Improved High Velocity Cold Compaction Processing: Polymer Powder to High Performance Parts

Bruska Azhdar

AKADEMISK AVHANDLING Som med tillstånd av Kungliga Tekniska Högskolan i Stockholm framlägges till offentlig granskning för avläggande av teknisk licentiatexamen onsdag den 21 september 2005, kl. 10.15 i sal B1, Brinellvägen 23, KTH, Stockholm. Avhandlingen försvaras på svenska.

LIST OF PAPERS

This thesis is based on the following papers 1. “Development of a High-Velocity Compaction process for polymer powders”, Bruska

Azhdar, Bengt Stenberg, Leif Kari, Polymer Testing, in press. 2. “Determination of springback gradient in the die on compacted polymer powders

during high-velocity compaction”, Bruska Azhdar, Bengt Stenberg, Leif Kari, Submitted to Polymer Testing.

ABSTRACT A uniaxial High-Velocity Compaction (HVC) process for polymer powder using a cylindrical, hardened steel die and a new technique with relaxation assist was tested with a focus on the compactibility characteristics and surface morphology of the compacted materials using various heights of relaxation assist device with different compacting profiles. Relaxation assist device was presented as a new technique to reduce springback, pull-out phenomenon and to improve the compaction process. The basic phenomena associated with HVC are explained and the general energy principle is introduced to explain pull-out phenomenon during the decompacting stage. In this study, polyamide-11 powders with different particle size distributions have been compacted with the application of different compaction profiles, e.g. different energies and velocities. It was found that the relative green density is influenced more by the pre-compacting (primary compaction step) than by the post-compacting (secondary compaction step). Experimental results for different compaction profiles were presented showing the effect of varying the opposite velocity during the decompacting stage and how to improve the homogeneous densification between the upper and lower surface and the evenness of the upper surface of the compacted powder bed by using relaxation assists, and the influences of the relaxation assist device on the process characteristics. It was found that the relaxation assist improves the compaction of the polymer powder by locking the powder bed in the compacted form. In addition, the relative times of the compacting stage, decompacting stage and the reorganisation of the particles can be controlled by altering the height of the relaxation assist. It was found that the high-velocity compaction process is an interruption process and that the delay times between the pressure waves can be reduced by increasing the height of the relaxation assist device. Furthermore, the first gross instantaneous springback and the total elastic springback are reduced. Two bonding strain gauges and a high-speed video camera system were used to investigate the springback phenomenon during the compaction process. Scanning electron microscopy (SEM) and image computer board Camera (IC-PCI Imaging Technology) were used to the study the morphological characteristics, the limit of plastic deformation and particle bonding by plastic flow at contact points, and pull-out phenomena. Keywords: high-velocity compaction; powder polymers; polyamide-11; relaxation assist; compactibility; relative green density; morphology; pull-out; springback.

DEFINITIONS

High velocity compaction: A high velocity compaction (HVC) of powder materials is

a dynamic process carried out with one or several strokes and the velocity from about 1 m/s to up to 100 m/s.

Pre- compaction step: This step is the primary step of the compaction process

which is carried out in order to drive out the air in the compacted powder bed and obtain a further densification of the powder.

Post- compaction step: This step is the secondary step of the compaction process

which is carried out in order to increase densification and particle-to-particle bonding and possibly obtain interlocking of particles.

Compacting stage: This stage is defined as the stage when material is

compressed and the density of the material increases to a maximum value. The time to reach the maximum pressure defines the compacting time.

Decompacting stage: After the maximum pressure has been reached and the

piston starts to withdraw the decompacting stage starts. The compressive stresses relax and the material expands due to the elastic energy stored in the material. The time to return from the maximum to the minimum pressure defines the decompacting time.

Total Work of Compaction: Total energy transferred to the powder by the piston

defines the total work of compaction (TWC).

Relaxation assists: Are defined as parts of the piston and regarded as

projectile supports. The pieces are constructed from the same material as the piston and the diameters are the same, but the lengths are different.

Relative green density: Is a ratio of the apparent density of the compacted

powder to the theoretical density of the powder.

LIST OF SYMBOLS

2A Surface area of particle (m ) d The diameter of the compacted powder bed (m)

D Relative green density d Equivalent particle diameter (m) eq E Kinetic energy (J) k F Hydraulic force (N) h The height of the compacted powder bed (m) h/d Geometrical factor

M Mass (Kg) n Number of particles s Distance (m)

oT C) Glass transition temperature (g

V Velocity (m/s) V Opposite velocity (m/s) B

Delay time (s) τ

TABLE OF CONTENTS

...............................................................................................1 1 INTRODUCTION

1.1 .................................................................................3 PURPOSE OF THE STUDY1.2 .........................................................................3 THEORETICAL BACKGROUND

1.2.1 ..................................................................6 The general energy principle

..............................................................................................8 2 EXPERIMENTAL

2.1 ....................................................................................................8 MATERIALS2.2 ..........................................................................8 EXPERIMENTAL PROCEDURE

2.2.1 ..........................................................................9 Relaxation assist device2.2.2 ..............................................................................10 Compacting profiles

2.3 .............................................11 ANALYTICAL TECHNIQUES AND EQUIPMENTS2.3.1 ....................................................................................11 Density analysis2.3.2 ...................................................12 Scanning electron microscopy (SEM)2.3.3 .....................................................................12 High-speed video camera

........................................................................13 3 RESULTS AND DISCUSSION

3.1 ...........................................................13 COMPACTIBILITY CHARACTERISTICS3.1.1 ................................................................13 The effect of pre-compacting3.1.2 ...............................................................14 The effect of post-compacting

3.2 ....................................................................16 RELAXATION ASSISTS ADVICES3.2.1 ............................................................................16 Pull-out phenomenon3.2.2 ..............................................................................18 Springback gradient3.2.3 Delay time, relative time of the pressure wave and relative green

density ...................................................................................................23 ................................................................................................25 4 CONCLUSIONS

