NSF Workshop on Polymer Processing, June 9-10, 2004

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NSF Workshop on Polymer Processing,June 9-10,
2004

• Hari Dharan
• University of California at Berkeley

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Present researchResin Transfer Molding

• New concept for resin transfer molding (RTM) in
  which tool articulation provides significantly
  faster mold fill time compared to conventional
  RTM processes.

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Outline

• Review of conventional RTM
• Darcys Law and its components
• Current analytical methods based on Darcys Law
• Example of one-dimensional analysis for mold fill
  time
• Drawbacks with conventional RTM
• Factors affecting permeability
• Tool articulation concepts
• Resin peristalsis by tool articulation
• One-dimensional analysis for mold fill time using
  articulated tool
• Advantages and drawbacks
• Some suggestions for future investigations

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Introduction

• In RTM, the mold is packed with a dry fiber
  preform in which the fibers are oriented in the
  desired directions for reinforcing the part.
• The preform is impregnated by resin injected
  through one or more ports in the mold. After the
  mold is filled, the resin solidifies by
  cross-linking and the part is removed from the
  mold.
• There are currently many analyses and computer
  programs that simulate the mold filling process.

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Prior Work

• Gonzalez, Castro and Macosko (1985) axisymmetric
  analysis 1-D (analytical and numerical)
• Coulter and Güçeri (1988) 2-D finite-difference
  code for isothermal flow
• Young, et al (1991) included variable
  permeability effects
• Bruschke and Advani (1993) non-isothermal flow
  using finite-element method
• Others Hieber and Shen (1980), Trochu and Gauvin
  (1992) numerical simulation issues

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Process description of RTM
Liquid resin
Major process steps
1. Prepare mold 2. Place fiber preform 3. Inject
liquid resin 4. Cure impregnated fibers 5.
De-mold and finish
Fiber reinforcements
In http//www.owenscorning.com
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Conventional RTM

• Complex shapes
• All surfaces finished in the mold
• Low cost for repetitive production
• Moderate production quantities

Key Benefits
Drawbacks

• Anisotropic flow leading to entrapped voids
• Long times to allow for resin flow and delayed
  cure
• Mold temperature control slow
• Restricted to low viscosity thermosetting resins
• Fiber wash

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Ideal RTM process

• Rapid mold fill (minutes, not hours)

• No voids or resin bypass regions

• No fiber wash for various preforms

• Applicable to a variety of resin systems
• hot melt epoxies, vinyl esters, thermoplastics

• Hybrid fiber preforms and embedded inserts

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Resin Infiltration Model

• Darcys law gt V - (S/?)(dP/dx)

SPermeability Tensor, mViscosity,
(dP/dx)pressure gradient

• Mold fill time can be reduced only by

1. Increasing permeability 2. Lowering viscosity,
or 3. Increasing inlet pressure
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1. Increasing Permeability

• Decrease fiber volume fraction
• Increase use of chopped and felt preforms
• Problem Lowers composite properties

2. Lowering Viscosity

• Low molecular weight resins
• High temperature injection
• Problems Process window and control issues
• Lower Tg, modulus, compressive strength

3. Increasing Pressure
Problems Preform distortion and fiber wash
Permeability reduction
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Permeability through Fiber Preform

• In-plane components (Sx, Sy) gtgt Sz

• Different types of preforms will have different
• compliance in the thickness direction resulting
  in
• different relative permeabilities
• (in-plane vs thickness).

• Compressible --gt Permeability f(P)

• Multi-scale permeable paths
• (Preform level / fiber bundle level)

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Articulated Approach
Major tooling and functional features

• Segmented upper mold
• Peristalsis-like flow propagation
• Squeeze flow through loose fibers
• Mechanical consolidation

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Articulated RTM
Loading point follows flow front
Result
Pressure gradient is kept from degrading at
constant load
Filling rate is expected to remain high
Key process scheme
1. Liquid resin is supplied onto loose dry
preform 2. Initial squeeze-down of upper mold
segments 3. Transverse infiltration is driven by
the first segment 4. Unloading of second
segment 5. Excessive resin volume is captured by
the unloading segment
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Analytical Approach
Objective

• Comparison of mold fill-time between
  conventional (C-) RTM and
• articulated (A-) RTM
• Investigation of segment controls as process
  parameters

• Unidirectional mold/ten segments
• Darcys eqn applied to each segment

Process model

• Transverse flow (w.r.t. laminate) is achieved by
• consolidation, longitudinal flow occurs
• through loose preform (higher S)
• Loose fiber volume fraction, Vo 0.58
• Sx/Sz100 at a given fiber volume fraction

Approach
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Mold filling analysis in C-RTM
Transverse flow rate
Longitudinal flow rate gradient
Total volume flow rate

• Two-directional flow is considered
• Longitudinal flow rate is a function of y and
  time

