V.M. Sglavo –CerMatEng-UNITN 2019 Extrusion Principlesof ... · 1995 –Ch. 25, 26 Ceramic Processing, 2nd Ed., M.N. Rahaman, CRC Press, 2017 –Ch. 10, 11. ... [6–8]. In the

V.M. Sglavo – CerMatEng - UNITN 2019

Extrusion

vol%

Principles of ceramics processing, 2nd ed., J.S. Reed, J. Wiley & sons, 1995

Principles of ceramicsprocessing, 2nd ed, J. S. Reed, J. Wiley & Sons, 1995 – Ch. 25, 26Ceramic Processing, 2nd

Ed., M.N. Rahaman, CRC Press, 2017 – Ch. 10, 11


Pressure distribution

Principles of ceramics processing, 2nd ed., J.S. Reed, J. Wiley & sons, 1995Introduction to the principles of ceramic forming, J.G. Heinrich, CFI, 2004

w/velocity

L

D

L

L


�

τ =r (P1 −P2)

2L

shear stress

flowvelocity

Introduction to the principles of ceramic forming, J.G. Heinrich, CFI, 2004


optimal sections forextrusion

gradientsà defects

Introduction to the principles of ceramic forming, J.G. Heinrich, CFI, 2004


Injection molding

Ceramic processing, M.N Rahaman, CRC Press, 2007

170°C– 240°C

40°C– 60°C


Examples

Ceramic processing, M.N Rahaman, CRC Press, 2007


Additive manufacturingNew technologies:• rapid prototyping• solid freeform fabrication• additivemanufacturingorthree-dimensional (3-D)printing

286 Ceramic Processing, Second Edition

11.2 POWDER METHODS

11.2.1 SELECTIVE LASER SINTERING

In selective laser sintering (SLS), parts are built layer by layer by scanning a laser beam over a thin layer of powdered material [1]. For polymers and some metals, interaction of the laser beam with the powder raises the temperature to the point of softening or melting. This leads to particle bond-ing and fusion of the particles to themselves, as well as to the previous layer, to form a solid phase with the requisite structure. Ceramic particles, as mentioned earlier, cannot be formed directly into

TABLE 11.1 Additive Manufacturing Techniques Used for Forming Ceramics Classified in Terms of the Feed Material Used

Feed Material Method Process

Powder Powder-binder mixture or binder-

coated powderSelective laser sintering (SLS)

Laser beam scanned on layer of material to soften binder and bind particles

Powder or dried powder from a slurry

Three-dimensional printing (3DP)

Binder solution sprayed on powder bed to bind particles

Particle-Filled Polymer Particle-filled polymer filaments Fused deposition

modeling (FDM)Extrusion of softened filaments through a heated

nozzleTape-cast sheets Laminated object

manufacturing (LOM)Sheets cut by laser beam and stacked

Suspension, Slurry, or Paste Particles dispersed in monomer

solutionStereolithography Laser beam scanned on suspension to polymerize

monomer solutionModerately dilute suspension

stabilized with organic additivesInkjet printing Printing of droplets from printer nozzle followed by

drying by evaporationConcentrated slurry or paste

stabilized with organic additivesRobocasting Extrusion of slurry or paste through a nozzle followed

by drying or gelationConcentrated slurry or paste

stabilized with organic additives Freeze extrusion fabrication (FEF)

Extrusion of slurry or paste through a nozzle and freezing in a cold chamber

1. Model or CT image

2. CAD file creation

4. As-formed object

3. Additivemanufacturing

machine

FIGURE 11.1 Basic approach of additive manufacturing.


286 Ceramic Processing, Second Edition

11.2 POWDER METHODS

11.2.1 SELECTIVE LASER SINTERING

In selective laser sintering (SLS), parts are built layer by layer by scanning a laser beam over a thin layer of powdered material [1]. For polymers and some metals, interaction of the laser beam with the powder raises the temperature to the point of softening or melting. This leads to particle bond-ing and fusion of the particles to themselves, as well as to the previous layer, to form a solid phase with the requisite structure. Ceramic particles, as mentioned earlier, cannot be formed directly into

TABLE 11.1 Additive Manufacturing Techniques Used for Forming Ceramics Classified in Terms of the Feed Material Used

Feed Material Method Process

Powder Powder-binder mixture or binder-coated powder

Selective laser sintering (SLS)

Laser beam scanned on layer of material to soften binder and bind particles

Powder or dried powder from a slurry

Three-dimensional printing (3DP)

Binder solution sprayed on powder bed to bind particles

Particle-Filled Polymer Particle-filled polymer filaments Fused deposition

modeling (FDM)Extrusion of softened filaments through a heated nozzle

Tape-cast sheets Laminated object manufacturing (LOM)

Sheets cut by laser beam and stacked

Suspension, Slurry, or Paste Particles dispersed in monomer solution

Stereolithography Laser beam scanned on suspension to polymerize monomer solution

Moderately dilute suspension stabilized with organic additives

Inkjet printing Printing of droplets from printer nozzle followed by drying by evaporation

Concentrated slurry or paste stabilized with organic additives

Robocasting Extrusion of slurry or paste through a nozzle followed by drying or gelation

Concentrated slurry or paste stabilized with organic additives

Freeze extrusion fabrication (FEF)

Extrusion of slurry or paste through a nozzle and freezing in a cold chamber

1. Model or CT image

2. CAD file creation

4. As-formed object

3. Additivemanufacturing

machine

FIGURE 11.1 Basic approach of additive manufacturing.287Additive Manufacturing of Ceramics

a dense solid by SLS because of insufficient matter transport by solid-state diffusion during the short time of laser scanning. Typically, the starting material consists of the ceramic powder mixed or coated with an organic binder.

