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CH5716
Processing of Materials
Ceramic Thick Film Processing
Lecture MC6 – Tape Casting
Tape Casting Background
• Doctor/Knife Blading long established in industry
•Blade scraped excess material from the moving substrate being coated •Paper – surface coatings
•Paints – coatings onto test substrates to assess opacity
•Polymers– coatings – rubber on to fabric
•First ceramic tape casting patent in 1952 for flat plates for electronic and
radio fields
•Introduction of moving carriers realised a truly mass manufacture and
continuous technique (mid 1950’s)
•Tape casting allows large area thinner layers to be produced •Too large/thin for pressing or extrusion
•Tapes can essentially be considered to be 2D objects
•Plasticisation also allowed for green post-processing •Shaping/Cutting
•Hole punching – electrical vias for multilayer ceramics
Applications for Thick film Components Telephones Cell Phones Pagers
Computers Modems Monitors
Keyboards Printers Photocopiers
Music Keyboards Guitars (electric) Microphones
Television VCR Laser Disc Players
Stereo CD Players Cassette Decks
Speakers Antennas (some) SEGA / Nintendo
Automobiles Video Cameras Disposable Cameras
Coffee makers Watches Washers/Dryers
Ovens/Ranges Digital Clocks Walkmans/Discmans
Irons Toasters Refrigerators
Airplanes Helicopters Missiles
Radar Sonar Satellites
Space Shuttle Christmas Ornaments pH meters
Ohmmeters Ammeters Intercom Systems
Oscilloscopes Scalpel Blades X-ray Machines
MRI machines Ultrasound Walkie Talkies
Metal detectors Golf Carts Motorcycles
Electric Toothbrushes Li-Ion Batteries Fuel Cells
Basically, if it contains electronics , very likely there will be a tape cast or
printed component somewhere. Probably both
The Basic Tape Casting Process
Casting
Head
Drying & Local
Ventilation
Casting Plate
Take up
spool
Carrier Film
Feed
Carrier motion
•Ceramic Slip (slurry) introduced to casting head
•Dragged under a preset gap between casting plate and doctor blade •This meters slip to a set thickness
•Solvent evaporates to form dry green tape
•Thickness can be 10’s to 100’s μm •Recent work has demonstrated tapes under 10μm
•Dry tape can be cut & shaped in green state
•Can have further layers applied •Lamination
•Casting
•Printing
•Technique widely used in mass manufacture of ceramics •Continuous and batch processes can be used
•Normally fixed casting head with moving carrier but moving casting head also seen at lab scale •Casting head normally sits on granite or tempered glass block for flatness •Casting plate either tempered glass or stainless steel
•Latter can be heated
Lab scale
The Tape Casting Machine
Industrial Scale
•Number of different carriers can be used •Polyester film (Mylar)
•Stainless steel belt
•Tempered glass
•Mylar and steel are moving carriers
•Glass is used in moving head designs
•Tape will generally stay attached to Mylar during rolling and storage •Protects tape, prevents sticking
•Steel belt common in continuous processes, tape to next process immediately on drying
•Glass for small scale lab batches, used soon after drying
The Casting Head – Doctor Blade
Slip
Reservoir
Doctor blade
Cast tape
Carrier film
Casting plate Casting direction
Micrometer
control
•Many variables on a theme •Mainly shape/geometry of blade
•Simplest flat bottomed <1cm thin
•Various materials used •Stainless or hardened steels
•Tungsten carbide, Al2O3
•Entire assembly rides on carrier •Minimise weight. normally Al construction
•Again surface contact must be smooth •Some coated with PTFE for low friction
•Easy dismantling for cleaning
•Gap between blade and carrier meters wet
thickness •Relative movement of carrier pulls material under blade
•Essentially a wiping process
•In theory a simple process but many
variables •Slip homogeneity, viscosity,
•blade height, reservoir height, blade geometry,
•carrier speed, wetting of slip on carrier
•Temperature, humidity
Factors Affecting Casting
Carrier motion
Hydraulic
pressure
•4 main factors affecting slip behaviour during casting •Rheology
•Hydraulic pressure
•Blade gap
•Carrier Speed
•Rheology defined during slip formulation
•Others are casting process variables
Blade
Gap
Hydraulic pressure
Carrier motion
Hydraulic
pressure
Friction
under
blade
•This pushes slip under the blade •Extent of this flow determined by slip viscosity
•This hydraulic pressure gives a slip velocity under the blade
•Occurs in both Newtonian and Non-Newtonian
•Welling often observed downstream in pseudoplastic slips
•Increase in apparent viscosity in lower shear conditions after the blade •Reduces flow rate
•Some polymer relaxation (lower influence)
•Increasing viscosity prevents sideways spreading during casting •Thixotropic behaviour will also influence this
Blade Gap
Carrier motion
Hydraulic
pressure
Friction
under
blade
•Increasing blade gap increases wet thickness •However not a linear correlation as viscosity effects also impact thickness
•Influences apparent viscosity through changing shear conditions
•Also affects influence of blade friction •Bigger gap – smaller influence of friction – higher slip velocity due to hydraulic pressure
•Higher slip relative to carrier velocity
•Increasing welling
•Therefore wet thickness may increase several percent more than the direct
increase in thickness
•Reverse is true for reducing blade height
Blade
Gap
Carrier Speed
V carrier > V Slip V carrier = V Slip V carrier < V Slip
•Also referred to as casting speed •Has a significant influence on tape thickness
•Only external applied force to slip
•In Pseudoplastic systems higher speed will result in lower slip viscosity
•Works in concert with hydraulic pressures •Wet tape thickness function of slip velocity and carrier velocity
