The Common Plastic Resins and What Each One Is For
Polymer grade selection fixes mechanical properties, tool shrinkage, cycle time, compliance limits, and final landed part cost across production runs.
Pseudoplastic Behavior
Meaning
This rheological phenomenon describes a fluid whose viscosity decreases as the rate of mechanical shear increases during flow. In the plastics industry, pseudoplastic behavior is a fundamental property of molten polymers that allows them to be pumped easily through small gates and thin sections at high pressures. As the screw moves faster or the orifice gets smaller, the polymer chains align parallel to the flow direction, which reduces the internal friction between the molecules.
This transition is essential for injection moulding because it allows thick, viscous resins to fill large cavities rapidly without requiring impossible amounts of torque. The measurement focuses solely on the relationship between shear rate and the resulting resistance to movement.
Molecular Alignment
In a stationary state, long polymer chains are typically tangled together in a random ball that creates high resistance when the material first starts to move. Once pseudoplastic behavior kicks in, these tangles begin to straighten out in response to the velocity gradients within the barrel. This realignment looks like dozens of tiny pieces of string lining up in the same direction, which creates clear pathways for other chains to slide past.
The more speed is applied, the more efficient this organization becomes, resulting in a dramatic drop in observed viscosity. This characteristic is different from simpler fluids like water where the viscosity remains constant regardless of how fast it is pushed. It allows for the production of parts with intricate details.
Moulding Advantage
Processors rely on this property to achieve low viscosity inside the cavity while maintaining enough thickness at the gate to prevent drooling once the pressure drops. Because of pseudoplastic behavior, an injection moulder can use lower melt temperatures and still successfully fill thin walled containers at high speeds. This relationship effectively broadens the processing window by making the flow manageable even when the resin would normally be too stiff to handle.
If the shear rate is doubled, the pressure required to move the melt increases by a much smaller amount. This efficiency minimizes the wear on the internal steel components of the machine by avoiding extreme pressure spikes. Consistency in this flow behavior ensures that each shot reaches the end of the mould at the same time.
Pressure Limitation
While this thinning helps with fill speed, it also means that calculations for tool design must use dynamic data rather than stationary measurements. Engineers look at graphs depicting pseudoplastic behavior to select the right gate sizes for each resin grade, as too small a gate will cause excessive shear heat. This heat can lead to localized degradation if the alignment of the chains is forced too aggressively.
Values on a static datasheet only give part of the story, whereas a capillary rheometer provides the full picture of flow under pressure. The shear thinning effect ends when the chain length is so short that there is no entanglement to remove, which often occurs with low molecular weight additives. Reliable moulding depends on this unique ability of long chains to slip more easily when challenged.