The ABCs of Tablet Tooling

Manufacturers producing solid dose products across industries like pharmaceutical, nutraceutical, confectionary, and more, are aware by now that when it comes to tablet tooling, there is no one-size-fits-all approach. No one product is the same, and for each production problem that arises, the solution must be found. Whilst there is no magic solution to addressing all the potential problems that can affect production, by working with a trusted tooling supplier, you can design and implement optimal punches and dies, tailored to your product. This can minimize the chance of encountering common issues such as sticking, picking, abrasion, and corrosion. When designing and ordering new sets of tooling, some key components to look at are steel choice, coating choice, and tooling designs such as multi-tips, shaped tips, and embossing. In short, you need to know the ABCs of tablet tooling.

Steels

Selecting the right steel for each application is key to the successful performance of your tablet compression tooling. Good steel selection will achieve an optimum balance of the following properties: toughness, abrasive wear resistance, adhesive wear resistance, high hardness levels, compressive strength, and corrosion resistance. Whilst satisfying all these criteria can seem like a tall order, high-quality steel should address many of these issues. Most suppliers of tooling offer a variety of steels that resist abrasion and corrosion. Deciding which to use can be overwhelming, but your supplier should offer guidance.

Tooling steel from a reputable manufacturer should be harder and more wear-resistant compared to equivalent steel. Tooling steel has very high toughness properties and a high strength profile, allowing it to achieve the high compression forces required to compress granules into tablets. Choosing quality steel is paramount to avoid tool breakages during production cycles. One option is electroslag re-melted premium steel (ESR), which is refined to reduce non-metallic inclusions and make it more homogenous. Its high chromium content also improves wear resistance. When choosing steels, you should consider the qualities of the product you are compressing, for example, if your product is very corrosive, stainless-steel punches are a great solution when corrosion resistance is a primary requirement.

Tungsten carbide extends the life of tooling by reducing premature die wear and die bore deformation. But while it offers high compressive strength, tungsten carbide can also be brittle,e and using it to tablet nutraceuticals can lead to premature punch tip fractures. In addition, certain formulations react with some grades of tungsten carbide, and when it’s used in dies, it can leave black spots on the tablet walls. However, some premium tungsten carbides are manufactured to not have this issue. When tableting, especially abrasive formulations, a powder metallurgy grade of steel may be a better choice for punches and dies. A specialist powder metallurgy grade steel with uniform carbide distribution throughout the steel structure and small carbide size provides extremely high wear resistance. This type of steel comes with a high wear resistance, extremely high compressive strength, and a reduced risk of formulation reacting to elements within the die material. It reduces die bore wear and ringing and leaves no black marks on tablets.

Coatings

Sometimes choosing the right steel is sufficient for avoiding problems, but other times production issues can still rear their ugly heads. This is where tool coatings can alleviate problems and maintain efficiency. Understanding the product being compressed is crucial when it comes to choosing a tool coating. By utilizing coatings, you can imbue tooling with properties such as improved wear resistance, increased corrosion protection, and guaranteed anti-stick properties. The majority of tablet tooling is still predominantly coated using hard chromium as it is a cheap treatment with reasonable corrosion and wear resistance. Because of this, it is a common choice within the pharmaceutical and nutraceutical industries. However, more technically advanced suppliers are moving to alternative coatings as they are aware of the disadvantages that come with only using hard chromium.

One of the major problems with hard chromium is the issue of microcracking. This develops during the plating process when the internal stress exceeds the tensile strength of the chromium, which is hard but brittle. These micro-cracks are problematic because they provide a porous route to the substrate that will, over time, allow aggressive tablet formulas or cleaning solutions to attack the steel beneath the coating, therefore reducing the tool's strength. Hydrogen embrittlement is another factor to consider when using hard chromium coatings. When it is applied to tooling, a percentage of hydrogen penetrates the substrate, which can decrease the steel’s working load by up to 20%, making it brittle and prone to unpredictable fracture, therefore decreasing the steel’s strength. To counter this effect, the plated tools undergo a baking process (known as de-embrittlement) that reduces but does not eradicate, this unwanted characteristic.

Despite its drawbacks, hard chromium still has its place in the industry, but it’s important to consider better alternatives, especially when combatting specific issues such as sticking. Sticking is the adherence of granules to the punch tip face and is one of the most common issues faced by tablet manufacturers. Common issues associated with sticking are tablet weight variation, reduced tablet output, and problems in downstream processes (e.g. coating). In short, it is a manufacturing nightmare. A popular choice for solving sticking, is chromium nitride (CN), an extremely smooth coating with excellent anti-stick properties, superb corrosion resistance,e, and good wear resistance. Another choice for solving sticking, especially where cost may affect decision-making, is polymer inserts. These are polymer or elastomer inserts cut to match the tip shape which reduces the sticking of the formulation to the punch face and are replaceable at a low cost. These are, however, only suitable for flat-bevelled edge and flat-faced punches. Oftentimes, it may seem hard to choose the correct coating, as each may appear to have its benefits and drawbacks, that’s why it’s always advised (if possible) to share your formulation and communicate your needs with your tooling suppliers as they will likely have experience dealing with similar issues, allowing them to guide you to the optimum choice for your product.

