Void Formation During Preform Impregnation in Liquid Composite Molding Processes Caleb DeValve and Ranga Pitchumani Advanced Materials and Technologies Laboratory Department of Mechanical Engineering Virginia Tech Blacksburg, Virginia 24061-0238 pitchu@vt.edu • http://www.me.vt.edu/amtl • (540) 231-1776 Presented at the FPCM 10, Ascona, Switzerland • July 12, 2010
Introduction ď ą General LCM Process:
1
2
4
3
1. Preform is laid up in the mold 2. Resin is forced into the mold and through the preform, exiting through ports on the opposite side of the mold 3. Resin is cured within the mold around the fibrous preform 4. Finished composite product is removed from the mold for use ď One challenge to processing is the entrapment of voids (intra-tow) and dry spots (macroscopic, inter-tow) in the preform, resulting in defective parts. ď A model for predicting air void entrapment and dry spot formation would be beneficial for forecasting limits on the appropriate processing conditions to design the process for voidfree mold filling. GOALS OF PRESENT WORK: To conduct numerical modeling of the transient resin infiltration of a three-dimensional preform weave architecture, and to derive design guidelines for effective processing
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Model Geometry PLAIN WEAVE FABRIC ARCHITECTURE: Fabric is modeled according to Owens Corning WR10/3010 Fiber bundle cross-sections are lenticular in shape and weave is contoured as a sine function The geometry is modeled so that there are three repeating unit cells along the direction of the infiltrating resin flow
4.5 mm 14 .0 mm
2.2 mm
Resin Flow Direction
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Model Formulation ASSUMPTIONS: Resin flow is Newtonian, incompressible, and laminar Fiber bundle can be modeled as porous media Fiber bundle permeability can be described by applying the analysis of fluid flow through aligned cylindrical beds Individual fibers have a diameter of approximately 10 microns Nesting effects (multiple layers of preform weave) are not considered
v u
1. Continuity:
t 2. Momentum : 3. Volume of fluid:
t
t
v u
0 vv uu
v u
p
v u
v uT
v Fsur
v Fpor
0
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Porosity-Permeability Relationship
(Annulus flow)
ď ą Existing analytical models were compared with numerical simulations, and ones with good agreement with the numerical simulations were used for the modeling. ď ą Using the permeability-porosity relationships for HEXAGONAL FIBER PACKING, a permeability tensor was projected onto the major Cartesian direction throughout the sinusoidal weave geometry in the numerical model.
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Parametric Study Variable
Parameter Values
Resin Mass Flow Rate [g/s]
1.0
2.0
4.0
Tow Porosity
0.3
0.4
0.5
TOW POROSITY: 0.3
ď Simulations were done using ANSYS FLUENT on a mesh of ~35k elements per unit cell (i.e., ~105k elements total) ď Simulation of ~15 s of flow required ~5 days on a Dell PowerEdge R610 with Dual Quad-Core 2.93 GHz Processors
TOW POROSITY: 0.5
Note that the air phase is represented by the solid green shading in order to visualize the air entrapment locations. The resin is represented by the empty shading.
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Simulation Results TOW POROSITY VARIATION COMPARISON (WITH CONSTANT RESIN FLOW RATE, 4 g/s): Tow Porosity of 0.30 0.50 s
0.0035 s
88.2% Air
16.0% Air
2.00 s
7.12% Air
Tow Porosity of 0.50
87.6 % Air
13.3% Air
4.84% Air
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Analysis of Simulation Results ď Tracking the air remaining in the flow domain over time for the different flow scenarios: 3.25 s First unit cell outlet completely
saturated
4.53 s Second unit cell outlet completely saturated
ď By comparing the percentage of air remaining in the domain over time with the onset of
the initial flow front completely saturating the outlet of the first unit cell, generalized metrics of the mold filling process can be defined as: 1. Void content in the tow 2. Additional process time needed for the entire unit cell to completely permeate with resin after the outlet is completely saturated 3. Excess resin wasted during this additional process time
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Void Entrapment VOID FORMATION FOR DIFFERENT OPERATING SCENARIOS: Defined as the percentage of air which remains in the first unit cell as the resin flow front completely saturates the outlet of the first unit cell, shown in the left figure
Shown in the right figure is the mapping of resin plane flow rate vs. tow porosities that lead to void-free mold filling and filling with void entrapment (shaded region) Provides for selection of line injection parameters for a given plain weave geometry (with its corresponding tow porosity), so as to attain void-free processing
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Excess Flow for Complete Fill
Max time of 400 s/m for a tow porosity of 0.30
0.17 m/s lower bound
Max. wasted resin of 700 cc/m for a tow porosity of 0.30
0.19 m/s upper bound
Slower resin velocities at lower porosities in general result in longer excess fill times, but less resin wastage
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Conclusions A three-dimensional transient simulation of resin flow in a plain weave architecture was studied accounting for the dual scale flow. The modeling provides for analysis of void formation in LCM. Design guidelines based on quantitative metrics of void formation and excess resin flow for complete saturation were presented. Acknowledgements The work was funded in part by the US. National Science Foundation through Grant No. CBET-0934008 and a GAANN Fellowship to Mr. DeValve from the US Department of Education through Award No. P200A060289
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