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Prepreg maximizes cargo-plane tooling accuracy.

Prepreg maximizes cargo-plane tooling accuracy

To maximize the dimensional accuracy of composite parts produced for the C-17 cargo transport, Auto-Air Composites, Lansing, MI, used Pre-Cat [TM] room-temperature-setting prepreg to build the 19 tools required for the plane's wingtip and dorsal fairing sections. The McDonnell Douglas contract calls for production of 210 shipsets from the tooling.

Ciba-Geigy Tooling Systems, Hawthorne, NY, introduced Pre-Cat epoxy-graphite prepreg in late 1988. Designed for thermal cycling up to 350 F, the prepreg can be laid up on a room-temperature tooling aid. It cures at room temperature to a self-supporting stage in four days, with no shrinkage or loss of tolerance.

Materials selection

The McDonnell Douglas C-17 cargo transport incorporates composite parts including epoxy-graphite wingtips and epoxy-aramid dorsal fairing for weight savings. Other benefits include abrasion and corrosion resistance.

Construction of composite parts that hold precise tolerances even in the complex geometries of wingtips and dorsal fairing necessitates careful choice of tooling materials. A primary consideration was that the tooling material have a coefficient of thermal expansion (CTE) matching the final parts--to minimize the risk of tool or part warpage and maximize part accuracy. On the C-17 project, wingtip parts must hold the loft of the pattern surface to within 10 mils.

John Scanlon, Auto-Air's director of engineering, explains the choice of Pre-Cat: "The Ciba-Geigy prepreg is formulated without volatile organic compounds (VOCs) and, as a result, the tools exhibit a better surface finish and fewer voids than other systems. The tools also are more stable and resistant to warpage during autoclave cycling."

Building the tools

For some of the C-17 tools, McDonnell Douglas supplied Auto-Air with plaster splashes and masters. From each master, Auto-Air constructed an epoxy-and-fiber-glass room-temperature tooling aid to obtain the needed tool contour. The tooling aid was released with three layers of Ren [R] RP 803 wax, and buffed to a hard finish.

To begin the tool, workers put down one layer of Ciba-Geigy CGL 1330-SPG graphite-reinforced surface ply. The surface ply was cut to fit rough tool contours and laid down with the veil side of the fabric against the pattern. For this first layer, all joints were butted. As tool-makers placed the material on the pattern, they smoothed the surface to remove gross air and then peeled off all poly-interleaf to prevent delamination and blistering.

The surface ply was followed by one layer of CGL 1330-3KG epoxy per 6-oz graphite prepreg laid down at a 45-deg orientation with joints overlapping by 1/4". Splices were staggered to prevent pieces of surface ply from separating during initial debulking.

Preparations for compaction of the first three layers of the tool began with one layer of peel ply placed directly on the laminate, followed by a layer of perforated Teflon and polyester breather. A vacuum bag was then put over the tool. A vacuum of 25" of mercury was pulled and held until 1/4"-dia circles of resin appeared throughout the breather. The bag, breather, and perforated Teflon were removed and new nonperforated Teflon and breather materials were put down. The tool was rebagged and held under vacuum overnight.

The next morning, the tooling procedure continued with four additional plies of CGL 1330-6KG laid down according to a Ciba-Geigy-recommended cloth-orientation schedule designed to ensure a balanced lay-up. Toolmakers compacted the laminate after layers 7 and 11, following the same vacuum-bag procedure used for the first compaction.

The final compaction followed laminate layer 16. After Auto-Air installed the bagging materials and vacuum bag on the tool, workers placed the tool on the pattern in the autoclave at 50 psi and 120 F for 12 hours to accelerate the cure. At that point, the tool was self-supporting; the pattern and bag were removed and the tool returned to the autoclave for postcure.

PHOTO : An Auto-Air toolmaker cuts Pre-Cat prepreg to prevent bridging.

PHOTO : An epoxy-graphite dorsal fairing for the C-17 is removed from the Pre-Cat prepreg tool.
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Jul 1, 1991
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