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TITLE: USE OF COMPACTOR IN POWDER

PROCESSING

ABSTRACT

The aim of the study is to analyse the application of compaction process used in powder processing in

several industries. Particle size is essential for the quality of products produced by a wide range ofindustries such as Cosmetics, agriculture, cement Industry. This report presents an in depth study ofthe powder compaction process and its importance. The contact behaviour at individual contactsbetween particles was analysed in detail. Compaction process was analysed mathematically bypredicting a steady-state mathematical model in order to give more description and understanding for

the mechanism of this process. Numerical investigation has been carried out on axisymmetriccylindrical parts using the finite difference method with relaxation technique to examine the physical

significance of constitutive model to produce the pressure gradients during compaction. This reportcontains the detailed literature review of soil compaction.

KEYWORDS : Compaction mechanism, application of compaction, powder processing, compactpowders, Finite difference method; Die compaction; Green density distribution; Aspect ratio;

Alumina powder; Relaxation technique

INTRODUCTION

Powder compactionis the pressure of powders into a geometric form. Pressing is usually performedat room temperature. This creates a solid part called agreen compact. The strength of this pressed,unsintered part, (green strength), is dependent on compactability, binders may be used to increasecompactability. Typically a green compact can be broken apart by hand but is also strong enough tobe handled, gently. The geometry of the green compact is similar to that of the final part, however,

shrinkage will occur during the sintering phase of the manufacturing process and must be calculatedin.

Amount of powder needed will be based on the bulk densityof the powder and the amount ofmaterialin the final part. Bulk density is the density of the loose powder by itself. Bulk density isimportant when measuring powder quantities. The effects of additives such as lubricants must alwaysbe calculated. For example, a green compact has a certain amount of lubricants and binders in it thatadd extra material. During sintering, these lubricants and binders are burned off. Their material is nolonger in the part after sintering and this must be a consideration.

To begin the manufacturing process, a certain amount of powder is filled into a die. Rate of die fillingis based largely on the flowabilityof the powder. Powders that flow readily can be poured at higherrates. Pouring can be an automated process.

Once the die is filled, a punch moves towards the powder. The punch applies pressure to the powder,compacting it to the correct geometry. A simple illustration of the pressing process is shown in figure

328.


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Punch and die surfacesare very important in powder manufacture. Some clearance between thepunch and die must exist in order for the punch to move within the die. Powder particles can becomestuck within this clearance, causing problems with the proper movement of machinery. In order toprevent powder particles from becoming lodged within this gap, clearance is designedto beextremely low. Clearance values between punch and die, used for powder pressing, are typically lessthan .001 inch. Most punch and die are made from hardened tool steels, the surfaces of which areground then polished, or lapped, in the direction of tool movement. Punches and die for more extremepowder processing operations may be made from tungsten carbide.

Amount of force necessary for a pressing operation is to a large degree based on material. Forexample, pressing aluminium powder generally requires lower force, while pressing iron powderrequires relatively higher force. Pressing force also depends uponpowder characteristics, additivesand desired density of the green compact. Friction force will oppose movement of particles duringpressing, therefore lubrication can reduce the required pressing force and also cause a more uniformdistribution of particles during pressing. Lubrication should be applied in the correct quantities.

Excessive lubrication will not all remain on particle surfaces, but will also collect in the interparticlespaces, (open pores), and prevent the proper compaction of powder. Pressing force is a function ofpressure over the area of the part perpendicular to the direction of pressing. Usually the press isvertical, in this case the horizontal plane of the part would be considered.

