Inorganic fillers and pigments are used in epoxy laminating compositions to control flow properties, increase bulk, reduce costs, and provide visual aid for the laminator. Proper choice and balance of fillers allow thorough wetting of the glass cloth and all filler particles, thus ensuring dimensional stability and
SINCE
their introduction in 1947 by Renaud Plastics, Inc., epoxy resin tools have become the primary standards for automotive tooling programs and are increasingly used for aircraft tooling. These tools are three-dimensional gages -a glass cloth-laminated skin supported by a light-weight rigid backing. T h e contoured surface is obtained from a master model, ordinarily carved by hand from hard wood. A precision tool is made by: spraying the master model with a parting agent to facilitate removal of the epoxy tool, applying a thin epoxy resin surface coating and allowing it to cure to a n initial tack-free gel state, laminating with glass cloth and epoxy laminating materials, using a hand layup technique, attaching the reinforced backing structure, and removing completed tool from the master pattern after room temperature cure. This tool can be used as the model for constructing precise epoxy duplications of the master model. Because most unmodified epoxy resins used in tooling industries exhibit roomtemperature viscosities too high for convenient hand laminating, low-viscosity diluents a r e generally added. Resins modified by unreactive diluents such as hydrocarbons, esters, and ketones show increased shrinkage and decreased dimensional stability. Modification of epoxy resins with low-viscosity epoxy modifiers gives rise to systems that react more completely and wet the glass cloth, fillers, and pigments better than with unmodified epoxy resins. T h e greatest variation in properties of a n epoxy tooling system may be effected by choice of curing agent (hardener). The hardeners used cause epoxy resins to convert to the thermoset state without formation of by-products. As oven or heat curves are inadvisable in making most precision tools, room temperaturecuring systems are preferred. T h e laminating compositions in most general use have a working life of 15 to 35 minutes in a 1-pound mass and
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produce rigid 6- to 8-ply laminates within 4 to 6 hours. This thickness requires rapidly curing compositions hardened by primary and secondary aliphatic polyamines. Catalytic curing agents, such as tertiary amines or other strong bases, generally fail to produce rigid laminates in the time required. Polyamides and liquid polysulfides cannot be used for most precision tooling, because the laminates are not rigid enough. Hardeners that produce laminates with high heat-distortion temperatures have not been applicable to precision tools because of high cure temperatures. Properties of cured epoxy resinhardener system are determined, not only by choice of hardener, but also by the resin-hardener ratio. Physical properties may change greatly as the ratio of resin to hardener is varied. A change of 5 parts of hardener per 100 parts of resin can reduce the heat distortion temperature 20’ to 25 C. from the maximum obtainable. As shown in the curve, insufficient or excess hardener reduces the heat distortion temperature. Precise control of resin-hardener ratio is essential for consistent performance of epoxy tooling compositions,
builder with completely -formulated epoxy tooling compositions. T h e performance of the precision tool is dependent also on reinforcing materials and method of construction. A plain, open-weave, balanced-strength glass cloth like H . G . 63 is more easily handled and more readily “wet out” in hand laminating operations than the thicker roving cloths or glass mats. T h e surface coating is used for best reproduction of minute surface details, and to provide a uniform surface for scribing reference lines. Good appearance, with sharp contrast for visual checking of reference lines, is required by the engineering d e p a r t i e n t for which it is built. A surface coating should contain sufficient pigment to form an opaque film l / 6 4 to inch thick. In such thin layers, the surface coating is generally based on a faster-curing resin-hardener system than for laminating. Thixotropic compositions are preferred for easy application and nonsagging on vertical and curved surfaces. Hard, dense, refractory materials properly incorporated in the surface coating give excellent wear resistance. T h e structural build-up, or backing structure, on the tool i s required to position the contoured surfaces with respect to fixed reference planes. Use of glassreinforced epoxy tubing for structural members results in a tool constructed entirely of reinforced epoxy resin, which exhibits a uniform coefficient of thermal expansion in all its members. A complete tool may be built in one department by the same workers, eliminating the transfer of work from one department to another. A method of obtaining the original master model, now in development, involves use of a n epoxy-based material which can be readily worked into the desired contours to form a rigid, dimensionally stable plastic master model. T h e authors acknowledge assistance in the preparation of this article by Harold E. Renaud, 6. D. Grant, and B. A. Graham, R e n Plastics, Inc.
J. W. GUYER H. L. THOMAS Ren Plastics, he.,
Lansing, Mieh.
”“1 I
INDUSTRIAL AND ENGINEERING CHEMISTRY
15
Division of Paint, Plastics, and Printing Ink 20
25
PARTS HAROENER/100
30 PARTS
35
RP-1100 RESIN
Chemistry, Symposium on Epoxy Resins, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