..............................................................................................26 5 FUTURE WORK

..............................................................................27 6 ACKNOWLEDGEMENTS

..................................................................................................28 7 REFERENCES

Introduction

1 INTRODUCTION

Powder compacting technology is used in the processing of a wide range of materials such as metals, ceramics, and polymers. This technique is effective for the processing of high melting temperature and high technology polymers such as polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), and polyamides (PA) [1-3]. The technique is especially suitable for high molecular weight polymers, such as ultra high molecular weight polyethylene (UHMWPE) [4, 5] and their blends [6, 7], for which conventional processing methods cannot be used because of their high melt viscosity. Many techniques can be used for processing powders at room temperature or elevated temperature. The most important are static pressing, dynamic pressing, hot and cold isostatic pressing (H & CIP), and the powder metallurgy technique (P/M). The most common method involves pressing the polymeric powder under single-ended pressing in a rigid metal die [1, 8]. The other techniques are suitable for processing polymers, polymer/polymer, and polymer/metal composites like high-velocity compaction. This technique appears to offer many interesting possibilities for conventional materials in specialized applications were conventional methods cannot be used. For example, since the technique does not require the polymer to flow on a macroscopic scale, heating is not needed as in a conventional melt flow process. This eliminates the need for thermal stabilizers, lubricating agents and other process aids, and the low process temperatures reduce the degradation risk and improve the potential for scrap recovery. The high-velocity technique is more energy-efficient and permits faster cycle times than conventional processing, since the relatively slow heating and cooling stages are eliminated from the cycle [6]. However, each technique has its advantages and disadvantages. Common problems are a low green density and an inhomogeneous density distribution in the compacted powder bed (compacted material). During the cold forming of plastics, the most important parameter is pressure. The disadvantage of the technique is the loss of pressure at the bottom of the die as result of friction between the plastic powder particles and the container wall [7]. In addition to the pressure loss effect, there are four main phenomena that can affect the high-velocity compaction process negatively: pull-out, springback, delamination and capping, all of which are due to a breakdown in the product during and/or after the decompacting stage. In the pull-out phenomenon, the top surface of the product becomes uneven, while in springback the dimensions of the product change. In

1

Introduction

capping, the top or cap of the product breaks off, and in the case of delamination the sample splits into a number of layers. These problems can be due to an initially non-uniform distribution of the powder within the die or to an excessive compaction speed. Earlier investigations have revealed that friction between the powder particles and the die wall is the major factor leading to structural variations within the compacted materials. Other investigations have revealed that the pressure transmission through the powder is important for the success and efficiency of the process. Crawford [9] has reviewed the different aspects of solid-state compaction of polymeric powders, and has discussed the effects of the various process parameters such as compaction pressure, compaction rate, and dwell time. Crawford and Sprevak [10] have studied the cold compaction of polymeric powders and have found that, with a cylindrical die and the geometrical factor h/d = 2, where h is the height and d is the diameter of the compacted powder bed, and using single-ended compaction, the pressure loss at the bottom of the die was 91% of the applied pressure at the top. They tried to reduce the pressure loss by using a tapered die to reduce the cross-sectional area, but the reduction was not as large as expected. The influence of various parameters such as the velocity of the cylindrical rod, the rod diameter and length, the granular particle size and the compaction pressure (by studying drag and compression force) in granular materials under high pressure was investigated by Zhou et al. [11], and they reported that the pressure loss at the bottom of the cylinder when h/d = 1.18 was about 61% of the applied pressure. In investigations of the dynamic compaction of biomaterial powders, Trécant-Viana et al. [12] showed that peripheral cracks, uneven surfaces and inhomogeneous densification are characteristic of this technique, regardless of which static primary compaction (pre-compacting) step is used before the dynamic secondary compaction (post-compacting) step. Canta and Frunza [13] studied the powder metallurgy technique during cold die compaction by using the container movement, and they reported that the effect of their new proposed technique (compaction by using container movement) on the density was similar to that of bilateral pressing. During the decompacting stage or an ejection of an object from a die, some degree of residual stress appears in the form of springback. Limit responses are related to cracking, delamination, and capping. Dimensional tolerance problems caused by large-scale, time-dependent springback limit the size of the compacted objects [14]. Several studies on springback and the dimensional changes in a compacted product, such as the effect of particle size, wall friction, glass transition temperature, and elastic recovery have been reported [15-20]. It is widely known that compacts which have been compacted by high pressure, cold compaction, and high-velocity compaction are non-uniform and contain both density and stress gradients [2, 6, 12, 21]. Most investigations have assumed that the

2

Introduction

compacted materials have a uniform density and a dimensional homogeneity in the die and they have focused only on the gross axial changes after the compacting process [22].

1.1 PURPOSE OF THE STUDY The powder processing technology and conventional powder metallurgical processing consisting of cold (room temperature) and high-pressure compaction, which is very well established for metals and ceramics, has been extended to certain specialty polymers that are difficult to melt [1, 5, 7, 13, 23, 24]. The disadvantages of the technique are the loss of pressure at the bottom of the die as result of friction between the plastic powder particles and the container wall, pull-out and springback. The fundamental mechanisms that control polymer compaction are not well understood, therefore this study has been performed to clarify the effects of the compacting conditions on the properties of the compacted products. The effects of compaction pressure, energy and velocity on compaction characteristics have been studied and efforts have been made to improve the compaction process.

1.2 THEORETICAL BACKGROUND In general, when a particulate material is placed in a rigid die at room temperature and is subject to pressure, there are a number of distinct stages in the process as the material is transformed from a loosely packed powder to a closely packed (possibly solid) material. When the powder is first poured into the die, its density is defined as the bulk density of the powder. This bulk density is a function of particle size and shape and it appears to have no influence on material’s compactability [9, 25]. When the compaction piston moves down onto the powder, the particles are reorganised in three stages: particle rearrangement, elastic and plastic deformation at contact points. In the particle rearrangement stage, the particles, which have been held up lightly by friction and bridges within the powder, move to lower positions at a low pressure until further piston movement is not possible without particle deformation. At this point, the density of the powder is referred to as the tap density [26]. During the next stage, sliding between particles and elastic compression at the contact points leads to a further densification of the powder. Under low stresses, the solid behaves essentially as an elastic body, i.e. it reverts to its original state on removal of the stress [27]. During the final stage, the downward movement of the compaction piston continues, the pressure increases and finally the material around the contact points is subject to plastic deformation and flows as shown in Fig.1. In the extreme case of a fully plastic response, there is no recovery of the original shape. At this point, the density of the material is referred to as the green density.

3

Introduction

15 kV X 350 50 µm

Fig. 1. Plastic deformations at contact points between particles on the bottom surface of compacted PA11 (20-30 µm) by pre-compaction at 60 J/g and post-compaction at 300 J/g. An early transition from elastic to plastic deformation is found in powders where the particle shape is irregular, since high stresses are created on small regions of contact between particles, and this means that the elastic limit is overcome under a relatively low compaction pressure [28]. Deformation processes that occur in the powder material under dynamic loading differ significantly from those that occur under static loading [4, 5, 23, 29, 30]. When the material is subjected to a dynamic load, the portion of the body that is exposed at the point of impact is stressed instantaneously while other portions may not yet have experienced the effect of the imposed impact, since the effect requires time to propagate through the body (Fig. 2. caption-b). This propagation occurs with a velocity which depends on the state and characteristics of the material. A high strain rate (compaction speed) is usually achieved by accelerating the impacting projectile via a hydraulic pump, a light gas gun or an explosive generator. In this investigation, a hydraulic power system was used to drive a piston [31]. The various parts of the compaction equipment are illustrated in Fig. 3.

