Ethylene Propylene Rubber Domain Morphology Tuning for Low Temperature Container Impact Survival
Tuning EPR domains to 0.4-0.8 microns with 45 percent ethylene content ensures sub-zero container shock survival at minus 30 degrees Celsius.
Melt Flow Rate Ratio
Meaning
The relationship between the flow rates of a polymer melt measured under two different test loads provides an estimate of the material’s shear-thinning behavior and molecular weight distribution. This melt flow rate ratio is calculated by dividing the flow rate under a high load by the flow rate under a low load. Polymer manufacturers and processors use this value to understand how a resin will flow under high-shear conditions like injection moulding compared to low-shear conditions like extrusion.
It assists in selecting the right resin for a specific process.
Molecular Structure
Resins with a wide range of chain lengths show more shear-thinning and a higher melt flow rate ratio than resins with a narrow range of chain lengths. This difference arises because long chains can disentangle more easily under high shear stress which reduces the viscosity of the melt. In contrast polymers with a narrow molecular weight distribution do not shear-thin as much.
Knowing this value helps moulders understand the molecular makeup of the polymer and how it will behave under different speeds. This molecular structure is determined during the polymerization process by the type of catalyst used.
Processing Behavior
The flow behavior of the melt during the filling phase of the moulding cycle is directly related to how the polymer responds to shear stress. A high melt flow rate ratio indicates that the resin will flow more easily through thin walls and complex gates as the injection speed increases. This ease of flow can reduce the pressure needed to fill the mould and help to prevent short shots in thin-walled parts.
However it can also increase the risk of flashing if the clamp force is not high enough. Moulders must balance injection speed with hold pressure to prevent these issues.
Material Quality
Fluctuations in this flow relationship can point to problems with resin consistency or degradation due to excessive reprocessing. When regrind is added to the virgin material the thermal history can cause chain scission which alters the melt flow rate ratio of the mixture. This change can make the process unstable and lead to variations in part weight and dimensions.
Moulders can use this measurement to check incoming batches and ensure that the material has not been degraded during storage or handling. It is also a valuable tool for troubleshooting moulding problems like inconsistent part dimensions. Maintaining a stable value is critical for ensuring a repeatable and reliable production process.
When the ratio is too low it can indicate that the material has high molecular weight and will be difficult to process which can lead to excessive wear on the injection screw and barrel. For these reasons keeping track of this value is a standard part of any quality control program.