Meaning
Material sciences focused on the flow and deformation of polymer melts under high temperatures and shear rates determine the behavior of resins during processing. In injection moulding and extrusion, melt rheology dictates how easily a polymer fills a mold and how the polymer chains orient in response to flow forces. This characterization is essential for design and process optimization, but does not describe the solid-state behavior of the finished parts.
Viscoelastic Flow
Polymer melts exhibit non-Newtonian behavior, meaning their viscosity decreases as the shear rate increases during processing. This shear-thinning behavior allows resins to fill thin-walled sections of a mold at high injection speeds that would be impossible at low flow rates. Measuring this behavior using capillary or rotational rheometers provides data for injection molding simulation software.
This data helps toolmakers size gates and runners to balance flow and prevent material degradation from excessive shear.
Processing Impact
Variations in melt rheology, often caused by batch-to-batch differences in molecular weight and viscosity or regrind content, can lead to process instability. If the viscosity is too high, the machine may experience short shots, where the plastic fails to fill the mold cavity completely. Conversely, low viscosity can lead to flash, where excess plastic leaks into the mold parting lines.
Material Characterization
Online rheometers can monitor the viscosity of the melt directly during production, providing real-time feedback on polymer quality. This monitoring is particularly valuable when processing recycled resins, where polymer degradation can cause sudden shifts in flow behavior. Adjusting the process temperatures in response to rheological changes helps maintain part consistency across the production run.
