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Research Papers

Refinement of the Thermal Press Curing Process for Advanced Composites

[+] Author and Article Information
Jaron Kuppers

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

Daniel Walczyk

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

1Corresponding author.

Manuscript received July 2, 2013; final manuscript received November 10, 2013; published online February 5, 2014. Assoc. Editor: Donggang Yao.

J. Manuf. Sci. Eng 136(2), 021014 (Feb 05, 2014) (12 pages) Paper No: MANU-13-1266; doi: 10.1115/1.4026043 History: Received July 02, 2013; Revised November 10, 2013

Thermal press curing (TPC) is an alternative process to autoclaving for consolidating and curing thermoset and thermoplastic prepreg composite parts by pressing them between a heated “curing mold” and a customized rubber-faced “base mold” that are engineered to provide uniform temperature and pressure conditions. A study was performed with a kayak paddle part made from eight plies of woven carbon/epoxy prepreg material and formed by double diaphragm forming (DDF). The study expounds on the narrow body of TPC knowledge around three main objectives: (1) to experimentally compare TPC cured parts to a benchmark autoclave process using a realistic part shape with fine geometrical details, (2) to evaluate the necessity of vacuum bagging of TPC cured parts, and (3) to characterize the robustness/sensitivities of pressure application during the TPC process by varying both the total pressure applied to the base mold and the location the hydraulic press ram applied pressure to the base mold. Maximum temperature and pressure variations around the target levels over the entire clamped tool surface were measured as 5.0 °C and 5.5%, respectively, both of which were well within the manufacturer's recommendations. The TPC part had fewer defects, was generally thinner, and had a higher fiber volume fraction than a comparable autoclaved part. Little difference was observed between the TPC parts made with and without vacuum bagging. Parts with too little pressure (90%) resulted in more thickness variation and defects than too much pressure (110%). Finally, TPC parts exhibit some thickness variation, as expected, when ram force is applied off the center of pressure (COP).

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Figures

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Fig. 1

Schematic of the TPC process for composite consolidation/curing; variable thickness elastomer mask creates hydrostatic surface pressure on composite when the mold set is compressed with a specific vertical force. (While this figure shows a 2D part shape for clarity, the actual process is intended for complex 3D shapes.)

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Fig. 2

CAD model of the kayak paddle (a) as a rendering and (b) original scanned surface (based on 3D scan of existing plastic paddle) with dimensions

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Fig. 8

Cutting and consolidation of the composite prepreg blank shape, (a) prepreg on the cutter bed, (b) preconsolidation of laminates on registration plate, (c) DDF apparatus with curing tool shown before forming, and (d) DDF apparatus during forming

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Fig. 7

(a) The finished base mold and (b) the rubber mask

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Fig. 6

CAD models of (a) the initial base mold, (b) the final base mold geometry (with sidewall features), (c) the curing mold side of the rubber mask, and (d) the base mold side of the rubber mask

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Fig. 5

Surface pressure contour plots for the initial rubber mask shape, after progressive iterations of the surface shape algorithm, and after the application of the boundary constraint

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Fig. 4

(a) top of the finished Kayak curing mold (with holes for registration pins as marked by black arrows) and (b) bottom of the mold (showing the pocket for heaters)

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Fig. 3

Manufacturer-specified autoclaving temperature, pressure, and vacuum schedule for curing the composite prepreg material

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Fig. 9

Kayak paddle TPC setup in a 10-ton hydraulic press

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Fig. 10

(a) COP with the x- and y-offsets and (b) vacuum-bagged part before TPC experiment

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Fig. 11

Eleven test points for thickness measurements, and approximate location where three-point beam bending specimen was removed from part; 0 deg prepreg ply orientation shown

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Fig. 12

TPC cured parts and defects, (a) example part under standard curing conditions and (b) worst case of the recurring void defect as a single large void

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Fig. 13

Autoclave cured parts and defects, (a) example part, (b) voids due to laminate wrinkling, (c) recurring void defect at point 11, and (d) void defect along the ridge of the kayak paddle blade

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Fig. 14

Deviations from average thickness of standard part (1.57 mm) for parts with varying applied pressure at 11 points shown in Fig. 11

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Fig. 15

Deviations from average thickness of standard part (1.57 mm) for parts with varying location of force application at 11 points shown in Fig. 11

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Fig. 16

Deviations from average thickness of standard part (1.57 mm) for autoclaved and vacuum-bagged parts at 11 points shown in Fig. 11

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