Common Tablet Defects That Can Be Avoided Easily
Tablet defects occur for a variety of reasons, but many can easily be avoided. To address defects, technicians involved in tabletting must have a full understanding of both the tabletting process and the materials used. Adequate training in the setting up and use of the machines is vital if the technician is to have the necessary skills to be able to fix equipment and minimize problems in production. A professional and experienced technician can turn an ordinary product into a high quality one whereas a novice or inexperienced operator cannot, and might even be afraid to adjust the machine in order to prevent or correct problems. When the equipment is properly operated the tablet production will go smoothly.
Problems such as tablet defects can cause the company loss of production time and sales, and therefore money. Common problems that afflict the tabletting industry include:
- Weight variation
- Double press or impression
Although there are several defects that affect the tablet making process, capping/laminating, hardness and sticking/picking are the three most commonly encountered.
These defects usually arise from:
- Problems with the unit upstream
- Problems with the tablet press
- Poor quality raw materials
- Defects caused by the formulation
- Milling process causes too many fines that result to capping, hardness, lamination and black spots
- Failure of compression
- Poor flow and compressibility of the powder
- Product is too dry or too wet preventing proper ejection
Ideally any issues with formulation, milling and processing before the tablet compression stage, should be identified well before the formulation progresses to the tablet press. If problems do only become apparent at the tablet press, it is also important that technicians are able to fix these with the minimal process downtime, thereby avoiding the design having to go back to the drawing board.
Below are the three common defects and how best to avoid them.
Capping happens when a fracture occurs at the top of the tablet and the top, or cap, separates itself from the body of the solid tablet. It is often caused by air trapped in the powder material during the compression stage. It can also arise because the press fails to compress the formulation due to the collection of powder fines.
When a tablet is compressed air is expelled from the powder granules allowing each of the particles to stick together. The press is designed to allow the air to dissipate during the compression process. As the air is released from the granules it can also push very fine dry granules fines outwards. These particles generally do not stick together and when the particles are pushed into the line of air being released near the cup and the tablet band, the fines prevent the granules from being compressed resulting in the tablet becoming fractured.
As the air is released when the upper punch die tip is raised, capping only occurs at the top of the tablet. If a fracture occurs in the lower part of the tablet, it is referred to as lamination, and is discussed below.
The faster the press speed, the more likely it is that capping will occur and simply reducing the speed of the press will often solve the problem. That said, there are other ways that this problem may be solved.
Poor formulation as well as bad processing practice can often cause capping. Dry blends can cause capping, as a low moisture content of the formulation tends to prevent the particles from blending with each another. The binder, which is added to a formulation to help bind the particles together, may not be adequate for the particular materials. In the event that capping does occur, it is therefore worthwhile reviewing the choice of binder. Improper mixing or blending can add air to the formulation, and inadequate mixing can also cause powders to segregate, leading to tablet capping. Over blending can itself cause problems so careful optimization of the blending process is required to ensure consistent quality of tablet production.
Capping is usually easy to fix either by increasing the dwell time (i.e., slowing down the production) or by careful analysis of the formulation and production process.
Lamination is the term used for a split in the tablet anywhere but at the top. Lamination is essentially the same as capping and with similar causes. It is, nevertheless, important to diagnose the lamination issue correctly to ensure that proper steps are taken to solve the problem. Lamination often occurs due to the over compression of the tablet. Too much compression can lead to the granules flattening out and thus preventing them from locking together. This can also happen when light or fine particles do not combine, as these particles do not compress well. To prevent this, the thickness of the tablet needs to be reduced, and/or the dwell time increased to allow the fine particles to combine. To increase dwell time, pre-compression can be employed or the speed of the tablet machine can be reduced. Another option is to use a tapered die rather than a perfectly cylindrical die bore. Tapered dies generally do not exhibit capping or laminating problems.
Steps in Eliminating Capping/Laminating
1.Punch Penetration. Check if the tablet press has an adjustable punch penetration. Adjusting the upper punch penetration depth in the die can allow better air exhaust, which can often solve the issue quickly. The punch penetration setting for pre-compression penetration does not have to be the same as for the main compression step.
2.Pre-compression. New models of high-speed rotary tablet press incorporate pre compression, which is an initial compression step before the main compaction step. Pre-compression is a means of compact the powder but at a lighter pressure than the main compression step. It is used especially if there are dry dust fines or particles – it binds the particles together using a lighter force, so that they cannot migrate out during the final compression step. If the tablet needs more dwelling time, pre-compression can be done twice with a higher force the second pre-compression step.
