Meaning
Unintended change in the flow behavior of a molten polymer during processing due to material degradation or contamination. Moulders monitor this property because it directly affects the fill times of the injection moulding process. This drift indicates changes in the polymer’s molecular weight.
Viscosity Change
The phenomenon occurs when polymer chains undergo either scission or crosslinking under high temperature and shear. Chain scission increases the melt flow rate, while crosslinking decreases it, making the resin more viscous and difficult to process. Sourcing teams must ensure the raw material contains sufficient thermal stabilizers to prevent melt flow rate drift during processing.
Process Variation
This shift in viscosity causes inconsistencies in the moulding process, such as changes in the cushion size and injection pressure. When the viscosity decreases too much, the polymer can flash out of the mould, while a viscosity increase can lead to short shots and incomplete parts. Moulders must run melt flow tests on incoming batches to ensure consistency.
Defect Genesis
Inconsistent flow rates lead to structural and aesthetic defects in the finished components. Parts moulded from degraded material often exhibit lower impact strength and poor surface finish. This variation increases scrap rates and production costs, reducing the profitability of the moulding run.
Implementing continuous process monitoring helps operators detect this flow drift before defects occur in the product.