Analysis of densification mechanisms of dry granulated materials

International audienceDry granulation by roll compaction is a continuum manufacturing process to produce granules with improved flowability which can further be easily used in tableting process. However, the granules are non-homogeneous in density and have non-spherical shapes which impact their densification behaviour during die-compaction. The aim of this study was to investigate both the densification mechanism and the failure strength of granules of microcrystalline cellulose (MCC) and mannitol using Cooper-Eaton and Adams models. For both materials, the Cooper-Eaton approach led to the quantification of fractional volume compaction by particle rearrangement and by plastic deformation respectively to explain the difference in densification behaviour of raw material and granules. Moreover, the model showed its ability to capture the effect of granule density and granule sizes and to differentiate the densification mechanisms of MCC as a plastic material and mannitol as a brittle material. The Adams model was used to compute the failure strength of single granule from in-die compression data. The obtained results of the granules were in the range [0.6–1.43  MPa]. However, regarding the effect of granule density, the model showed mixed results indicating that the model is not representative of the studied granules which are not spherical and have a relatively wide range of sizes, nevertheless, the model was derived for near spherical particles with a narrow size distribution

Compaction behavior of binary mixtures

Symposium on Powder Science and Technology - Powders and Sintered Material, Albi, FRANCE, MAY 23-25, 2007International audienceThe compaction behavior of binary mixtures of lactose and a functional excipient containing 98% of microcrystalline cellulose and 2% of silica was investigated by experiment. The densification process and the role of each component in the compaction behavior were examined by analyzing the effect of the composition on the stress transmission to the powder bed in axial (stress transmission) and in radial (stress transfer) directions. Unlike the behavior of a single component where the applied pressure is predominantly transmitted in the axial direction to the powder bed for densification, the increase of the composition of lactose up to 50% in mixtures increases the stress transfer that becomes greater than the stress transmission. However, for the compaction behavior of mixtures with 50% of lactose, two opposite behaviors were found, below and above a pressure around 70 MPa. Die-wall friction showed also a change at this pressure due to a smoothed contact surface with wall. By increasing the composition of lactose (>50%), the behavior of mixtures becomes dominated by the behavior of lactose where the transmission is better than the transfer. This study demonstrates that the analysis of the compaction behavior of mixtures by recording stress transmission, stress transfer and wall friction give pertinent information about the role of each component in the densification and Could be, to a certain extent, help to the selection of excipient for powder formulation

Wall friction and its effects on the density distribution in the compaction of pharmaceutical excipients

International audienceThe effect of powder-die wall friction during the compaction of pharmaceutical excipients has been investigated for three modes of lubrication: lubricated die, non-lubricated die and with the lubricant mixed with the powder. Coulomb friction is assumed and the wall friction coefficient was evaluated from the transmission ratio (applied pressure/transmitted pressure), the transfer ratio (radial pressure/axial pressure) and the aspect ratio (height/diameter of tablet). The friction coefficient of three pharmaceutical excipients was measured with respect to the relative density of the tablet by means of an instrumented press. It was found that the behaviour of the friction depends on the powder and the lubrication mode. Mixing the powder with a lubricant reduces the friction with respect to that of the lubricated die, but the evolution of the friction coefficient with the densification is different. The effect of the wall friction on the axial density distribution in the tablet was investigated by experiment and by modelling. The model was based on Janssen-Walker analysis coupled with the Heckel equation. For comparison, only the single action compaction in a non-lubricated die was considered. It was found that the measured and predicted axial density decrease from the top to the bottom of the tablet. Moreover, the predicted and measured density had the same tendency, but different values. However, the analysis should not be applied to the compaction of the powder mixed with lubricant because no physical parameter for this mode of lubrication is taken into account in the model