Tooling Design - Multi-tips

A popular choice for increasing tablet output is multi-tips. Multi-tip punches increase the number of tablets produced per turret rotation, which increases the production rates of existing presses. They have transformed the production of tablets into a process that allows manufacturers to deliver them quickly, efficiently, and at a lower production cost. Successful implementation of multi-tip tooling can reduce the need to invest in expensive tablet presses, therefore reducing overall capital spend. Multi-tipped tools are punches with more than one tip, ranging from a simple two-tip punch to one with an average of 26 tips per punch face. Most presses can accommodate multi-tip tooling, but any press being planned for multi-tip production must be in good working order with no significant wear in the turret area, including punch guides, keyways, and die pockets.

It is important to look at the formulation being compressed when using multi-tips as it must have the correct characteristics to work successfully. Granulation flow is important as there will be the need for increased fill. If flow into the die is not at an optimum level and the correct speed, then the use of multi-tips will be affected and productivity increases will not be as high. Particle size is another important factor. If there are high percentages of fines, problems typically experienced when using single-tip tooling can be multiplied. The particle size must be within certain limits for the tooling to work effectively; however, with modification to features of the tablet design, like tip length and types of undercut, and changes to the working tolerances, multi-tips can be accommodated. Multi-tips can produce most tablet designs, but this is influenced by the press and tool-type capabilities. You must look at the tablet size to evaluate the number of tips to fit the punch. Some shapes such as ovals can be more challenging to arrange.

If your requirement for tablet manufacture is to increase production capacity within a short time frame, multi-tips are the best solution. With a combination of correctly designed tablets and well-manufactured multi-tip tooling using the correct raw material and coatings, the use of multi-tips is the obvious answer to greater productivity.

Tooling Design - Extended Dwell Heads

Some formulations are extremely difficult to compress effectively and particularly sensitive to how this crucial element of compression is performed. They require extended time under peak compression to obtain the required tablet hardness and prevent any problems. Dwell time plays an important part in determining if one can produce a tablet successfully, especially in those incorporating formulations that are challenging to compress. Not all formulations are dwelled sensitive as some will compress effectively at any speed. However, many are very vulnerable to even the slightest change.

Understanding dwell sensitivity is instrumental in the production of solid dose forms. Every formulation is different and requires its own carefully calculated compression time. If this is not factored into the production, the results can be detrimental to the bottom line, with product quality, time to make,t and cost implications all affected.

Slowing the tablet press down to achieve an increase in dwell time is a common way to help in increasing compression. This however leads to a reduction in tablet production and has a detrimental effect on production in terms of output and cost. Equally, over-pressuring of the punch can cause the failure of the tool and will damage the press, again affecting production.

The most practical answer is using extended dwell tooling that can be used without slowing the press. This tooling is designed specifically with an elliptical head form, which has an extended head flat. The diameter of the flat area on the top of the punch head is extended allowing the time it is in contact with the compression roller to be longer. An elliptical head flat design can increase the dwell time by up to 50% over a standard punch head, promoting improved granule bonding.

Design Details

Tablet design also affects susceptibility to wear, and a good design helps maintain high overall quality. If the tablet requires a punch with a deep concave face, it will wear quickly around the steepest areas because, during compression, the granules will tend to move laterally across the concave area and erode the tip face. That can lead to capping and delamination. In such cases, consider using a flatter tablet profile or one specially designed to reduce the effects of abrasive wear. Using a compound radius is often helpful. Also consider the gap between the periphery of the punch and the die wall — if abrasive particles get sandwiched there, they’ll accelerate the wear of both the die bore and punch tip. That can cause wear rings to form in the die, and the concave face of the upper punch will gradually curl inward to form a claw, causing flashing.

Tip Embossing

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 consider the practicality of tablet manufacture. Caution is required during the design process when applying branding to your tablet. For tablets with a logo, the design and placement are very important. The tablet designer should always seek to maximize the face area to avoid picking and lack of distinction. However, be aware that problems may arise if too much of the face area is used, leading to embossing distortion and weakness in the tooling.

The correct font style is also very important to avoid tableting problems such as “picking.” Picking is a compressed granule that has adhered to the detail on the punch face, resulting in the “picking out” of parts from the tablet 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 granules. Selection of the right font style can also help to avoid coating problems, tooling failures, and lack of distinction.

Author Details

Rob Blanchard, Research Development & QS Manager- I Holland

Since joining I Holland in 2003, Rob Blanchard has been instrumental in the development of the company’s surface treatments and coatings for tablet compression tooling. He coordinates I Holland’s collaborations with academic research bodies and develops new products and services to bring to market.

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