Cooling Time calculation theory and formulas

[MB]

I need to create an application to calculate cooling time and
injection time for injection moulded parts. Does anybody know of a
good formula and theoretical information to do it accurately? Most
books and literature calculate cooling times using a Wall thickness Vs
Cooling Time graphic. I find it very inaccurate as it does not factor
the material. I ran a mould flow analysis for a part using different
materials and the cooling times where all over the place (from 11 sec
for PP to 60 for POM). Using the graphic the cooling time would be
38s.

Thanks,

Mauricio B

[Rolf Wissmann]

go to slide 23 of this ->
http://www.me.gatech.edu/jonathan.colton/me4793/injection.pdf
where you will see that thermal diffusivity of the polymer is key to cooling
time, or
go to -> http://dettenberger.de/index.php?option=com_kzb&Itemid=45 where you
can enter your data (in case you are able to read German ...)

[Bob Hatch]

Mauricio, Cooling time calculations depend on so many factors. Yes,
most of us use wall thicknesses as a basis, but the heat from shear
and conductive heat is going to cause dramatic variations in cycle
time due to the lack of input heat control by the processing
technicians.

My own personal experience tells me that most materials will have a
cooling rate within 10% of each other, but this is true only if the
heat applied to the material is on the low end of the manufacturers
temperature recommendations.

Acetal is actually a very fast cycling material unless you run the
barrel heats up in the low 400F set point range which is more like a
430F melt temperature most of the time.

Another huge factor in cycle time is how quickly the mold heat can be
removed through the traditional water circuits. Eliminating jumpers
and quick disconnects is a good way to speed up cycle times for most
materials, more especially for acetal.

You can also look at latent heat of fusion charts and determine the
removal rate of heat from a particular material if you want to zero in
on problems with different injection molding materials.

Lastly, I size the nozzle orifice and style of nozzle, the sprue
bushing "O" diameter, main and sub runner diameters to remove
restrictions to flow and eliminate pressure losses so I can run parts
with the lowest possible barrel heat settings. This in turn gives me
the fastest cycles for a given material.

Good luck with your search. Bob Hatch