Friday, 6 November 2020

Rule of Thumb for Plastics Injection Moulding: 20% Material Viscosity Variation over Time


In this blog post, I present to you another helpful rule of thumb for plastics injection moulding.
Viscosity is one of the important parameters for running consistent injection moulding operation. In general, polymer viscosity is a function of temperature, pressure, time, and shear rate. Polymer melts have a non-Newtonian flow behavior (shear thinning).
Moulders specify the viscosity range of the polymer compounds received by the material supplier. Most often the so called Melt Flow Index (MFI) is used. Based on the MFI results, upper and lower viscosity borders are defined. In general, 20% viscosity variation can be expected by the material itself [1]. This variation is already there, although the moulder did not yet start processing or damage the material during processing.
Cavity pressure sensors in injection moulds are a crucial part for monitoring viscosity changes over time. It could be shown that viscosity correlates with cavity pressure. If the viscosity increases, cavity pressure decreases. This in turn may produce short shots, smaller parts or even sink marks [1].
Dimensional part variations, flash, warp, sink marks, and short shots are all linked to viscosity changes. If such moulding problems occur and you have a viscosity tracking in place, then it is easy to pin-point viscosity changes and you can take immediately counter measures.
Thank you and #findoutaboutplastics
Herwig Juster

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