Force for industrial powder manufacture typically varies between 10,000 lbs/in2, (70 MPa), and

120,000 lbs/in2, (800 MPa). Parts for this type of manufacture are mostly small, (under 5 lbs), andpress requirements are typically under 100 tons. Mechanical presses with capacities on the magnitudeof a few hundred tons are usually adequate for most powder processing operations. Hydraulicpresseswith capacities of several thousand tons are sometimes used for work requiring more force.Double action presses, with opposing top and bottom punches, are commonly used, but for morecomplex parts multiple action presses may be employed. Punch speed must be regulated. Faster

compaction of the work can result in higher productivity, however if the punch speed is too high, airmay become trapped in the pores and prevent the part from compacting correctly.

Mechanics of Pressing

Compaction of a part is dependent upon the actions of the powder particlesduring pressing. When apowder is first filled into a die it is at its bulk density or the density of loose powder. As thecompression of the powder occurs its volume decreases and its density increases, until it reaches thefinal volume and density of the green compact,(green density). The fully pressed part will still containporosity and the green density will be lower than the true density of the material.

Interparticle pores and particle surfaces are an important consideration in the pressing of powders.Surface films on the powder particles should be avoided. These materials such as oxides can be


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rubbed off during pressing and occupy interparticular vacancies, preventing proper compaction of thepart. In the first stageof powder pressing density is increased by a rearranging of the individualpowder particles. Spaces, bridges and gaps are eliminated, and density increases due to a moreefficient packing of the particles. This initial stage provides relatively lower resistance and the densityof the powder rapidly increases with applied pressure. Contact points between powder particlesbecome established. As compression continues, increasing forces act between these contact points.Cold pressure welding occurs at contact points between particles. Cold pressure welding is a type ofbonding that happens during powder pressing, it helps give the green compact structural integrity sothat it may be processed further.

The second stagehas no definite starting point but is characterized by plastic deformation ofparticles. Stress between powder particle contact points causes material deformation. Contact areasare increased, interlocking and plastic flow of particles occurs, volume decreases and density

continues to rise. Material movement is increasingly opposed by friction and the work hardening ofthe metal powder. Unlike the first stage, the rate the density is increasing will decrease as pressurecontinues to rise. Density will continue to increase until the maximum density of the pressed powder,or the green density, is reached. Although greatly reduced, interlocking space still exists extensively

in the green compact.

Much of this space is still an interconnected network of pores, mainly open pores. The density of the

compact relative to the applied pressure varies with different processing factors. A typical relationshipbetween pressure and compact density is shown in figure 329. Note how the rate of density increasedrops off between the initial repacking stage and the second plastic deformation stage.

Ideally the density increase would occur uniformly throughout the compact. However, due mainly tofriction and part geometry, variations in density can be a significant problem in powder manufacture.

These problems increase with part complexity. Lubrication can help mitigate friction, providing amore even flow during compaction. Over lubricating should be avoided. Another method to create a

more uniform density in the pressed compact is to use additional punches with separate movements.Multiple action pressing of powders is common industrial practice with more complex parts.

The principle of the design of a powder pressing operation using one, two, or multiple actions, isbased on the way that powders compress. Less pressure within the powder material will result in lesscompaction and lower density in that area. Pressure within the powder decreases with distance fromthe punch surface.For a single punch that would mean through the thickness of the part.


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Why compaction?

TO PRODUCE UNIFORM BLENDS OR MIXTURES

Mixtures of various discrete particles tend to classify in transport or handling because of differences

in particle size, shape and density. The Chilsonator can produce granules of uniform consistency

which eliminates segregation and facilitates consistent analysis.

TO PRODUCE A UNIFORM PARTICLE SIZE RANGE

The particle size range of the product can be selected to suit individual requirements and varied

according to individual needs.

TO CONTROL DUST

Dust is generally a wasteful and obnoxious form to handle. Cross contamination and product loss can

be eliminated.

TO ADJUST FLOW PROPERTIES

Granular materials flow more easily and resist bridging and caking. Higher flow rates and more even

fill can be achieved in many cases.

TO CONTROL BULK DENSITY

Increased bulk density may be desirable for storage, transport or packaging. Marked increases in bulk

density can usually be achieved and controlled within certain limits.

