5: Moulding Flashcards
What is the mechanism of moulding of thermosets?
-Energy (heat, photons, electrons, ect) applied to hardener to create reactive species
-Combined with base resin (monomers/oligomers)
-Exothermic crosslinking reaction creates thermoset polymer
Describe the Kamal-Sourour curing reaction model for thermoset resins
Degree of cure:
-increases with time
-converges to 100% as time tends to infinity
Cure rate:
-increases with degree of cure
-as exothermic reaction rate decreases with increased degree of cure, the rate slows
What does Differential Scanning Calorimetry measure for thermoset resins?
Heat flux to or from the resin while temperature is raised
-First drop step in curve indicates glass transition point
-Later peak shows exothermic heat flux (curing reaction)
-With increasing time the glass transition temperature increases, and rate of curing reaction (exothermic energy) decreases
Explain the thermoset resin processing chain (storage, moulding, curing, post-curing, cooling)
Storage:
-Glassy uncured resin is stored at low temperature (solid resin)
Moulding:
-Temperature increases above glass transition (resin becomes liquid)
Curing:
-Cross-linking reaction forms single molecule polymer network (gelation), cross-linking continues
Post-Curing:
-Additional heat applied
-Higher degree of cross-linking (increasing mechanical properties)
Cooling:
-Rubbery to hard solid as temperature is below glass transition
How does the glass transition temperature vary between the liquid resin and crosslinked resin for thermoset matrices?
-Glass transition of the liquid resin is lower than the crosslinked resin
What are the 3 stages of thermoset resin processing?
A-Stage:
-Early chemical reaction (resin + hardener mixed)
-Liquid at rtp
-used as “out of the bucket” composite manufacturing
B-Stage:
-Intermediate reaction (partially cured but not gelled)
-Tacky at rtp, so frozen for storage to halt the reaction until use
-Used in prepreg laminates (activated by heat)
C-Stage:
-Resin is solidified (finished component
Explain the importance of viscosity for thermoset resins
-Viscosity is correlated to permeability (depends on shear rate, however negligible at low flow rates)
How does viscosity vary during the processing of thermoset resins?
-Decreasing viscosity with increasing temperature
-Increasing viscosity with increasing degree of cure
-Viscosity, degree of cure and temperature all depend on time
-Low viscosity increases wet-out (full impregnation), so set the moulding temperature accordingly
-Can determine an appropriate processing window
How does volumetric shrinkage vary between non-isothermal cure and isothermal cure in thermosets?
-Volumetric shrinkage is greater for non-isothermal cure, resulting in higher residual stresses
-Increased volume change leads to increased risk of deformations
How is temperature controlled for non-isothermal and isothermal thermoset reactions?
Non-isothermal:
-Temperature is not controlled (Temp increases due to exothermic reaction)
Isothermal:
-Temperature is controlled (To reduce volumetric shrinkage)
How does modulus vary with temperature during crosslinking of a thermoset?
-For cured resin; high glass transition temperature is related to high degree of cure
-If temperature is very high then the cross-linked resin will degrade
Why is temperature control during moulding of thermosets important?
-Controls viscosity (ability of wet-out)
-Maximise the degree of crosslinking (improving mechanical properties)
-Minimises the residual stresses
How do stress/strain plots vary for processed thermosets tested at different temperatures?
-Glass transition here is 90 degrees, reflected by the graph (transition from brittle to more ductile as temperature increases)
What is the structure of amorphous thermoplastics?
-Entangled chain molecules
What is the structure of semi-crystalline thermoplastics?
-Chain molecules are partially structured in lamellae
-Amorphous phase between lamellae
-Lamellae arranged in larger structures (spherulites)
Explain the thermoplastic matrix processing chain (storage, moulding, cooling)
Storage:
-stored at room temperature
Moulding:
-Temperature increases above glass transition, formable then liquid (for amorphous)
-Temperature increases above glass transition and melt temperature (higher energy needed to break crystalline phase) (for semi-crystalline)
Cooling:
-Cool below glass transition, hardens (brittle)
Explain the importance of viscosity for thermoplastic matrices
-During moulding, flow depends on viscosity
-No effect of viscosity on degree of cure (as no cross-linking reaction occurs)
-Shear rate dependence is important at high flow velocities (eg. injection moulding)
-Viscosity varies as a function of temperature
How does specific volume vary with temperature for thermoplastics?
