School
School of
of Mechanical
Mechanical
Engineering
Engineering
FACULTY OF ENGINEERING
FACULTY OF ENGINEERING
Thin Film Coating & Spreading
Flows
Harvey Thompson, Nikil Kapur, Jon Summers,
Mark Wilson & Phil Gaskell
School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK
School of Mechanical
School
of something
Engineering
FACULTY OF OTHER
FACULTY OF ENGINEERING
Overview:
1. Introduction
2. Brief Review of Industrial Coating & Drying Flows
1. Self-metered processes
2. Pre-metered processes
3. Drying
3. Droplet formation and spreading
4. Modern simulation techniques
5. Conclusions
School of Mechanical
School
of something
Engineering
FACULTY OF OTHER
FACULTY OF ENGINEERING
School of Mechanical Engineering:
•
5** rated under the Government Research Exercise:
•
3 multidisciplinary research institutes:
1. Institute of Medical and Biomedical Engineering
2. Institute of Engineering Thermo-fluids, Surfaces and Interfaces
3. Institute of Engineering Systems and Design
Broad groupings designed to allow cross-fertilisation of research areas
•
Excellent research staff and facilities
School of Mechanical
School
of something
Engineering
FACULTY OF OTHER
FACULTY OF ENGINEERING
Core Strengths
•Combustion
•Thin Films and Fluids
•Tribology
•Corrosion
Growth Areas
•Engineering Optics/Metrology
•Biomimetics
•Microfluidics
•Computational Simulations
School of Mechanical
School
of something
Engineering
FACULTY OF OTHER
FACULTY OF ENGINEERING
Interferometric sensing:
Full field shape / colour / texture
Ultrahigh dynamic range distance metrology (to 1: 1011)
High bandwidth 10MHz
Flow Metrology:
Optics and
Flow
Diagnostics
Multiphase / multi-constituent flows
Mixing
GDI sprays
Micro-reactors
4D flow metrology:
Time varying turbulent flow
In-cylinder automotive applications
Biophotonics:
Signal processing
4D sensing fluorophor labelled cells
School of Mechanical
School
of something
Engineering
FACULTY OF OTHER
FACULTY OF ENGINEERING
Coating and Printing
Numerical and experimental investigations of industrial processes
Free Surface Flows and Wetting
Thin
Films
and
Fluid
Flow
Droplet motion on surfaces
Spreading films on heterogeneous surfaces
Microfluidics
Microfluidic device characterisation
Mixing
MicroPIV
Flow Modelling and Simulation
Multiscale modelling: molecular dynamics / lattice Boltzmann
Computational Fluid Dynamics for Engineering Applications
Engineering Simulation Tools
Rheology and Fluid Characterisation
Review of Industrial Coating
Flows
Industrial coating flows have several key stages:
Fluids preparation, coating, drying and winding.
Key distinction in coating stage: self-metered, pre-metered
Liquid supply
Coater
Dryer/Curer
Substrate
Winder
Self-Metered Processes
Self-metered Coating Processes: where the wet film
thickness is controlled by the process itself as opposed to
controlled flow rate to the coater – hence self metering.
Simple example: roll coating
Substrate
Bath
Self-metered processes
Many different forms
Just a few basic principles
Fighting the same sets of
problems
Substrate
Bath
Self-metered processes
Pick up some liquid – Dip Coating
Split it between rolls
Get the split ratio right
Hope there’s no ribbing, barring or
runback
Hope we have a wide coating window
Self-metered processes
Liquid Pick-up:
Key parameter is Capillary number:
For vertical pick-out (Wilson 1982):
H lift
 U lift  

 0.944
 g 
1/ 2
Ca 1 / 6
Can be modified to take account of
angle of pick-up and effect of plate.
Simple tools aid coating design
Self-metered processes
Reservoir-fed Reverse Roll Coating:
Industrially-important variation of the robust reverse roll
configuration used for manufacturing a variety of films and foils
s r or
Reservoir
Wiper
Coated Substrate
Self-metered processes
Reservoir-fed Reverse Roll Coating: Comparison with Experiment
S=0.1
S=0.5
Self-metered processes
Controlling position of
wetting line position is key.
Wetting line position vs
speed ratio:
Hydrostatic head is
important, as is the
variation of dynamic
wetting angle with metering
speed.
