4: Preforming Flashcards
What is preforming?
The processes used to convert intermediate products into suitable shapes for moulding (3D form from 2D intermediates)
What are some material joining techniques?
-Inter-ply stitching
-Adhesive binders (powder or solvent)
-Welding (thermoplastics)
What is the importance of preforming?
-Either manual or automated
-Directly influences: Cycle time (max parts/annum), cost and quality
-Typically involves head and/or pressure
-Affects impregnation with resin: Fibre volume fraction, Local fibre orientations, Layup sequence and Defects (wrinkles)
-Allows parts integrations (seamless joining of multiple parts)
What is the influence of preform compaction on resin flow?
-Over compaction of the preform affects resin flow, and therefore the fibre volume fraction which can be plotted as a function of compaction pressure
-Resin “race-tracking” (voids) occurs at fibre bends due to high pressure, leading to dry spots
What materials are used in Hand layup 3D preforming?
-Dry fabrics - woven or NCF
-Prepregs
How suitable is Hand layup 3D preforming for production volumes?
~1000 parts per annum
-Good for prototyping or small production runs
Why is high volume production expensive for Hand layup 3D preforming?
-Requires skilled labour
-Multiple tools used in parallel
-Long production and layup time
(used in aerospace, motorsport or marine)
Why does Hand layup 3D preforming have poor repeatability?
-Dependent on the experience of the operator
-Fabric shear during shaping on the tool
-High scrap rate (material wastage ~40%)
Why do some plies need to be ‘darted’ when designing for manufacture?
-Darting (small cuts to reduce wrinkles) conforms the plies to the curved tool
-However, stress concentrations are created (account for these in calculations)
-For “black metal” designs (unsuitable for automation)
List some advantages and disadvantages of using humans during the layup process
Advantages:
-Highly dexterous (layup movements)
-Good at visually spotting errors
-Adaptive (can perform multiple roles)
Disadvantages:
-Expensive, skilled labour
-Variability (due to user error)
-Can take short cuts
-Motivation required for repetitive tedious tasks
List some advantages and disadvantages of using robots during the layup process
Advantages:
-Strong
-Repeatable and more consistent
-High precision
-Fast
-Inexpensive
-Continuous work
-Able to work in dangerous/harsh environments
Disadvantages:
-Limited feedback
-Cannot take corrective action if there’s an error
-Not as fast as expected
-Limited movement envelope (hard to achieve straight line due to 6Dof)
-Dedicated to one role
-Requires skilled programmers
What are properties of Automated Tape Laying (ATL) 3D preforming?
-High cost
-High quality
-Prepreg tapes (continuous unidirectional 8”-12” wide) robotically placed on tool surface
-Simple 3D or 2D shaped parts achievable
-Automatic debulking of thermoset prepregs
-In-situ consolidation of thermoplastic prepregs
-Allows: tailored orientation, more intelligent use of materials, reinforcement of other substrates
-Wastage is dependent on size/shape of component & fibre orientations
What are challenges of Automated Tape Laying (ATL) 3D preforming?
-Void content increases with line feeding speed
-Expensive feedstock materials (due to required precision)
-Expensive machines (~£2mil+)
-Limited to simple geometries (due to size of placement head)
-Limited curvature (hard to shear wide tapes due to buckling)
-Limited accel/decel rates of machines
-Long machine setup times
-Lack of CAE design tools
-Layup quality depends on: humidity and age of prepreg
What are properties of Automated Fibre Placement (AFP) 3D preforming?
-Places individual tows, increasing precision
-Lays up to 32 tows at once
-AFP achieves greater precision than ATL
-More suitable for shorter courses
-Achieves moderate double curvature at slow speeds
-Lower material wastage than ATL
-Tow is more likely to fail in tension than tape
What are limitations of robotic grippers (needle gripper and vacuum gripper) for automated preforming?
Needle gripper; can damage plies
Vacuum gripper; limited by how much it can pick up)
What is/are the forming mechanism(s) for metal forming?
-Plastic deformation (material thinning)
What is/are the forming mechanism(s) for composite fabric forming?
Fibres are inextensible
-In-plane shear (trellising) is the primary mechanism
-Shear resistance (due to: fibre crimp constraining yarn rotation, friction between yarns (, tension of stitch for NCFs))
What are the challenges for 2D to 3D preforming?
-Wrinkling (reduces mechanical properties due to stress concentrations)
-Thickness uniformity (particularly around male corners)
-Fibre breakage (tool will close if pressure is too high)
What is the mechanism and measure for macroscale wrinkling (ply folds) in a fabric?
