Rheology of Fluoropolymer Melts

Rheology of the molten fluoropolymers is of critical importance in processing these polymers. Fluoropolymers, and generally thermoplastic materials, must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin (in flow) exceeds the critical 

There are a few possible ways to eliminate melt fracture, such as reducing the velocity of melt, increasing the critical velocity, and reducing heat losses (see Table 6.1). These remedies are limited in the way that they affect the design of the mold, fabricated part properties, and degradation of the polymer. 

Knowledge of flow of the melt at different temperatures and shear rates is required in the fabrication processing of the resins. Melt viscosity of these polymers is a function of shear rate. For example, pol5mier viscosity changes as extrusion rate is varied. In addition to melt viscosity, melt flow rate (MFR), also known as melt flow index (MFI), is a customary flow characteristic of resins. MFR is defined as the mass of molten polymer (in grams) that flows through the die/orifice of a rheometer in ten minutes and is inversely proportional to the melt 

Reduce the velocity of melt Enlarge runners, gates, or cavities. Slow ram speed. Polymer degradation. Premature melt freeze. 

Increase the critical velocity Increase melt and/or mold temperature. Polymer degradation. Ejectabihty of the part. 

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