Tooling Applications of Epoxy Resins Steel, kirksite, and aluminum tools being replaced with tools made of epoxies c
I N C R E A S I N G quantities of plastics are being applied in tooling. Phenolics have been used. for models and dies; polyester laminates for checking tools, drill fixtures, and trim fixtures. Introduction of epoxy resins cured at room temperature has given added impetus to the change-over from metal to plastic tooling. The greatest advantages of plastic tooling are found where massive tooling and frequent tooling changes are common-as in the aircraft and automotive industries. Many tools formerly made of kirksite, steel, and aluminum are currently made of epoxies. Because of difficulties inherent in the use of phenolics and polyesters, progress was relatively slow until the epoxy resins appeared. These improved strength, shrinkage control, handling, and general versatility, and are replacing not only phenolics and polyesters, but conventional metals in many types of tool.
Tooling Production Two basic operations are used in the manufacture of epoxy tools: laminating and casting. I n laminating, a layer of cloth (generally glass) is wetted by brushing with a fluid epoxy resin formulation. Alternate layers of cloth and resin are used to produce the desired thickness. The laminate is cured a t room temperature for 4 to 24 hours, depending on resinhardener combination and thickness of laminate; then further operations are performed. I n casting, t h e prepared resin formulation is poured into a mol& and allowed to cure.
In general, a plastic tool is molded in five steps.
1. Parting agent is applied to mold surfaces. 2. ' A restriction box of metal sheet or plate, fiberboard, or plywood, sealed and coated with parting agent, is built up around the mold. 3. The epoxy resin is formulated to meet specific requirements. 4. For most tooling work, cures are at room temperature. For hot forming tools (matched dies, etc.) oven cures or postcures (depending on hardener system) are necessary. 5 . For some large tools low-cost cores of wood, plaster, metal, or air foams are economical. Prime necessities for successful production and utilization are: handling and application ,techniques ; knowledge of epoxy resin formulation; understanding of physical limitations of epoxy resins; and proper tool design. Production techniques are readily learned. Formulation is based on chemical knowledge of epoxy reactions and on experimentation. Often metal tool designs must be completely revised to accommodate epoxy tools.
and built up to ten '/d-inch thick layers. A steel I-beam reinforcing structure provides stiffness. The mold is filled with dry gravel. Epoxy casting resin is poured over the top of the gravel and allowed to seep in. When the gravel is thoroughly saturated with resin, a ' / p inch leveling layer is poured to provide a flat surface, and the assembly is cured overnight. The finished stretch form is removed from the mold and the surface is cleaned. This tool required approximately 150 man-hours to build; in kirksite it would have required twice as long. The casting technique was developed to produce a material of high strength with a minimum of resin. The major problem in large mass casting had been the cure exotherm. By the technique described, maximum temperature during cure was below 200' F. and cure was ashieved in 12 hours a t room temperatare. The same procedures, with slight variations, were used to produce a n experimental drop hammer die.
The utilization of formulation and production techniques for making high strength mass castings may be illustrated by the manufacture of a 41-cu. foot production stretch form.
The plaster mold was prepared with parting agent and a n epoxy surface coat was applied. Two layers of dry glass cloth were placed and the mold was filled with dry gravel. The casting resin was poured, allowed to seep in, and leveled. The assembly was cured overnight and the female die removed from the mold. Vertical surfaces were waxed to part thickness, parting agent was applied, and a male was cast in a similar manner.
The plaster mold is coated with paste wax and polished. Epoxy surface coat is applied and allowed to gel, Glass cloth and .epoxy laminating resin are applied
After making more than 100 parts, the die was in as good condition as a t the start of the test.
Techniques
VOL. 49, NO. 7
JULY 1957
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type of cure desired, cure time, mass, application, etc. 3. Flexibilizers give impact strength, resilience, hardness, etc.
4. Fillers, properly chosen, increase strength, and reduce shrinkage, cure exotherm, resin costs, and thermal expansion. 5 . Diluents. Commercially available unmodified epoxy resins have viscosities of 5000 cp. or more. To attain fluidity or use more filler, reactive diluents are added.
Disassembled mold shows foamed plastic part
The minor variations in formulation and procedure for making the stretch form and drop hammer die are indicative of the need for understanding the capabilities and limitations of available materials. Drop hammer die materials require considerably higher impact strength than those used for stretch forms. Stretch forms require a relatively hard surface; drop hammer dies, a relatively flexible surface. Drop hammer dies have more complex contours than stretch forms. Another type of epoxy tool currently in use is a 27-pound mold for manufacturing isocyanate foam parts.
Formulation
All RAC epoxy resin formulations contain two or more of five classes of ingredients.
1. Epoxy resin. For tooling work, liquid resins are desirable. 2. Curing agent should allow for full cure at room temperature. For most applications liquid agents of the amine type are used, depending on
The specific effect of materials used in formulating tooling resins can be learned only by testing and using them in various combinations. Proper formulation can yield tooling resins varying from hard abrasive, wear-resistant, nonmachinable, to soft, flexible, impact-resistant materials. Impact strength of unreinforced epoxy resins can be varied from 0.5 to over 30 foot-pounds per inch. Compressive yield strength will vary from practically 0 to 20,000 pounds per square inch. Epoxy resins do not supply a panacea for all tooling ills, but in the hands of a cooperative team of formulators and tooling men, they simplify and reduce costs of many tooling operations. LAWRENCE R. SPARROW Republic Aviation Corp., Farmingdale, N. Y.
Division of Paint, Plastics, and Printing Ink Chemistry, Symposium on Epoxy Resins, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
A foamed part of triangular cross section, approximately 12 inches long, containing circular indentations on its two major faces, was required. A wood model was made and coated with parting agent. Arform”,wasbuilt around the part, with one face and one end free for reproduction. A layer of surface coat was brushed on and allowed to gel. An epoxy-glass laminate was built up and allowed to gel. An epoxy casting resin was placed and allowed to cure. Two other mold sections were made in a similar manner, using embedded steel dowel pins to locate the completed mold sections. The self-cure of the foamed plastic part is exothermic. As the cast plastic mold did not allow for rapid dissipation of the heat produced, a 2-hour mold cycle was required. A tool using the same type of laminate shell, but with a paper honeycomb core, yielded a mold weighing 8 pounds, with the desired heat-transfer characteristics, and reduced the in-mold production cycle to 30 to 45 minutes.
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Die of laminated epoxy resin shows no wear after more than 100 parts have been made
INDUSTRIAL AND ENGINEERING CHEMISTRY