4

Introduction

Fig. 2. Pressure waveforms recorded by two strain gages (upper and lower-strain gauge) (a) First reflecting waveform, compacting and decompacting stage, (b) Time-delay between upper strain gage and lower strain gage. (c) The delay time τ between the pressure waves.

Fig. 3. Diagram of the compaction testing cell equipment.

5

Hammer

sMAX

Bottom load cell (Plug)

Bottom relaxation assist

Polymer powder bed

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Top load cell

Top relaxation assist Signal-amplifiers

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Introduction

1.2.1 The general energy principle In engineering mechanics, the general energy principle may be stated as: the work done on a system is equal to the sum of the changes in kinetic energy and potential energy and the losses. The kinetic and potential energies are stored within the system and are recoverable. All other energy forms, such as friction, are considered to be losses. When friction is present, the work done depends on the path taken. Further experience of the physics of the problem suggests that other measurable changes may be taking place [32, 33]. In the first place, a local vibration and a change in temperature are expected. It is possible that a change such as melting of the material may take place. In mechanics, these latter changes generally represent a loss to the system, but such 'losses' may be regarded as 'gains' in the potential energy in the material [34, 35]. In addition to the bulk material properties of the powder, each powder particle is defined by its particle mass and velocity. The compaction process is defined by four additional system-state parameters: the kinetic energy Ek, the hydraulic force F, the mass M, and the velocity of the stroke V, and the opposite velocity during the decompacting stage (Fig. 2. caption-a), boundary velocity VB. B

The total energy transferred by the piston, the Total Work of Compaction (TWC), and absorbed by the material powders may reflect the particle behaviour, e.g. the elasticity, viscoelastic flow, brittle fracture and breaking of bonds in the material [36-38]. This can be calculated directly as:

, (1) ∫= MAX

0 dTWC

ssF

where F is the hydraulic force on the piston and s the distance between the bottom surface of the hammer and the top surface of the piston. The TWC is equal to the change in kinetic energy:

2 d

2MAX

0

MVsFs

=∫ , (2)

= swhere V is the velocity of the stroke at s MAX. This kinetic energy is transferred to the

compacted powder bed during the compaction stage [39]. At a low strain rate (low velocity compaction), the particles follow the common cyclic limit conditions in the horizontal plane (x and y-directions). Under these conditions, there is more opportunity for the particles to move, rotate and slide past each other [20]. At a high strain rate (high-velocity compaction), the motion in the z-direction is more energetic. Under this condition, there is less opportunity for the particles to move, rotate and

6

Introduction

slide past each other. The bottom wall reflects the particles, the powder bed, to begin the decompacting stage with the inverse velocity, the boundary velocity VB. The piston ultimately returns toward its original position and the particles, the powder bed, follow it. The movable top wall of the mould takes over in this case. However the top wall of the mould moves faster than the compacted powder bed, due to the elastic response of the compacted material. A large elastic response and non-homogeneous particle velocities can, furthermore, lead to delamination, capping, and pull-out problems.

B

7

Experimental

2 EXPERIMENTAL

2.1 MATERIALS ® Polyamide-11 powders (Rilsan Polyamide-11, ARKEMA A/S) were used as received. The

powders had been sieved prior to delivery to produce grades with different particle sizes. The particle size distribution of each powder was measured by manual counting of particles imaged on micrographs. The areas of approximately 500 particles were measured using imageJ 1.33u (National Institutes of Health, NIH, USA). The measurements were processed and the equivalent particle diameter deq i for particle i was calculated according to

πi

iAd 4

eq = , (3)

and the arithmetic mean particle diameter according to

∑=

=n

iid

nd

1 eq

1 , (4)

where n is the total number of the particles and Ai is surface area of particle i. The mean diameter of the particles is assumed to represent the average particle size of each sieved fraction. The mean particle size groups were (20-30, 40-45, 75-85 and 104-114 µm) and the mean particle sizes were 22, 42, 82 and 110 µm, as shown in Fig 4.

2.2 EXPERIMENTAL PROCEDURE The powders were compacted uniaxially in a 25 mm diameter cylindrical die using a Hydropulsor powder compacting machine (Hydropulsor AB, Sweden). The dynamic compaction system fired a 25 mm diameter and 80 mm long projectile (piston) with projectile supports (relaxation assists devices) to the unlubricated die.

8

Experimental

2.2.1 Relaxation assist device In this study, two new components are introduced as a relaxation assist device to improve the compaction process as shown in Fig. 5. The relaxation assists are parts of the piston and they are regarded as projectile supports. They are constructed of the same material as the piston, and the diameters are the same but the lengths are different. The relaxation assists used had lengths equal to 12, 37 and 60% of the length of the piston. The function of the relaxation assist device is to lock the powder bed in the compacted form during the compaction process. Fig. 4. Particle size distributions and equivalent diameter for polyamide-11 aspects by scanning electron microscopy (SEM).

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uenc

y (n

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ticle

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Upper relaxation assist

Lower relaxation assist

Fig. 5. Diagram of the relaxation assist devices.

9

Experimental

2.2.2 Compacting profiles Compaction profiling was carried out on a HYP35-4 testing machine equipped with two voltage amplifiers to amplify the output voltage from the load cells. The compaction cycle used to drive the compaction process is a one-ended compression (one piston moving) process operating over a range of velocities. Projectile velocities were measured and energies were calculated using equation (2). The pressures during the compaction process were measured by two strain gauges positioned on the piston and on the plug (Load Indicator AB, Sweden) [40]. Samples were prepared using four different compacting profiles in order to study of the compression characterizations: 1. Samples were compacted via a single stroke, one downward post-compacting. This compacting profile is referred to as P . post-down 2. Samples were compacted via a single downward pre-compacting stroke followed by one constant downward post-compacting stroke. This compacting profile is referred to as Ppre + P . post-down 3. Samples were compacted via a single downward pre-compacting stroke followed by one constant downward primary post-compacting stroke followed by one constant secondary upward post-compacting stroke. This compacting profile is referred to as P + Ppre post-down + P . post-up

4. Samples were compacted via a single downward pre-compacting stroke followed by one constant upward primary post-compacting stroke followed by one constant secondary downward post-compacting stroke. This compacting profile is referred to as P + P +pre post-up P . post-down The average strain rate in each case was calculated by dividing the compaction velocity by the initial height of the powder bed and the compacting energy (energy per mass) in each case was calculated by dividing the total energy transferred via the piston by the weight of the powder bed. The velocity and kinetic energy of the stroke against the distance between the bottom surface of the hammer and the top piston are shown in Fig. 6.

10

Experimental

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Fig. 6. The velocity and kinetic energy of the stroke plotted against the distance between the bottom surface of the hammer and the top piston.

