Why is Good Tablet Design so Important and how to Achieve it?

Detailed design of pharmaceutical and nutraceutical tablets is essential in order to produce robust tablets with bespoke designs. Tablet manufacturers should not overlook tablet design because it is key to the quality of the end product.

“With sufficient fore-thought and consideration, most potential tablet making problems can be eliminated at the early stage of design.” I Holland.

Good tablet design is extremely important; it has an impact upon anti-counterfeiting, tooling strength, tablet coating, durability and functionality. Good tablet design helps to avoid downstream manufacturing problems such as tablet sticking, picking, lamination, capping and premature tooling failures. It is essential to consider these at the beginning of the process, ensuring a problem free, high quality, end product, so getting all these factors right is imperative to a ‘good’ tablet and to mazimize the efficiency of the tablet manufacturing process.

Several elements need to be considered when designing a tablet, including shape and profile, breakability, tablet/tooling performance, branding and finally anti-counterfeiting.

Tablet Shape and Profile

The first thing to consider is the tablet shape followed by an optimum tablet profile. 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. The shape plays a decisive role in the final quality of the tablet.

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 of which there are many options, including flat faced, bevelled edge, single radius / double radius (compound) cup, ball or pill. The type of profile required is influenced by several factors; the granule, embossing requirements, coating process, packaging and the company’s branding. Should a large amount of branding be required, a profile with a larger surface area is preferred, such as a shallow cup or flat bevel.

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, whether it is film or sugar coated, present challenges for the tablet designer. The complexity of the coating process is vast. Many of the variables are within the manufacturer’s control but expert tablet design can help eliminate some potential problems.

Typically, the centre of a tablet, the core, 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 to the core. This vulnerability, caused by mechanical stress during coating, can be reduced by avoiding very deep concaves and ensuring a robust design. For film coated tablets, double radius profiles are the preferred choice.

For shallow tablets with hard, sharp edges, the coating process will damage the exposed edge of the tablet which can result in chipped edges and sometimes cracks within the coating. 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, which will roll in the coating pan. Another benefit of the double radius design is that it can accommodate most marking and branding requirements (i.e. de-bossing, breakline, logo), because it increases the usable surface area available for this.

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 because the coating does not fully follow the contours of the marking on the tablet core, but bridges over leaving a void under the coating) and Infilling (when too much coating material has filled the detail making it indistinct) during film coating. Possible causes for this can include:

1) Inadequate adhesion of the film coating – the coating supplier should be consulted to improve the adhesion characteristics of the coating.

2) Inappropriate marking design where the angle may be too acute or too deep (bridging) – The marking should be re-designed in consultation with the tooling supplier. It can also be due to the stroke or section of the embossing being too wide or too shallow (in-filling).

3) In appropriate coating procedure i.e. spray rate, drying time etc – the coating supplier should be consulted.

Another problem is twinning, tablets sticking 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 and eliminates the potential for twinning.

Tablet Breakability

Good tablet design will enable the tablet to be broken easily and accurately, ensuring that when the tablet is broken, the required tolerance for dosage is achieved. 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.

FIG 1 – The tablet on the left shows a D type breakline, a more rounded cross‐section causing an uneven break. The tablet on the right shows an optimised cross section, the breakline has a small radius and penetrates deep into the profile of the tablet to effectively achieve an optimum break.

Figure 1

Not all breaklines are functional breaklines for example short bisects (Fig.2 – B type or H type) which are mainly used for aesthetic purposes and will not break evenly. But combinations of a G type and D type breakline, applied to opposite sides of a tablet with the correct physical dimensions are considered the best options for successful tablet division.

Fig 2 Tablet Breaklines

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.

The breakline should penetrate into the tablet whilst maintaining an optimised radius and angle. A larger radius usually makes the breakline less effective.

Where a breakline is functional and present on both sides of the tablet, alignment of the breakline on the upper and lower punch tips is critical and requires the turret to have a lower key facility. Also pay attention to breaklines that stand above the punch tip edge. Upper and lower punches must be set correctly for effective tablet ejection and take‐off. If not set correctly damage can occur to the breakline on the punch tip resulting in catastrophic failure and damage to the tooling and the tablet press. It can also cause severe chipping of the tablet on ejection.

