Issue link: https://www.e-digitaleditions.com/i/1022930
26 September 2018 Tablets & Capsules dor and confirm that any coating you use won't react with, wear off into, or absorb any ingredient of the formulation you're tableting. Coating types The three most common coating types are electrostatic, physical vapor deposition (PVD), and chemical vapor deposition (CVD). Electrostatic coatings were the most commonly used coatings for early tablet tooling, while PVD and CVD are more modern technologies. Electrostatic coating. Electrostatic coatings include chrome, flash chrome, and hard chrome. To apply the coating, the vendor places the steel tooling into a tank filled with the coating material and charges the tank with electricity. The electric charge attracts and bonds the coating material to the steel. An electrostatic chrome coating can provide twice the wear resistance of standard tool steel on average. Note that this doesn't mean that a chrome-coated tool will necessarily last twice as long as a standard uncoated tool. A problem with electrostatic coating is that the coat- ing material tends to ball up in the punch cups, leaving more chrome on one side than on the other, while PVD and CVD coating processes tend to result in a more even coating layer. Another potential problem with this method is that the chroming process can sometimes cre- ate a phenomenon called hydrogen embrittlement, in which the steel around the edge of the punch cup hard- ens. This reduces the toughness of this thin perimeter and limits the amount of compressive force the punch tip can withstand without fracturing. PVD coating. Common PVD coatings include titanium carbo-nitride (TiCN) and chromium nitride (CrN). For PVD coating, the uncoated tools are placed in a tank in a coating machine along with gases and the coating material. For example, to apply a TiCN coating, the coating mate- rial would be a titanium plate. The machine raises the tem- perature inside the tank to approximately 825° to 900°F and strikes the titanium plate with an electrical arc, which atomizes the titanium molecules. The atomized titanium molecules then mix with the gases and adhere to the steel tool surface, creating a very thin coating, a fraction of the thickness of a human hair. While thin, the coating is glass- like and very hard and resistant to material sticking to its surface. A TiCN or CrN coating and can provide ten times the wear resistance of standard tool steel. CVD coating. A CVD coating, such as diamond-like coating (DLC), is applied in a similar manner to PVD coatings except the coating forms as a result of a chemi- cal reaction between the gases and the heated tool sur- face. CVD coating requires much higher temperatures than PVD coating, however, so only certain tool steels can withstand the process without becoming weakened or embrittled. The tablet design and surface features can also influence whether a steel can handle the heat involved in the coating process. There are many more coating types available and numerous companies that specialize in their application. Again, talk to your tooling vendor for ideas about which coatings can be used in your industry, with your prod- uct, and with your particular tool. Starting with the base tool steel is always best. Select the steel that will pro- vide the longest tool life for your product and then determine whether a coating can further enhance the tool's performance. Evaluating your tooling While optimizing the tool steel and coating can make your tooling last longer, the benefits will vary from prod- uct to product. A tool's service life ends when it no lon- ger makes high-quality, marketable tablets. This is sub- jective to some degree and depends on the company's requirements and quality controls. When evaluating tool performance, it's important to have a good baseline. For example, if you know how long the tooling lasts when you use a standard S7 punch and a standard D3 die, you can compare that with how long the tooling lasts when you make changes. Tooling ven- dors look at how many tablets a set of punches can make before the tablet parameters fall outside of QC require- ments. That gives the vendor a baseline from which they can recommend alternative steels and/or coatings to improve on that performance. Vendors use a similar pro- cess to evaluate dies and determine, for example, whether carbide-lined dies (which often produce more than 100 million tablets before wearing out) are appropriate. Have your tool vendor evaluate your tooling and determine whether you have a good baseline and can begin documenting performance information or whether you should you look at replacing those tools and starting over. It's a good idea to evaluate the condition of your press equipment as well, because a poorly performing press can contribute to premature tool wear. Photo 2: A coating can improve wear resistance or reduce sticking and picking without affecting the tooling structure.