2.3 ANALYTICAL TECHNIQUES AND EQUIPMENTS

2.3.1 Density analysis The relative green density D calculated as the ratio of the apparent density of the compacted polyamide-11 to the theoretical density of polyamide-11 [41] was used as a measure of the degree of compression [2]. The apparent density was calculated by applying the Archimedes principle using n-hexane as the liquid medium, according to

hexane-n ofDensity hexane-nin Mass-air in Mass

airin Mass densityApparent ×= , (5)

the density of n-hexane being taken as 0.66 g cm−3. A Mettler Toledo AE100 balance and a Mettler density determination kit 33360 were used for the measurements.

11

Experimental

2.3.2 Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) was used to study the morphological characteristics, particle size distribution, the limit of plastic deformation and particle bonding by plastic flow at contact points. The instrument used was a Jeol JSM-35CF from Jeol, Japan. SEM is a frequently used technique in materials science research. Its strength lies in the combination of imaging with high resolution and a large depth of focus together with the possibility of obtaining chemical and physical information.

2.3.3 High-speed video camera A FASTCAM-ultima APX High-Speed Video Camera System with an electronic shuttering system up to 120,000 frames per second (fps) and PFV (Photron FASTCAM Viewer, Photron Inc., USA) were used to study the relationship between the springback and movement of the piston during the compaction process. The relative dimensional changes were calculated by relating the position of piston at each pressure wave, which is related to the thickness of the powder bed including the elastic deformations of the piston parts, to the first pressure wave during the compaction. Compaction of a sequence of pressure waves reveals the springback gradient in the die. The reorganization times were calculated by measuring the time from the first contact between the hammer and the piston to the first rise of pressure recorded by the top strain gauge. This time is governed by the ability of the particles to move, rotate and slide past each other before permanent deformation occurs. The time to reach the maximum pressure defines the compacting stage and the time to return to the minimum pressure defines the decompacting stage. The relative times for the compacting stage, the decompacting stage and reorganisation of the particles were calculated as fractions of the total time of the pressure wave. The time for no pressure between two consecutive waves defines the delay time τ, as shown in Fig. 2 caption-c. For each experiment, ten independent samples prepared under the same conditions were tested. All the powders were initially dry-mixed for ten minutes at room temperature in order to obtain a homogeneous particle size distribution before compaction.

12

Results and Discussion

3 RESULTS AND DISCUSSION

3.1 COMPACTIBILITY CHARACTERISTICS

3.1.1 The effect of pre-compacting The effect of the pre-compacting stroke on the relative green density is shown in Fig. 7. Compacting by a single stroke, Ppost-down, of polyamide-11 is insufficient to achieve an adequate density at energy level less than 100 J/g, as shown in Fig. 7(a). One stroke occurs under a relatively short time, and this is not sufficient to reorganize and deform the particles both elastically and plastically. Since powders have different initial densities, they have different initial adequate densities [1]. Compacting with different pre-compacting energies followed by a constant post-compacting energy (300 J/g), P + Ppre post-down leads to a higher density than the single stroke, Ppost-down profile. This is attributed to an increasing densification and particle-to-particle bonding and possibly to an interlocking of particles with increasing pre-compacting energy. To raise the pre-compacting energy, with constant mass of the stroke, is not always an advantage and the density sometimes develops unpredictably as shown in Fig. 7(b). As the pre-compaction energy increases the compacting time decreases, and the particles-particularly the coarse particles- have less time to move and slide past each other. As a result, the porosity between powder particles increases and finally the apparent density decreases.

13

Results and Discussion

ig. 7. The relative green density plotted against compaction energy of a two-gram ompacted PA11 (different particle size distributions) by: (a) One downward post-ompacting, Ppost-down profile, (b) A single downward pre-compacting stroke followed by one onstant downward post-compacting (300 J/g), Ppre + Ppost-down profile.

.1.2 The effect of post-compacting he distributions of force within the powder mass and the pressure loss during compaction ave a major influence on the density distribution and on the quality of the compacted aterial.

the geometrical die esign or the geometrical factor.

irection on the relative green density is shown in

rofile, and 0.63% higher than when compacting by a Ppre + Ppost-down profile. The

FCcc

3Thm In this investigation attempts were made to compensate for the pressure loss by changing the post-compacting quantity and direction using different compacting profiles such as Ppre + Ppost-down + Ppost-up and Ppre + Ppost-up + Ppost-down, without changing eitherd

The effect of post-compacting quantity and dFig. 8. The Ppre + Ppost-down + Ppost-up compacting profile leads to a higher relative green density, as shown in Fig. 8(a). The maximum relative green density increase achieved by this compacting profile was 17.5 % higher than that achieved when compacting by a single stroke, Ppost-down presults achieved with a Ppre + Ppost-up+ Ppost-down compacting profile are shown in Fig. 8(b). The maximum relative green density increase achieved with this profile was 17.6 % higher than that achieved by a single stroke, Ppost-down compacting profile and 0.66 % higher than that achieved by a Ppre + Ppost-down + Ppost-up compacting profile.

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14

Results and Discussion

Fig. 8. The relative green density plotted against compaction energy of two-gram

ompacted PA11 (different particle size distributions) by: (a) A single downward pre-ompacting stroke followed by one constant downward primary post-compacting stroke (300 /g) followed by one constant secondary upward post-compacting stroke (300 J/g). Ppre + post-down + Ppost-up profile, (b) A single downward pre-compacting stroke followed by one onstant upward primary post-compacting stroke (300 J/g) followed by one constant condary downward post-compacting stroke (300 J/g). Ppre + Ppost-up + Ppost-down profile.

g the pre + Ppost-up+ Ppost-down profile than when using the Ppre + Ppost-down + Ppost-up profile.

he relative green density was influenced more by the pre-compacting effect than by the post-ompacting quantity and direction. The surface morphology, density distribution and the

energy was ca.100 J/g. This reflects the fact at the particle behavior is independent of particle size and the geometrical h/d factor.

CcJPcse The difference in total relative green density achieved by pre-compacting followed by a single post-compacting and by pre-compacting followed by double post-compacting was very small, regardless of the post-compacting profile used. Changing the pressure direction leads to evenness, homogeneity, and densification on the surfaces as well as the density distribution in the compacted powder bed. The pressure and density distribution between the upper and lower surface were more uniform when usinP Tcquality of the surfaces of compacted powder beds were influenced to a greater extent by the post-compacting direction. The maximum relative green density was achieved by all the compacting profiles when the pre-compacting th The pressure loss for a geometrical factor of h/d = 1.12 at the bottom of the die during the secondary post-compacting was 35.3% of the applied pressure at the top with the Ppost-down + Ppost-up profile and 21.5% with the Ppost-up+ Ppost-down profile.

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15

Results and Discussion

3.2 RELAXATION ASSISTS ADVICES

al parameters of the machine. The disadvantage of the increasing the distance between the hammer and the top surface of the piston is that it leads to an increase in the compaction velocity at constant hydraulic force and

ty of the particles to move, rotate and r time available for this process [24].

h n by the particles which have more contact points per a r than that by the larger particles. This increase in the

n early transfer from elastic to plastic deformation and plastic flow occurs in powders where

ster than the compacted material.

e stress relaxation.