TO CONTROL PARTICLE HARDNESS

The characteristics of particle hardness can sometimes be adjusted to suit the product needs. Crush

strength and disintegration can be important properties brought under more rigid control.

TO IMPROVE SOLUTION OR DISPERSION RATES

Granular materials absorb liquids more readily than do many powders. Therefore, granular materials

will dissolve or disperse more easily and quickly. Under proper conditions, some materials can also be

adjusted to sink or float as desired.

The Effect of Particle Size Distribution in Cold Powder

CompactionThe contact behaviour at individual contacts between particles was analysed in detail by Storkers et

al. and Storkers where spherical indentation of solids was described constitutively by power law

creep, plastic flow theory and general viscoplasticity. In a recent study Skrinjar et al. also derived

approximate but accurate relations describing contact between dissimilar particles in a fairly general

situation. The accuracy of these relations in a compaction situation have been investigated by large

scale finite element simulations of compaction of particles forming regular lattices.

DEM (discrete element method) will be used as the preferred numerical method with particle

interaction determined through the contact law presented by Storkers et al. Frictional effects at

particle contact, as well as particle rotation, will be accounted for while the material behaviour is, forconvenience but not out of necessity, restricted to rigid perfect plasticity. The particle size distribution


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is assumed to be well described by a (truncated) normal distribution characterized by an average

particle size and a standard deviation.

Numerical method

In DEM, originally developed by Cundall and Strack , each particle is modelled as a separate objectand the local contact forces between the particles, which will be discussed in a separate section,

determines the macroscopic properties.

The numerical simulation can be divided in two stages:

one where a realistic random assembly of particles is generated by simulating the filling

process

one where the actual pressing of the powder compound is made.

To calculate the positions xi(t) and velocities vi(t) of particle at a given time t, Newtons second law

applied to each particle were integrated numerically using an explicit Verlet type algorithm

with Fi(t) = Fi(t)^N + Fi(t)^T being the sum of forces from all contacts (tangential and normal forces)

acting on particle at time t and mi is the mass of the particle. The rotation of particle i, hi and the

angular velocity xi is calculated in a similar manner by

with Mi the sum of all moments from tangential forces acting on particle i and Ii = (2mi*Ri^2) /5, the

moment of inertia of particle i.

In order to ensure numerical stability of the explicit integration method, it was shown by Cundall and

Strack [24] that a maximum time step Dt of


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is required where mmin is the mass of the smallest particle and k is the contact stiffness. With realistic

values of mass and stiffness, a compaction simulation would require in the order of 10^10-10^11

iterations.

During the compaction process, data like macroscopic pressure and average number of contacts per

particle are registered together with the relative density D. The relative density is calculated as thevolume of the particles inside a box with walls placed one mean radius inside the aggregate walls

divided by the volume of the box to avoid the effect from the lower packing density close to the walls.

The compressed particles are shown in Fig. 1

Contact Forces between Powder Particles

To obtain a correct macroscopic response of the powder compact, the computation of the contact

forces between the particles (and between particles and walls) is critical. By assuming spherical and

rigid plastic particles the problem can be solved using formulae from Storakers et al. and Storakers.

Some of these results will be recapitulated below. The particles are restricted to follow a power-law

plasticity model

where c^2 is an indentation invariant depending only of the power law exponent M and can be

approximated with good accuracy by c(M)^2 = 1.43 exp(-0.97M) as derived by Biwa and Storakers

and Fleck et. al. The effective radius, R0, is defined by

1/Ro = 1/R1 + 1/R2


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The unloading of the contacts is assumed to be elastic with neglected adhesive forces and follows a

Hertzian relationshipby

A sketch of the contact behavior is shown in Fig. 2. The tangential force Fij(T) is modelled to be

either in a state of gross sliding, modelled by Coulomb friction, or in a sticking state with tangential

stiffness kt.

