Decreasing Energy Consumption during Injection…

I was at a company recently and the engineer was confused about how to reduce energy consumption during injection…

I know higher injection velocities reduce variability, but a lower injection speed uses less pressure and thus reduces the overall energy consumption… right?

My Response
Actually, a higher velocity reduces overall energy consumption during injection even though the injection pressure is increased. Increasing the injection velocity decreases the material viscosity through shear thinning. As a result, the overall energy required to push the final volume of polymer necessary to fill the mold is decreased.
In an actual example performed by one of our trainers, the data from a molding process which was filled at two different times is shown below:
Slow Fill:
Fill time = 3.15sec
Transfer Pressure = 10,460psi
Fast Fill:
Fill time = 0.83sec
Transfer Pressure = 11,910psi
Although the faster fill time has a higher pressure at transfer, the molding machine applied the injection pressure for 27% of the time necessary to fill the mold at the slower speed. When you multiply the fill time by the transfer pressure you get the following effective (or relative) viscosity:
Effective Viscosity = t x psi
Slow Fill = 32,950 psi-s
Fast Fill = 9,890 psi-s
Assuming the pressure profiles are similar in shape, it can be reasonably proposed that the faster fill consumed approximately 30% of the energy necessary to fill the mold at the slower speed.
Additional Thoughts
The additional benefit to filling faster is that the polymer fills the mold and begins to cool faster. In this case the overall cycle time could drop as much as 2 seconds due to the higher injection viscosity.

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