Die Wall Pressure Research Articles

Effect of lubrication on the compaction properties of pharmaceutical excipients as measured by die wall pressure

The effects of lubrication on the compaction properties of pharmaceutical powders were evaluated by measuring several compaction parameters, including maximum die wall pressure (MDP), residual die wall pressure (RDP), and RDP/MDP, which is a novel parameter for evaluating the compaction properties of materials. In tableting model powders lactose and mannitol, a higher RDP/MDP and a tendency toward capping were observed. The addition of a small amount of internal lubricant to either powder decreased RDP/MDP and inhibited the capping tendency. When external lubrication was applied to the powders, similar improvements in these compaction properties were observed. However, the tensile strength of the mannitol tablet prepared under external lubrication conditions was much greater than that of the tablet prepared with 0.5% internal lubricant. On the other hand, the hardness of lactose tablets was almost the same irrespective of the lubricating method. These phenomena were explained by the plastic deformation property of mannitol and by the fragmentation of lactose particles during compression.

Tabletting of Solid Dispersion Particles Consisting of Indomethacin and Porous Silica Particles

We attempted to make the rapidly dissolving tablet (Tab) containing solid dispersion particles (SD) with indomethacin (IMC) and porous silica (Sylysia350) as carrier prepared by using spray-drying technique. Rapidly dissolving tablet was formulated with mannitol as a diluent and low substituted hydroxypropylcellulose (L-HPC) or partly pre-gelatinized starch (PCS) as a disintegrant. The percent dissolved from Tab (SD) was higher than that of tablet containing physical mixture (PM) at 20 min. Nearly 100% of drug in Tab (SD) was dissolved within 60 min, while the drug dissolution of Tab (PM) was not completed at the same time period. In addition, the tensile strength of Tab (SD) was much higher than that of Tab (PM). Adding L-HPC in Tab (SD) (Tab (SD-L-HPC)), the percent dissolved from Tab (SD-L-HPC) at 5 min became much higher than that from Tab (SD). The dissolution profile of IMC from Tab (SD-L-HPC) was almost the same irrespective of the compression pressure, while the tensile strength of tablet increased with increasing the compression pressure. In comparing the compaction property of these tablets by observing the ratio of residual die wall pressure (RDP) to maximum die wall pressure (MDP) (RDP/MDP), it was found that addition of L-HPC in the tablet formulation improved compactibility. In case that PCS was formulated as disintegrant, Tab (SD-PCS), similar improvement in the dissolution profile and tensile strength was observed, though the dissolution rate of IMC from Tab (SD-PCS) was slightly lower than that from Tab (SD-L-HPC) irrespective of the compression pressure.

Die wall pressure measurement for evaluation of compaction property of pharmaceutical materials

The aim of this study was to evaluate the compaction property of several pharmaceutical materials by measuring the die wall pressure. The profile of die wall force during tabletting process was measured with the compaction process analyzer (TabAll). Several compaction parameters such as maximum die wall pressure (MDP), residual die wall pressure (RDP) and pressure transmission ratio (PTR) from upper punch to lower punch were calculated. The ejection pressure (EP) of tablet compacted was also measured as a parameter for sticking property of the compacts. The profile of die wall force observed was classified to the typical two types, a small type and a large one. Partly pre-gelatinized starch (PCS), cornstarch and low substituted hydroxypropylcellulose ( L-HPC) were the small type, while crystalline lactose, ascorbic acid and potassium chloride were the large type. The die wall force of crystalline lactose remarkably increased at the ejection of tablet and then capping was observed. RDP value of PCS, cornstarch, L-HPC was smaller than that of crystalline lactose, ascorbic acid, potassium chloride. As the higher pressure transmission ratio from upper punch to lower punch means a good compressing property of the powder, we proposed that RDP/MDP is a useful parameter for evaluating the compaction property of powders. Although potassium chloride which is strongly plastic deformable powder showed the highest RDP value among the powders tested, the RDP/MDP value was lower than that of crystalline lactose or ascorbic acid and the tensile strength of resultant tablet of potassium chloride was much higher than these powders.