MATHEMATICAL SIMULATION OF COLD DIE COMPACTION

Compaction process was analysed mathematically by predicting a steady-state mathematical model in

order to give more description and understanding for the mechanism of this process. Numerical

investigation has been carried out on axisymmetric cylindrical parts using the finite difference method

with relaxation technique to examine the physical significance of constitutive model to produce the

pressure gradients during compaction. The pressure distribution model is then typically coupled with

empirical functions relating pressure and density to obtain a green density distribution at all nodes in

the green compacts. The model has addressed the influence of frictional forces acting at the powder

and die walls interfaces which dissipate the applied pressure throughout the compact. The effect of the

compact geometry has a similar effect on the uniformity of green pressure and density distributions

through the compact. It was found that a small aspect ratio resulted in a more uniform distribution

than a higher aspect ratio. Therefore, the model seems to work better for the lower aspect ratio. The

constitutive model predicts accurately the pressure and density distributions during compaction

process.

The die wall was modelled as a rigid surface made of elastic material. During compaction at room

temperature, the punches move at a slow rate, dynamic effects were neglected and no thermal effect

was considered.

Two dimensional steady state model is in the radial and depth direction. It is necessary to use the

finite difference approximation to the partial differential equation system (PDEs), so that the problem

could be solved by a computer. The steady state pressure distribution is governed by the Laplaces

equation and we get the generalised governing pressure distribution equation as:


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Numerical solution of finite difference equations

Elliptic PDEs are usually solved with approximate method. The most commonly employed approach

is GaussSeidel iteration (Shima and Saleh; 1993). The finite difference equation is applied at each

node, node by node. With each calculation, the updated value from the previous node is used. The

calculation may also incorporate the method of relaxation. The calculations are continued until

repeated iterations fail to change the pressure distribution within a specified amount. This procedure

will eventually converge on a stable solution. Over relaxation is often employed to accelerate the rate

of convergence by applying the following formula for each iteration:

where jip , and old jip , are the values of Pi,j from the present and the previous iterations respectively

and w is a weighting factor which is set between 1 and 2.

Prediction density gradients

In spite of high degrees of surface finish on the tool set, friction exists between the powder and the

tool components. Axial forces applied by the compaction load cause radial forces to be generated atthe die walls. A differential pressure distribution during compaction produces a density gradient in the

green parts. To predict the relative green density of the compact, the following equation was used:-

pho =pho0 + k*(p^1/3)

where pho is a relative density, pho0 is an initial relative density, P is an applied pressure and k is a

constant reflect variations in material properties such as hardness and plasticity. k appears to be a

material constant. Soft and ductile powders have higher k values than hard and brittle powders.

DISCUSSIONS

Numerous analysis and models have been found in the literature for investigating and predicting thecompaction behaviour of ceramic powders. It is important to note that such diagrams pertain only to

the average properties of powder compacts. To better understand the significant issue of density

variations within pressed powder compacts, models of pressure distribution which are obtained from

an applied compacting pressure have been developed and implemented into a finite difference

method.

The processing of an axisymmetric component has been simulated. Coupling pressure distribution

functions with pressure-density relationship allows the development of model to predict density

distribution.

Following results were obtained:


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Effect of coefficient of friction

Note that the corners near the moving portion of the die, where the pressure builds up are the highest.

The pressure at the edges decreases with depth from the surface to the bottom of compact.

Effect of aspect ratio


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Figures 7 and 8 show the effect of compact geometry on axial compacting pressure with axial

distance in alumina green compact as a function of aspect ratio with a radial to axial pressure ratio

equal to 0.25, the coefficient of friction at die walls is equal to 0.5 at different compacting pressure

69, 138, 306 and 500 MPa. At an aspect ratio equal to 0.38, the pressure at the bottom of the compact

is nearly 82% of the applied compacting pressure which transmitted to the bottom of the compact as

shown in figure7. For aspect ratio h = 2, only 36% of applied pressure is transmitted to the bottom of

the compact as shown in figure 8.