The effects of ambient temperature on the compaction of pharmaceutical powders

International audienceThis article presents an experimental study of the effects of raised ambient temperature in dies and punches on the compaction of pharmaceutical powders. The experiments use an instrumented hydraulic press having a temperature-controlled enclosure allowing the ambient temperature of die and punch to be varied from 20 to 57 degrees C. A pharmaceutical powder was compacted at temperatures in this range and mechanical parameters, such as stress transfer ratio, stress transmission ratio, and die-wall friction, were analysed to examine the effects of heat transfer between tools and powder. In particular, it is shown that increasing the environmental temperature of die and punch increases the transfer ratio and the die-wall friction. The radial pressure is also slightly increased at the first stages of the compaction. However, the stress transmission is reduced by increasing the temperature. This may indicate an increase of shear stress. It is also observed that the particles undergoing compaction are `softened' by increase of the temperature. This softening is certainly due to rise in temperature of the powder generated by the compaction and by the heat flux transfer between the die and the tablet. It is suggested that these effects could be important in industrial tablet production installations without air conditioning and thus subject to variations in ambient temperature

Rôle du poteyage et de la température initiale du moule sur les sollicitations thermomécaniques des moules permanents de fonderie

International audienceIn the casting industry metal moulds or dies are used more and more. The reason is that they allow a fa st cooling rate o f the solidifying part, hence allowing higher productivity, finer microstructure and higher mechanical properties. In most cases the die is made out o f steel and reacts with the liquid cast metal. The usual solution is to cover the moulding surface with a coating or spray. Depending on the casting technology, the coating is sprayed every cycle or every 8 to 10 hours o f production. A second but nonetheless important effect o f the coa ting is its thermal effect. The coating acts as a thermal barrier and protects the die against thermal shocks. The topic o f the present paper is to assess this function o f the coating. During a casting cycle, the coated die and the molten metal are briefly in contact during the very first moments and then an air gap may form and separate them apart. During the first stage, an intense heat is tran sferred from the m elt to the die. H eat flu x den sities fro m 0.5 MW/m2 up to 10 MW/m2 have been reported in literature. The intense heat transfer gene rates high temperature heterogeneity into the die. The corresponding dilatation heterogeneity is responsible fo r internal stresses into the die, so called thermal stresses. They are usually compressive stresses on the hot surface. It will be shown in this paper that the moulding surface o f the die suffers the most stress. The stresses can be high enough to cause yielding o f the steel at high temperature. Because the steels in use fo r dies have a high yield stress at high tempera ture the plastic deformation remains small. However it is a cyclic plasticity because the same phenomenon occurs at every casting cycle. We believe that this plasticity in warm conditions is responsible fo r residual tensile stresses in cold conditions (i.e. nearly isothermal conditions). This phenomenon is rather classical in most thermal stresses problems [ 1, 2], A t the lifetime scale o f the die, the moulding surface is cyclically stressed in traction at low temperatures and compression at high temperatures leading to a fatal cracking. In a first approach we suggest measuring temperatures within the die during a casting cycle. From this measurement it is possible to estim ate the thermal stresses, assuming that the stresses remain below or at least close to the yield stress o f the die materials. This assumption is usually fair. Indeed, if the plastic deformation were large during every cycle, the die would never last much longer than a few thousands cycles. If this ever occurred, it would not be a great advantage fo r the casting factory nor its client. Other materials should be sought as a first priority. From the estimation o f thermal stresses, it would be delicate to foresee the mecha nical behaviour o f the materials o f the die in fatigue condition. Some materials tend to harden (copper alloys, f cc materials) while others tend to soften (heat treated martensite steels [4])[5], Instead o f trying to guess the behaviour, the method that we suggest is to perform a thermo mechanical fatigue test (TMF). This TMF test consists o f applying measured temperature and evaluated strain/stresses history to a mechanical testing sample [6], The most relevant tempe rature and stress history is, o f course, the one corresponding to the moulding surface o f the die. This test will provide information on the materials behaviour and some relevant data about the lifetime o f the die. This paper provides an example o f this method. The thermal data was obtained from a gravity casting experiment [3] that is described in the first part. The second part deals with the evaluation o f the thermal stresses and the third part shows some results from the TMF testing. Throughout the paper the influence o f the coating nature and o f the die initial temperature is examined.Cet article traite de la fatigue thermique subie par les outillages de mise en forme. Pour appréhender le problème d’endommagement des outillages, il est nécessaire de bien connaître les conditions de transfert de chaleur et d’évaluer les contraintes thermo-mécaniques subies par l’outillage. La prévision de la durée de vie et l’évolution de la plasticité cyclique qui précède la fissuration est difficile car certains matériaux ont tendance à s’écrouir alors que d’autres ont tendance à s’adoucir. Nous proposons le recours à des expériences de fatigue thermo-mécanique (TMF) pour appréhender cette prévision. Par un exemple emblématique, nous montrons comment il est possible d’aborder le problème. Le cas considéré est la fonderie gravité en coquille d’acier. Nous mettons en évidence l’influence de paramètres process sur le transfert de chaleur et sur les contraintes thermo-mécaniques subies par le moule pendant un cycle de coulée. Les paramètres retenus sont la température initiale du moule et la nature du poteyage. Des essais de fatigue thermo-mécanique ont été menés et les résultats sur 1000 cycles sont exposés