Mechanism:
-Transverse yarn compaction (shear locking)
Measure:
-Shear angle (locking angle)
What is the mechanism and measure for mesoscale wrinkling (bundle loops) in a fabric?
Mechanism:
-Longitudinal yarn compression (yarn buckling)
Measure:
-Fibre compressive stress (critical buckling stress)
What is the mechanism and measure for gaps/voids (laddering) in a fabric?
Mechanism:
-Intra-ply yarn spacing (intra-ply over-slippage)
Measure:
-Fibre tensile strain (fibre spacing)
What is the mechanism and measure for Stitch damage in a NCF?
Mechanism:
-Longitudinal stitch extension (stitch rupture)
Measure:
-Stitch stress (stitch strength)
What are the 3 main components in the 2D to 3D press forming process?
-Punch (male die)
-Die (female die)
-blank holder (applies tension to the fabric)
What are the 4 stages in 2D to 3D press forming process?
-Tool opened, release agent is applied, blank is transferred to the mould
-Blank holder clamps the fabric stack around the edge of the cavity
-Male punch is driven by a hydraulic press (preform heated to activate binder)
-Tool is opened & preform is ejected
What are the press forming variables on the forming tool?
-Blank holder arrangement
-Blank holder pressure
-Minimum radii
-Surface finish (COF)
-Temperature
-Closure rate (prepreg only)
What are the press forming variables on the fabric blank?
-Material format (orientation, areal density, fibre constraints)
-Binder or resin content
-Ply shape
-Ply layup sequence relative to the forming tool
-Darts or slits
Give an outline of single diaphragm forming (SDF)
-Typically used in aerospace to form long structures (spars <20m)
-Only suitable for single curvature components (folding)
-Single sided tool, with silicone membrane forming bag
-Heat applied by infra-red lamps
What are the process steps for double diaphragm forming (DDF)?
DDF suitable for complex, double curved components
-Fabric plies sandwiched between diaphragms
-Vacuum drawn between 2 diaphragms to clamp the fabric
-Diaphragm heated (or raised to heating units) to activate the binder
-Quickly lowered onto contact tool (die)
-2nd vacuum (independent of the first) applied between lower diaphragm & die
-Preform cools below Tm of binder
-Vacuum removed, top diaphragm released first to prevent distortion
What are the double diaphragm forming variables on the diaphragm?
-Material type
-Stress/strain profile & temp/time relationship
-Diaphragm thickness
-COF between diaphragm & fabric
-Vacuum level
What are the double diaphragm forming variables on the diaphragm?
-Minimum radii
-Surface finish (COF)
-Temperature
What are the double diaphragm forming variables on the risers &/or intensifiers?
-Shape
-X-Y position relative to the tool
-Height relative to the tool
-Material (stiffness)
What are the double diaphragm forming variables on the fabric blank?
-Material format (orientation, areal density, fibre constraints)
-Ply shape
-Ply position/orientation (relative to forming tool)
-Binder/resin content
-Darts or slits
Provide an overview of matched tool forming
-High CAPEX
-Non-isostatic pressure (problems for near vertical faces)
-High material wastage (additional blank material required for gripping
-Blank holder controls material draw-in and tension
-Preforms can be formed using multiple operations
-Blank holder used to locate/hold preform prior to forming
Provide an overview of Single Diaphragm Forming (SDF)
-Low CAPEX
-Isostatic pressure
-Net-shape
-Limited control over material draw-in or fabric wrinkling
-Preforms can be formed using multiple operations
-Intensifiers can be placed inside (or incorporated within) the diaphragm
-Minimal spring-back (depends on release agent/film)
-Difficult to locate preforms prior to forming
Provide an overview of Double Diaphragm Forming (DDF)
-Low CAPEX
-Isostatic pressure
-Net-shape
-Pressure between diaphragms can be regulated to control tension
-Lower membrane prevents multiple forming operations
-Difficult to incorporate intensifiers:
Lower membrane (wrong side of preform)
Upper membrane (may impede fabric slippage/draw-in)
-Problems removing preform (spring back of lower diaphragm)
-Lower diaphragm used to transport preforms
Give an overview of the braiding Direct Textile Preforming method
-Interlacing of fibres about a mandrel (internal tube)
-May-poling action used to create prismatic shape (tubes with geometrical changes)
-Must be strong enough for braiding forces & resin injection
-Commonly biaxial (10 to 85 degrees) or triaxial
Fibre angles controlled by:
-Rotational speed of fibre carriers
-Axial speed of mandrel
-Number of fibre carriers
-Mandrel diameter
Give an overview of the braiding complex geometries possibilities
-Concave/convex sections
-Square/circular sections
-Care required to avoid fibre bridging (control speed)
-Ply drops (change in number of layers) produced by reversing braid direction
-Displacers used to create holes, keeps fibres continuous, decreasing stress concentrations (1.8x higher strength)
-Superior fatigue performance compared to filament winding (due to interlacing)
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What are some advantages of braiding?
