Trevor Higgins

	Tablet tooling, usually made from hardened and tempered tool steel, is subjected to a highly demanding process. The tooling is rotated through a cam track system at high speeds, exerting high friction forces. High cyclic compaction force is also applied to the punch heads via the precompression and main compression rollers to form what can sometimes be aggressive granules and powders into the final tablet. The delicate punch tips are extremely vulnerable to accidental damage at various stages of this process. This article highlights real-life examples of the more common types of damage or wear to the tooling and explains how these problems could be avoided. 


	One of the most common issues experienced is handling damage. This is usually accidental and can occur at several stages of the process such as: unpacking the tooling, loading/unloading the tools in or out of the tablet press, tool cleaning/maintenance procedures, and storage/transportation. If this damage occurs, usually in the form of nicks or burrs to the delicate punch-tip land area (see 

), it can lead to the production of poor quality tablets and probable further damage to both the tooling and the tablet press. With a better understanding and appreciation of the delicate nature of the tooling and with good tool care, maintenance, storage, and handling procedures, problems can be reduced.  


	Inappropriate operating conditions can occasionally lead to catastrophic failure during use of the punches and dies. Again, these issues relate to the delicate nature of the punch tip. The detail in the punch tip is usually described as the cup; this cup forms the profile of the tablet and can contain detail such as embossing or break lines, which effectively reduce the strength of the punch tips. Such problems can manifest themselves in small, difficult-to-detect cracks, chipping of the punch-tip edges, or even a full piece of the tip edge breaking away (see 

). These issues may look insignificant to the naked eye, but because high cyclic loading is applied to the cup many thousands of times per running hour, the tip edge is flexing and bending within the clearance between the punch tip and die bore. If the conditions are not optimized, this can eventually lead to fatigue or brittle fracture, causing the punch tip to fail. These types of failures can be avoided by careful punch and tablet design, correct metallurgical condition, and using the appropriate compaction force.


	Aggressive formulations can subject the punch tips to adverse effects such as abrasion, pitting, or corrosion when under compression, as shown in 

. The tools are manufactured from hardened and tempered tool steel, yet due to the demanding process, can deteriorate quickly if the tool material is not optimized to suit the formulation being compressed. Certain granules are extremely hard and abrasive and can scratch, wear, and impregnate the steel surface. Other granules can contain corrosive elements that react with the steel. Understanding the nature of the product to be compressed and carefully selecting a material and operating conditions can reduce these effects.


	Die wear is also quite a common occurrence, especially when compressing hard, abrasive granules and powders. Die wear is also known as “die ringing” because the wear takes place in the area of the die bore where the tablet is compacted. The wear appears in the bore as a double ring (see 

) due to the upper and lower punch tips flexing and bending within the clearance between the tips and the die bores during full compaction. The granule reacts between these surfaces leading to compressive and abrasive wear. When the wear becomes significant, it will lead to problems such as tablet capping or delamination and increased tablet ejection forces. This type of wear can be reduced by using different zones in the die bore where the tablet is compacted or by the selection of a harder, more wear-resistant die material.

	Incompatability of geometry between the punch-head profile and tablet-press cams will lead to premature head wear. The punch heads are subjected to high cyclic loadings, typically 20 kN to 50 kN, many thousands of times per hour of running. If the punches are running tight for any reason, there will be resistance between the punch heads and the cams and rollers, creating high frictional force. This will lead to premature wearing of the heads and eventual fatigue and total breakdown of the metal (see 

). An additional effect of this breakdown will be contamination in the tablet press by the metal particles, leading to expensive damage to tooling, press cams, and compression rollers. This type of damage can be avoided by close monitoring of the tooling condition and good tooling and tablet-press maintenance procedures.


	Another common sight is damage to punch keys (see 

), which is usually the result of handling damage during loading and unloading the tooling in or out of the tablet press. The punch key is an extra component fitted to punches for producing non-round tablets and for punches with multiple tips (usually upper punches) to facilitate accurate alignment of the upper punch tip into the die bore. The key is usually manufactured from a slightly softer material than the body of the punch, which may make it more prone to damage if not handled correctly during loading and unloading. Damage to keys can occur during storage and transportation of tooling, tool cleaning, repair and polishing, or wherever the keys can contact a hard surface. Damage to the keys can be reduced by having a good understanding and appreciation of its function and by taking extra care during the handling, maintenance, and setting processes.


	The problems discussed in this article are quite common, although the damage is not always to the extent shown in the images in this article. Damage and wear to tooling result in time-consuming and costly mistakes. If the correct procedures are put in place at the beginning of the process and the correct quality tooling is used, problems like the ones discussed will simply not happen. Employing good care and maintenance practices with both the tooling and the tablet press is crucial.


	Trevor Higgins is technical director at I Holland, trevor.higgins@iholland.co.uk, tel. +44 (0) 1159726153. All figures are courtesy of the author.      

When referring to this article, please cite as T. Higgins, “Preventing Tablet Tooling Problems,” 

Articles

Tooling can be damaged by poor handling or problems in process design or material choice.

Preventing Tablet Tooling Problems

Poor tooling condition can cause many tableting problems, but these problems can be minimized or eliminated 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.

The seven steps are summarized as follows:

 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., inside 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.

 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.

 Light surface wear, corrosion, and damage on tooling can be repaired and polished to a useable 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.

 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 semiautomatic, 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.

 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.

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

 Tooling storage and transportation should be specifically designed to maximize security and safe handling to minimize damage and deterioration. There are different ways to 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.

For examples of how the seven-step process can be applied to real tooling problems, read the article by Trevor Higgins in the upcoming February 2013 issue of 

Trevor Higgins is a technical expert at I Holland,

Adopting a seven-step process resolves tableting problems.

Improving Tablet-Press Tooling Maintenance and Storage

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.

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.

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.

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.

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.

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.

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.

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.

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.

 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.

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. 

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).