Self-metered processes
Negative Gaps: Deformable roll
coating
One roller rubber
Great way to make really thin coatings
…
… if you’re not too bothered about
coat quality
Depends critically on rubber, which
can change with time or between
batches
Set load or (-ve) gap
Three-roll pan reverse
Elastomer
Web
Back-up
Metering
Applicator
Three-roll reservoir reverse
Applicator
Metering
Web
Back-up
Self-metered processes
A PRE-SET GAP is specified and the separation of the roll centres is set
by the adjustment of mechanical stops.
A LOAD is specified and the separation of the roll centres is set by
applying a force across the roll pair.
Deformable roll coating can be very complex – requires sophisticated
Finite element analysis
BUT simple models can be developed which do a good enough job for
practical design – take account of roll speeds, viscosity, load,…
Self-metered processes
Gravure Coating is increasingly
popular – small film thicknesses
(a few microns) and good
stability
Fluid properties
Direct Gravure nip
Web
• viscosity, surface tension
Web & roll speeds
Doctor
blade
Gravure roll
Doctor blade position
Gravure cell
Note - web tension and wrap don’t
affect transfer!
Reservoir
Self-metered processes
Gravure Cell Shape is Key
• Quadrangular
• Pyramidal
• Laser engraved ceramics
• various shapes
• also QCH
• trihelical
Volume (microns)
Density (lines per inch /lines per cm)
Screen angle
Photographs of gravure roll surfaces
Self-metered processes
The film thickness & pickout is also sensitive to fluid properties,
doctor blade pressure and roll speed.
Need for accurate predictive models!
Pre-metered processes
You deliver just the right amount of liquid to the web through,
for example, a slot.
Pre-metering is used to smooth the liquid surface without the
need to throw away or recirculate any liquid.
You know your flow rate Q m3/s and the line speed U m/s –
wet film thickness is then Q/U.
Pre-metered processes are often used in high precision
applications.
Pre-metered processes
Slide-bead
Slot
Curtain
and others
Slot coating
Slot Die
Coated Substrate
Roll
Slot coating is a versatile method for applying single layers to a web.
Examples include photosensitive materials, such as photo-resist, magnetic
suspensions, waxes, inks, silicon, rubber and foams and hot melt
adhesives, in addition to low viscosity melts of alloys, metals and
organic materials
Controlling slot coating
Slot coating is affected by:
Lip shape
Lip land length
Land length
Angle
Angle of slot
Type of backing (roll/web)
Radius of backing roller
Gap
Roller
Radius
Web speed
Wet
Thickness
Slot coating
Q = UH
Simple models useful – may need more
detailed analyses of the velocity or pressure
field – can use Finite Elements or other
CFD methods.
SLOT
DIE
static contact line
upstream meniscus
Typical streamlines in a
slot coating system.
S

D
dynamic contact line
downstream
meniscus
G
U
H
web (wrapped around roll of radius, R)
shows eddies etc…
BUT simpler models are often effective…
Ld
A happy slot
Pressure curve
A nice downstream meniscus
A healthy balance of pressure
Downstream
meniscus
Upstream meniscus under
control …
… able to absorb fluctuations
Upstream
meniscus
Too much pressure and your
pump has problems, too little
and you’re out of control
Unhappy slots
Upstream overspill
Unstable inflow
Ribbing
Slide coating
Slide Coating
popular in the photographic industry for producing multi-layer
coatings
Cascade
Substrate
Slide coating
Typical Finite
Element Grid
Slide coating
upper free
surface
Streak-Line Formation
internal
interface
(a) 
S = 25
web
o
recirculation region
(b) 
S = 45
o
lower free surface
Curtain Coating
High impingement speed of
falling curtain enables high
coating speeds - up to ~600m/m
Very versatile due to large gap
Not so mechanically demanding
Highly robust against lines
Predictable performance
Can coat several layers at once
Curtain Flow Zone
Simple and highly predictable
V
0
impingement velocity
V
2
 V02
 2 g X  X 0 
X0
X
But can be inherently unstable – minimum
flow rates to avoid break-up of curtain
V
Curtain Impingement Zone
An unwanted heel can form
This can trap particles
and bubbles - causing lines
Can also entrain air
Defects Caused by Feed Flow
Slot exits often used to supply pre-metered coatings such as slide and
curtain
Defects Caused by Feed Flow
Back-wetting of uppermost slot may occur during start-up – can lead to
defect-causing solids deposits due to degradation in recirculation regions.
