40207_25 - TOOLING FOR COMPOSITES J erry L. C adden and...

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TOOLING FOR COMPOSITES 25 Jerry L. Cadden and Paul F. Sadesky 25.1 INTRODUCTION The manufacture of composite details and assemblies requires that some kind of accurate repeatable tool surface be provided, indexed to an engineering database or reference model and be capable of withstanding repeated expo- sures to the cure cycle environment of high temperatures and pressures. Once the specific manufacturing process has been selected (i.e. vacuum bag lay-up or resin transfer molding), decisions regarding tolerances, heat up rates, coefficients of thermal expansion, tool longevity etc. influence the construction of the tool from an engineering design and material selection standpoint. Individual composite parts or details will require a variety of sup- port tooling beyond the initial cure tool, such as master model reference patterns, trim or router tools, precision hole location drill tools, assembly fixtures, ply locating templates and other shop aids. Planning must ensure that a point of reference is established that will con- trol all tooling in any one part family. This will guarantee that critical dimensional tolerances are maintained within the relationship between different tools supporting the fabrica- tion of one composite detail or assembly. In addition, coordination between various com- posite details will ensure that interchangeability or replacement is main- tained throughout entire structures. There is an extensive list of materials which can be uti- Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7 lized for tooling, but no one material solves all of the problems, particularly when factors such as cost, longevity and tolerances are con- sidered. The primary objective of any tool for composite fabrication is to make an accurate repeatable part, within the confines of the process parameters defined by the composite material supplier and the detail performance characteristics of the end use customer. Design of the initial tool becomes the most pressing initial issue of tooling for composites. 25.2 TOOL DESIGN BASICS 25.2.1 COEFFICIENT OF THERMAL EXPANSION One of the most critical parameters in the design of tooling for composites is the differ- ence between the coefficient of thermal expansion (CTE) of the tool being designed and of the composite detail being fabricated. During the cure cycle of the composite lay-up on a tool, the lay-up expands during the heat up cycle. The specific rate of expansion is directly related to the type and combination of resin or matrix and fibers or reinforcement used. The tool will also expand and contract at a specific rate determined by the material and construction techniques utilized. If the CTE values for the laminate and the tool dif- fer significantly, stresses may result in the laminate causing the occurrence of dimen- sional, strength and part stability problems.
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40207_25 - TOOLING FOR COMPOSITES J erry L. C adden and...

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