. A correctly selected and applied blended land provides benefits to handling, loading, setting, tooling strength and the visual appearance of the tablet.

Tablet branding

When considering the visual appearance of the tablet, think about the type of font and logo. Typefaces and designs must take into account practicality of tablet manufacture. Failure to consult with an expert tablet design team could result in a product that looks good on paper but is not practical to produce.

For tablets with a logo, the design and placement are very important. The tablet designer should always seek to maximise the face area to avoid picking and lack of distinction. However, problems may arise if too much of the face area is used leading to embossing distortion and weakness in the tooling.

As a general guide the embossing should sit below the landed edge of the punch tip. If this guide is not followed then the embossing will be unprotected and prone to damage, causing further downstream problems.

The correct font style is very important to avoid problems such as ‘picking’, when compressed granule adheres to the detail on the punch face. To reduce picking the best practice should be to design font styles that have large open counters and no sharp corners, which could act as a trap for granule. When the font has been chosen it is important to ensure clarity of definition. The profile of the embossing is equally important to reduce picking to ensure good coating and tooling strength.

‘Sticking’ is another major issue in the design and manufacture of tablets. It differs to picking in that it is granule adherence to the punch tip face. This is not normally associated with design, however when picking occurs on a tablet this in turn can result in sticking on the punch tip face by providing a key to which further granule will adhere.

A coating can be used on the punch tip or a higher press force could be applied to prevent sticking. Punch tips could also be polished in the MF automated polishing machine, or, a design related solution would be to change the cup profile to reduce the depth.

Anti-counterfeiting techniques

As counterfeiters become more technologically advanced, basic tablet designs are more easily re‐produced. The purpose of an anti-counterfeiting feature is usually to enable the authentication of an item, and to act as a deterrent to anyone considering counterfeiting a product. An expert tablet designer can employ techniques to make this more difficult. These are not always visible to the naked eye but ensure that a branded tablet can be identified as an original. Examples include altering the thickness of the embossing in places, changing the angle of the lettering, or simply by having the logo on different inclines.

Conclusion

Good tablet design is imperative. It is important to consult with an expert tablet designer as early in the process as possible, who can ensure that tablet designs are not only unique and visually appealing, but are also robust and producible in a rigorous tablet manufacturing environment. By making just a few simple changes to a design it can stop future problems from picking and sticking to counterfeit issues.

Steve Deakin and Steve Osborn are with I Holland.






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