Melt Viscosity Drift

Viscosity Variation

Monitoring the pressure required to fill the mold at a constant speed reveals when the viscosity of the resin is changing. A decrease in viscosity allows the material to flow more easily, which can lead to flashing or over-packing of the cavities. Conversely, an increase in viscosity makes the material harder to push, which often results in short shots or visible sink marks on the surface of the part.

Material Influence

Feedstock consistency plays a major role in preventing drift during a long production cycle. Virgin resins with a narrow molecular weight distribution are generally more stable than recycled materials or blends with high levels of regrind. Additives like lubricants or flame retardants can also influence the stability of the melt if they are not evenly dispersed throughout the batch.

Process Window

Control systems on modern moulding machines can compensate for minor drifts by automatically adjusting the injection pressure or the switch-over point. However, these adjustments only work within a defined window, and extreme drift usually requires the operator to stop the machine and investigate the root cause. The boundary of successful compensation is reached when the change in viscosity starts to affect the physical properties or the appearance of the finished product.