Ceramic Pressing

Question 1

Uniaxial Pressing

Answer 1

Low moisture containing powders

Binders and lubricants to aid compaction

Question 2

Uniaxial Pressing Advantages

Answer 2

Simple to automate
Economic
Good dimensional tolerances

Question 3

Ceramic pressing Disadvantages

Answer 3

Shape limitations
Die-wear
Pressure/density variations
Cracking

Question 4

Common defects of ceramic uniaxial pressing

Answer 4

Laminations caused by die wall friction and pressure gradients
Capping due to friction and spring back during pressure removal
Green density gradients cause non-uniform shrinkage

Question 5

Wet-Bag Isostatic Pressing

Answer 5

Wet-bag process

• Fully isostatic
• Mould pressurized in a water/oil mix
• Eliminates pressure gradients
• Batch process

Question 6

Dry-Bag Isostatic pressing

Answer 6

-Air hydraulic pressure, quasi-isostatic, automated

Uniform density , require “green” machining, outside dimensional tolerances are poor

Question 7

Dry-Bag Isopressing of Zirconia Automotive Oxygen Sensor

Answer 7

Multi step process
Mandrel provides internal bore and shape
Final green machining operation used to provide outside shape and dimensions
Firing shrinkage taken into account in dimensions
Also used in spark-plug manufacture

Question 8

Slip Casting

Answer 8

Slip: High solids content (50-80%) suspension - water or organic liquid used
Clays naturally form slips
Mould is of plaster (gypsum) and absorbs the water - drain or solid casting

Question 9

Dispersed system equation for slip casting

Answer 9

n = no (1+2.5v)

Question 10

Viscosity- Deflocculation Behaviour Slips

Answer 10

Agglomerated particles
Dispersed particles flow better
Achieved by controlling surface properties of slip
- Surface charge (particles repel one another)
- Polymers (steric interference)

Question 11

Water distribution during casting

Answer 11

Dewatering of a slip creates a water concentration gradient

Question 12

Characteristics of Slip Casting mold

Answer 12

Gypsum reaction creates fine acicular crystals with high porosity
Capillarity draws water into mold
Mold must be dried and are recycled
Porous plastic mould now used with vacuum casting

Question 13

Tape Casting-Production of Ceramic Sheets

Answer 13

“Doctor-blade” process uses a slip often in an organic fluid
Blade controls thickness
Thin continuous sheets cast on polymer film for capacitor dielectrics
Organic fluid evaporates using hot air

Question 14

Ceramic extrusion

Answer 14

Plastic forming operation
Excellent for clays
Non clay ceramics require addition of plasticisers
Shear thinning behaviour preferred for ceramics
Used to produce long shapes which can be sectioned
Produces bricks, tubes, insulator and catalyst supports

Question 15

Flow Characteristics of Plasticised Ceramics

Answer 15

Viscous flow behaviour:
tau = K gamma ^n
na = K gamma ^(n-1)
tau = shear stress 
n = 1 for newtonian flow
*Shear thinning preferred i.e a decrease in viscosity with increasing shear rate

Question 16

Piston Extruder

Answer 16

Used for technical ceramics
Die can be tapered to improve flow behaviour
Extrusion best if ceramic is made shear thinning

Question 17

Extruded Honeycomb automotive catalyst support

Answer 17

Cordiertie
Metallic catalyst applied to high surface area ceramic
Resists high temperature
Reduces NOx and CO emissions

Question 18

Ceramic Injection Moulding

Answer 18

Adapted from polymer processing
50-80% ceramic with polymer plasticizer
Polymer must be removed  after molding 
-burnt out
-porosity generated burins burn out
-special polymers developed that are soluble in water
Very high production rate
Die-wear problems with abrasive ceramic