School of Mechanical Engineering FACULTY OF ENGINEERING Thin Film Coating & Spreading FlowsHarvey Thompson, Nikil Kapur, Jon Summers, Mark Wilson & Phil Gaskell School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK
School of something FACULTY OF OTHER School of Mechanical Engineering FACULTY OF ENGINEERING • Overview: • 1. Introduction • 2. Brief Review of Industrial Coating & Drying Flows • Self-metered processes • Pre-metered processes • Drying • Droplet formation and spreading • 4. Modern simulation techniques • 5. Conclusions
School of something FACULTY OF OTHER School of Mechanical Engineering FACULTY OF ENGINEERING • School of Mechanical Engineering: • 5** rated under the Government Research Exercise: • 3 multidisciplinary research institutes: • Institute of Medical and Biomedical Engineering • Institute of Engineering Thermo-fluids, Surfaces and Interfaces • Institute of Engineering Systems and Design • Broad groupings designed to allow cross-fertilisation of research areas • Excellent research staff and facilities
School of something FACULTY OF OTHER School of Mechanical Engineering FACULTY OF ENGINEERING Growth Areas Core Strengths • Combustion • Thin Films and Fluids • Tribology • Corrosion • Engineering Optics/Metrology • Biomimetics • Microfluidics • Computational Simulations
School of something FACULTY OF OTHER School of Mechanical Engineering FACULTY OF ENGINEERING Interferometricsensing: Full field shape / colour / texture Ultrahigh dynamic range distance metrology (to 1: 1011) High bandwidth 10MHz Flow Metrology: 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 Optics and Flow Diagnostics
School of something FACULTY OF OTHER School of Mechanical Engineering FACULTY OF ENGINEERING • Coating and Printing • Numerical and experimental investigations of industrial processes • Free Surface Flows and Wetting • 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 Thin Films and Fluid Flow
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
Substrate Bath 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
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): • Can be modified to take account of angle of pick-up and effect of plate. • Simple tools aid coating design
Reservoir Wiper Coated Substrate 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
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.
Three-roll pan reverse Elastomer Web Back-up Metering Applicator Three-roll reservoir reverse Applicator Metering Web Back-up 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
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,…
Gravure Coating is increasingly popular – small film thicknesses (a few microns) and good stability Fluid properties viscosity, surface tension Web & roll speeds Doctor blade position Gravure cell Note - web tension and wrap don’t affect transfer! Self-metered processes Direct Gravure nip Web Doctor blade Gravure roll Reservoir
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 Self-metered processes 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 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 • Angle of slot • Type of backing (roll/web) • Radius of backing roller
Slot coating Simple models useful – may need more detailed analyses of the velocity or pressure field – can use Finite Elements or other CFD methods. Typical streamlines in a slot coating system. BUT simpler models are often effective… shows eddies etc…
A happy slot Pressure curve • A nice downstream meniscus • A healthy balance of pressure • Upstream meniscus under control … • … able to absorb fluctuations Downstream meniscus 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 Coatingpopular in the photographic industry for producing multi-layer coatings Slide coating Cascade Substrate
Slide coating Typical Finite Element Grid
Streak-Line Formation Slide coating
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 • impingement velocity • But can be inherently unstable – minimum • flow rates to avoid break-up of curtain
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)
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.
(a) (b) (c) (d) (e) (f) (a) (b) (c) (d) (e) (f) (g) (h) Droplet Coalescence Droplet coalescence is of fundamental importance for a variety of applications Experiments Lubrication Theory
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