•In industrial continuous processes will maintain constant reservoir level •Therefore hydraulic pressure remains constant
•In fixed slip volume systems, hydraulic pressure will reduces as slip is consumed
Dual Blade System
Slip
Carrier Film
Tempered Glass Plate
Doctor Blades
•In small lab systems variations in slip reservoir viscosity is manageable
•In bigger non-continuous systems it can become significant
•The effects of changing hydraulic pressures can be minimised by using a dual blade
system •Back (left hand) metering blade meters slip into casting pool
•Front (right hand casting blade controls final wet thickness
•Aim to minimise difference in height between casting pool and wet tape
•Reservoir depth now not so critical as back “metering” blade merely supplies slip to
casting pool
Casting pool
Metering blade
Casting blade
Multi Layer Tapes- Simultaneous or Multiple Casting
•Multiple casting heads can be used to layer up different tapes
•Complex casting heads and blade settings
•Care needed in control of slip properties •Maintain good interfacial bonding
•Avoid diffusion and mixing
•Useful for building up tape thicknesses of single materials
•Sequential multiple casts can also be used
Carrier Motion
Triple casting Head
•Commonly used for building up thicknesses •Thicker single material
•Multilayer devices
•Important in MLCCs
•Layers brought together in green tape once
fired result in monolithic ceramic part •Lamination function of pressure temperature and
time
•Plasticiser selection important •type I for bonding,
•type II for conforming to features
•Warm Isostatic pressing often used
lamination method •Layered parts placed in sealed bag then immersed
in water (or water glycerine mix)
•Pressure & heat applied, typically 5000-1000psi and
70-90°C for 3-10 minutes
•Even pressure distribution
•Good dimensional control
•Multiple parts processed in single operation
Multi Layer Tapes- Lamination
Drying •Complex field in its own right – larger scale casting about drying
• Downstream of casting head is essentially a large drying chamber
•Drying is a one sided process controlled by 2 significant parameters •Rate of solvent evaporation at surface
•Rate of solvent diffusion through the body of the tape to the surface
•Second is the rate limiting step
Surface Evaporation •Controlled by airflow and temperature
•Generally fast – skin formation
•Higher solvent partial pressure slows skinning
•Airflow opposite to casting direction
•Airflow calculations important •Avoid explosive mixtures
•Especially when heated
Solvent Diffusion •This is slower process
•Once skin forms slows further
•To improve diffusion underside heating
sometimes used
•In combination with unheated outer airflow
•Promotes diffusion while trying to avoid
too rapid evaporation •Best case when diffusion = evaporation
•Worst when evaporation much faster than
diffusion
Air Flow
Casting Direction
Drying Issues Drying results in tape shrinkage
This is mostly in z direction
This is can occur due to packing of ceramic particle and polymer reorientation
Shrinkage can occur in x and y directions, can cause issues –
most often in the cross cast direction
Curling – Edge, Full Body, Gulls Wing, Edge with release, Reverse •Often top layer shrinking more than underlying layers
•Good particle to particle adhesion
•Thickness, Particle size dist, carrier interface, will determine which type
•Addition of type II plasticiser, Heating drying air to soften tape, Slowing solvent evaporation
Cracking – Occurs when the particle/particle adhesion less than the carrier adhesion
“Centre” Cracking- can occur down length of tape •May be offset if tape inhomogeneous or defects present
•Minimise defects & improve homogeneity
•Increase binder content, additions of Type I & II plasticiser, slow drying rate
Crows feet cracking – often propagate from defects Can be more prevalent in thicker tapes – lower binder levels higher drying gradients
•Eliminate defects
•Type II plasticiser addition to help dissipate stress concentrations round defects
Further Drying Issues
Aqueous systems can also have casting & drying issues
High surface energy can dewet from polymer carrier films •Careful matching surface energies (high surface energy carrier)
•Use of wetting agents in tape slip
•Increase solids loading increasing SG, viscosity pseudoplasticity
Much slower drying than non-aqueous •Can be an issue in continuous processing
•However explosive mixtures less of an issue
•More flexibility in application of heating
Many other tape & drying effects exist but outside scope of this lecture series Refer to Mistler & Twiname or other relevant literature for full discussion of these
However many issues at the final stage of the process can be addressed by the first – slip formulation
Surface Defects
•Wrinkling, Mudflat Cracking, Orange Peel
•Surface effects •Do not penetrate full depth of tape
•Often a sign of two layer drying
•Some effects (such as wrinkling) may be transient
•Slow down surface drying rates •Reduce air temperatures or flow rates
•Skin retarding agents
•Addition of Type II plasticisers – Allow for plastic deformation & creep in outer layer
Still a developing Technology- Freeze Casting
J.D. McCrummen MSc Thesis Montana State Univ. 2008
•Based on standard tape casting process
•Aqueous based solvent system
•Caster modified to include freezing bed
•Ice crystals grow through tape acting as a
pore former
•Once fired creates a skeletal structure for
infiltration of catalytic species
Attraction of microstructure immediately
apparent Natural grading due to the nucleation and growth
of ice crystals
Current area of research
Interest in effects of process variables on
morphology of ice crystals Freezing temperature, casting speed, tape
thickness, solvent systems,