-Shrinkage during cooling is greater for semi-crystalline polymers than for amorphous polymers
-Effect of pressure: higher the pressure, the curves shift towards smaller specific volume and higher glass transition temperature
-Thermoplastic heating process is generally reversible (unless quenched)
How does volumetric shrinkage affect the material properties?
-Higher volumetric shrinkage results in large residual stresses, leading to deformations
How does Quenching (rapid cooling) of semi-crystalline thermoplastics affect the structure?
-Reduces the formation of lamellae and therefore remains an amorphous structure after quenching
-Upon reheating, the lamellae can recrystalise
How do mechanical properties of thermoplastics vary with temperature?
Amorphous:
-Hard & brittle below glass transition (high modulus)
-At glass transition, material is “rubbery” (reduction in modulus)
-Liquid above glass transition, therefore can only be used below the glass transition temperature
Semi-Crystalline:
-High modulus below glass transition
-Small reduction in modulus at the glass transition temperature (crystalline phase is unaffected)
-Significant drop in modulus at melt temperature as crystalline phase is disrupted
Why is the control of temperature important during the moulding process of thermoplastics?
-To control viscosity
-Important to control the cooling rate to manage the degree of crystallinity and glass transition temperature
How does controlling of pressure help during the moulding process of thermoplastics?
It helps compensate for shrinkage
Explain the wet-layup open mould process
-Apply release agent to mould surface
-Add layer of reinforcement mat/fabric
-Apply resin (“wet-out”) with a brush/roller
-Consolidate with roller to fully distribute the resin and remove air
-Allow to cure
-Demould and trim
What is a gelcoat in the wet-layup open mould process
Additional resin layer which defines visible part surface properties
What are the advantages of the wet-layup open mould process
-Low investment costs
-Flexible (easy production of most geometries)
What are the disadvantages of the wet-layup open mould process
-Time consuming
-Labour intensive
-Low repeatability of accurate parts (due to user errors)
-Poor dimensional control
-Relatively low fibre volume fractions (can be increased if vacuum bagged after impregnation)
What are the applications of the wet-layup open mould process
-Marine craft
-Large, lightly loaded structures
-Short production runs/one offs
-DIY/hobby applications
-Repairs
Explain the spray-up open mould process
-Mix of thermoset resin (polyester) & fibres (glass), chopped in a hand-held gun, sprayed onto the mould tool
-Left to cure at rtp
-Use of single sided mould (good surface finish on one side)
-100-500 parts per year
What are the advantages of the spray-up open mould process
-Uses roving (low cost material form)
-High deposition rates possible
-Not as labour intensive as wet lay-up
-Can be automated
-Suitable for large components
What are the disadvantages of the spray-up open mould process
-Operator is exposed to styrene emissions
-Low reproducibility (random fibre orientations)
-Poor dimensional control
What are the characteristics of Autoclave processing?
-Heated pressure vessel, isostatic pressure normal to the surface
-Connective heating (temperature control)
-Connections to vacuum pump
What intermediates are typically used in the autoclave process?
-Processing of lay-ups from thermoset prepregs
-B-stage thermoset resin & reinforcements already combined
What are the Autoclave process stages?
-Prepreg laid on tooling surface
-Lay-up enclosed in vacuum bag
-Tooling with lay-up is moved into an autoclave where it is cured
What does applied pressure & vacuum achieve in the Autoclave process?
Lay-up consolidation
What does applied Heating achieve in the Autoclave process?
Induce resin cure
What is the ideal tooling material used in the Autoclave process?
Same material as the laminate (same thermal expansion to prevent warping)
In practise, what is the tooling material used in the Autoclave process?
-Usually metal (high dimensional stability and more durable)
What is the purpose of cut-outs in the tooling used in the Autoclave process?
Allows sufficient airflow to control thermal expansion
How is heat capacity of the tool material important in the Autoclave process?
Affects the rate of heating to cure temperature
What are the components of the vacuum bag in the Autoclave process?
