Meaning
Physical instabilities occur when a less viscous fluid is injected into a more viscous one, causing the interface between them to break into irregular, finger-like patterns. Viscous fingering is a common defect in processes like gas-assisted injection moulding or when injecting a low viscosity resin into a high viscosity melt. This phenomenon leads to an uneven distribution of material and can compromise the structural integrity and appearance of the final part.
Interface Distortion
Boundary layers between the two fluids become unstable due to small variations in pressure or surface tension. During viscous fingering, the thinner fluid finds a path of least resistance and pushes through the thicker one rather than displacing it evenly. This results in a jagged or branching pattern that is difficult to control and can lead to thin spots or voids in the part walls.
Mobility Ratio
Differences in the flow properties of the two materials determine the severity of the instability. A high mobility ratio, where the injected fluid is much thinner than the displaced one, increases the likelihood of viscous fingering. To prevent this, engineers often select materials with more similar viscosities or adjust the injection speed and temperature to bring the flow properties closer together during the critical phase of the process.
Injection Defect
Quality issues on the production floor often stem from this unpredictable movement of the melt. Viscous fingering in a multi-shot moulding process can cause the core material to break through the skin, leading to a visible flaw on the surface of the part. This defect is particularly problematic in transparent or high-finish components where any internal irregularity is immediately visible to the consumer.
Precise control of the injection profile is the most effective way to eliminate these patterns in automotive interiors.