Relationship between single and bulk mechanical properties for zeolite ZSM5 spray-dried particles

In this work typical mechanical properties for a catalyst support material, ZSM5 (a spray-dried granular zeolite), have been measured in order to relate the bulk behaviour of the powder material to the single particle mechanical properties. Particle shape and size distribution of the powders, determined by laser diffraction and scanning electron microscopy (SEM), confirmed the spherical shape of the spray-dried particles. The excellent flowability of the material was assessed by typical methods such as the Hausner ratio and the Carr index. This was confirmed by bulk measurements of the particle–particle internal friction parameter and flow function using a Schulze shear cell, which also illustrated the low compressibility of the material. Single particle compression was used to characterize single particle mechanical proper-ties such as reduced elastic modulus and strength from Hertz contact mechanics theory. Comparison with surface properties obtained from nanoindentation suggests heterogeneity, the surface being harder than the core. In order to evaluate the relationship between single particle mechanical properties and bulk compression behaviour, uniaxial confined compression was carried out. It was determined that the Adams model was suitable for describing the bulk compression and furthermore that the Adams model parameter, apparent strength of single particles, was in good agreement with the single particle strength determined from single particle compression test

Experimental and numerical analysis of homogenenity over strip width in roll compaction

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Analysis of strain stress state in roller compaction process

Symposium on Science and Technology of Powders and Sintered Materials (STPMF 2009), Montpellier, FRANCE, MAY 25-27, 2009International audienceThe process of drawing and densification of powdered microcrystalline cellulose by roller press in a steady state operation is analyzed using a 2D modeling with the finite element method and the modified Drucker-Prager Cap model as material behavior. Distributions of process variables in contact surface between powder and roller such as pressure, shear stress and relative speed were predicted and used to analyze the basic mechanisms of the transport and the densification of powder between rolls. The results show clearly the existence of three contiguous zones: a phase where the powder is drawn between rolls by a sliding mechanism, a sticking phase where the powder is transported with the same velocity as the roll and where the densification by deforming the powder bed is achieved under the increase of roll pressure that reaches its peak before the neutral angle. The formed compact is then expulsed out of the gap by a slip mechanism resulting from the change of the sign of the shear stress. The predicted density distribution between the rolls, shows a gradual increase. The density reaches its maximum before the neutral point and shows values in agreement with the density of strips prepared with an instrumented roll press. The effect of varying the material parameters on the maximum pressure and the nip angle s also investigated. Beyond the description of the basic mechanisms of roller compaction, this modeling shows a real potential of the optimization of the roller compaction process

Analysis of strain stress state in roller compaction process

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