-Vacuum film: seals lay-up from outside air
-Breather: non-woven felt, allows airflow when a vacuum is applied
-Barrier film: prevents resin from permeating into breather
-Bleeder: non-woven felt, absorbs excess resin
-Peel ply: porous fabric (allows excess resin to permeate into the bleeder), prevents sticking between the bleeder and laminate (leaves a textured surface)
-Prepreg lay-up: Resin infused reinforcement fibre
-Mould release: prevents the laminate from bonding to the tool
-Tooling: provides 3D shape for prepreg to conform to
-Sealant tape: seals lay-up from outside air
All components must withstand the curing temperature (high disposable waste process)
Explain the autoclave cycle steps
-Increase temperature to reduce resin viscosity (rate determined by thermal mass (heat capacity) of tooling & lay-up)
-Resin flows at reduced viscosity, vacuum removes trapped air
-Ramp up to cure temperature, pressure increases to compress residual voids
-Hold pressure & temperature until curing is complete
-Cool down to demoulding temperature, vent to ambient pressure
What are the advantages of the Autoclave process?
-High part quality (good mechanical properties due to low void content)
What are the disadvantages of the Autoclave process?
-High cost (high energy, high setup costs and high material costs)
-Labour intensive
What are the process steps for Liquid Composite Moulding (LCM)?
-Dry fibre reinforcement placed in tooling
-Compaction pressure is applied
-Dry fibre is impregnated with liquid resin, pressure gradient is applied
-Composite is left to cure, and removed from the tooling
What materials are usually used in Liquid Composite Moulding (LCM)?
-mats/fabrics
-Preforms from fabrics
-Thermoset resins
How does resin flow velocity affect the product in Liquid Composite Moulding (LCM)?
-Impregnation amount
-Time required for complete impregnation (wet-out)
What material properties are desirable for good resin flow for Liquid Composite Moulding (LCM)?
-High permeability of the reinforcement
-Low viscosity of the fluid
-High pressure gradient
How does permeability vary with Volume fraction for Liquid Composite Moulding (LCM)?
-Increasing Vf reduces permeability
-Can vary with fibre orientations
How do you achieve low viscosity of the resin for Liquid Composite Moulding (LCM)?
-Preheat the resin prior to injection
-Heat the tool prior to injection
However, this accelerates the curing process, increasing viscosity over time
Why are molten thermoplastics unsuitable for Liquid Composite Moulding (LCM)?
Viscosity is too high, can be achieved with injection moulding, but is uncommon
How can poor mould design affect Liquid Composite Moulding (LCM)?
Can result in zero pressure gradients (stalled resin flow (dry spots))
Explain the Resin Transfer Moulding (RTM) process variant of Liquid Composite Moulding (LCM)
Stiff matched (dual) tooling, Positive pressure and/or vacuum applied (~10bar pressure difference)
-Preparation: produce preform, apply mould release and place dry preform in cavity
-Injection: Mix resin and hardener, inject resin into cavity
-Curing: Heat mould to accelerate curing reaction
-Demoulding: Demould cured part and finish component
What are the properties of the Resin Transfer Moulding (RTM) process variant of Liquid Composite Moulding (LCM)?
-Cycle time: ~hours
-Production rate: ~30,000ppa
-Good surface quality on both sides
-Fibre volume fraction: ~50%
-Geometrically complex components with high levels of parts integration
What occurs if the mould is not stiff enough in the Resin Transfer Moulding (RTM) process variant of Liquid Composite Moulding (LCM)?
Preform compaction pressure and resin injection pressure can result in deflection.
Affects: thickness, Vf and permeability
What are the properties of the High Pressure Resin Transfer Moulding (HP-RTM) process variant of Liquid Composite Moulding (LCM)?
-Cycle time: <10min
-Injection pressure: ~100bar)
-Faster impregnation
What are the issues with the High Pressure Resin Transfer Moulding (HP-RTM) process variant of Liquid Composite Moulding (LCM)?
-High tool closing force required
-Sealing of mould tool is required
-Fluid pressure can deform or displace the preform
-Expensive
Explain the Vacuum Infusion (VI) process variant of Liquid Composite Moulding (LCM)
-Stiff tooling on one side, vacuum bag on the other side of the component
-Resin flow is driven by vacuum only from a resin reservoir (resin trap prevents resin flow to the pump)
-Components are cured at room temperature
What are the components of the vacuum bag in the Vacuum Infusion (VI) process variant of Liquid Composite Moulding (LCM) process?