3.Slow the Press Down. By decreasing the speed of the press, the dwell time or the time that the tablet is under pressure is increased. By slowing the press and extending the dwell time, air is allowed to evacuate, giving the particles time to bind themselves to each other, leading to a controlled hardness once tablets are released. It is important, however, to make sure that the dwell time is not too long as that may lead to particles becoming too dry and the tablets laminating. Clearly extending dwell time is one method of solving the capping issue but it needs to be managed so that it does not introduce a laminating issue.
4.Tooling Design. The design of the tablet tooling can influence capping and laminating. A dome headed tool can extend dwelling time and changing the cup depth and radius can help make air release faster during compression stage. Additionally, adding a taper to the die can help evacuate trapped air during compression and reduce the tendency of the tablet to cap. Tapering can also reduce the force that contributes to the tablets’ tendency to laminate. Remember that proper care and maintenance can eliminate other problems such as j-hook, compression wear rings within the die and premature punch tip wear.
Sticking and Picking
The sticking defect occurs when the granules of a formulation become stuck to the face of the press punch. Picking, on the other hand, happens when the granules stick to the design embedded in the punch tip such as in lettering or logos. Both sticking and picking result in defective tablets. A visual inspection is usually carried out as an element of quality control, and may identify sticking and picking. Visual inspection, however, is time-consuming and can decrease the yield production – but many manufacturers have no option. As the batch reaches the compression stage, the operator must adjust the press to comply with the product’s characteristic designs. The tablet press setup and operation, tooling and maintenance can each affect the quality of the product. There are also times when the granules not fully dried. That is, they may be dry and hard on the outside but moist or wet on the inside. This can seriously affect the quality of the tablet as the poorly dried particles can break open during compression and stick to the surfaces of the punch press. If this happens, it is important to check the granule drying process.
Steps to Improve and Eliminate Sticking:
1.Adjusting the compression force. The compression force can affect the product in that increasing the force may cause sticking for over-granulated products and decreasing force may result in the particles sticking to the punch face than to each other.
2.Increase pressure. If picking is observed, one the punch pressure can be increased to encourage the particles to bond with each other rather than stick to the punch.
3.Pre-compression. Use of extended dwelling time through pre-compression or slowing the press speed makes the granules stick together rather than on the punch face.
4.Polishing. Polishing the punch face may help with the sticking problem.
5.Proper lubricant mix. It is important to mix the lubricant properly as over mixing can result to the lubricant being ineffective in preventing sticking or picking.
Sticking can happen anytime during production, but most often happens during the initial setup of the press. Although sometimes unpredictable, it can also be a frequent occurrence, and what may be an acceptable level of sticking for one product may not be for another. To determine the likelihood of sticking, it pays to know the product and formulation properties such as the moisture content and size distribution of the particles. Having said that, it is only when the formulation is in the tablet press that sticking will become apparent.
To prevent sticking during pressing, the hardness of the tablet can be increased by making it thinner. Another solution is to increase the dwell time to make sure that the wet granules stick to the other granules instead of the punch face. Sticking or picking may also occur when the powder blending is incomplete. When the lubricant in the formula is not blended well with the granules they stick to the punch face. If improving the blending does not solve the sticking or picking problem, polishing the cup surface may help. Other causes of the problem may be associated with the product itself, the upstream process, operation of the press or the tooling, or a combination of these factors.
Products that have high compressible formulations tend to turn out high quality tablets. In the event that sticking occurs with these formulations, experienced operators sometimes increase the compression pressure significantly. This shock method may work for limited products but is not recommended as a long term solution as it can overload the punches leading to damage of the tooling, and machine. Again, it must be emphasized that experience operators understand the limitations of the tablet press and would know the best approach to solving a sticking or picking problem.
Another problem with tablet production is variation in the hardness of the tablet. A soft tablet can cause a multitude of problems not only with the press but also with product consistency, film coating process and packaging which can sometimes lead to product recall.
The first step in addressing issues of tablet hardness is to check the press before investigating the upstream processes
1.Weight Control. Maintaining the weight of the tablet is the key to controlling its hardness; fluctuations in the weight cause changes in the hardness of the tablet. Accuracy in the control of tablet weight with respect to a target weight is therefore especially important. A tablet that is lighter than the target tends to be soft tablet whereas a heavier tablet tends to be harder. If there are variations in the bulk density, then the die filling will not be consistent, resulting in wide variations in tablet weight.
2.Scraper blade and die fill. The scraper blade is often overlooked as a item that wears, and should be replaced regularly. If the product is very abrasive it may be that the scraper blade should be changed on a daily basis. For other regular products, the blade may last for several months. Blade lifetime is therefore dependent on the product type and how well the blade is maintained.