What happens if the material viscosity changes in injection moulding process?

Generally when we say higher viscosity then one would probably need higher injection pressure resulting in higher clamping force that is required. Regarding Jetting in my experience it is mostly affected by the location of the gate. Generally, plastic should be injected against the metal rather than in open cavity to avoid jetting. The effect of the viscosity on jetting I can only conjecture that it might be reduced very slightly. Regarding process parameters the jetting is normally controlled by reducing the injection speed.

In the injection moulding process, the mould cavity is filled by pressure-driven flow, in fact the material is injected very fast into a mould through small gates. The melt is at high temperature in the filling phase and some viscous heating also occurs due to friction. This increases the melt temperature even more and thus change (lowers) the viscosity. The gate dimension so effect a lot the material behavior.

As viscosity is a function of pressure, the pressure dependence of viscosity may become important in this kind of process. In addition to shear also extensional deformation has a significant effect, due to viscoelasticity.

Furthermore, to optimize the mould filling phase, in which the hot polymer is injected into a mould cavity at lower temperature with an overpressure, it is necessary to determine the relationship between pressure, volume and temperature. Polymers can change in density if pressure is applied especially when they are in the molten state and also have a significant change in density with temperature.

In principle you won't have a reproducibility of the process so, low quality, high wrong part percentage, higher production cost. The moulding process needs to be more stable as possible so the tool, the injection moulding machine, the rheological feature of the resin need to be more stable as possible.