-Vacuum film: Seals lay-up from outside air
-Flow medium: Allows resin distribution
-Peel ply: Porous fabric allowing resin exchange between flow medium and reinforcement
-Reinforcement: dry reinforcement fabric
-Mould release: Prevents laminate from bonding to tooling
-Tooling: Provides 3D shape for finished part
-Sealant tape: Seals lay-up from outside air
What are the advantages of the Vacuum Infusion (VI) process variant of Liquid Composite Moulding (LCM)?
-Good surface quality on one side
-Tooling costs are lower than for Resin Transfer Moulding (RTM)
-Suitable for manufacture of large components (eg. ship hulls, wind turbines)
What are the disadvantages of the Vacuum Infusion (VI) process variant of Liquid Composite Moulding (LCM)?
-Slow process
-Poor dimensional control
-Lots of consumables used
-Long cycle time (slow curing due to long infusion and low pressure gradient)
Explain the Filament Winding Process
-Continuous dry fibres are wetted in resin (excess removed) and deposited on a rotating mandrel
-Mandrel removed once cured leaving a hollow component
-Any fibre can be used
-Fibres positioned along a pre-determined pattern (CNC controlled by guide linear speed and mandrel rotation speed)
What are the available patterns and applications for the Filament Winding Process?
Hoop:
-For internal pressures (pipework)
-Open ended
Helical:
-For torque applications (Driveshafts)
-Open ended
Polar:
-For internal pressures (pressure vessels), metal liner required internally preventing gas permeation
-Closed ended
What are the fibre path properties for the Filament Winding Process?
-Fibre path must be geodesic (shortest possible line due to tension), to prevent tow slipping
-Non-geodesic paths are possible if a tacky material is used
-Possible fibre paths are constrained with double curvature geometries
-Fibre angle must be adjusted for changes to mandrel diameter (avoids fibre bridging)
How does fibre tension affect the properties in the Filament Winding Process?
-Tension translates to radial compaction pressure
-Affects fibre volume fraction
-Affects void content
What are the advantages for the Filament Winding Process?
-High Vf (good for structural parts)
-Metallic fasteners can be integrated (ends of component)
-Highly automated
-Highly reproducible
-High deposition rates (~50kg/hr)
What are the disadvantages for the Filament Winding Process?
-Geometric restrictions
-Poor dimensional accuracy (surface quality) on the outer surface (can be mitigated using heat shrink after manufacture)
What are the applications for the Filament Winding Process?
-Pipes (hoop pattern)
-Drive shafts (helical pattern)
-Pressure vessels (polar pattern)
-Wind turbine blades
-Railway carriages
Explain the Pultrusion moulding Process
-Manufacture of continuous profiles (extrusions)
-Fibre bundles wetted (resin bath) & passed through a heated die (for curing)
-Mostly unidirectional rovings/tows (can use mats/fabrics)
-Typically glass fibre & thermoset resin (polyester)
-Reduced friction in the die due to polyester shrinkage (easier demoulding)
-Fully automated process
What are the properties in the Pultrusion moulding Process?
-Diameter: ~3-150mm
-Speed: ~1-2m/min
-Pulling speed dependent on material & cross-section
-Influenced by: heat-up, curing & cooling
-Wall thickness: ~1-76mm
What are the advantages of the Pultrusion moulding Process?
-High productivity
-Low labour content
-Precise cross-sectional dimensions
-Good surface finish
-Homogeneous fibre distribution
-High Vf
-Minimal fibre crimp & curvature
What are the disadvantages of the Pultrusion moulding Process?
-Geometric limitations (extrusion shapes)
-Complex (expensive) die designs
-High capital equipment cost (minimum 500ppa)
Explain the Thermoplastic Stamping moulding process (preparation, heating, consolidation, cooling, demoulding)
Preparation:
-Apply release agent to mould
-Untreated blank constrained in a spring-frame
Heating:
-Controlled heating (avoids polymer degradation) melts thermoplastic prepreg tapes/sheets (heaters/oven)
Consolidation:
-Transfer blank (2D) to matched tool, temperature below Tm
-Apply pressure to drape/form (3D)
Cooling:
-Solidify thermoplastic material (cooling rate determines residual stress & crystallinity)
Demoulding:
-Eject part and finish component (suitable for shell structures with uniform thickness)
Give an overview of Compression Moulding
-Discontinuous fibres & thermoset resin are pressed by heated tool, left to cure
-Short cycle time (high volume production; >100,000ppa)
-Hydraulic press with moulded tool (50-150 bar)
-Isothermal process (component is hot-demoulded)
Explain the Compression Moulding process
-Fibre/matrix pellet (charge) cut and weighed
-Charge is placed in the preheated matched tool (male + female dies)
-Tool rapidly closed and pressure is applied
-Charge flows to fill extremities of the tool
-Matrix material cross-links (due to heat)
-Tool is opened, ejecting the part
-Tool must be cleaned before next cycle
What are the tooling conditions for the Compression Moulding process?
