The ability of the material to flow is measured by filling a mold with the plastics material under a specified condition of applied temperature and pressure with a controlled charge mass. The flow tests are used as a quality control test and as an acceptance criterion for incoming raw materials.
Factors Affecting Flow
Resin Types. All resins flow differently because of basic differences in the structure of the polymers. For example, melamine formaldehyde exhibits longer flow than urea formaldehyde. Phenolics, because of the variety of resin types, enable the molder to select the flow best suited for a particular design.
Type of Fillers. The small particle size of wood fl our, mica, and minerals creates less turbulence and less frictional drag during mold filling. The size of the glass fibers, short or long, can adversely affect the flow.
Degree of Resin Advancement. The degree of advancement is generally controlled by the resin manufacturers. Molders can advance resin polymerization with oven or radiant heat or electronic preheating.Storage Time. All resins have a natural tendency to polymerize in storage, causing partial precure which reduces flow. An exception might be polyester in which catalyst decomposition slows or prevents curing, which increases flow duration.
Spiral Flow of Low-Pressure Thermosetting Compounds (ASTM D 3123)
The spiral flow of a thermosetting molding compound is a measure of the combined characteristics of fusion under pressure, melt viscosity, and gelation rate under specific conditions. The test requires a transfer molding press, a standard spiral flow mold, and a thermosetting molding compound. The molding temperature, transfer pressure, charge mass, press cure time, and transfer plunger speed are preselected as specified. The preconditioned compound is forced through a sprue into a spiral flow mold. Once the curing is complete, the part is removed and the spiral flow length is read directly from the molded specimen. Compounds are classified as low (1–10), medium (11–22), and high (23–40) plasticity.
Cup Flow Test (ASTM D 731)
Molding Index of Thermosetting Molding Powder. This test is primarily useful for determining the minimum pressure required to mold a standard cup and the time required to close the mold fully. The preconditioned and preweighed material is loaded into the mold. The mold is closed using sufficient pressure to form a required cup. The pressure is reduced step by step until the mold cannot close. The next higher pressure and time to close the mold is reported as the molding index of the material.