Tablet ejection forces

Ceramic materials such as partially stabilized zirconia can also be used as die liners. Ceramics offer high wear and corrosion resistance and lower tablet ejection forces than either steel or carbide due to their low coefficient of friction. 

Some presses are equipped with strain gauges at key points in the overall feed—compress—eject cycle. Thus, these measure compression and ejection forces. Tight specifications for punch lengths and weU-designed and prepared granulations have led to better control of variations in tablet weight. In fiiUy automated presses, weight variations are adjusted by computer. 

The force of tablet ejection from the die, Fr, is a function of both and the residual die wall force, RD WF, that exists after decompression. As the friction decreases, one will obviously see a corresponding drop in Fr. It is important to remember here that it is desirable for Fr to be as low as possible so that minimal damage is imparted to both the tablet and the tooling. 

One should note that BC represents a highly elastic material as little plastic deformation or brittle fracture has occurred. Also, sharp differences between the slope CD and DE are indicative of weak, or failed, tablet structures. The RDWF estimated from these plots can provide a good indication of the ejection force. More detailed treatments of such studies are now in the open literature, to which the interested reader is referred [118-120],... 

Most capsules are filled on piston-tamp machines. These are fully automatic fillers in which pistons or tamping pins lightly compress the individual doses of powders into plugs (sometimes referred to as slugs ) and eject the plugs into the empty capsule bodies. The compression forces are low, often in the range of 50 200 N, or about 50-100-fold less than typical tablet compression forces. Hence, the plugs frequently will have the consistency of very soft compacts and will not be able to be recovered intact from the filled capsule. 

Within the realm of physical reality, and most important in pharmaceutical systems, the unconstrained optimization problem is almost nonexistent. There are always restrictions that the formulator wishes to place or must place on a system, and in pharmaceuticals, many of these restrictions are in competition. For example, it is unreasonable to assume, as just described, that the hardest tablet possible would also have the lowest compression and ejection forces and the fastest disintegration time and dissolution profile. It is sometimes necessary to trade off properties, that is, to sacrifice one characteristic for another. Thus, the primary objective may not be to optimize absolutely (i.e., a maxima or minima), but to realize an overall pre selected or desired result for each characteristic or parameter. Drug products are often developed by teaching an effective compromise between competing characteristics to achieve the best formulation and process within a given set of restrictions. 

The use of compaction simulators was first reported in 1976. Since then, a variety of simulators have been developed. Hydraulic simulators, as well as mechanical simulators, are available to characterize raw materials, drug substances, and formulations, as well as to predict material behavior on scale-up. The appeal of simulators is due to the fact that they purport to provide the same compaction profile as experienced on a tablet press while using only gram or even milligram quantities of powders. Compaction simulators can achieve high speeds, as would be experienced on a production tablet press, and can be instrumented to measure a variety of parameters, including upper and lower punch force, upper and lower punch displacement, ejection force, radial die wall force, take-off force, etc. Summaries on the uses of simulators and tablet press instrumentation can be found in (19,20). 

Another example of the use of DOE during pilot studies is the study of factors affecting the ejection and take-off force. Measurement of ejection force and take-off force will determine if the formula is sufficiently lubricated. Ejection force is measured as an indication of the release of the tablet from the die wall forces, and the take-off force related to the adhesion forces to the punch face. Take-off force is an appropriate measurement to determine if a formula has a tendency to stick. Based on these designs, an optimal formula, including the lubricant level, and process can be predicted for scale-up (Figs. 12 and 13). 

Figure 12 Factors affecting ejection force of a tableting mixture prepared by wet granulation.

Ejection forces are relatively higher for roller compaction tablets, although similar to direct compression. Ejection forces are usually 100-400 N, depending on the tablet weight and shape, as shown in Figure 26. 

Figure 26 Ejection force profiles of tablet cores made from ribbons compacted at three different forces.

In sampling ten tablets, the following parameters were studied the lubrication index (R), the ejection force (F0), the residual force (Fr) and the cohesion index (CI=ratio of the pressure applied to the upper punch and the tensile strength) proposed by Guyot [17]. 

One very common beneficial interaction involving an excipient is the interaction between magnesium stearate and the metal of tablet punches and dies, or the equivalent parts on a powder encapsulation machine. Magnesium stearate is an example of a boundary lubricant. As such it has a polar head and a fatty acid tail. It is believed that the polar head of the magnesium stearate is oriented toward the die wall or tablet punch face. In these ways it is able to reduce the ejection force (the force required to eject the tablet from the die after compaction) and prevent sticking to the punch faces. The other boundary lubricants, e.g., calcium stearate and sodium stearyl fumarate, will also function in a similar manner. However, the so-called liquid film lubricants function in a very different manner (19). 

In general, one should be aware that tablet press instrumentation involves the use of strain gauges or piezoelectric transducers to provide a voltage signal proportional to the force applied for the compaction operation. Let us say we can measure forces, such as those applied to the granulation by the punches, that applied to the die wall, that required for tablet ejection, etc. With tlie use of other transducers we can also measure distance. With the measurement of force and distance, we can calculate work, energy, etc. 