-Very high quality tool surface finish (for class A composites and fast cycles)
-Hardened & chromed P20 steel for tooling
-Locally hardened sections for critical wear areas (shear edges)
-Uniform mould temperature and fast thermal response (heat transfer is critical)
-Air (flash) gap required (~0.2mm)
What induces fibre alignment (driven by shear) during the Compression Moulding process?
-Initial shape of the charge (%tool coverage)
-Position of the charge
-Geometry of the tool (divergent=perpendicular to flow, convergent=parallel to flow)
-Tool closing speed
How can mechanical properties vary in components manufactured by the Compression Moulding process?
-Fibre alignment (longitudinal alignment to applied load is optimal)
-Weld lines (where 2 fronts meet, leads to a stress concentration)
What are the advantages of the Compression Moulding process?
-Highly automated
-Short cycle time (<10mins)
-High production volume (>100,000ppa)
-Complex geometries (with closed dimensional control)
-Low material wastage (net shape components)
-Class-A surface finish
-Simple mould designs (no gating)
What are the disadvantages of the Compression Moulding process?
-Highly variable (weld lines, fibre/matrix separation, agglomerations)
-Non-uniform consolidation pressure (determined by die geometry)
-Limited fibre volume fraction (<50%), varies with charge/pellet type
-High tooling costs
Explain the Injection Moulding process steps
How is shear stress calculated during Injection Moulding?
Shear stress = Viscosity * Shear rate
How is shear stress rate calculated during Injection Moulding?
Differential of velocity with respect to displacement
Explain the relative velocity and shear rate against through-thickness position for Newtonian and Non-Newtonian behaviour
What are the advantages of the Injection Moulding process?
-Very high production rates (>250,000ppa)
-Highly automated
-Complex geometries
-Good tolerances on small parts
-Minimum material wastage
What are the disadvantages of the Injection Moulding process?
-High tooling cost
-Only suitable for high production volume
-Varied thickness & cooling rates leads to warping
-Large undercuts cannot be executed
-Melt flow distance limited in plane
-Limited mechanical properties (short/discontinuous fibres)
Why is Non-Destructive Testing (NDT) used?
-To identify and characterise damage within and on the surface without altering the part
-To be used after visual & dimensional inspection
Why does Non-Destructive Testing (NDT) carry substantial costs?
-Staff
-Equipment
-Disruptions to production
How does cost of inspection and probability of defect detected vary with inspection methods?
-Probability of an undetected defect decreases with each inspection process
-Cost of inspection increases with each inspection process
What defects are aimed to be detected by Non-Destructive Testing (NDT)?
-Delamination
-Cracks
-Voids (porosity)
-Resin richness
-Contaminants (foreign objects) within the laminate
-Ply misalignment (incorrect layup)
-Fabric wrinkles
-Fibre bridging
-Incomplete bond lines in adhesive joints
-Core crush in sandwich panels
What are visual inspection methods in Non-Destructive Testing (NDT)?
-Look for surface pitting (suggests internal voidage)
-Add surface coatings (contrasts show irregularities)
-Fluorescent matrix material (contrasts under UV light)
-Dye application (permeates and developed to draw to the surface, inspected under UV)
What is Dimensional Inspection used for in Non-Destructive Testing (NDT)?
-First article inspection (check manufacturing equipment is installed correctly)
-Quality control (ensure components are within tolerance)
Why is Dimensional Inspection used in Non-Destructive Testing (NDT)?
-Compare dimensions against original CAD model
-Check for changes in dimensions (due to shrinkage)
What are the methods of Dimensional Inspection used for in Non-Destructive Testing (NDT)?
Contact:
-Coordinate measuring machine (CMM)
Non-Contact:
-Structured white light 3D scanner
-3D laser scanning
What determines what method is used for Dimensional Inspection used for in Non-Destructive Testing (NDT)?
