Today's KNOWLEDGE Share : Weld-line weakness

Today's KNOWLEDGE Share

Enhance your understanding of polymer behavior and their impact on end-use performance with this comparison of two polymers exhibiting distinct rheological characteristics.

Due to differences in molecular structure, these polymers diverge in their non-Newtonian behaviors, particularly in terms of viscosity.

The polymer associated with the red curve shows a significantly wider molecular weight distribution, leading to the absence of a Newtonian plateau in typical measurement windows.

This plateau is shifted out of view and could be observed at very low shear rates.

Interestingly, these polymers have identical viscosities at molding rates but display drastically different Melt Index values, which is a low shear-rate single-point viscosity measure.

In injection molding, the weld-line strength is critical.

The polymer represented by the red curve exhibits longer re-entanglement times, resulting in inherently weaker weld-lines.

In contrast, the blue curve's Newtonian plateau signifies fewer components with long relaxation times, enabling rapid inter-diffusion of polymer chains at weld interfaces.

To effectively identify weld-line weaknesses, conduct creep or fatigue tests, as they are more revealing than classical tensile tests.

Research shows that moderate re-entanglement can recover adequate stress at break in standard tensile testing.

source:Vito leo