Calculating Cooling Time...

tc = [(h^2)/()(π^2)] ln |(4/π)[(Tm-Tw)/(Te-Tw)]|

In the above specified formula , what is "n" ?

Kindly explain

Vishal

Good question,

'ln' actually refers to the exponential function on your calculator. This is typically represented as 'ln' on your scientific calculator (or windows calculator). In essence, you calculate everything on the right hand side of the equation and apply that as the exponent of the left half of the equation.

-Andy

Hi Andy,
Thanks for clearing the doubts.
1> So by calculating cooling time with the given formula, can one understand that the part has reached the ejection temperature ? hopefully yes but would request your views ?
2> Now this formula speaks about the cooling requirements of the plastic parts.
3> I have also come across the cooling requirements for the injection mold, meaning the heat generated by the polymer need to carry away by the cooling media like water or oil or heaters
I would like to share the same , If the cooling media is water
We need to calculate first,
Btu - is the heat generated by the molten polymer (British Thermal Unit)
Btu - P x (t1 - t2) x S x h
P - Plastic part weight in lbs
t1 - melt temp. degree F
t2 - mold temp degree F
S - specific heat of plastic Btu/lb/degree F
h - latent heat of fusion in Btu/lb
Once we know the Btu, one understand the amount of heat to be carried away by the water
Now we know, water can absorb Btu by the above formula, so
Btu = W (T2 - T1) x S = (water will take this much Btu to cool the mold)
W - Weight of water per hour , lbs
T2 - mold temperature , degree F
T1 - water temperature , degree F
S - specific heat of water = 1
Now we know the W, weight of water and thus we can circulate the amount of water throygh MTC in the mold to maintain the specific mold temperature during each cycle.
Request to have your views on above
Best Regards
Vishal

Hi Andy,

tc = [(h^2)/()(π^2)] ln |(4/π)[(Tm-Tw)/(Te-Tw)]|

In the above frmula, the 2 vertical lines on left of |[(4/π)..... and on right of .....(Te-Tw)]|
is indicating that we have to calculate the derivaties ? Request to clarify

Best Regards

Vishal...

The vertical lines represent the calculation to which the ln function is applied. I am not sure why parenthesis are not commonly used, but most of my guides and handbooks use the vertical lines to represent this.

There are many more complex calculations used to approximate cooling time. This represents the fastest the heat can theoretically leave the polymer. In reality, this process will be less effective if the mold cannot remove the heat from the mold efficiently. Such equations which take the mold materials and water line layouts into consideration are significantly more complex and are best performed using mold filling software which can take many more factors into consideration.

-Andy

Andy,

Vertical lines is used therefore because you can't count ln with negative number. Vertical lines make number positive, so you can calculate ln-natural logarithm.

Best regards
Milan

Hello Andy.

What is thermal difussivity and how I can calculate it?

Milan

Hello Milan,

Wikipedia does a better job of answering this one than I could:
http://en.wikipedia.org/wiki/Thermal_diffusivity

-Andy

Andy,

Thanks, but I tried to find specific heat and thermal conductivity in material sheet but nothing was there. Where I should find these parameters for my specific material?

Best Regards
Milan