Radial die wall pressure in compaction of ceramics

Radial die wall pressure in compaction of ceramics

Radial Die Wall Pressure during Ceramic Powder Compression and Unloading.

Radial Die Wall Pressure during Ceramic Powder Compression and Unloading.

Analysis of the Mechanics of Die Drawing Polypropylene Through Strain Rate Controlled Dies

The die drawing of polymers in the solid state is now an established process for achieving molecular orientation in drawn products and an enhancement of their mechanical properties. A mechanics analysis is presented of the drawing of polypropylene through strain rate controlled dies, where the strain rate is either constant, increasing at a uniform rate or decreasing through the die. The stress in an element of material being drawn is determined from equilibrium considerations and application of the Tresca yield criterion. Flow stress data are deduced from uniaxial stress-strain-strain rate tests and applied to the problem using equivalent stress-strain concepts. Results are presented of axial stress, flow stress and die wall pressure throughout the dies and show that polypropylene can prematurely leave contact with the die wall, particularly in decreasing strain rate dies at high drawing speeds. Increased friction of the die wall and increased drawing speed cause this ‘loss of contact’ point to move progressively downstream in all die cases. Theoretically predicted drawing loads show acceptable agreement with experimental test results when low values of friction coefficient are assumed.

Application of a newly defined capping index in evaluation of the compressibility of pharmaceutical powders

The compressibility of pharmaceutical powders was evaluated using the capping index which was newly defined as the ratio of the residual die wall pressure to the binding strength of compacts. As the compaction pressure was increased, the residual die wall pressure increased while the binding strength reached a plateau, and the resultant capping index increased. These facts explained the capping behaviors during tableting under various compaction pressures. In the case of a binary powder mixture, the residual die wall pressure changed in a simple manner with a change in powder composition while the binding strength changed in a complex manner. It was found that the capping tendency of these mixtures could be correlated with the capping index. It was also found that the addition of polymeric powders was effective to prevent capping.

Characterization of die wall pressure to predict capping of flat- or convex-faced drug tablets of various sizes

Die wall pressure during compaction of drug powders was measured to elucidate the mechanisms involved in capping. More than 200 kg/cm 2 of die wall pressure remained after decompression, whereas in the case of capping, the die wall pressure rapidly dropped at the final stage of the decompression process. These results suggested that capping was the fracture of compacted powders caused by high residual die wall pressure. The extrapolated residual die wall pressure of convex-faced tablets was higher than that of flat-faced ones. For the convex-faced tablets, the thinner ones indicated a higher residual die wall pressure. These results coincided with the capping tendency seen in drug tablets and should pave the way for a better design of tablet formulation.

Introduction of a new index for the prediction of capping tendency of tablets.

Capping within a tablet was quantitatively estimated by using the capping ratio, defined as the ratio of the tablet strength to that of the uniformly compacted tablet.Capping index was also newly defined as the ratio of the extrapolated residual die wall pressure to the binding strength of compacted powder. It was found that capping occurred when the capping index exceeded unity. This fact indicated that capping can be regarded as cracking within a tablet caused by high residual die wall presure. The requirements to achieve uniform compaction were clarified from these results.It was found that the extrapolated residual die wall pressures of convex-faced tablets were related to those of flat-faced ones. It was also found that tablets consisting of more elastic powders had a lower ersidual die wall presure.

The role of binders in the prevention of capping within a tablet.

The role of a dry or wet binder in the prevention of capping within a tablet was evaluated by considering the binding strength of compacts and the residual die wall pressure during compression.Capping of bucetin tablets was not prevented by adding dried colloidal silica, which increased the binding strength and the residual pressure. However, the addition of dried low-substituted hydroxypropyl cellulose, which increased the binding strength but decreased the residual pressure, prevented the capping.The effect of addition methods of a low viscosity grade of hydroxypropyl cellulose and α-starch was also investigated for bucetin and ascorbic acid tablets. The compacts showed the greatest binding strength when the binders were added in a wet state. However, the residual die wall pressure did not depend on the addition method. These results suggest that the addition of a binder in the wet method is effective for the prevention of the capping.