3.2.1 Pull-out phenomenon The degree of plastic deformation and particle bonding by plastic flow at the contact points depends on the amount of energy that can be absorbed by the powder particles. In the high-velocity compaction process, kinetic energy is proportional to the applied pressure. It is possible to increase the kinetic energy by increasing the hydraulic force or by increasing the distance between the hammer and the top surface of the piston. It is difficult to increase the hydraulic force without changing the design and/or initi

hammer mass. As the piston velocity increases; the abilislide past each other decreases as a result of the shorteNevert eless, the energy absorptiosurface rea, i.e. small particles, is greateenergy absorption at contact points results in plastic deformations and plastic flow although the effective dwell time is reduced. Athe particle shape is irregular, because high stresses develop on small contact areas between particles. This early transfer cases the elastic limit to be overcome at a relatively low compaction pressure, generally in the upper part of the powder bed. These particles form contorted and deformed particle layers in the compacted powder bed. When the piston moves down, the top wall of the compacted powder bed also moves down (compacting stage) to a point of maximum compaction, and this is followed by the inverse movement which begins the decompaction stage. The piston ultimately returns toward the original position and the compacted powder bed follows it. During faster compaction, the withdrawal time of the piston is shorter than the natural relaxation time of the compacted material and the piston moves back fa Different post-compacting profiles, i.e. different energy distributions between the upper and lower parts of the compacted powder bed, lead to different movements of the particles and the particle layers. The use of relaxation assists reduce the difference between these velocities in the powder bed and gives the compacted powder bed a more homogeneous opposite velocity during the decompacting stage and probably more time to slow down th Particles which have inadequate contact points with other particles will be less obstructed. They can withdraw more freely than the deformed particle layers and start the pull-out phenomenon. In addition, the deformed particle layers withdraw more rapidly than the other parts of the compacted powder bed and start the capping phenomenon. The effect of compacting with different energies and different velocities on the morphology of the top

16

Results and Discussion

surface of the compacted material, without relaxation assists and with relaxation assists is

ig. 10. The top surface of compacted PA11 (20-30 µm) with relaxation assists by: (a) Low nergy, 180 J/g and velocity 4,83 m/s, (b) Higher energy, 300 J/g and velocity 6,23 m/s.

shown in Fig. 9 and Fig. 10 respectively.

(a) (b)

Fig. 9. The top surface of compacted PA11 (20-30 µm) without relaxation assists by: (a) Low energy, 180 J/g and velocity 4,83 m/s, (b) Higher energy, 300 J/g and velocity 6,23 m/s. (b) (a) Fe

17

Results and Discussion

3.2.2 Springback gradient uring the compaction process, the polymer powder undergoes a high compressive stD rain hich ultimately leads to consolidation and densification of the material. The process can be ivided into two stages, compacting and decompacting stages. In the compacting stage, the aterial is compressed and the density of the material increases to a maximum value. After e maximum pressure has been reached and the piston starts to withdraw (the decompacting age), the compressive stresses relax and the material expands due to the elastic energy stored the material. During this springback stage, the density decreases. Most of the springback

henomenon occurs during the decompaction stage, within the die, rather than after ejection 2].

typical example is shown in Fig 11. This figure shows the pressure wave forms, the motion f the hammer and of the piston when compacting PA11 (mean particle sizes ≈ 22 µm) with e geometrical factor h/d = 0.18 and strain rate 1.15×103 s−1 without relaxation assists. uring the decompacting stage, the hammer returns to its original position and the piston llows it, as a result of the release of elastic energy stored in the powder bed. The new

osition of the piston is higher than the initial position before the stroke with the same gap to ottom surface of the hammer as shown in Fig. 11 regions a and b. Immediately after the first

1 region a, of the initial volume of the powder bed before compaction. In this volume, during e delay time, the polymer powders are less restricted and dimensional changes occur which

nd on the characteristics of the powder material and their kinetic energies. The hammer aw d the piston moves back faster than the compacted powder e ation time of the compacted material. During each delay

wdmthstinp[4 AothDfopbpressure wave, the compaction process is interrupted and there is a delay between the successive pressures wave. In this case, the delay time between the first peak and the second

eak was 12.2 ms. During the first delay, the compacted volume was maximally 195%, Fig. p1thdepewithdr s faster than the piston, anbed du to the longer natural relaxperiod, there is a springback, until the end of the pressure wave. The first instantaneous springback is difficult to repair during the following pressure waves because the pressure decreases from peak to peak until the process ends.

18

Results and Discussion

Fig. 11. The pressure waveforms, the dynamic motion of the hammer and the piston plotted

gainst time for two-gram compacted PA11 (mean particle size ≈ 22 µm and geometrical ct

everal studies have proposed that high-velocity compaction (HVC) is a continuous process. he complex nature of the pressure waves within the compacted materials has not been onsidered [43] and the process analysis has been limited to the first peak of the pressure aves [44, 45].

owever, the results of this study reveal that the high-velocity compaction process is not a ontinuous process, but rather that it includes several interruptions that are likely to have a reat influence on the compacted material. Instead, the subdivisions of the high-velocity ressure waves have an important function and they should not be underestimated. The nature f the multiple waves and peak-to-peak delay times at different strain rates are shown in Fig. 2.

afa or h/d = 0.18) by the strain rate 1.15×103 s−1 without relaxation assists. STcw Hcgpo1

-10

0

0,00 0,02 0,04 0,06 0,08 0,10

time (sec)

Dis

ta

0

250

500

Pre

ssur

e10

20

30nc

e (m

m)

750

1000

(MP

a)

Motion of the hammer Motion of the piston Pressure on the top load cellPressure on the bottom load cell

Positioa

n of the top surface of the powder bed before compactionb

c

19

Results and Discussion

Fig. 12. The pressauge) for PA11 by: (

ure waveforms recorded by two strain gauges (upper and lower-strain a) Velocity 4,82 m/s and strain rates (1×103, 0.4×103 and 0.25×103 s−1)

r mean particle size ≈ 42 µm, ( b) Velocity 5,3m/s and strain rates (1.15×103, 0.5×103 and .3×103 s−1) for mean particle size ≈ 80 µm, (c) Velocity 6.2 m/s and strain rates (1.5×103,

