Research Papers

Silicone Rubber Properties During Consolidation/Curing of Advanced Composites Using Specialized Elastomeric Tooling

[+] Author and Article Information
Paul Malek

Center for Automation Technologies and Systems,
Rensselaer Polytechnic Institute,
110 8th Street,
Troy, NY 12180
e-mail: paulvmalek@gmail.com

Daniel Walczyk

Fellow ASME
Center for Automation Technologies and Systems,
Rensselaer Polytechnic Institute,
110 8th Street,
Troy, NY 12180
e-mail: walczd@rpi.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received November 12, 2014; final manuscript received April 10, 2015; published online September 9, 2015. Assoc. Editor: Donggang Yao.

J. Manuf. Sci. Eng 138(2), 021002 (Sep 09, 2015) (7 pages) Paper No: MANU-14-1586; doi: 10.1115/1.4030432 History: Received November 12, 2014

Specialized elastomeric tooling (SET) is a patented process that can replace autoclaving for consolidating and curing advanced thermoset and thermoplastic composite parts. The process mimics autoclave conditions of uniform pressure and temperature by clamping an uncured laminate or sandwich structure with known force between a temperature-controlled lower tool and an engineered, rubber-faced upper tool. Several published studies involving small- to medium-sized parts have shown that SET provides equal or better quality, but at fraction of the energy, waste, and capital and consumable costs. The elastomer of choice for the upper tool is a castable, platinum-catalyzed silicone rubber because of its high working temperature, high tear strength, and negligible shrinkage. To date, there is limited understanding about the properties of silicone rubber subjected to high temperature and compression for long periods of time over multiple cycles. This paper discusses recent work that characterizes silicone rubber under these conditions for design and simulation purposes. Compression testing performed per ASTM-D575 exhibited linear behavior at 125 °C (typical processing temperature for epoxy resins), whereas tensile testing (per ASTM-D412) at the same temperature exhibited strain softening. To show the repeated effect of compression on rubber properties (i.e., mimic multiple loading cycles on rubber mask) at typical process temperature (125 °C) and pressure, a fatigue testing apparatus was custom designed and fabricated. Over repeated cycles between 0 and 1.35 MPa (typical consolidation pressure for advanced composites), silicone rubber exhibited slight hysteresis and a minor stiffening effect that appears to plateau at a particular modulus. Static and kinetic frictional coefficients, also used in modeling, between silicone rubber and several materials commonly used in SET ranged from 0.5 to 2.4 (per ASTM-D1984). Finally, pressure injection and in-line mixing of uncured rubber resulted in significantly less entrained air bubbles (and resulting surface defects in contact with composite part) than the current standard practice of hand mixing. Results are applicable to both SET and any advanced composite forming or curing/consolidation processes involving rubber-faced tools.

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Walczyk, D. , Hoffman, C. , Righi, M. , De, S. , and Kuppers, J. , 2013, “Consolidating and Curing of Thermoset Composite Parts by Pressing Between a Heated Rigid Mold and Customized Rubber-Faced Mold,” U.S. Patent No. 8,511,362.
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Fig. 6

Friction testing setup on Instron 5848 Microtester

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Fig. 5

Fatigue testing apparatus

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Fig. 4

Temperature-controlled constraining rings used for fatigue testing

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Fig. 3

(a) Compression testing and (b) tensile testing setups in Instron Environmental Chamber

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Fig. 2

Stress–strain plots for silicone rubber in (a) tension and (b) compression at room (24 °C) and elevated (125 °C) temperatures. Power law curve-fit is provided for high temperature tension as an example.

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Fig. 1

2D schematic of SET curing process

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Fig. 7

Side view of friction sled with test surface

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Fig. 8

Stress–strain curves for rubber in compression at 125 °C when loaded at various strain rates for specimens shaded gray in Table 1

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Fig. 9

Comparing stress–strain plots for silicone rubber subject to compression–relaxation cycling at 125 °C for one loading cycle to (a) 1000, (b) 2000, (c) 3000, (d) 4000, and (e) 5000 cycles

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Fig. 10

Plot of rubber compressive elastic modulus, Ec, versus number of compression–relaxation cycles, N, in fatigue tester with approximate linear fit

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Fig. 11

Plot of dissipated energy per unit volume, U, versus number of compression–relaxation cycles, N, with approximate linear fit

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Fig. 12

Cross-sectional view of difference in area density of internal bubbles (17 versus 3) for (a) hand-mixed sample and (b) dispensing gun sample, respectively




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