残留壁面圧と結合強度による粉末の圧縮成形性の評価

The compressibility of powders was evaluated by using residual die wall pressure and measuring the binding strength of the compacted powders.The binding strength was reflected in a saturated curve as the compaction pressure increased. However, the residual die wall pressure increased in proportion to the compaction pressure. These phenomena explained the capping behaviors during tableting.In the binary mixed powders, the binding strength changed in a complicated way, but the residual die wall pressure changed simply with the composition of the mixture. The ratio of these two values was defined as the capping index, and the actual capping tendency was defined as the capping ratio. Good correlation was observed between this index and ratio.

An evaluation of the techniques employed to investigate powder compaction behaviour

Employing 4 model powders, Avicel, Dipac, mannitol and paracetamol and a single-punch tablet machine, the most common techniques used to investigate powder compaction behaviour are compared and assessed. The use of tablet tensile strength rather than crushing force is found to bias the interpretation of lower punch work (LPW 1)/hardness profiles when compacts of very different thicknesses are produced. Radial versus axial pressure cycles, maximum die wall pressure and lower punch work on recompression (LPW 2) are demonstrated to be insensitive comparative measurements of compaction behaviour. Further, the latter suffers from considerable interpretive difficulties. Stress relaxation measurements are employed to verify data obtained from residual die wall pressure determinations and Heckel plots. Thus, the use of LPW 1 /crushing force profiles, residual die wall pressure versus axial pressure and elastic recovery versus LPW 1 plots are simple and efficient techniques that assist in defining powder compaction behaviour

Rotary Press Utilizing a Flexible Die Wall

A die with a flexible wall was constructed and evaluated on a specially modified instrumented rotary tablet press. The design permits an inward deflection of the die wall by a side punch, which rolls past a side compression roll during compression-decompression. The side compression roll is instrumented to monitor the applied side compression roll forces. On decompression, return of the die wall to its original position permits release of residual die wall pressure. The decreased residual die wall pressure can decrease fracture and capping of tablets for problem formulations. The performance was tested on three experimental formulations. For these formulations, tablets made in a conventional die exhibited severe capping problems. However, most tablets compressed in the special die were superior. With proper adjustment of punch and die wall compression forces, excellent tablets could be manufactured. The merits of the special die and modified tablet machine are substantiated, although this initial design did not provide adequate die wall pressure for all formulations. Further engineering efforts could result in practical production equipment.

Correlation Between Hysteresis Loop Areas of Lower Punch and of Die Pressures Versus Upper Punch Pressures

In tablet compression a series of authors (1-11) have employed plots of die wall pressure as a function of applied pressure to elucidate the mechanism of bonding in the compact (Fig. 1). The so-cal...

Compression cycles in tableting

When powders are compressed to make a tablet, the powder is placed within a die and compressed by two punches. In general the act of compression is considered to occur in five stages: (a) first the powder rearranges to its closest packing, then (b) the particles deform elastically. (c) Beyond the elastic limits the particles will either fracture or deform plastically, and either of these processes leads to interparticulate bonding, i.e. gives rise to a tablet. (d) After release of the upper punch there is a relaxation of stresses in the bonded mass, and this is followed by (e) ejection of the tablet from the die. It is conventional to describe these processes by monitoring either the die wall pressure ( F) or the lower punch pressure ( P ℓ) as a function of the upper punch pressure ( P). These two types of cycles are conventionally described in the literature by assuming the solid to be a non-porous body. The cycles exhibit a hysteresis loop, and it is shown here that the consequence of considering the body nonporous throughout is that the hysteresis area of the cycles is either a quadratic or a linear function of the maximally applied pressure.