Fig 3 illustrates a breakline that stands above the edge of the tip.

Normally setting is gauged from the overall length of the tool but adjustment may be required to prevent: Tablet chipping during take‐off, De‐capitated tablets at ejection, or upper and lower punch tips coming into contact with each other.

Tooling Performance

Tablets are becoming more complex and exotic in both shape and profile for brand identity and marketing. As the tablets become more complex so does the tooling which increases the demand for tooling strength, durability and overall performance. This has to be a major consideration when designing a tablet.

Fig 4 shows a 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 behavior of engineering structures. It is used to calculate deflection, stress and strain to determine fatigue limits of both material and design.

Figure 4 – FEA Analysis of a punch tip.

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 as shown in the image.

An expert designer will be able to add appropriate strengthening features to the design such as blended lands and profile changes. They should also ensure that any embossing detail is not in direct proximity to the high stress areas. Optimising the embossing position on the tablet face can be of benefit to the strength of the design rather than detrimental.

One of the most important features of any tablet design is the blended land. 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.

The answer is to always include a blended land as when applied correctly, it will optimise tablet and tooling strength and performance. When applying a blended land it is important that the designer considers all these factors. The size of the land should be appropriate to the design of the tablet.

Fig 5 – Blended land

Fig 5 shows 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 correctly selected and applied blended land provides benefits to handling, loading, setting, tooling strength, the visual appearance of the tablet and ultimately, your brand.

Tablet Branding

When considering the visual appearance of the tablet, it is important to think about the type of font and logos used for branding. Typefaces and designs must take into account practicality of tablet manufacture.

Caution is required during the design process when applying branding to your tablet. 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, be aware as problems may arise if too much of the face area is used leading to embossing distortion and weakness in the tooling.

Fig 6 shows 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.

Figure 6
Occasionally, when the need to exceed the safe zone is required, the best practice is to ensure the detail is spaced far enough away from the edge of the tip. 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 during the tableting process.

If a company decides to have a common theme of identification on their tablets, it is important to take into account how that identification will scale up or down, according to the size of the tablets to be produced. When designing logos, applying identification and other detail, think about the smallest tablet that is likely to be produced, as this reduces the working area available. Where company or product logos are required on the tablet face the complexity of the logo should be considered. If logos have been designed for advertising, packaging etc. they may need to be adapted and modified to have less detail. This can often be achieved with very little detriment to the final appearance of the tablet.

The correct font style is very important to avoid tabletting problems such as ‘picking’. Picking is compressed granule that has adhered to the detail on the punch face, resulting in ‘picking out’ of parts from the tablet face.

Fig 7 is a classic example of picking. For identification purposes this product requires heavy embossing contained in a small area. This design creates powder traps, these areas are prone to picking. In this instance, the center of the 6 has been completely picked out. Therefore it is important to use techniques to prevent this from occurring. 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. Selection of the right font style can also help to avoid coating problems, tooling failures and lack of distinction. Fonts with curved sides can be used to increase spacing between the letters. By using curved characters the potential for granule traps in tightly spaced embossing can be reduced. It is also essential that the spacing between the embossing is not too small generating further powder traps.

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, ensure good tablet coating and tooling strength.

The best practice is for the stroke angle to be between 70 to 80 degrees. The stroke depth should be 50% of the width and the stroke break radius blends 30% of the stroke depth (Fig 8).

Fig 8

Another technique used to minimise picking is a reduced counter. The counters, which are sometimes referred to as islands, are very vulnerable to picking and granule can easily get trapped in these areas on the punch tip face. The counter is modified to increase the surface area by reducing stroke depth, which will minimise the tendency for the product to pick.

A tapered peninsula (Fig 9) can also be applied. The taper is applied to blend between the surface of the tablet face and the stroke angle. This method softens the profile reducing the risk of powder being trapped in the shaded areas shown in the image.

This maintains definition without reducing the overall stroke depth of the embossing and will assist you in keeping a clear brand identity.

Fig 9

‘Sticking’ of course is another major issue in the design and manufacture of tablets. Sticking differs to picking in that it is granule adherence to the punch tip face, rather than in and around the embossing. 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.