Fill Time, Viscosity & Molding Economics

A friend emailed me this great question the other day regarding the relationship between fill speed and pressure…
The pressure required to fill a particular mold went up from 950 psi at 1 in/s to 1050 psi at 3 in/s. 

I understand the higher pressure needed to hit the higher speeds, but shouldn’t this be offset by the significantly reduced viscosity?

Does it hold true that anytime you increase the fill velocity, you see an increase in fill pressure… all things being equal?
My Response
The pressure required to fill will increase because the viscosity change will not completely outweigh the pressure losses… The overall energy consumption does drop considerably during fill since the pressure to fill is being appllied for only one third the amount of time!
It is very likely that a rheology curve would demonstrate that the 1 in/s fill is on the left hand side of the shear thinning transition region and the 3 in/s is on the right hand side of this region.
Think of it in vehicular terms… Your 1 in/s is like pushing a large pickup (higher resistance to movement) at 20 miles per hour, while your 3 in/s is like pushing a compact car (lower ristance to movement) at 60 miles per hour. It takes more gas per minute to move the smaller car at the higher rate, but it takes less time and you consume less gas overall getting to your final destination.
Additional Thoughts
Always keep in mind…the reduced viscosity due to shear thinning will actually save you money… making your processes much more economical since the overall energy consumption to fill the mold as well as the time are reduced.
Additionally, the drop in viscosity will also reduce the pressure required to pack out the mold cavity during 2nd stage. In the long run, all these changes can make a big difference in the productivity and efficiency of your facility.

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