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www.ctechinnovation.com
Radio Frequency for Rapid, Energy Efficient Heating of Polymers and
Composites
Presentation Summary
• Introduction to dielectric heating• Benefits of dielectric heating• Dielectric heating of polymers and composites• C-Tech process development
• RF welding of medical devices• RF forming of polystyrene packaging• RF curing of composite materials
• Conclusions
What is Dielectric Heating?
Dielectric Heating; Process where an alternating RF wave heats a dielectric (insulating) material
Dielectric Heating
• Electrical (Ionic) Conduction
• Joule heating• Significant at low (RF)
frequencies• Dipole Rotation
• Dominant mechanism at higher frequencies (e.g. microwave)
• Molecules align to the applied field
Two Fundamental Heating Mechanisms
Why use dielectic heating?
•Energy dissipated throughout material
• Volumetric Heating
Dielectric Heating
Conventional Heating
• Heat must diffuse through material
• Creates temperature gradient
Benefits of dielectric heating
• Rapid; high heating rates achievable• Uniform; volumetric heating reduces temperature
gradients• Selective
• Use of RF transparent materials (e.g. tooling) gives selective heating of target material
• Energy efficient
50Ω RF; advantages
7
RF Generator
Matching networkcontrol unit
Power meter
Matchingnetwork
Computer
RF Applicator
RF power cable
kW 50Ω
50Ω
• Fixed operational frequency; complies with regulations
• Process control + accurate control of power input
• System flexibility; readily tunable
Dielectric heating of polymers and composites
Ability to heat by RF or microwave depends on dielectric properties of the specific polymer; some are transparent, some heat rapidly.
Heat in RF RF Transparent
Epoxies; adhesives and matrix in composites PTFE
Polyurethanes; e.g. adhesive curing Polypropylene
PVC; applications in RF welding Polystyrene
Phenolics; used as wood glues with rapid RF cure Polyethylene
Polyamides
PVDF
RF Transparent polymers can be heated indirectly through use of dopants or heat transfer media (e.g. steam)
• Research and development of new industrial applications of dielectric heating
• Process Evaluation & Quantification of Benefits
• Design of RF applicators, tooling and control systems
• In house build of prototype equipment
C-Techs capabilities
9
C-Tech development of RF processes
• PVC Welding; e.g. for medical devices• Moulding of expanded polymer products; e.g.
polystyrene packaging• Curing of composites; GFRP e.g. for wind turbine,
automotive or structural parts
Benefits of RF welding• Heat focused on weld area; volumetric and selective heating
• Rapid process (both heating and cooling)
• Joint production under clean conditions
RF Welding
11
Applications• Thermoplastics e.g. PVC
• High value e.g. Medical devices
C-Tech; welding of medical devices
12
Medical device
• Development of precision tooling
• Build of test equipment
• 50Ω systems with PLC control provide accurate control and delivery of power to weld, reports any weld errors
• High part reproducibility
• Low scrap rates
RF heating of expanded polystyrene (EPS)
EPS products currently produced by steam-chest moulding• Rapid process• Very high energy and water usage• Expensive aluminium tooling
Aims of RF process• Production of comparable products• Equivalent processing time • Significantly reduce energy and water usage• Use of low cost polymeric tooling
RF EPS Moulding approach
• EPS is RF transparent; addition of water provides indirect heating of beads
• Use of RF press provides sealed system with compression of beads
• Use of polymer moulds; RF transparent so heat focused on product
Fill mould with beads & add water
mould in press Press closure compresses beads; RF applied
Water/Steam Consumption
Energy Consumption
Steam Chest Moulding 50‐100Kg 50‐100KWh
RF Moulding 5‐6Kg 8KWh
Process Benefits
15
Based on requirement for moulding 1m3 of EPS at 25kg/m3
Moulded products formed
• 20secs processing time
• Densities 20-25kg/m3
• Compressive stress comparable to standard EPS
Curing of composite materials
Current process; autoclave curing• Slow heating (≈3°C/min) due to slow heat transfer from surface• High energy process• Expensive equipment and tooling• Not suitable for rapid throughput; often limiting factor for
industrial productionAims of RF process• Consolidation pressure achieved by press• Rapid heating (>10°C/min)• Low energy processes• Cost effective alternative to autoclave• Utilise cheap and simple polymer moulds• Suitable for rapid throughput processing
RF Press for composites
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Project Involved design and build of RF press system for composite curing
• 8 tonne maximum closing force
• Controllable closing pressure
• 5KW RF power (13.56MHz)
• Monitor process temperatures
• Potential to operate in automatic mode to achieve defined cure cycles in terms of time, temperature and pressure
Press Specification
18
• Using ‘pre-preg’. Sheets of glass fibre pre-impregnated with partially cured epoxy resin.
• Rapid heating of pre-preg to cure temperature (120-180°C). Target >10°C/min
• Compression of pre-preg (autoclave use 5-7bar) to consolidate and reduce void content.
Curing Trials; Glass fibre composites
19
• Good Heating Rates can be achieved; up to 20°C/min
• Heating uniformity appears good
• Cured parts produced; property evaluation required
Early Trials
20
Polymer Tooling System
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• 2 part matched tooling; attached to RF press plates
• RF transparent polymer construction
• Enables compression of part during curing
• Porous surface and vacuum outlet
• Evaluation of mechanical properties
• Identification of preferred process conditions; pressure, time and temperature
• Production of industrial demonstrator part
• Clarification of benefits; process time and energy use
• Extension of work with glass fibre composites to carbon fibre composites.
Ongoing Composite Work
22
Conclusions
• RF Provides a versatile method for a range of polymer heating applications
• Development of new RF systems and tooling can provide cost effective processing systems
• Main system advantages• Rapid, selective heating of target product• Uniform heating• Use of cost effective tooling systems• Good process control• Energy Efficient
