Tabletting equipment: common problems and how to avoid them

Trevor Higgins. Director at I Holland.We regularly deal with tabletting equipment problems and malfunctions, and find that the most common cause is misuse or improper handling and transportation, which often leads to accidental damage to tablet punches (including nicks and dents to the punch tip edges). Premature wear to the punch heads through incorrect operation/set up of the tablet press is also a common problem, as is picking and sticking of the product to the punch faces leading to defective tablets.

Recommendations

To prevent or reduce the occurrence of these common problems, I have three key recommendations for manufacturers:

• Take great care: most tip edge damage occurs through careless handling of the punches when setting in the tablet press, during repair, measurement, polishing or general transportation. Although the tools are made from hardened tool steel and can withstand high compression force and wear, the tip edges can be very delicate and should be treated as if they were glass. Even the slightest contact with another punch, press part or any hard surface can bruise or nick the fine tip edge and this damage can then lead to premature fatigue failure of the punch. Great care should be taken when loading the tooling into the tablet press, polishing and measuring equipment. A suitable transport system, which keeps the tooling separated at all times, should be adopted.
• Maintenance is essential: premature head wear will occur if the punches are not operating freely within the punch guides, cam tracks and through the compression rollers. Punches can run tight if the lubrication is insufficient or if the guide-ways become contaminated by residual product mixed with the lubricant. Tightness can also occur in the case of shaped and multi-tipped punches if they are not aligned to the dies correctly; the tips will bind in the dies and cause undue force between the punch heads, cams and rollers. If the tips are damaged in any way they will also bind in the dies and the result will be similar. Top quality tooling and press maintenance is essential. If the tooling or the press are not kept in pristine condition, damage may occur to either or both.
• ‘Picking’ and ‘sticking’ are often used to mean the same thing, but are actually different defects. ‘Picking’ can be described as compressed material that has adhered to detail on the punch face, resulting in ‘picking out’ parts of the tablet face. This can usually be resolved by re-designing the embossing on the tablet. There are many subtle techniques to achieve this and a better quality tooling supplier will advise on this. ‘Sticking’ is described as general granule adherence to punch faces and die bores, and is not influenced by embossing, logos etc. Sticking is generally a product formulation issue, but in some cases can be reduced by certain tooling materials and surface coatings/conditions, or by changing the tablet hardness/density. Again, your tooling supplier should be able to give advice.

Seven steps to maintenance best practice

Tool maintenance is essential for problem-free production of tablets and to obtain the best service life of both the tooling and the tablet press. There are many systems and equipment choices available, but the ‘Seven Step’ approach can be adopted as a logical guide to best practice for tool maintenance.

Step 1. Clean: When the tooling is removed from the tablet press it should be thoroughly cleaned to remove all traces of product, lubricant and contamination. Whilst there are many systems available to clean the tooling, there are three key elements to consider: i) the cleaning should be thorough and remove all contamination from embossing, keyways, holes etc.; ii) once the tooling is clean it is vulnerable to oxidation and should be protected with corrosion inhibitors; iii) the tooling should be handled carefully during the cleaning process as accidental damage may occur if punch tips make contact with each other or the cleaning equipment.

Step 2. Assess: Once the tooling is clean we can then assess the condition. The condition can show if the process is running well or not; various types of damage and wear can give clues to problems with the press or tooling. The assessment is usually carried out by visual inspection of the tooling under magnification — for which there are systems available from a simple magnifying eyeglass to a sophisticated high magnification camera inspection system where images can be stored and transferred.

Step 3. Repair: The assessment will help to decide if any repair is necessary. Repair can be carried out on lightly damaged tips and minor head wear. Equipment is available in various forms, but the person performing the repair must be well trained and experienced in these techniques.

Step 4. Measure: Tooling should be measured to check for natural wear during the compaction process and also after any repair work. The essential measurement is the critical working lengths of the punches because this controls the thickness, weight and dosage of the tablets. Other measurements include the tip size and die bore size/condition. Again, it is important that the personnel are well trained in this procedure.

Step 5. Polish: Keeping the punch tip faces polished to a high degree of surface finish will maintain the tooling life and reduce problems, such as sticking and capping. Polishing can be carried out manually using polishing motors or automatically using special polishing machines.

Step 6. Lubricate: When the tooling is polished and ready for future use, it is essential to protect it from oxidation. Various types of rust inhibitors are available, including food safe options.

Step 7. Store: The safe storage and transportation of tooling is also essential to maintain the condition of the tooling and prevent damage to the delicate punch edges. There are various systems available, including custom-designed storage boxes and cabinets with special insert trays for keeping the tooling separated. These systems are also good for inventory control and tool management.

Selecting the right equipment

When selecting tablet equipment the following factors are key:

• Good quality: to produce high‑quality tablets, the equipment in the tablet making process should be of high quality, especially the tablet tooling because this is a mirror image of the tablet. Many production problems such as picking, sticking and capping can be eliminated by using superior tooling. The lifetime of the press and tooling will also be enhanced.
• Standardisation: standardisation of tooling is important for utilising the inventories of tooling for various products and tablet presses.
• Expert product and after sales support: quality equipment suppliers will offer a training and technical support service to troubleshoot problems and design improvements.