Properties of Molded Phenolic
Phenolic was the first plastic material molded by Rebling. Since 1961, Rebling has continued to mold this heat resistant thermoset plastic using both the transfer and injection molding processes.
The ability of phenolic to maintain its mechanical and electrical properties at high operating temperatures is the principal reason for its use today. When properly molded and post cured, some phenolics can withstand a 550°F continuous operating environment. Rebling utilizes several industrial ovens for post curing operations.
Prices for phenolic compounds start at $1.00 per pound and are attractively priced compared to expensive high temperature thermoplastics. This temperature resistant material is used in applications such as handles for cookware, car ashtrays, rocket motor housings, electrical motors components such as brush holders, and wherever resistance to high temperatures is required.
Phenolic molding compounds are available in single and two stage compounds. Single stage compounds are produced without ammonia and are ideal for insert molding metal components where metal corrosion is a concern. Phenolic molding compounds are available in numerous formulations where both the filler and the percentage of filler are varied to alter the properties of the molded part. Mineral and glass fillers are used to improve heat resistance; cotton flock fillers are used to improve impact strength; and glass fillers are used to improve mechanical strength and stiffness. Specialty fillers such as carbon fiber and lubricants such as PTFE are also available.
Phenolic molding compounds are available in limited colors. Most are available in black and brown, some also in red and green.
Many grades of phenolic have UL 94V 0 approval and a UL Relative Thermal Index typically 150°C, some compounds to 180°C.
While the cost of these compounds is attractive, processing times are significantly higher than thermoplastics. Cycle times can range from 30 seconds to several minutes depending on part geometry and material selection. Unlike thermoplastics which transform from a viscous liquid and solidify by cooling down in the mold, phenolic materials undergo an exothermic chemical reaction or cure. After molding, the parts must go through a deflashing operation also adding to the final part cost. Phenolic materials cannot be reground and reprocessed.
Phenolics have excellent chemical resistance to organic and halogenated solvents such as carbon tetrachloride. They are attacked by inorganic bases.
Design of Molded Phenolic Parts
Draft and tolerances should be specified on part drawings indicating maximum and minimum sizes for sections at each end of the taper. The drawing should also indicate whether or not tapers are included in the tolerances and note which end of a taper has a critically fixed size. Gate locations should be identified as well as any areas that are to be free of ejector pin marks.
Maintaining a uniform cross sectional thicknesses to avoid sink marks by coring is not as critical for thermoset phenolics, since they do not readily show sink marks. However, parts with heavy cross sections require longer cycle times and are subject to increased warpage.
Rebling design engineers are prepared to provide design assistance to insure the successful outcome of your project.
Phenolic parts can be bonded or mechanically assembled. Metal inserts can be molded in or pressed in after assembly. Decorating is typically limited to silk screening.