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Novel Technique to Improve High-Velocity Cold Compaction: Processing of Polymer Powders and Polymer-Based Nanocomposite High Performance Components
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Compaction of polymer powders and polymer-based nanocomposites by uniaxial high-velocity cold compaction (HVC), by high-energy ball milling (HEBM) and using a novel technique, relaxation assists, was investigated with a focus on the process parameters, the compactibility characteristics, surface morphology and friction. The basic phenomena associated with HVC are explained and the general energy principle is introduced to explain the pull-out phenomenon, springback gradient, delay time, relative time of the pressure wave, and stick-slip phenomenon during the compaction process. Experimental results for different compaction profiles, different particle size distributions and different milling system for polymer-based nanocomposite are presented, showing the effect of varying the process parameters on the compacted material; the compactibility in the compacted bed, the uniformity of the compacted surface, the pull-out phenomenon, the springback gradient, the stick-slip phenomenon and the homogeneity of the dispersions of nanoparticles in the polymer powders in the solid state. It was found that the high-velocity compaction process is an interruption process and that the opposite velocity and pressure loss during the compaction process have a major influence on the quality of the compacted material. The relaxation assist device is a novel technique that has been successfully developed to improve the compaction process. 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 assist device leads to an improvement in the compaction of powders, polymer powders and polymer-based nanocomposites by giving a more homogeneous opposite velocity and a better locking of the powder bed in the compacted form during the compaction process with less change in dimensions in the case of both homogeneous and heterogeneous materials. If the movement of the particles is restricted the powder bed attains a higher density and the total elastic springback is minimized. In addition, there is a more homogeneous dispersion of nanoparticles in the case of a heterogeneous material. A much better transfer of the pressure through the powder bed and a smaller loss of pressure lead to a more homogenous stick-slip of the particles and a higher sliding coefficient due to the overall friction during the compaction process.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2006:31
Keyword [en]
polymer powders, nanocomposites, high-velocity compaction, high-energy ball milling
National Category
Production Engineering, Human Work Science and Ergonomics
Identifiers
URN: urn:nbn:se:kth:diva-4133ISBN: 91-7178-459-4 (print)OAI: oai:DiVA.org:kth-4133DiVA: diva2:10879
Public defence
2006-11-03, E2, Lindstedsvägen 3, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20100630Available from: 2006-10-06 Created: 2006-10-06 Last updated: 2010-06-30Bibliographically approved
List of papers
1. Development of a High-Velocity Compaction process for polymer powders
Open this publication in new window or tab >>Development of a High-Velocity Compaction process for polymer powders
2005 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 24, no 7, 909-919 p.Article in journal (Refereed) Published
Abstract [en]

The High-Velocity Compaction (HVC) process for powder polymers has been studied, with a focus on the compactibility characteristics and surface morphology of the compacted materials, with and without relaxation assists, by increasing compacting quantity and direction. The basic phenomena associated with HVC are explained and the general energy principle is introduced to explain pull-out phenomena during the decompacting stage. Polyamide-11 powders with different particle size distributions have been compacted with the application of different compaction profiles, e.g. different energies and velocities. Scanning electron microscopy (SEM) and image computer board camera, (IC-PCI Imaging Technology) have been 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. The relative green density is influenced more by the pre-compacting (primary compaction step) than by the post-compacting (secondary compaction step). The pressure and density distribution differences between the upper and lower surface are not uniform. Projectile supports or 'relaxation assists' are presented as a new technique to reduce pull-out phenomenon. Experimental results for different compaction profiles are 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.

Keyword
high-velocity compaction, polyamide-11, powder polymers, compactibility, green density, morphology, relaxation assist, pull-out
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-6226 (URN)10.1016/j.polymertesting.2005.06.008 (DOI)000232677300015 ()2-s2.0-25844462783 (Scopus ID)
Note

QC 20100506

Available from: 2006-10-06 Created: 2006-10-06 Last updated: 2017-12-14Bibliographically approved
2. Determination of springback gradient in the die on compacted polymer powders during high-velocity compaction
Open this publication in new window or tab >>Determination of springback gradient in the die on compacted polymer powders during high-velocity compaction
2006 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 25, no 1, 114-123 p.Article in journal (Refereed) Published
Abstract [en]

A uniaxial high-velocity compaction process for polymer powder using a cylindrical, hardened steel die and a new technique with relaxation assist was tested with various heights. The influences of the relaxation assist device on the process characteristics are discussed. Two bonded strain gauges and a high-speed video camera system were used to investigate the springback phenomenon during the compaction process. It was found that the relaxation assist improves the compaction of the polymer powder by locking the powder bed in the compacted form. It is shown 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. The delay times between the pressure waves are also strongly dependent on the strain rate. If the height of the relaxation assist device is increased, the first gross instantaneous springback, and the total elastic springback, are reduced. In addition, the density of the powder bed is increased. The relative times of the compacting stage, decompacting stage and the reorganisation of the particles can be also controlled by altering the height of the relaxation assist.

