Ejection force

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. 

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],... 

Fig. 10 Illustration of the dosing-disk filling principle (A) view looking down on the dosing disk (B) side view (projected) showing progressive plug formation. Note the placement of strain gauges on the piston to measure tamping and plug ejection forces (see text). (From Ref. 37.)...

Fig. 17 Effect of powder bed height, piston height, and compression force on plug ejection force in an instrumented Zanasi LZ-64 automatic capsule-filling machine (pregelatinized starch lubricated with 0.005% magnesium stearate). Note that the first point of each curve is precompression. Piston height (mm) , 15 -jlf, 14 , 13 , 12. Powder bed height (mm) heavy line, 30 light line, 50. (From Ref. 51.)...

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). 

Characterize the drug substance regarding deformation properties, compactibility, sticking tendency, ejection force, etc. 

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). 

Statistically Significant Main Effects (p<. 10) in Order From Strongest to Weakest for Ejection Force (6kN) Magnesium Stearate Lubricant... 

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

Another force measurement to monitor is the ejection force during the compression event. Figure 15 shows the ejection force monitored during the compression run. The ejection force was approximately SOON throughout the compression run. This demonstrated that the blend was lubricated adequately. No picking/sticking was observed on the punch surfaces after 225 minutes. 

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). 

Ejection force vs. time curves to observe shape and peak force to evaluate lubrication efficacy... 

Eased on plug ejection forces, a relatively lower level of lubricant (about ) is sufficient for H K machines as compared to Zanasi. 

Schwartz [23] suggests special consideration for the compression process. Press speed for material that compact by plastic deformation, overmixing of lubricant in the force feeder, heat buildup on long compressions runs, material abrasiveness, and tooling care are important variables for consideration. Dwell time and compression and ejection forces are other variables identiLed for monitoring process. 

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. 

Percentage wetting agent Ejection force Mixture... 

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. 

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