-Low wastage (<10%)
-High degree of automation
-High deposition rates
-Cycle time is independent on mandrel complexity
-Most efficient when braiding straight sections
-Suitable for structural applications (bending, torsion, crash)
What are some disadvantages of braiding?
-Batch process (limited mass of fibre on bobbins)
-Long setup times & downtime initiating & terminating a part
What are the 3 styles (Z binder yarn pattern) of 3D weaving?
-3D angle interlock
-3D orthogonal
-3D layer-to-layer
What is a 3D weaved fabric?
Similar to 2D weaving, with the inclusion of high stiffness/strength fibres in the through-thickness (Z) direction (0.5-10%)
What are properties of a 3D weaved fabric?
-High delamination fracture toughness
-High impact damage tolerance
-Higher through-thickness electrical & thermal conductivity
How do z-fibres affect the in-plane performance of the laminate?
Depends on % content
-up to 25% reduction in stiffness
-up to 50% reduction in strength
What are some limitations of 3D weaving production?
-Long machine setup time (due to number of creels)
-Limited preform width dur to available yarn ends
-Available preform width gets narrower as preform becomes thicker
How can more complex geometries be produced using 3D weaving?
-Strategic removal of z-binder yarns in some regions, then hollow out
What are some 3D weaving applications?
-Stiffeners for aerospace panels (T and pi cross-sections)
-Nodes for civil applications
-Turbine blades
-Fan casings
Explain Embroidered fabric placement
-Reinforcement fibres are embroidered onto a substrate (fabric or polymer film)
-Net-shape with no waste
-Fibres can be steered through tight radii
-Used to reduce stress concentrations around holes/notches
-Stitches improve through thickness permeability
-Limited fibre deposition rates, parallel embroidery head can be used on a single part
-Standard tows can be used (minimising cost)
Give an overview of random fibre products
-Low performance applications (usually marine)
-Low fibre volume fractions due to high bulk factor
-Difficult to form from sheet (risk of tearing as unable to shear uniformly in-plane)
Give an overview of Directed Carbon Fibre Preforming (DCFP)
-Chopped fibres & resin randomly sprayed onto part (perforated screen)
-Easy to automate (high precision not required)
-For semi-structural components
-Preforms moulded using liquid resin methods
What are the advantages of Directed Carbon Fibre Preforming (DCFP)?
-Rapid cycle time (5-10 mins)
-Low cost of raw materials
-Low use of intermediates
-Low wastage (2-5%)
-Complex geometry
-Parts integration
-Large parts (2-3m)
-Locally variable thickness
What are the manufacture process steps of Directed Carbon Fibre Preforming (DCFP)?
-Fibre and binder deposited onto perforated screen/tool
-Heating and compression of preform (activates powdered binder)
-Cooling (freezes preform)
-Demoulding (extracts preform part)
Explain the Directed Carbon Fibre Preforming (DCFP) chopper gun
-Fibre fed in by rollers into a pressure roller and chopper (number of blades determines the fibre lengths)
-Randomly oriented fibres
-Tuneable fibre deposition rate
-Scalable process
-Max volume fraction ~55%
What are the challenges of Directed Carbon Fibre Preforming (DCFP)?
-Heterogeneous (different properties in different regions) as it’s not a laminate
-Many variables (orientation, tow size, fibre length, local Vf changes)
-Local stiffness & strength variations due to architecture
-Fibre homogeneity affected by fibre length and tow size
-Material stiffness & strength dependent on thickness (scale effects)
What effect does thickness have on modulus for Directed Carbon Fibre Preforming (DCFP)?
Exponential curve, rapidly increasing modulus with thickness then plateauing
What effect does volume fraction have on modulus for Directed Carbon Fibre Preforming (DCFP)?
Positive linear relationship
What effect does volume fraction have on UTS for Directed Carbon Fibre Preforming (DCFP)?
Positive parabolic curve
Discuss the permeability of Directed Carbon Fibre Preforming (DCFP)
-Local permeability is affected by heterogeneous fibre architecture
-Global permeability ~10x lower than for quasi-isotropic laminate of the same Vf
-Higher injection pressures required (risk of washing)
-Irregular resin flow (risk of dry spots in final moulding)
-Variability makes tool design & process simulation difficult