0.6×103 and 0.35×103 s−1) the mean particle size ≈ 100 µm. In this study, a relaxation assist device has been used to reduce the instantaneous springback and to increase the understanding of the springback gradient in a die. The lengths of the assists were 12, 37 and 60% of the length of the piston. The upper relaxation assist was located above the polymer powder bed, below the piston, where the top strain gauges was positioned, and the lower relaxation assist was located below the polymer powder bed, above the plug, where the bottom strain gauges was positioned. The function of the relaxation assists is to lock the powder bed during the compaction process. The compaction process with and without the relaxation assist device is shown schematically in Fig. 13. Without the assists, particles can withdraw more freely; start the dimensional changing, during decompacting stage. The pressure waveforms, dynamic motion of the hammer and the piston when compacting PA11 (mean particle sizes ≈ 22 µm) with the geometrical factor h/d = 0.18 and strain rate 1.15×103 s−1 with relaxation assists (12, 37 and 60% of the length of the piston), respectively, are shown in Fig. 14 a, b, and c. During compaction, all collisions between different parts of the piston, the relaxation assists, and the hammer were elastic without permanent deformation.

he ability to absorb energy in an elastic collision without permanent deformation is a aterial property. During the decompacting stage, the lower relaxation assist relaxes rapidly

upper relaxation assist. The high speed camera showed that the position of the piston after the

gfo0

Tmand strikes the powder bed upwards due to the pure elastic energy which is stored in the lower relaxation assist during the compacting stage. The piston withdraws faster than in the case of compacting without the relaxation assist due to the elastic collision between the piston and the

0

2 5 0

5 0 0

0

2 5 0

5 0 0

7 5 0

0 ,0 0 0 ,0 2 0 ,0 4 0 ,0 60

2 5 0

5 0 0

a

Pres

sure

(MPa

)

0

2 5 0

5 0 0

7 5 00

2 5 0

5 0 0

7 5 0

0 ,0 0 0 ,0 2 0 ,0 4 0 ,0 60

2 5 0

5 0 0

7 5 0

P re s su re o n th e u p p e r p is to n P re s su re o n th e lo w e r p is to n

D e la y t im e s

T im e (s e c )

b

0

7 5 0

2 5 0

5 0 0

0

2 5 0

5 0 0

7 5 0

1 0 0 0

0 ,0 0 0 ,0 2 0 ,0 4 0 ,0 6

c

7 5 0

0

2 5 0

5 0 0

20

Results and Discussion

first pressure wave, during the first delay time, was 61% higher in the case of the relaxation assist with a height 37% than that without the use of relaxation assist, and the delay time

etween the first peak and the second peak decreased from 12.2 ms to 11.58 ms with the 37%

he relative dimensional changes during the sequence of the pressure waves for two-gram ompacted PA11 (mean particle sizes ≈ 22 µm) with and without relaxation assists (12, 37

60% of the length of the piston) are shown in Fig. 15. The first instantaneous springback was found to increase with decreasing height of the relaxation assists. During the first pressure wave, the dimensional change takes place more rapidly without than with the relaxation assists. Furthermore, the final dimensional change was greater without than with the relaxation assists.

brelaxation assist and to 8.58 ms with the 60% relaxation assist. The collision between the upper relaxation assist and the powder bed is not elastic, because the permanent plastic deformation which occurs within the powder bed during the compacting stage. The upper relaxation assist probably relaxes against the powder bed, and is in direct contact with the powder bed. Consequently, the powder bed is held between the two relaxation assists without any gap during the delay time. This locking action improves the compacting process by reducing the expansion of the compacted volume until the powder bed is subjected to the next pressure wave. Fig. 13. Compacting and decompacting stage: (a) Without relaxation assists, (b) With relaxation assists.

(b) (a)

Tcand

21

Results and Discussion

Fig. 14. The pressure waveforms, the dynamic motion of the hammer and the piston plotted against time for two-gram compacted PA11 (mean particle size ≈ 22 µm and geometrical factor h/d = 0.18) by the strain rate 1.15×103 s−1 with relaxation assists (a) 12%, (b) 37% and (c) 60%.

-10

0

10

20

30

0 0,02 0,04 0,06 0,08 0,1

time (sec)

Dis

tanc

e (m

m)

0

250

500

750

1000

Pre

ssur

e (M

Pa)

Motion of the hammerMotion of the pistonPressure on the top load cellPressure on the bottom load cell

Position of the top surface of the powder bed before compaction

-10

0

10

20

30

0 0,02 0,04 0,06 0,08 0,1

time (sec)

Dis

tanc

e (m

m)

0

250

500

750

1000

Pre

ssur

e (M

Pa)

Motion of the hammer Motion of the pistonPressure on the top load cellPressure on the bottom lod cell

Posit ion of the top surface of the powder bed before compaction

a

b

c

-100,00 0,02 0,04 0,06 0,08 0,10

0

0

10

20

30

time (sec)

Dis

tanc

e (m

m)

250

500

750

1000

Pre

ssur

e (M

Pa)

Motion of the hammerMotion of the pistonPressure on the top load cellPressure on the bottom load cell

Position of the top surface of the powder bed before compaction

22

Results and Discussion

0

10

20

30

Fig. 15. The relative dimensional change plotted against the sequence of pressure waves for

o-gram compacted PA11 (mean particle size ≈ 22 µm and geometrical factor h/d = 0.18) by e strain rate 1.15×103 s−1 with and without relaxation assist (12, 37 and 60% of the length of e piston).

.2.3 Delay time, relative time of the pressure wave and relative green density

he delay time, relative time of the pressure wave components (compaction stage, ecompaction stage and reorganisation time) and the value of the relative green density are lotted against the height of the relaxation assist in Fig. 16.

ith increasing height of the relaxation assists, the pressure wave requires more time to ansfer through the piston and the upper relaxation assist. Furthermore, the ability of the articles to move, rotate and slide past each other before some permanent deformation take lace increases. As a result, the polymer powder bed reaches a higher relative green density.

onsequently, with the better particle rearrangement and more closely packed material the eight of the powder bed is decreased and the pressure wave requires less time to reach the aximum value. As a result, the relative time of compacting stage decreases (the time needed

timheight of the relaxation assists.

1 2 3 4 5 6 7 8 9 10

Rel

ativ

e di

men

sion

al c

hang

e (%

)

Number of the pressure wave peak (n)

The percentage of the relaxation assists 0% 12% 37% 60%

twthth

3

Tdp Wtrpp Chmto reach the maximum value of the pressure related to the time of the pressure wave) with increasing height of the relaxation assists. During the decompacting stage, more time is needed to release the elastic energy stored in the upper and lower relaxation assists during the compacting stage. As a result, the relative e of decompacting stage increases with increasing

23

Results and Discussion

The de e elastic ollision between the lower relaxation assist and the plug which means that the locked powder ed is in a higher position when the powder bed is subjected to the next pressure wave and the lastic collision between the upper relaxation assist and the piston which causes the piston to ithdraw rapidly and meet the hammer in a shorter time. As a result, the powder bed has less me to relax and change its dimensions before it is subjected to the next pressure wave. By voiding the first instantaneous springback the powder bed attains a higher density as shown the Fig. 16. Ultimately, the interruption of the process decreases and the powder bed laxes during the whole compacting time with less change in dimensions, as shown in Fig.