Compression speed The formulation should be compressed at a wide range of compression speeds to determine the operating range of the compressor. The adequacy of the material s flow into the dies will be determined by examining the tablet weights. Is a force feeder required to ensure that sufficient material is fed into the dies Compression/ejection force The compression profile for the tablet formulation will need to be determined to establish the optimal compression force to obtain the desired tablet hardness. The particle size/size distribution or level of lubricant may need to be adjusted in order to have a robust process on a high-speed compressor. 

The pharmaceutical industry produces tablets almost exclusively on rotary tablet presses from pilot plant to commercial manufacture. The output from different tablet presses may range from a few thousand tablets per hour to more than 1 million tablets per hour. By design, the compression event occurs using three parts a die, lower punch, and upper punch. The dies and punches are mounted on a rotating turret. The shape of the die controls the shape of the tablets, while the distance between the lower and upper punch tips at the maximum compression force determines the thickness of the tablets. The tablet compression process is divided into three steps powder filling into the die, compression, and tablet ejection from the die.85... 

Third, after the tablet is compressed, the upper punch is withdrawn from the die and lower punch moves upwards to eject the tablet. Successful ejection of tablets without chipping or sticking requires sufficient lubrication of the powder blend so there is minimum adhesion between the tablet and the die wall. Lower ejection forces are preferred during tablet production to avoid unnecessary mechanical wear on the tablet press. 

From their investigations, it appears that the fluidity of combined powders with chitin and chitosan was greater that that of the powder with crystalline cellulose. The reported hardness of the tablets follows the order chitosan tablets >MCC> chitin. In the disintegration studies, tablets containing less than 70 % chitin or chitosan have passed the test. Moreover, the ejection force of the tablets of lactose/chitin and lactose/chitosan was significantly smaller than that of lactose/MCC tablets [301]. However, no reports are available on CDR formulations using these studies. 

Besides upper punch force, lower punch force, die wall force [63-65], ejection force [66], and tablet scraper force can be measured. Die wall force measurement will be discussed separately. 

The most often measured force is the upper punch force. For the eccentric machine it is the force which controls densification for rotary tableting machines upper and lower punch forces have ideally the same values. Schmidt et al. [67] measured force with a single punch of a rotary tableting machine. Ejection force is visible as a small lower punch signal which occurs shortly after the end of one compaction cycle. It is measured by lower punch instrumentation but needs more resolution. A review of force measurement is given by Bauer-Brandl [68]. 

Some basic parameters can be directly read from the curves. For the force values upper and lower punch forces and ejection forces should be mentioned, and for the time values contact time should be mentioned. Deduced parameters such as pressure and normalized contact time can be calculated and further statistical data are often used for characterization (Table 2). Due to the different shapes of force-time curves from eccentric tableting machines compared with those from rotary tableting machines, some parameters can only be calculated from eccentric machine data and some can only be calculated from rotary machine data. 

Compaction simulators are single-station machines capable of mimicking the in-die compaction event that occurs on a rotary tablet press in real time. Simulators have been used to predict material behavior on scale-up and to evaluate various compaction parameters (punch force, ejection force, displacement, speed, etc.). Hydraulic compaction simulators (ESH) as well as mechanically driven machines (i.e., Presster and Stylecam ) are available for such studies. 

After the compression and consolidation of the powder in the die, the formed compact must be capable of withstanding the stresses encountered during decompression and tablet ejection. The rate at which the force is removed (dependent on the compression roller diameter and the machine speed) can have a significant effect on tablet quality. The same deformation characteristics that come into play during compression play a role during decompression. 

After decompression, the tablet remains in the die until it is ejected. During this time, a residual die wall force is exerted by the tablet on the die wall. Tablet ejection is defined by three stages ... 

The time that elapses from point D to point F is known as the consolidation time. This is the time interval when a force is detectable at the upper punch. The contact time is the period when the upper punch is in contact with the original particles or the tablet (point D to point H). The residence time is the period when a force is detected at the lower punch (point D to point J), which ends on tablet ejection. 

Salpekar, A.M. Augsburger, L.L. Magnesium lauryl sulfate in tabletting effect on ejection force and compressibility. J. Pharm. Sci. 1974, 63 (2), 289-293. 

Formulation and process optimization can be done statistically with the use of experimental design for estimates of the best processing parameters and excipient and lubricant levels. Controllable variables in tableting are mainly the precompression and compression forces and tablet press speed, as well as the formulation component levels. Response variables include the ejection force, tablet hardness and friability, dissolution rate, and drug stability. The purpose of an experimental design is to perform a series of experiments in... 

Punches usually have a longer lifetime than dies if they are made from the same material that is why manufacturers often choose different steel qualities. The surface of a die bore can become smoother or rougher during its use. The ejection force does not— apart from the composition of the tableting material and the compaction force—necessarily depend on roughness of the die wall, but to a high degree depends on the metal type of the... 

Reduced ejection forces can help to avoid both lamination and capping and give tablets a shiny surface... 

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