Prediction pressure

Density of green compacts Figure 17 shows the relationship between the compaction pressure with

relative density for simulation and experimental results. It can be seen that the relative density

increases with increasing compacting pressure; the pores decrease with increasing the compacting

pressure. The pressure-density relationship shows that there is initially a large increase in density

during the low pressure applied. This may be due to the initial rearrangement of particles. As the

density of the part increases, the curve levels are indicating not much change in density when pressure

is applied. It is appeared that the first region at low pressure is due to particles rearrangement. The

second region appears to be a linear, where a brittle fragmentation occurs. The third region shows the

change in density with increasing pressure.


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APPLICATIONS

Soil Compaction

Pharmaceutical Industry

Polymer compaction Beauty Industry

Cement Industry

In the previous reports these above mentioned industries are discussed in brief. Now, I have presented

detailed study of soil compaction.

COMPACTION IN SOIL

Compaction is a process that brings about an increase in soil

density or unit weight, accompanied by a decrease in air

volume. There is usually no change in water content. The

degree of compaction is measured by dry unit weight and

depends on the water content and compactive effort (weight of

hammer, number of impacts, weight of roller, number of

passes). For a given compactive effort, the maximum dry unitweight occurs at an optimum water content.

Compaction purposes and processes

Compaction is a process of increasing soil density and removing air, usually by mechanical means.The size of the individual soil particles does not change, neither is water removed.

Purposeful compaction is intended to improve the strength and stiffness of soil. Consequential (oraccidental) compaction, and thus settlement, can occur due to vibration (piling, traffic, etc.) or self-weight of loose fill.

Objectives of compaction

Compaction can be applied to improve the properties of an existing soil or in the process of placingfill. The main objectives are to:

increase shear strength and therefore bearing capacity increase stiffness and therefore reduce future settlement decrease voids ratio and so permeability, thus reducing potential frost heave


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Factors affecting compaction

A number of factors will affect the degree of compaction that can be achieved:

Nature and type of soil, i.e. sand or clay, grading, plasticity

Water content at the time of compaction Site conditions, e.g. weather, type of site, layer thickness Compactive effort: type of plant (weight, vibration, number of passes)

Types of compaction plant

Smooth-wheeled roller Grid roller Sheepsfoot roller Pneumatic-tyred roller Vibrating plate Power rammer

Construction traffic, especially caterpillar-tracked vehicles, is also used.

Laboratory compaction tests

The variation in compaction with water content and compactive effort is firstestablished in the laboratory. Target values are then specified for the dry densityand/or air-voids content to be achieved on site.

Dry-density/water-content relationship

The aim of the test is to establish the maximum dry densitythat may be attained for a given soil with a standard amount

of compactive effort. When a series of samples of a soil arecompacted at different water content the plot usually shows adistinct peak.

The maximum dry densityoccurs at an optimum

water content The curve is drawn with axes of dry density and water

content and the controlling values are values read off:rd(max) = maximum dry densitywopt= optimum water content

Different curves are obtained for different compactive efforts.

Explanation of the shape of the curve

For claysRecently excavated and generally saturated lumps of clayey soil have a relatively high undrainedshear strength at low water contents and are difficult to compact. As water content increases, thelumps weaken and soften and maybe compacted more easily.

For coarse soils

The material is unsaturated and derives strength from suction in pore water which collects at grain
http://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTSMOOTHhttp://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTGRIDhttp://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTSHEEPhttp://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTTYREhttp://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTPLATEhttp://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTPLATEhttp://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTTYREhttp://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTSHEEPhttp://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTGRIDhttp://environment.uwe.ac.uk/geocal/SoilMech/compaction/compaction.htm#PACTSMOOTH


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contacts. As the water content increases, suctions, and hence effective stresses decrease. The soilweaken, and is therefore more easily compacted.