5.

ig. 16. The delay time, the relative time of the pressure wave components (compaction stage,

8

11

13

14

lay time decreases with increasing height of the relaxation assists due to thcbewtiainre1

12

10

9

Fdecompaction stage and reorganisation time) and the value of the relative green density plotted against the height of the relaxation assists (the height of the relaxation assist related to the height of the piston).

0

20

40

60

80

100

0% 10% 37% 60%

89.29 %

87.52 %

87.17 %

Relaxation assist

Relative green density

87.16 %

Delay time

y tim

Del

ae

(ms)

Compacting time Decompacting time Reorganisation time

oeo

re

time

(%)

gani

satio

n re

lativ

mpa

ctin

g, d

ecom

pact

ing,

rC

24

Conclusions

4 CONCLUSIONS

High-velocity compaction process for polymer powder and a new technique with relaxation ssist reen ensity depends on nature of the particle behavior regardless of particle size and geometrical haracteristics. The relative green density is more influenced by pre-compacting step haracteristics than by the post-compacting step characteristics. However, the difference in lative green densities achieved by pre-compacting followed by single post-compacting and

y pre-compacting followed by double post-compacting is very small, regardless of the post-ompacting profile was used. Furthermore, the pressure and density distribution difference etween the upper and lower surfaces is influenced more by post-compacting quantity and irection than by the pre-compacting characteristics.

he relaxation assist device leads to an improvement in the polymer powder compaction rocess by locking the powder bed in the compacted form during the compaction process. onsequently, the movement of the particles and the particle layers during the decompacting age can be reduced and the powder bed attains a higher density. By increasing the height of e relaxation assist, the first gross instantaneous springback is reduced, and total elastic ringback is minimized.

he high-velocity compaction process is a non-continuous process, the delay times between e pressure waves are strongly dependent on the strain rate, and this delay can be decreased

y increasing the height of the relaxation assists. Furthermore, the relative times of the ompacting stage, decompacting stage and reorganisation of the particle can be controlled by ltering of the height of the relaxation assists.

a device leads to higher relative green density than by static pressing. The initial gdccrebcbd TpCstthsp Tthbca

25

Future Work

quations and models are needed that can orrelate the process-ability of material in large-scale manufacturing. These models must be

5 FUTURE WORK

The high- velocity cold compaction (HVC) processing of polymers is a relatively complicated process. The properties of the final products can vary over a wide range though suitable changes in the process variables. Although the reported work illustrates the effect of these process variables and parameters on the properties of the final products, some more work needs to be done to investigate the structure development during processing so as to establish processing—structure—property correlations for the solid-state processing of polymers. There is need to develop predictive correlations between molecular structure, solid-state properties and mechanical properties. Mathematical ecvalidated to ensure efficacy, accuracy and consistency.

26

Acknowledgements

6 ACKNOWLEDGEMENTS

First and foremost, I would like to acknowledge my supervisors Professor Bengt Stenberg and

y is cknowledged for brilliant ideas, helpfulness and I have learned a lot during our discussions.

would also like to mention all people at the department of Fibre and Polymer Technology nd thank everyone for help with various sorts of problems, especially Mattias Lokander, aria, Stina, Karin, Jovan, and Johan. The administrative staff are thanked, especially arbara Karan, Maria, Margareta, Inger, Maria, Viktoria and Ove. Richard Olsson is thanked r his fruitful discussions during late nights.

he Swedish Agency for Innovation Systems (VINNOVA), Grant 2002-02177, is gratefully cknowledged for financial support. I would thank Dr. Jan Asplund at Lund University/ NIVA for coordinating of this project. All partners of (VAMP30) are thanked for the aluable discussions and good times at our meetings.

Anthony Bristow is thanked for the rapid and skilful linguistic corrections.

would like to thank Lidingöhem, Rikesbyggen, especially Ulla Sundberg, Linnéa Ståhl, and ans Hall for their support and help with a new apartment during the difficult moments of

writing this thesis. I would finally like to thank my wife Sayran, my daughter Naz, my father, my brothers and sisters, and the rest of my friends outside the department.

Professor Leif Kari. Professor Bengt Stenberg at the Department of Fibre and Polymer Technology, School of Chemical Science and Engineering is acknowledged for persuading me to start working in this project and for accepting me as his PhD student. His never ending optimism, enthusiasm and continuous support have been very important for me during this period. My co-supervisor Professor Leif Kari at the Department of Aeronautical and Vehicle Engineering, School of Engineering Sciences at the Royal Institute of Technologa Professor Ulf W. Gedde ”The Angel” is gratefully acknowledged for his unbounded and continuous help whenever I needed it. I aMBfo TaUv J. I H

27

References

7 REFERENCES

1. D. M. Bigg, A Study of the Effect of Pressure, Time, and Temperature on High-Pressure Powder Molding. Polymer Engineering and Science, 1977. 17(9): p. 691-699.

2. J. J. Relly and I. L. Kamel, Characterization and Cold Compaction of Polyether-etherketone Powders. Polymer Engineering and Science, 1989. 29(20): p. 1456-1465.

3. Jun Yang, S.L., Xingyuan Guo, Yucheng Luan, Wenhui Su, and Jingjiang Liu, Structural Studies of Nylon 1010 Treated at Atmospheric and High Pressures. Polymer Journal, 2001. 33(10): p. 821-824.

der Processing of Ultra-High Molecular Weight Polyethylene

t Polyethylene. Polymer Engineering and Science, 1980. 20(11): p. 747-755.

nd Nickel

4. G. W. Halldin and I. L. Kamel, Pow

I. Powder Characterization and Compaction. Polymer Engineering and Science, 1977. 17(1): p. 21-26.

5. R. W. Truss, K.S.H., J. F. Wallace, and P. H. Geil, Cold Compaction Molding and Sintering of Ultra High Molecular Weigh

6. Mikko Karttunen and Pekka Ruuskanen, The Microstructure and Electrical Properties of Mechanically-Alloyed Copper-Polymer Composites. Materials Science Forum, 1998. 269-272: p. 849-852.

7. J. J. Relly and I. L. Kamel, Cold compaction of Polyether-etherketone aPowder Blends. Polymer Engineering and Science, 1989. 29(20): p. 1446-1455.

8. N. A. Adamenko V. N. Arisova and A. V. Fetisov., Structure and properties of fluoroplastic and ultra high molecular weight polyethylene produced by shock-wave pressing. International Polymer Science Technology, 2001. 28(1): p. T/64-T/68.

28

References

9. Crawford, R.J., D.W. Paul, and D. Sprevak, Solid phase compaction of polymeric powders. Effects of compaction conditions on pressure and density variations. Polymer, 1982. 23(1): p. 123-8.

10. Crawford R. J. and Sprevak D., Cold Compaction of Polymeric Powders in Tapered Dies. European Polymer Journal, 1984. 20(5): p. 441-446.