-Reflectivity of surface (unsuitable for scanning methods)
-Compliance of material/structure (unsuitable for contact methods)
-Colour of object (Black doesn’t work for white light scanning)
-Scale of object (contact unsuitable for small, complex parts)
Explain Non-Destructive Testing (NDT) Tap testing method
-Tap every point with a coin/hammer
-Acoustic response differs for local material stiffness
-Clear sound (indicates stiffness, structure intact)
-Dull sound (reduced stiffness, delamination or void)
What are the advantages of the Non-Destructive Testing (NDT) Tap testing method?
-Very simple and cheap method
What are the disadvantages of the Non-Destructive Testing (NDT) Tap testing method?
-Results rely on inspector perception (however, advanced test rigs exist)
Explain Non-Destructive Testing (NDT) Ultrasonic methods
-High frequency sound waves (1MHz-50MHz) detects internal flaws
-Coupling medium is required (eg. water allows acoustic transmission to the part)
Either reflection method:
-Material surface or internal defects
-Transducer transmits and receives the signal
-Amplitude and delay of reflection are measured to give an image
Or transmission method:
-Transmitter and receiver on opposite sides of the specimen
-Measure amplitude of the transmitted signal giving an image
Explain the A-scan Ultrasonic Non-Destructive Testing (NDT) method
-Amplitude of the pulse is recorded against time
-Information at a single point
-Defects cause an additional echo appearing as a signal
-Depth of defect determines echo delay and pulse strength
Explain the B-scan Ultrasonic Non-Destructive Testing (NDT) method
-A-scans recorded while transducer moves on the specimen surface
-Gives cross-section information on depth of defects
Explain the C-scan Ultrasonic Non-Destructive Testing (NDT) method
-Most popular (gives attenuation as a function of 2D position)
-No depth information (only defect detection)
-Used in reflection or transmission set-up
-Benchmark required to interpret scan data
What are the advantages of the C-scan Ultrasonic Non-Destructive Testing (NDT) method?
-Most common NDT inspection technique
-Easily detect voids as air greatly affects sound attenuation
-Fast scanning speeds
-High resolution
What are the disadvantages of the C-scan Ultrasonic Non-Destructive Testing (NDT) method?
-High equipment costs
-Skilled operator required
-Some composites are hydrophilic, therefore water coupling can cause issues
Explain the Radiography-x-Ray Non-Destructive Testing (NDT) method
-Non-contact NDT method
-Short wavelength electromagnetic radiation (0.03nm-3nm)
-Transmission degree depends on: Material density, thickness and composition
-Difficult to distinguish between fibre/resins (similar density)
-Voids & cracks are detectable due to air pockets
-Glass fibres (good absorbers) used as tracers
in CFRP parts
-Safety precautions are required
Explain the x-Ray Computed Tomography Non-Destructive Testing (NDT) method
-X-Ray measurements are taken at different angles (virtual slices)
-Specimen typically rotated in front of x-Ray source
-3D reconstruction to visualise internal features
-Relatively slow process, computing power required
-Laboratory environment required
Explain the Thermography Non-Destructive Testing (NDT) method
-Analysis of thermal flow from excitation of an object
-Thermal energy propagation within object influences surface temperature over time
-Thermal cameras used to visualise temperature field
-Thermal conductivity affected by material structure
-Detects: fibre architecture irregularities (orientation, Vf, ect.), Delamination and voids
What is the Externally Applied Thermal Field (EATF) Thermography Non-Destructive Testing (NDT) method?
-Heat source is applied directly to the laminate
-Heat distribution on the opposite surface maps through thickness diffusivity
What is the Stress Generated Thermal Field (SGTF) Thermography Non-Destructive Testing (NDT) method?
-A small cyclic load is applied to the structure
-Temperature distribution is generated
Explain the Structural Health Monitoring Non-Destructive Testing (NDT) method
-In-service monitoring of structures
-Used for large stiffness driven components (wind turbine blades, bridges, yacht masts)
-Optical fibre & sensors (can create stress concentrations) are embedded (strain and temperature affects the wavelength of transmitted light)
Describe the Kamal-Sourour curing rate model for thermoset resins
Curing rate:
-has a maximum at a certain degree of cure
-Increases with increasing temperature