For bothAt relatively high water contents, the compacted soil is nearly saturated (nearly all of the air has been

removed) and so the compactive effort is in effect applying undrained loading and so the void volumedoes not decrease; as the water content increases the compacted density achieved will decrease, withthe air content remaining almost constant.

Expressions for calculating density

A compacted sample is weighed to determine its mass: M (grams)The volume of the mould is: V (ml)Sub-samples are taken to determine the water content: wThe calculations are:

Dry density and air-voids content

A fully saturated soil has zero air content. In practice,even quite wet soil will have a small air content

The maximum dry density is controlled by both thewater content and the air-voids content. Curves fordifferent air-voids contents can be added to the rd/ wplot using this expression:

The air-voids content corresponding to the maximum dry density and optimum water content can be

read off the rd/w plot or calculated from the expression.

Effect of increased compactive effort

The compactive effort will be greater when using aheavier roller on site or a heavier rammer in the

laboratory. With greater compactive effort:

maximum dry density increases optimum water content decreases air-voids content remains almost the same.


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Effect of soil type

Well-graded granular soils can be compactedto higher densities than uniform or silty soils.

Clays of high plasticity may have water

contents over 30% and achieve similardensities (and therefore strengths) to those oflower plasticity with water contents below

20%. As the % of fines and the plasticity of a soil

increses, the compaction curve becomesflatter and therefore less sensitive to moisturecontent. Equally, the maximum dry density

will be relatively low.

Interpretation of laboratory data

During the test, data is collected:

1. Volume of mould (V)2. Mass of mould (Mo)3. Specific gravity of the soil grain (Gs)4. Mass of mould + compacted soil - for each sample (M)5. Water content of each sample (w)

Firstly, the densities are calculated (rd) for samples with different values of water content, then rd/ wcurve is plotted together with the air-voids curves.The maximum dry density and optimum water content are read off the plot.The air content at the optimum water content is either read off or calculated.

Specification and quality control

The degree of compaction achievable on site depends mainly on:

Compactive effort: type of plant + No of passes

Water content: can be increased if dry, but vice-versa

Type of soil: higher densities with well-graded soils; fine soils have higher water contentsEnd-resultspecifications require predictable conditionsMethod specificationsare preferred in UK.

End-result specifications

Target parameters are specified based on laboratory test results:

Optimum water content working range, i.e. 2%Optimum air-voids content tolerance, i.e. 1.5%For soils wetter than wopt, the target Avcan be used, e.g.10% for bulk earthworks

5% for important workThe end-result method is unsuitable for very wet or variable conditions


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Conclusion

The effect of particle size distribution in powder compaction has been analysed using the

discrete element method. One clear result is that the size distribution has a very small

influence on the compaction pressure when the size distribution is small. This is anencouraging result from a practical point of view due to the fact that detailed knowledge

about the size distribution is not required in order to successfully simulate powder

compaction. With an appropriate particle size distribution and with rotational degrees of

freedom of the particles accounted for, it is possible to capture both the evolution of the

average number of contacts and macroscopic pressure also at larger size distributions.

The simulation of predicting pressure shows experimental data has an excellent agreement

with computational results obtained using the above discussed models.

The compact geometry has effect on uniformity of green pressuredensity distributions

through the compact. Small aspect ratio has a more uniform than higher aspect ratio.

Small aspect ratio has a more uniform than higher aspect ratio due to low density variations in

low aspect ratio. Therefore the model seems to work better for the lower aspect ratio.

Friction plays an important role in determining pressure distributions. The pressure at die wall

decreases gradually from top to the bottom of the compact because of the pressure gradient.

The variations in axial compacting pressure with axial distance increasing with increasing the

coefficient of friction. Increasing in radial pressures gives rise to increased friction forces at

die wall, therefore the difference in maximum and minimum in pressure values between top

and bottom of compact increase.

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