Back wetting at the upper slot: (a) experimental, (b) CFD prediction
Defects Caused by Feed Flow
Geometrical modifications to slots – effect of a curved diffusor
(Schweizer (1988))
Diffusor can also remove downstream eddy
CFD Slot Optimisation
CFD used to identify a more practical solution:
Merging flow out of slot exits – effect of chamfering lower corner
Chamfer can remove
eddies in both liquid
layers simultaneously!
Film Drying
Film drying is often the limiting factor in industrial coating systems:
In drying, two processes must occur simultaneously:
(a) The transfer of energy (heat) from the surrounding environment to the product in
order to evaporate the surface moisture
(b) The transportation of solvent held within the
product to its surface where it can be removed
by process (a)
Liquid supply
Coater
Dryer/Curer
Substrate
Winder
Film Drying
For aqueous coatings, most of
drying is in the Constant Rate
Period.
For solvent-based coatings,
most drying is in the Falling Rate
Period.
The Drying Curve
For aqueous coatings, most of
drying is in the Constant Rate
Period.
For solvent-based coatings,
most drying is in the Falling Rate
Period.
Practical Dryer Design
Air Floatation Dryers
Double-sided floatation dryers usually have nozzles in a
staggered arrangement on opposite sides with a stable,
sinusoidal web profile.
Practical Dryer Design
Understanding web stability is crucial
A typical nozzle design show below –
two angled jets separated by a Coanda
plate.
Practical Dryer Design
Accurate predictions require sophisticated numerical
models (Computational Fluid Dynamics):
But simpler models can also be extremely useful.
Practical Dryer Design
Aqueous coating, 30% solids
By weight
Single sided drying,
Temp = 125oC,
h = 100 W/m2K
Single zone – 4m in length
No recirculation
Water not completely evaporated by end of zone
Practical Dryer Design
Effect of more volatile solvent
Methanol, 30% solids
by weight
Single sided drying,
Temp = 125oC,
h=100W/m2K
Single zone – 4m in length
No recirculation
All methanol evaporated after 1.1m.
Droplet Formation and Spreading
Droplet flows are increasingly important – coating of electronic
circuits, spray deposition onto leaves, heat mitigation in
circuits, ink-jet printing.
At Leeds, have developed range of experimental and
numerical methods for analysing droplet flows: formation,
coalescence and migration.
Droplet Formation – Ink Jet Printing on Textiles
Courtesy of S.L. Turner & T.P. Comyn,
University of Leeds
Droplet Formation – Ink Jet Printing on Textiles
CFD predictions agree well with experiment – very useful
design tool.
Droplet Coalescence
Droplet coalescence is of fundamental importance for a variety of applications
Experiments
(a)
Lubrication Theory
(a)
(b)
(c)
(d)
(e)
(f)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Droplet Migration on Complex Surfaces
Droplet motion on chemically- and topographically patterned surface
Through a small trench
Droplet Migration on Complex Surfaces
Droplet motion on chemically- and topographically patterned surface
Through a larger trench
Droplet Evaporation
Droplet evaporation – formation of classical ‘coffee-ring’
Lattice Boltzmann Method
The Lattice Boltzmann Method (Jon Summers, Mark Wilson)
•Capabilities to deal with complex physics are increasing
rapidly.
• Can deal with flow in very complex geometry
• Can couple up to continuum and couple down to molecular
Rayleigh-Taylor Instability
Fluid-fluid interface as heavier fluid on top plunges into lighter fluid below
Droplet dynamics
Animations by Alan Davies
Passive mixing with blocks.
Electric field and fluid flow both solved using LBM. Very
useful for microfluidic flows in micro-channels.
Electric field
Species concentration
Slug in a pore-throat
Lattice Boltzmann very powerful for porous media flows
Heterogeneous surface causes snap-off
Conclusions
Coating and Drying phenomena can be very complex
Often best optimised by combination of targeted experiments,
simple models and/or more complex numerical models
Simple models can often be produced which capture the
essence of the process.