, Slow drag in granular materials under high , Nonlinear, and Soft Matter Physics, 2004.

12. Trecant-viana, M., et al., Dynamic compaction of biomaterial powders. Journal de

.

.

16. S.L. Rough, The effects of wall friction on the ejection of pressed

17. ering and Science Proceedings, 1999. 20(2,

Materials & Equipment and Whitewares): p. 51-63.

18. yl alcohol on granule compaction. Journal of the American

19. to

20. ar pectin for tableting operation using a compaction simulator. International Journal of Pharmaceutics, 1998. 161(2): p. 149-159.

11. Zhou, F., S.G. Advani, and E.D. Wetzelpressure. Physical Review E: Statistical69(6-1): p. 061306/1-061306/7.

Physique IV, 1997. 7(C3, International Conference on Mechanical and Physical Behaviour of Materials under Dynamic Loading, 5th, 1997): p. C3/3-C3/6.

13. Canta, T. and D. Frunza, Friction-assisted pressing of PM components. Journal of Materials Processing Technology, 2003. 143-144: p. 645-650.

14 G. W. Halldin and I. L. Kamel, Powder Processing of Ultra-High Molecular Weight Polyethylene I. Powder Characterization and Compaction. Polymer Engineering and Science, 1977. 17(1): p. 21-26.

15 Sinka, I.C., J.C. Cunningham, and A. Zavaliangos, The effect of wall friction in the compaction of pharmaceutical tablets with curved faces: a validation study of the Drucker-Prager Cap model. Powder Technology, 2003. 133(1-3): p. 33-43.

Briscoe, B.J. andceramic parts. Powder Technology, 1998. 99(3): p. 228-233.

Perry, C.R. and W.M. Carty, Effect of plasticizer on compaction behavior and springback defects. Ceramic Engine

Nies, C.W. and G.L. Messing, Effect of glass-transition temperature of polyethylene glycol-plasticized polyvinCeramic Society, 1984. 67(4): p. 301-4.

Sugimori, K., S. Mori, and Y. Kawashima, Characterization of die wall pressurepredict capping of flat- or convex-faced drug tablets of various sizes. Powder Technology, 1989. 58(4): p. 259-64.

Kim, H., G. Venkatesh, and R. Fassihi, Compactibility characterization of granul

29

References

21. Tavernor, A.W., et al., Improved compaction in multilayer capacitor fabrication. Journal of the European Ceramic Society, 1999. 19(9): p. 1691-1695.

22. Linda W. Vick and Ronald G. Kander, Ambient temperature compaction of

23. Crawford R. J., Effect of Compaction Rate During the Cold Forming of Polymeric

24. Maclaine, An investigation of the effect of the punch velocity on the compaction properties of ibuprofen. Powder Technology, 1993. 74(2): p.

25. id granules at different temperatures. Advances in Polymer Technology, 1998. 17(1): p.

26. Crawford R. J. and Paul D. W., Cold compaction of polymeric powders. Journal of

27. r, and D.M. Heyes, Molecular dynamics simulations of granular compaction: the single granule case. Journal of Chemical Physics, 2003.

28. -Phase Compaction of Polymeric Powders. European Polymer Journal, 1981. 17(10): p. 1023-

29. Teizo Maeda and Shin-ichi Matsuoka, Study on Cold-compaction Molding of Polymeric .

30. ials. 2001. 510 PP.

32. Kumar, V. and S.H. Kothari, Effect of compressional force on the crystallinity of directly ):

33. Ek, R., et al., Crystallinity index of microcrystalline cellulose particles compressed into

polycarbonate powder. Polymer Engineering and Science, 1997. 37(1): p. 120-127.

Powder. Polymer Engineering and Science, 1982. 22(5): p. 300-306.

Marshall, P.V., P. York, and J.Q.

171-7.

Qiu, D.Q. and P. Prentice, Influence of pressure on bulk density of polymer sol

23-36.

Materials Science, 1982. 17(8): p. 2267-80.

Sanchez-Castillo, F.X., J. Anwa

118(10): p. 4636-4648.

Crawford R. J. and Paul D. W., Radial and Axial Die Pressures During Solid

1028.

Powders. Journal of The Faculty of Engineering, 1975. XXXIII(2): p. 192-226

Nesterenko, V.F., Dynamics of heterogeneous mater

31. Haddad, Y.M., Mechanical behaviour of engineering materials. Vol. 2, Dynamic loading and intelligent material systems. 2000: Kluwer Academic. 484 PP.

compressible cellulose excipients. International Journal of Pharmaceutics, 1999. 177(2p. 173-182.

tablets. International Journal of Pharmaceutics, 1995. 125(2): p. 257-64.

30

References

34. Harrison, H.R.N., T., Principles of Engineering Mechanics (2nd Edition). 1994: EArnold. 270 pp.

dward

iley &

, y (heat)

determinations of compact unloading and ejection. International Journal of

37. is, C., Fluid Dynamics - Theory, Computation, and Numerical Simulation. 2001: Kluwer Academic Publishers. 675 pp.

38. . 1996: McGraw-Hill. 1563 pp.

41. Ibos, L., et al., Thermal behavior of ferroelectric polyamide 11 in relation to

42. Klemm, U. and D. Sobek, Influence of admixing of lubricants on compressibility and

43. Al-Hassani, S.T.S., P.W. Tennant, and M.A. Sarumi, Stress wave method for measuring rgy,

1989. 32(3): p. 204-8.

44. ty and properties. Advances in Powder Metallurgy & Particulate Materials, 2002: p. 4/111-

45. , M. Kejzelman, and I. Hauer, High density PM components by high velocity compaction. Advances in Powder Metallurgy & Particulate Materials, 2002: p.

35. Harrison, H.R.N., T., Advanced Engineering Dynamics. 1997: Arnold/John WSons. 301 pp.

36. DeCrosta, M.T., et al., Thermodynamic analysis of compact formation; compactionunloading, and ejection II. Mechanical energy (work) and thermal energ

Pharmaceutics, 2001. 213(1-2): p. 45-62.

Pozrikid

Avallone, E.A.B., T., III, Marks' Standard Handbook for Mechanical Engineers (10th Edition)

39. Beatty, M., F., jr., The principles of engineering mechanics. Vol. 1, Kinematics the geometry of motion. 1986: Plenum. 396 PP.

40. McMillan, G.K.C., D.M., Process/Industrial Instruments and Controls Handbook (5th Edition). 1999: McGraw-Hill. 1200 PP.

pyroelectric properties. Journal of Polymer Science, Part B: Polymer Physics, 1999. 37(7): p. 715-723.

compactibility of uranium dioxide powders. Powder Technology, 1989. 57(2): p. 135-42.

elastic moduli of powder compacts in tension and compression. Powder Metallu

Skagerstrand, A., High velocity compaction of metal powders, a study on densi

4/117.

Skoglund, P.

4/85-4/95.

31

References

32