As illustrated in Figure 11.2, a layer of the material is rolled onto the build platform from the reservoir of the delivery system. Then, a computer directs the laser to raster onto the powder bed, softening the binder and binding the particles together. After one layer is built, the platform is lowered, another layer of material is rolled onto the platform, and the computer directs the laser to build a second layer. The process is repeated until the entire object is built. Loose powder within the as-formed object is removed (e.g., by mechanical vibration) and, following binder removal, the article is sintered to produce a dense ceramic phase.

Commercial SLS machines commonly require the use of a considerable amount of powder in the reservoir of the delivery system. When only a few prototypes are required from an expensive powder, the cost of a considerable amount of starting powder needed for this technique can be high. The technique also requires an additional step to remove loose powder trapped within the as-formed structure. The surface roughness of the as-formed object often increases with increasing particle size of the powder, and the scale of the structural features that can be formed by this technique depends on the resolution of the laser beam.

1 1.2.2 THREE-DIMENSIONAL PRINTING

In three-dimensional printing (3DP), complex-shaped parts are formed by sequentially depositing a thin layer of ceramic powder followed by printing or spraying a binder solution to fix the powder in place and to selectively define the geometry of the part [4]. The thin layer of powder can be formed by roll compaction (Figure 11.3), but more homogeneous particle packing and higher packing den-sity can be obtained by deposition from a well-dispersed slurry (e.g., through a nozzle 100– 200 µ m in diameter) followed by drying. After application of the binder solution to fix the powder, the layer is heated to remove excess liquid (typically an aqueous liquid). Once a single layer is complete, the sequential slurry and binder deposition processes are repeated until the part is completed. Then, the binder is cured to develop adequate strength in the object and the unwanted powder within the struc-ture is redispersed in a liquid to recover the part. Finally, the shaped part is heated to decompose the binder and sintered to produce a dense ceramic phase.

The ability to form a ceramic part successfully via the 3DP process is dependent on a few key factors. One factor is the structure of the powder layer. The particle packing density and packing

Scannersystem Laser source

Closed chamber

Fabricated part

Powderdeliverysystem

Buildcylinder

Rollermechanism

FIGURE 11.2 Schematic of the selective laser sintering (SLS) technique.288 Ceramic Processing, Second Edition

homogeneity are often limited when a dry powder is used. More homogeneous particle packing and a higher particle packing density can be obtained by using a suspension. The colloidal properties of the suspension and the drying process control the structure of the deposited powder layer. Another factor is the interaction of the binder solution with the powder layer. This interaction should be optimized to control the shape uniformity of the as-formed part. A third factor is redispersion of the unwanted powder from within the structure to retrieve the printed part. The chemistry and col-loidal properties of the slurry should be controlled to achieve a consolidated powder that is easily dispersible.

11.3 PARTICLE-FILLED POLYMER METHODS

11.3.1 FUSED DEPOSITION MODELING

Fused deposition modeling (FDM) is a technique that builds plastic objects by extrusion of a polymer filament through a heated nozzle (Figure 11.4). The technique is essentially a hot extrusion process. In FDM of ceramics, the feed material consists of a particle-filled polymer filament [5]. A ceramic– polymer mixture, composed typically of 50– 60 vol.% particles, a few percent dispersant, and an organic binder system, is first extruded to form filaments with a diameter of ~2 mm. Then, the filaments are fed into a computer-controlled heated extrusion head (100˚ C– 150˚ C). Extrusion of the plastic mixture through a nozzle (diameter in the range 0.25– 1.0 mm), according to a computer-controlled pattern, is used to form the object layer by layer. The as-formed article is subjected to binder burnout and sintering steps to produce a dense ceramic phase.

Ceramic– polymer mixtures used in FDM should be optimized to produce desirable flow prop-erties for the hot extrusion process. The filament should have enough flexibility to allow winding and unwinding of continuous lengths on a spool, as well as sufficient stiffness to act as a piston for extrusion of the molten material through the fine-diameter nozzles. Good adhesion between each deposited layer is also required.

FDM has been used for the production of ceramics and ceramic particle-filled polymers for a variety of structural, electroceramic, and bioceramic applications [6– 8]. In the production of dense ceramics, the removal of the large amount of polymeric binder as cleanly as possible from the green article can be a limiting step, particularly for large articles. Inadequate processing of the ceramic– polymer feed material and limitations of the deposition process can lead to internal and surface flaws that degrade the strength of the final sintered article.

Print headRoller

x, y rail

Powder bed

z elevatorPowder

tank

FIGURE 11.3 Schematic of the three-dimensional printing (3DP) technique.

V.M. Sglavo –CerMatEng-UNITN 2019 Extrusion Principlesof ... · 1995 –Ch. 25, 26 Ceramic Processing, 2nd Ed., M.N. Rahaman, CRC Press, 2017 –Ch. 10, 11. ... [6–8]. In the

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