Machine Design


Alkyd molding compounds are based on unsaturated polyester-type resins, which are combined with crosslinking monomers, catalysts, reinforcements, lubricants, and fillers. The formulations are similar to those of thermosetting polyesters but with lower amounts of monomers.

Alkyds are part of the group of materials that includes bulk-molding compounds (BMCs) and sheet-molding compounds (SMCs). They are processed by compression, transfer, or injection molding. Fast molding cycles at low pressure make alkyds easier to mold than many other thermosets.

Alkyds are furnished in granular compounds, extruded ropes or logs, bulk-molding compound, flake, and puttylike sheets. Except for the putty grades, which may be used for encapsulation, these compounds contain fibrous reinforcement. Generally, the fiber reinforcement in rope and logs is longer than that in granular compounds and shorter than that in flake compounds. Thus, strength of these materials is between those of granular and flake compounds. Because the fillers are opaque and the resins are amber, translucent colors are not possible. Opaque, light shades can be produced in most colors, however.

Properties: Low-moisture absorption and excellent dimensional stability and electrical properties are the outstanding characteristics of most alkyd molding compounds. For electrical-grade materials, absorption can be as low as 0.5%. Alkyds are relatively low-loss materials, especially the mineral-filled and glass-filled grades. Those containing cellulose may have higher loss characteristics and "drift" with temperature and humidity changes.

Molded alkyd parts resist weak acids, organic solvents, and hydrocarbons such as alcohol and fatty acids; they are attacked by alkalies. Glass and asbestos-filled compounds have better heat resistance than the cellulose-modified types. Depending on type, alkyds can be used continuously to 350 °F and, for short periods, to 450 °F. Alkyd molding compounds retain their dimensional stability and electrical and mechanical properties over a wide temperature range.

Halogen and/or phosphorus-bearing alkyd molding compounds with antimony trioxide added provide improved flame resistance. Other flame-resistant compounds are available that do not contain halogenated resins. Many grades are UL-rated at 94V-0 in sections under 1/16 in. Flammability ratings depend on specific formulations, however, and can vary from 94HB to V-0. Flammability ratings also vary with section thickness.

Applications: High-impact grades of alkyd compounds (with high-glass content) are used in military switchgear, electrical terminal strips, and relay and transformer housings and bases. Mineral-filled grades, which can be modified with cellulose to reduce specific gravity and cost, are used in automotive ignition parts, radio and TV components, switchgear, and small appliance housings. Alkyds with all-mineral fillers have high moisture resistance and are particularly suited for electronic components. Grades are available that can withstand the temperatures of vapor-phase soldering.

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