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Picking may occur when granulation becomes imbedded on a punch surface and does not freely release when the compressing event has finished.
At 7 am, a product is prepared for compression on a tablet press. The operator obtains the physical properties (thickness, hardness and weight) and begins to compress the batch. Shortly afterwards, some of the upper punches begin to pick.
The operator stops the press, removes the punches and polishes the tips. The punches are reset into the punch bores and compressing resumes. Soon the same, or even more, punches pick again. Polishing resumes. At 8:30 am, after three incidents of picking and polishing, the press finally goes into steady state and the batch completes compressing at 2 pm. No picking from 9 am onwards. Sound familiar?
In solid dosage manufacturing, picking, capping and lamination are the most common defects seen in the compressing suite. Because lamination is related to the material being compressed (i.e., formulation related) and capping is a result of nonoptimized tablet press operation, more research has been dedicated to understanding and overcoming these problems.1–5
Picking, however, is more related to process and industrial practice, and as such has received less rigorous attention. An exception to this situation is an excellent article published earlier this year where the root causes and the corresponding corrective actions addressing the general picking issue were thoroughly discussed.6 Important picking issues and their resolution were presented dealing with specific picking situations that commonly occur in the course of a compressing event. Unfortunately, the article stopped short of highlighting and addressing a unique and specialized picking problem that is often encountered at an initial press start up. This article will offer five alternative procedures to address this and, hopefully, prevent the unique picking associated with press start up.
Picking may occur when granulation becomes imbedded on a punch surface and does not freely release when the compressing event has finished. There are three separate and distinct mechanisms that allow this defect to happen:
Excessive moisture. Excessive moisture in the final blend prevents a clean break between the powder and punch surface at the end of the compressing cycle. Some formulas, such as herbal nutritional supplements, contain a large quantity of moisture. If the product must be a tablet and not a hard shell capsule, some of the procedures described in this article will be beneficial for such applications too.
Punch closure. The punches never really meet close enough to produce an adequately hard tablet and thus, material is allowed to adhere to the punch surface. This may also be described as a mismatch between the product hardness specification and the size/shape of the tablet.
This particular problem may be rectified technically (for some products) by compressing the tablet at a higher hardness value. This situation is not usually regarded as picking, but as incomplete tablet formation. Assuming that this solution is not possible, then prepicked punches and specialized surface coatings for the punch tips are available.8,9
Scratched punch tip surface. Nicks and cracks in the punch tip surface attract and hold small amounts of moisture after cleaning. This promotes picking at press start up and after periods of press shutdown when the same product remains on the press. To avoid this defect, the punch surface must be perfectly polished and the powder to be compressed must not contain an excessive amount of moisture. If either of these two requirements are not achieved, then picking may occur at start up. This is the problem that I will focus on in this article.
The picking observed at initial press setup and in steady state manufacturing are different — picking at steady state is usually caused by either a sub-optimized blend cycle that does not evenly distribute the lubricant, segregation of the granulation before it is introduced into the die or an inadequate quantity of lubricant in the formula. This is very different than picking at start up. Picking has more than one root cause, so it is important to determine if the picking is resolved by technique or is constant no matter what process change is brought to bear. As a subset condition under the technical term 'picking', the picking seen intermittently throughout a batch compression is separate and distinct from the picking observed at press start up and will not be covered in this article.
Two common practices are responsible for picking at press start up:
Leaving product on a tablet press for extended periods of time. Tablet presses require lubrication of the tooling used to manufacture tablets. The punches may heat up, sometimes by many tens of degrees centigrade, if the situation changes around them or they are insufficiently lubricated. If these conditions are not controlled, the product itself may heat, pick or even extrude. As tablet presses cool, they may attract moisture both on metallic surfaces and in the product.
The higher the temperature, the longer the press is idle, the longer the setup and the more moisture the die table and tooling will collect. At press shutdown, with product not fully vacuumed off, a press cools during a break, lunch period, company meeting or (worse) overnight and pulls back all the moisture it gave up during operation.
Depending upon the circumstances, the nature of the material being compressed and how severe the heating is, a restart of the press may be very problematic and result in the classic sticking situation described in the opening paragraph. By avoiding these press shutdown situations, a vigilant management team avoids this unique picking situation, and the quality and efficiency problems associated with it.
The punch preparation task is not fully preparing the punches for pick-free compressing. How can polished tooling, fully prepared for use, cause granulation to pick at start up? Polishing involves grinding the tips to remove damage caused by the compressing event. Abrasive materials may accelerate the damage (wear), but at some point all tooling must be reconditioned and repolished. This function is conveniently performed after a batch campaign if the tooling wear is not excessive. The tool reconditioning involves the application of a grinding material, sometimes a diamond paste, combined with either brushing using nylon brushes and felt bobs followed by a rotary white buffing wheel for final polishing.
It is important to note that experts disagree slightly on whether to use a white buffing wheel for the final polishing.7,8 After polishing the tooling is returned to the production floor or stored.
Most tooling arrives on the floor fully polished with a mirror-like finish. Protocol and standard operating procedures (SOPs) then sometimes dilute all these efforts. Many firms require a final 'cleaning' of the punches with isopropyl alcohol shortly before punch installation. This allows the residual moisture from the alcohol to collect in the micro pits on any punch surface.
This is the primary cause of picking at press start up. Unfortunately, wiping away the alcohol with a cloth or with a tissue-like surface does not remove all the moisture. Once the punches are installed, the residue can cause picking on press start up.
This practice focusses us on two key questions: how are punches best prepared and are there optimum ways to prepare punches to prevent picking?
There are five basic methods of tooling preparation used to minimize or eliminate picking at press start up. Tablet presses are used by many industries and one procedure may be more attractive to that industry than the others. These procedures and the advantages/disadvantages of using each are shown in Table 1. In all these procedures, it should be noted that alcohol is not used to sanitize the tooling at the time of installation on the tablet press.
Table 1 The five main methods of protecting the tooling during installation and press start up.
While proven effective and applicable for resolution of the physical problem at hand, all five alternatives are to some degree unattractive to the pharmaceutical industry. Other industries frequently use one or more of these practices as necessary, often with different practices for different products.
The unique picking that is sometimes very problematic at press start up may be reduced or eliminated by selecting any one of the five options described. Of the five practices, option five — compressing tablets significantly harder followed by a return to specification target — is perhaps the most readily acceptable practice in pharmaceutical solid dosage compression. This procedure eliminates short-term picking at start up and allows for effective segregation of discarded tablets. It also avoids the use of polishing compound at press start up and the need for excessive use of magnesium stearate in the tablet press room.
Fred A. Rowley is a chief lecturer at Solid Dosage Training Inc., USA.
1. R. Kuppuswamy et al., AAPS PharmSci. 3(3), Article 5 (2001).
2. E. Shotton and D. Ganderton, J. Pharm. Pharmac. 13(suppl.), 144T–152T (1961).
3. W.M Long and J.R. Alderton, Powder Metall. 6, 52–72 (1960).
4. E.N. Hiestand et al., J. Pharm. Sci. 66, 510–519 (1961).
5. E. Shotton and B.A. Obiorah, J. Pharm. Sci. 64, 1213–1216 (1975).
6. M. Tousey, Tablets & Capsules 2(6), 52–60 (2003).