Solving Tableting and Tooling Problems

Tablet manufacturing problems and tablet-tooling failures are generally the result of one or a combination of the following:

• Difficult product/granule

• Inappropriate tablet design

• Incorrect tablet-press setup and operation

• Inadequate tablet-press maintenance

• Poor condition of tablet tooling.

Poor tooling condition, in particular, can cause many problems. These problems can be minimized or eliminated, however, by adopting a simple, seven-step, tool-care process. The seven-step process is a logical, planned, and professional approach to tooling maintenance and storage that has been adopted by many companies around the world as a standard operating procedure (SOP).

GLOWIMAGES/GETTY IMAGES

The seven-step process

1. Clean.When tooling is removed from the tablet press, it should be thoroughly cleaned and dried to remove any oil or product residue, especially from difficult-to-reach areas (e.g., in embossing and keyways). Equipment used for this process could include ultrasonic cleaning and automated washing processes. It is essential, however, that the process does not cause corrosion of the tooling material, and to that end, a corrosion inhibitor should be used.

2. Assess. Punches and dies should be visually inspected under magnification for signs of damage, wear, or corrosion and to validate the cleaning process. This assessment will establish if the production process is running well or not. The assessment will also give clues to problems with the tablet press or tooling itself and indicate if any tooling maintenance is required. Typical equipment includes high magnification lenses and microscopes.

3. Repair. Light surface wear, corrosion, and damage on tooling can be repaired and polished to a usable condition. Worn tips can result in poor quality tablets and inferior embossing definition, which can lead to picking and sticking. Equipment such as a motorized chuck and double-ended polishing motor are used in conjunction with polishing accessories. Repair should be carried out by well-trained and experienced maintenance technicians to ensure that the tooling is not taken out of tolerance limits. Repair should not be carried out on coated tooling as this may remove the coating from the punch.

4. Measure. Measuring is essential after repair to ensure that crucial tooling dimensions have been maintained within an acceptable working tolerance. The equipment for measuring can range from simple hand-held micrometers, vernier callipers, and height gauges to semi-automatic, computerized, digital gauging systems. Measuring should be carried out at regular intervals, even if repair has not been necessary, to check for natural wear during the compaction process. The essential measurement is the critical working length of the punch as this controls tablet thickness, weight, and dosage.

5. Polish.Controlled, light polishing on a frequent basis will ensure that the tools are maintained to a smooth finish, helping to maximize tooling life and reduce problems such as sticking and capping. Remedial, manual polishing can be conducted using double-ended polishing motors with nylon polishing wheels and fine abrasive compounds. A more controlled and consistent process is the automated, drag polishing system, which relies less on the skill and experience of the operator. This also allows for up to 70 punches to be polished in one 20-minute cycle, thus providing efficiency gains. Recleaning after polishing is not required provided that a food-grade polishing media and paste is used during this operation.

6. Lubricate. Lubrication is important to protect the tooling and aid smooth operation. A range of oils and greases can be recommended for different applications.

7. Store.Tooling storage and transportation should be specifically designed to minimize damage and deterioration. There are different ways of safely and securely storing tablet tooling; these range from storage cabinets to custom-designed storage containers. Whichever system is adopted, it is important that the tooling is separated to avoid contact and that the tooling condition does not deteriorate during storage.

Application of the seven-step process will have a direct impact on the reduction of many common tablet and tooling problems, thus resulting in a better quality tablet, and can provide direct cost saving for the tablet manufacturer.

Applying the process to tableting problems

Common problems can be identified and resolved by using the seven-step process.

Damage to the punch-tip edge.Punch-tip edges are very delicate, and damage in the form of nicks and bruises can cause burrs and occasionally chipping (see Figure 1). These defects can lead to more serious failures, such as punch-tip breakage. Experience tells us that approximately 80% of damage to punches and dies is usually caused accidentally when handling the punches during the production and tool-care processes. Damage can occur, for example, when loading or unloading the tools in the tablet press, during transportation of the tooling, during storage, or while cleaning or polishing the tools. By adopting Steps 1 (Clean) and 2 (Assess) of the seven-step process, this type of damage can be identified and, by repeating the assessment throughout the process, we may recognize the main causes. If the damage is only slight, it may be possible to use Step 3 (Repair) to repair the damage, by removing the burrs and repolishing the tip edges to a good usable condition, thus ensuring consistent tablet appearance.

Figure 1: Typical handling damage to a delicate punch-tip edge.

Head wear.Those areas of a punch that are in contact with other parts of the tablet press and are subjected to high-speed, frictional and compaction forces are typically prone to wear. An example is the head "dwell" flat, which is in contact with the precompression and main-compression rollers (see Figure 2). Premature wear should be investigated. Wear may be attributable to various causes, but commonly it is due to punch tightness and/or poor lubrication. Again this can be identified during Step 2 (Assess). Any light wear can be removed by remedial polishing through Step 3 (Repair). Polishing should be followed by measurement of the critical working length during Step 4 (Measure) thereby ensuring that the punches are within an acceptable tolerance range for good tablet weight and thickness control. Wear to the head angle from the cam track can also be a result of tight punches or lack of lubrication, and this wear can also be monitored and repaired as above.

Figure 2: Typical head wear.

Die-bore wear.Die-bore wear or "ringing" (see Figure 3) is another common defect that can lead to problems such as tablet "capping" and tablet-ejection issues, which result in wasted product and reduced tablet output. Ringing is caused by abrasive wear and deformation from continuous forces acting on the face of the die bore. This damage is accelerated when compressing hard and abrasive granules found in some formulations, particularly vitamins and minerals. This wear can be identified using visual inspection during Step 2 (Assess), but precise equipment is also available to measure and monitor this wear Step 4 (Measure). Physical die-condition monitoring can result in early diagnosis before the defect has a negative effect on tablet output.

Figure 3: Typical die-bore ringing.

Sticking and picking. When in a new condition, the surfaces of tablet-punch faces are usually polished to a high mirror finish. These surfaces, however, can deteriorate over a period of time due to the continuous compaction of granules, which can lead to tableting defects such as "picking" and "sticking" (see Figure 4) brought about by adhesion of the product to the now rough finish. This deterioration can be identified during assessment (Step 2, Assess). The product can be cleaned off with an efficient cleaning system (Step 1, Clean), and the finish can be improved/repaired by using a remedial polishing process as in Step 3 (Repair). With frequent light polishing using an automated polishing system (Step 5, Polish), a smooth and efficient surface can be maintained.

Figure 4: Sticking (i.e., product adhering to the punch tip face).

Conclusion

The seven-step process is a logical sequence to follow when tooling has been used and removed from the tablet press. Each of the steps can be conducted in isolation, if necessary, and go some way towards extending tooling life, while at the same time allowing for the planned and scheduled replacement of tooling.

Trevor Higgins is a technical expert at I Holland, tel. 44 0 115 9726153, email@iholland.co.uk.