Meaning
The change in the flowability of a polymer melt between its initial state and after a processing step provides a measure of thermal or mechanical degradation. This melt flow rate shift is calculated by comparing the flow index of the virgin resin with that of the moulded part or the recycled regrind. An increase in the flow rate usually indicates that the polymer chains have been broken into shorter segments during the high heat and shear of the process.
Conversely, a decrease may suggest that the chains have crosslinked into a larger network. The measurement is used to monitor the stability of the production process and the quality of the material. It applies to all thermoplastics that can be repeatedly melted and solidified.
Thermal Degradation
Exposure to excessive heat in the barrel of an injection moulding machine can cause the molecular structure of a plastic to break down. The melt flow rate shift quantifies this damage by showing how much easier the material flows after being processed. If the residence time is too long or the temperature settings are too high, the polymer undergoes scission and loses its mechanical strength.
This degradation often leads to parts that are brittle or have poor surface finishes. Technicians use this data to optimize the machine settings and ensure that the material is not being overcooked. Monitoring the shift helps in identifying the safe limits for processing a specific resin grade.
Protecting the polymer from thermal damage is a requirement for maintaining long term part performance.
Molecular Change
The physical properties of a plastic part are a direct consequence of the length and arrangement of its molecular chains. A significant melt flow rate shift indicates that the underlying chemistry of the material has been altered. When the chains become shorter, the material’s ability to resist impact and environmental stress cracking is reduced.
This change is not always visible to the eye but can lead to catastrophic failure when the part is in use. By tracking the flow rate before and after moulding, manufacturers can verify that the material remains within its design specifications. This analysis is especially important when using regrind, as every heat cycle adds more damage to the molecular structure.
Reliable molecular data supports the production of high quality and durable plastic goods.
Scrap Management
Reusing leftover material from the moulding process requires a careful balance between economy and quality. The melt flow rate shift provides a way to determine how much regrind can be safely mixed with virgin resin without compromising the part. If the shift is too large, the percentage of recycled material must be reduced to maintain the required mechanical properties.
This data helps in setting the internal quality standards for the handling and reuse of production scrap. It also allows for the identification of batches that have been contaminated or severely degraded. The cost of regular testing is offset by the ability to maximize the use of recycled material while avoiding part recalls.
Effective scrap management ensures that the manufacturing process is both sustainable and profitable.
