Thermoset Materials

Thermoset plastics, primarily Epoxy, Phenolic and DAP, have been molded by Rebling for over 50 years. Rebling uses both the transfer and plastic injection molding
processes for thermoset moldings.

During the molding cycle, thermoset plastics undergo an irreversible chemical reaction or cure brought on by heat, pressure and time. Once cured, they cannot be reshaped by melting and reprocessing as is the case for thermoplastics. Cured thermosets possess a highly cross-linked chemical structure enabling them to retain their mechanical and electrical properties as well as dimensional control over a wide temperature range. All thermosets molded by Rebling undergo a "deflashing" process after molding. This deflashing operation eliminates the undesirable excess material that by design is evident on parting lines and vent passages.

Epoxy molding compounds are characterized by their long flow length and the ability to flow through thin cross-sections, filling minute details. This flow characteristic is particularly beneficial for encapsulation as there is less pressure required on the material during the molding cycle and therefore less pressure on the internal components of the device to be encapsulated. The outstanding dielectric properties of epoxy combined with the low pressure molding requirements are the reasons epoxies are frequently specified for encapsulation of electrical devices, semi-conductor devices, transformers and other electrical/electronic apparatus. Epoxies have a recommended shelf life as received from the material supplier. To insure maximum shelf life, Rebling stores epoxy molding compounds in refrigerated storage units. Epoxies are available formulated with various fillers including glass and mineral at various concentrations.

Phenolic, the oldest polymer, is known for its excellent high temperature resistance. When properly post cured, some heat resistant phenolics can withstand up to a 550°F operating environment. Phenolic molded parts exhibit a glossy finish and a high surface hardness. As with other plastic materials, phenolics can be compounded with various fillers to alter the materials properties. These include wood flour, minerals and glass fiber reinforcement. Phenolics are used in applications such as rocket motors, electrical motor brush holders, handles for cook wear and wherever plastics need to operate at elevated temperatures.

Diallyl Phthalate, commonly referred to as DAP, exhibits outstanding electrical insulating properties particularly at high humidity and high temperatures environments. While not as heat resistant as phenolic, DAP can operate continuously at temperatures as high as 450°F. DAP is typically available in flame retardant grades including ratings of UL 94V-O in cross sections as small as .062 inches. End uses include coil bobbins, connectors, and circuit breakers.

Rebling has a significant amount of insert molding experience employing epoxy, phenolic and DAP thermoset resins. Rebling can provide a turnkey service including design and procurement of mold(s), inserts and molding compounds, as well as post curing, deflashing and secondary operations. Rebling also has preforming equipment to compress the molding compounds into solid shapes suitable for dielectric preheating. Mold temperatures for thermosets typically are in excess of 300°F, so inserts must be able to withstand this molding temperature and the increase in temperature caused by the exothermic reaction that takes place during the cure cycle. Electrical devices incorporating solder connections and tin plated metal components are examples of inserts that could be damaged during the molding cycle if temperatures are too high.

Rebling engineers are available for consultation and recommendations on thermoset materials, processes and critical molding requirements.