A reliable solution to check tablet weight variation

Auto Weight Controller is an electro-mechanical system that derives its decision-making data from the statistical process control outputs making it the most reliable solution for determining weight variation

When David A Aaker, the eminent marketing guru at the University of Berkely, interviewed 248 CEOs of various high tech industries and asked them to name the strategic competitive advantages of their businesses, he could list down 31 of them. Of these, a majority of CEOs voted “reputation for quality” to be the primary reason of their survival. The second most boasted virtue was “customer service and product support.” The third was the “brand name” and so on. Down the lane, the 31st merit was “good distributor relations.”

Today, in general, “quality” is the uppermost concern of all major companies. This is particularly true in case of pharmaceutical companies since they cater to health of the society. This article focuses on the quality control issue of compressed tablets.

Though local machines are quite good in performance, when it comes to the various new formulations demanding international standards and the specific output requirements, sophistication becomes mandatory. Sophistication helps us to have control on the parameters of tablet compression in terms of accuracy in thickness, hardness, weight, surface finish etc.

Newer formulations have constraints on the mixing of sufficient binders and other facilitating components with the active ingredients. This makes most of the conventional machines incompetent in handling such powders efficiently. While using conventional machines, the tablet operators have to be very skilled to manipulate various levers and adjusters on the machines. They have to conduct several trials and errors before coming up with a standardised product. These trials incur wastage of time, raw materials and throughput - this is especially true in case of companies, which often venture into a variety of products.

To alleviate these losses, many companies are now switching to machines, which control important parameters such as weight, automatically. Weight control is most crucial since it decides the dose a patient takes. An operator working with a new powder does three major steps to run a new formulation on a common tablet press. He tries to do several trial and errors to set up the optimal mechanical positioning of dosing cam adjustors for getting the correct tablet weight, hardness and thickness.

Secondly, he regularly picks up samples to ensure that the individual tablet weight is acceptable. Thirdly, he keeps on adjusting the dosing cam position using levers or gears, while the machine is running — if the weight of tablets is not correct.

Auto Weight Controller (AWC) is a vigilant servant, which eliminates these procedures efficiently. The initial trial and error time is significantly reduced by means of statistical process control techniques built into the AWC. This makes the tablet press easily adaptable to new formulation or a different blend of the same formulation.

Secondly, the AWC makes sure that the weight of tablets is regulated very efficiently throughout the tedious eight or nine hour shift. Thus, by the end of the day, we get a batch of tablets with consistent weight — assured of not a single tablet with over or under weight. In the normal operation the weight is checked at the end of a definite period (say 15 mts) and there is no guarantee that the tablets produced between any two such weigh check will have the right weight. In this case, we have to rely on conventional sampling and make a statistical guess to evaluate the weight regularity of tablets of a batch. Whereas in an AWC, the moment an unusual tablet is found during a run, it is eliminated ensuring a zero error batch output.

Weight variations in a tablet press occur due to a variety of reasons. It may be because of a small piece of tablet in the die hole, different flow properties of different batches, special not-so-uniform formulations, humidity and related clogging of powders, non-appropriate speed of the force feeder paddle, some sticky properties of the powders etc.

Since weight variation could be due to various reasons, it is not economically feasible to address each issue and cater to each of them separately by attaching separate gadgets on the machine. This will make the machine inordinately sophisticated and costly. The most reliable and practical solution for such difficult-to-predict weight variation is to incorporate an AWC, which is a rapidly iterative electro-mechanical system that derives its decision-making data from the statistical process control outputs.

An AWC essentially works on a simple principle. We know that the weight of the tablet is decided by the position of dosing cam of the tablet press. Dosing is the process by which the filled die hole is eliminated of excess powders. The cam in which punch head passes through, during this process of this elimination is called the dosing cam. The lower the dosing cam, the lesser the lower punch pushes out the powder and more weight the tablets accrue.

For a specific tablet thickness, the punch head has to negotiate a specific penetration into the die, at the compression point — with respect to the lower punch position at the point of compression. Now, for this particular penetration depth, the build-up of force on the punch tip (or on the head or on the pressure roller), during the process of compression, depends on the mean density of the powder in the die. The more the material in the punch the more resistive force the punch undergoes during compression. This is a crude approximation of Boyle’s law (PV = Constant).

In case of gases the pressure volume variation as per this law is a pure hyperbola (at a constant temperature) but for powders, the curve would be something like what is seen in the figure 1. For example, if the punch head of a machine capable of a 10 ton main compression will feel only 0 ton on its head if there is no powder. Here the mean density of the powder in the die hole is 0. If there is some powder in the die hole and hence a non-zero mean-density, then the reactive force will build up.

Thus the force on the punch tip is generated by the reaction from the powder in the die hole. Now, for a particular fill depth, we know that the mean-density of the powder decides the weight of the tablet produced. Thus if we are able to measure the force on the punch we can very well calculate back the weight of the tablet or in other words, the reactive pressure (which is force per unit area) on the punch head is directly proportional on the weight of the tablet). This is the principle of operation of an AWC.

Such a system, in some European machines or Sejong is incorporated by means of a load cell. This load cell is, in principle, a piezo electric transducer, which translates pressure to electrical voltage. Such voltages are then amplified and digitized. These digital outputs are internally converted (calibrated) to the weight of tablets. These values are processed by statistical techniques. The real time output of these analyses eventually decides whether the dosing cam has to be lowered to accommodate more powders or vice versa.

Based on such decisions, the servomotor under the dosing cam is given the necessary commands to bring the cam up or down.

Though the principle sounds easy, the design requirements of such a system are very demanding. For example, the load cell should be consistent under the environmental electrical and mechanical noises, which distort the actual signals of high impact forces.

Once the consistency is achieved, the user may decide and program through the interactive monitor of the AWC on:

a) the upper and lower limit of allowable pressures (control pressures)

b) the upper and lower limit of pressures which lead to the faulty tablet to be ejected (ejection pressure)

c) the upper and lower limit of pressures when the machine has to stop (stopping pressure).

Thus AWC is a feedback circuit in which the error signal is the difference between these programmed limits and the real pressure generated at the punch tip during a run. The activity of the AWC is to adjust the dosing cam in such a way that these errors are minimised.

For details contact:
Parle Tools International
5th floor, Vyom Arcade, Vir Baji Prabhu Deshpande Marg, Vile Parle (E) Mumbai-400 057
Tel: 022 26827005. Fax: 022 2683 4866
E Mail: info@parletablet.com
Website: www.parletablet.com