Meaning
Rheological behavior describes the phenomenon where the viscosity of a fluid decreases as the rate of shear strain applied to it increases. Injection moulders rely on non-newtonian shear thinning to fill thin-walled cavities quickly and with lower injection pressures than would otherwise be required. This characteristic is typical of molten polymers because their long molecular chains align along the flow direction when forced through narrow gates.
Melt Rheology
Polymeric melts consist of randomly coiled, entangled molecules that resist movement when subjected to low flow velocities. Under high flow rates, the molecules disentangle and slide past one another with less resistance, resulting in a drop in apparent viscosity. The degree of non-newtonian shear thinning is a property that varies among resin types and grades, with materials like polycarbonate exhibiting less thinning than polystyrene under the same shear conditions.
Process Consequence
Understanding the flow behavior under shear enables moulders to optimize fill times and control gate wear. If the shear rate remains too low because of slow injection speeds, the melt viscosity will remain high, resulting in incomplete filling of the mould. Conversely, excessive shear can generate localized frictional heating, causing thermal degradation and discoloration of the polymer.
Tooling Constraint
Gate design and runner layout must be engineered to exploit this behavior. Incorrect gate placement can result in uneven viscosity distributions across the cavity, leading to internal stresses in the moulded part.