Keyword
high-velocity compaction; powder polymers; polyamide-11; relaxation assist; strain rates; relative green density; morphology; springback gradient
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-6227 (URN)10.1016/j.polymertesting.2005.09.002 (DOI)000235262700017 ()2-s2.0-31444445807 (Scopus ID)
Note

QC 20100506

Available from: 2006-10-06 Created: 2006-10-06 Last updated: 2017-12-14Bibliographically approved
3. Determination of dynamic and sliding friction, and observation of stick-slip phenomenon on compacted polymer powders during high-velocity compaction
Open this publication in new window or tab >>Determination of dynamic and sliding friction, and observation of stick-slip phenomenon on compacted polymer powders during high-velocity compaction
2006 (English)In: Polymer testing, ISSN 0142-9418, E-ISSN 1873-2348, Vol. 25, no 8, 1069-1080 p.Article in journal (Refereed) Published
Abstract [en]

Dynamic friction, sliding friction, and the stick-slip phenomenon have been studied on compacted polymer powders during high-velocity compaction. It is particularly important from a practical point of view to distinguish the stick-slip mechanism and the sliding mechanism which occur concurrently. A practical experimental system has been successfully developed to study the dry frictional force and to measure the sliding coefficient between the polymer powder particles and the die wall during high-velocity compaction. Two new components have been introduced as relaxation assists to improve the compaction process by reducing the frictional forces. It was found that the relaxation assist device leads to an improvement in the polymer powder compaction process by giving a more homogeneous opposite velocity and a better locking of the powder bed in the compacted form with less change in dimensions. The subsequent movement of the particles can be reduced and the powder bed attains a higher density with a minimum total elastic spring-back. The relative time of the stick-slip phenomenon during the compacting stage is also reduced so that the time needed to transfer from an intermittent stick-slip state to a smooth sliding state is reduced and the powder bed slides smoothly. It was found that the dynamic, dry frictional force is intermittent (stick-slip mechanism) at low compaction rates but that at high compaction rates is becomes more smooth (sliding mechanism). Both mechanisms depend on the nature of the powder and on the compaction conditions. At the beginning of the compaction stage, the sliding coefficient decreases due to an increase in the radial to axial stress ratio until the maximum pressure has been reached. During the reorganization stage, more time is needed for large particles to move, rotate and slide due to their relatively large diameter and mass. As a result, the reorganization stage is extended and the stick-slip phenomenon is observed more with increasing particle size. Much better transfer of the pressure throughout the powder bed and less loss of pressure lead to a higher sliding coefficient due to the overall friction during the compaction process. It was found that the sliding coefficient is proportional to the density. A more homogeneous density distribution in the compacted powder and a smaller pressure loss during compaction has a major influence on the sliding coefficient and on the quality of the compacted material

Keyword
high-velocity compaction; relaxation assist; opposite velocity; frictional force; stick-slip phenomenon; sliding coefficient
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-6228 (URN)10.1016/j.polymertesting.2006.07.009 (DOI)000242782200011 ()2-s2.0-33750942720 (Scopus ID)
Note

QC 20100630

Available from: 2006-10-06 Created: 2006-10-06 Last updated: 2017-12-14Bibliographically approved
4. Polymer-nanofiller prepared by high-energy ball milling and high velocity cold compaction
Open this publication in new window or tab >>Polymer-nanofiller prepared by high-energy ball milling and high velocity cold compaction
2008 (English)In: Polymer Composites, ISSN 0272-8397, E-ISSN 1548-0569, Vol. 29, no 3, 252-261 p.Article in journal (Refereed) Published
Abstract [en]

High-energy ball milling using comilling in a solid state by low-temperature mechanical alloying to prepare nickel-ferrite (NiFe2O4) nanopowders and ultrafine poly(methyl methacrylate) (PMMA), dispersing nanoparticles in a polymer matrix, and a uniaxial high-velocity cold compaction process using a cylindrical, hardened steel die and a new technique with relaxation assists have been studied. The focus has been on the particle size distributions of the nanocomposite powder during the milling and on the surface morphology of the nanocomposite-compacted materials after compaction with and without relaxation assists. Experimental results for different milling systems are presented showing the effects of milling time and material ratio. It was found that a longer mixing time give a higher degree of dispersion of the nanopowder on the PMMA particle surfaces. Furthermore, with increasing content of NiFe2O4 nanopowder, the reduction of the particle size was more effective. Different postcompacting profiles, i.e. different energy distributions between the upper and lower parts of the compacted powder bed, lead to different movements of the various particles and particle layers. Uniformity, homogeneity, and densification on the surfaces in the compacted powder are influenced by the postcompacting magnitude and direction. It was found that the relaxation assist device leads to an improvement in the polymer powder compaction process by reducing the expansion of the compacted volume and by reducing the different opposite velocities, giving the compacted composite bed a more homogeneous opposite velocity during the decompacting stage and reducing the delay time between the successive pressure waves.

Keyword
LAYERED SILICATE NANOCOMPOSITES; COMPOSITE; POLYPROPYLENE; CONDUCTIVITY; EXTRUSION; POWDERS
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-6229 (URN)10.1002/pc.20353 (DOI)000253401700003 ()2-s2.0-41449115349 (Scopus ID)
Note

QC 20100630

Available from: 2006-10-06 Created: 2006-10-06 Last updated: 2017-12-14Bibliographically approved

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