Internal friction of compressed pharmaceutical powders observed in terms of the die wall pressure.

The internal friction of powders was examined during tablet preparation for lactose and phenacetin as base materials and for potassium chloride, potassium bromide and sodium chloride as model samples. The coefficient of internal friction decreased with the porosity, even if the voids were filled by fragmentation of particles or by plastic flow. When the porosity had reached zero, the state of the whole bed of powder was found to be plastic, where the coefficient of internal friction was zero, for magnesium stearate, phenacetin, potassium bromide and potassium chloride. The plastic behavior of magnesium stearate and phenacetin was considered to be related to the capping of tablets containing them. For lactose granules, the porosity did not reach zero even at a compacting pressure of 4000 kg/cm2, the maximum used in this work, because of its crystal hardness. The coefficient of internal friction was large and decreased only gradually with the porosity. Mixtures of two components exhibited intermediate internal friction, corresponding to the composition. However, when hydroxypropyl cellulose (HPC) was added to phenacetin powder as a binder, HPC increased the yield stress from 2000 to 3100 kg/cm2 even at a content of only 2% w/w on a dry basis. In contrast, lubricants (magnesium stearate and talc) added to granules had little or no effect on the internal friction, although this small amount was sufficient to reduce the wall friction.

Instrumentation and calibration of a single-punch press for measuring the radial force during tableting

A single-punch tablet press was instrumented for radial force measurement during tableting by cutting out segments from the die, leaving 5.0 mm of die wall, and by applying strain gauges to these thinner parts. Active horizontal gauges were connected to temperature-compensating vertical gauges to Torm a Wheatstone bridge with two fixed resistors in the bridge amplifiers. The instrumentation was dynamically calibrated at tableting speed (30 rpm) with rubber-like materials lubricated with molybdenum disulphide (Molykote). The die wall signals were reproducible and increased linearly with the applied pressure. The signals increased with increasing height of the compact. The position of the lower punch had a marked influence on the response. When the lower punch was in a position 10.2 mm below the upper surface of the die, the signal at constant pressure was proportional to the cross-sectional area of the compact. At this position the die wall pressure could be calculated from the regression line of signal/cross-sectional area vs applied pressure. This regression was the same for all tested rubber materials and the estimated relative standard deviation of single determinations at 100 MPa was about 3%.

Effect of Physical Properties on Compression Characteristics

The transmission of force to the die wall was measured by a piezoelectric sensor, and the compression cycles of lactose granules of different shapes were compared. In addition, a nearly spherical fraction of spray‐dried lactose was similarly compared with a crystalline sample. Better tablets were formed when the conversion of axial to radial pressure was high and the residual pressure on the die wall remained after removal of the top punch. With acetaminophen and phenacetin, the pressure on the die wall was low, as was the residual pressure, and capping occurred in both cases. With direct compression acetaminophen, higher die wall pressure was produced and capping did not occur. It is considered that these results can be explained by the ease with which the more nearly isodiametric particle can rearrange under pressure and by the elastic properties of the solid.

Physics of Tablet Compression XIV. Lateral Die Wall Pressure During and After Compression

Measurements of pressure exerted by various substances on the die wall during and after compression of tablets have shown that the information derived can be related directly to the ease of formation and ejection of these tablets. The side wall pressure has been found to decay at a measurable rate after the normal compressional force is abruptly removed. Similar studies also have been carried out on preformed tablets subjected to repeated compression.

Physics of Tablet Compression XIII: Development of Die-Wall Pressure During Compression of Various Materials

A method for determining the pressure transmitted to the die wall during tablet compression is described. Basically, the method involves strain-gauge measurement of die expansion and the relation of this to the transmitted pressure. The pressure transmitting behavior of a number of compounds, both organic and inorganic, have been studied; some correlations between observed response and physical properties are suggested. Limited studies on the effect of the addition of a lubricant to the crystals are also reported. It was noted that change in the magnitude of transmission to the die wall was not a simple linear function of lubricant concentration.