NSF Workshop on Polymer Processing,

June 9-10, 2004

Hari DharanUniversity of California at Berkeley

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.

Outline

• Review of conventional RTM– Darcy’s Law and its components– Current analytical methods based on Darcy’s 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

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.

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

Process description of RTM

Fiber reinforcements

Liquid resin

1. Prepare mold

2. Place fiber preform

3. Inject liquid resin

4. Cure impregnated fibers

5. De-mold and finish

In http://www.owenscorning.com

Major process steps

Conventional RTM

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

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

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

Resin Infiltration Model

• Darcy’s law => V = - (S/)(dP/dx)

S:Permeability Tensor, :Viscosity, (dP/dx):pressure gradient

• Mold fill time can be reduced only by :

1. Increasing permeability

2. Lowering viscosity, or

3. Increasing inlet pressure

1. Increasing Permeability

2. Lowering Viscosity

3. Increasing Pressure

• Decrease fiber volume fraction • Increase use of chopped and felt preformsProblem: Lowers composite properties

• Low molecular weight resins • High temperature injectionProblems: Process window and control issues

Lower Tg, modulus, compressive strength

Problems: Preform distortion and fiber wash Permeability reduction

Permeability through Fiber Preform

• In-plane components (Sx, Sy) >> Sz

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

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

• Compressible --> Permeability = f(P)

Articulated Approach

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

Major tooling and functional features

Articulated RTM

Loading point follows flow front

Pressure gradient is kept from degrading at constant load

Filling rate is expected to remain high

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

Key process scheme

Result:

Analytical Approach

Process model • Unidirectional mold/ten segments• Darcy’s eqn applied to each segment

• Comparison of mold fill-time between conventional (C-) RTM and articulated (A-) RTM

• Investigation of segment controls as process parameters

Objective

Approach • Transverse flow (w.r.t. laminate) is achieved by consolidation, longitudinal flow occursthrough “loose” preform (higher S)

• Loose fiber volume fraction, Vo =0.58

• Sx/Sz=100 at a given fiber volume fraction

Mold filling analysis in C-RTM

Transverse flow rate

Longitudinal flow rate gradient

Total volume flow rate

t

zVA

y

PSAq f

foz

oz

1

t

xV

x

PS

z

q ff

xx

1

zx qqq

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

Nomenclature : In the next page

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

Nomenclature

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

ooz

oz V

tPSVAq

1

21

'

Case 1:

Volume flow rate per unit width by transverse flow

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

S’x : Longitudinal permeability at Vo

S’z : Transverse permeability at Vo

This equation is limited to when the transverse flow front reaches the bottom of preform

Case 2:

Volume flow rate per unit width by longitudinal flow

After the transverse flow is completed in case 1

tc : Time when the transverse flow reaches bottom

(when hf = h’)

h’ : Preform thickness at Vo

S’x : Longitudinal permeability at Vo

S’z : Transverse permeability at Vo

112

3

122/3

1f

cfo

h

httD

hVq

oox

V

PSD

11 o

oz

V

PSD

12 o

ozf V

tPSh

1

2

where

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 segments0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1Normalized time (t/tr)

Mo

ld-f

ill

rati

o

Articulated process

Conventional RTM

Result: Mold fill time for A-RTM is 6 % of C-RTM fill time

Effect of segment load on mold fill time

• tr: Mold-fill time

for C-RTM

• P(segment load)=xPo

• 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

0

0.1

0.2

0.3

0.4

0.5

0 1 2 3 4 5Pressure (P/Po) for articulated process

Mo

ld f

ill

tim

e (t

/tr)

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.

1

1.2

1.4

1.6

1.8

2

2 4 6 8 10Number of segments

Mo

ld f

ill

tim

e (t

/ts)

Summary of results

• Mold-fill rate does not decelerate during the process

• Mold fill time is increased by increasing segment load

• Fewer segments result in slightly slower mold filling

3-4 segments can increases mold fill time effectively

• Mold filling is much faster than in conventional RTM

Advantages of Articulated Tooling

Fast filling --> 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

Control of articulated segments is straight-forward using computer-controlled servoactuators (with load andposition feedback)

Proposed concept could be coupled to low presssure injection machines

A-RTM Tooling

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

Potential applications of A-RTM

Articulated RTM Tooling

Schematic of axi-symmetric process

Center piston(segment)

First ring(segment)

Second ring(segment)

Servohydraulic equipped A-Tooling

Schematic drawing of cross-section configuration of axi-symmetric process

Servo hydraulicpump

Inlet hole

Upper mold segment

Drawbacks of Articulated Tooling

Expensive tooling

Probably restricted to small parts < 1 m2 ?

Potential for fiber damage by articulated segments needs to be assessed and eliminated by tool force control.

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.

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)

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.