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 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