Nomenclature
In the next page
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Nomenclature
qx Longitudinal volume flow rate per unit
width qz Transverse volume flow rate per unit
width t Time xf Longitudinal flow front,
fn(z) zf Transverse flow front. Ao Inlet area
in C-RTM Po Constant inlet pressure in
C-RTM Vf Fiber volume fraction Sx
Longitudinal permeability at Vf Sz Transverse
permeability at Vf ? Viscosity
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Mold filling analysis in A-RTM
Resin flow in A-RTM
Two-stage flow

• Case 1 Initial stage when the first segment is
  in motion
• Transverse down-flow through the loose preform
  only
• Longitudinal flow through the squeezed preform
  is restricted

• Case 2 After the transverse flow is completed
  in case 1.
• Longitudinal flow is driven by the excessive
  resin squeezed
• by segment motion.
• Transverse flow is achieved by the
  consolidation
• of the wet loose preform

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Case 1
Volume flow rate per unit width by transverse
flow
This equation is limited to when the transverse
flow front reaches the bottom of preform
Az Area of a segment in contact with
preform Po Constant inlet pressure in
C-RTM Vf Fiber volume fraction Vo Fiber
volume fraction of loose fiber Sx Longitudinal
permeability at Vo Sz Transverse permeability
at Vo
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Case 2
Volume flow rate per unit width by longitudinal
flow After the transverse flow is completed in
case 1
where
tc Time when the transverse flow reaches
bottom (when hf h) h Preform thickness
at Vo Sx Longitudinal permeability at Vo Sz
Transverse permeability at Vo
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Mold filling simulation results
Mold fill ratio with time

• tr Mold-fill time
• for C-RTM
• Po Inlet pressure
• for C-RTM
• P(segment load)Po
• Ten segments

Result Mold fill time for A-RTM is 6 of C-RTM
fill time
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Effect of segment load on mold fill time

• tr Mold-fill time
• for C-RTM
• P(segment load)x?Po
• Ten segments

• For P/Po 1, mold fill time (t/tr)0.06
  relative to C-RTM
• Lower pressures than this still result in
  faster mold fill times
• relative to C-RTM

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Effect of number of segments

• ts Mold-fill time
• for ten Segments
• P(segment load)Ps
• Mold length is constant

• For ten segments, mold fill time (t/ts)1.
• Four segments result in a 40 increase in mold
  fill time.
• This is still only 8 of C-RTM mold fill time.

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Summary of results

• Mold-fill rate does not decelerate during the
  process

• Mold fill time is increased by increasing
  segment load

• Mold filling is much faster than in conventional
  RTM

• Fewer segments result in slightly slower mold
  filling

3-4 segments can increases mold fill time
effectively
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Advantages of Articulated Tooling
Fast filling --gt Mass-production Fiber
distortion and wash-out can be avoided Locally
trapped air pockets can be removed Robust process
relatively insensitive to resins,
preform characteristics and temperature Mechanical
design of tool is complicated but probably not
significantly more than for a typical injection
molding tool
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Potential applications of A-RTM
Mass production of large and complex parts Tool
motion control can be used to tailor local
permeability for various types of
reinforcements Multi-resin systems High
temperature resins can be used, including
high-viscosity systems Complex stitched preforms
can be used
A-RTM Tooling
Control of articulated segments is
straight-forward using computer-controlled
servoactuators (with load and position
feedback) Proposed concept could be coupled to
low presssure injection machines
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Articulated RTM Tooling
Schematic of axi-symmetric process
Center piston(segment)
First ring(segment)
Second ring(segment)
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Servohydraulic equipped A-Tooling
Schematic drawing of cross-section configuration
of axi-symmetric process
Servo hydraulic pump
Upper mold segment
Inlet hole
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Drawbacks of Articulated Tooling
Expensive tooling Probably restricted to small
parts lt 1 m2 ? Potential for fiber damage by
articulated segments needs to be assessed and
eliminated by tool force control.
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Future work
Conduct simple experiments using axisymmetric
segmented mold Study preform characteristics
using preg fiber bundles for thermoplastic
matrices. Couple to front-end of injection
molding machine.
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Issues to be addressed

• Modeling of conventional processes should predict
  defects and flow paths.
• Adjustments in conventional processes should be
  applied to eliminate predicted defects, increase
  process speed, widen range of allowable
  parameters (viscosity, molecular weight,
  temperature, reaction times).
• Transfer to industry via interactive programs
  (instrumented prototype machinery, correlation
  with models, validate improved processes)

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Modeling issues

• Dynamic modeling
• Pressure gradient-dependent permeability
• Thermoplastic processing extension to continuous,
  oriented fibers.
• Controlled porosity, fiber orientation and
  distribution achieved by mold and process design.
• Preform and fiber placement machine design.