Justifying a chiller

[Valentin Leung]

All,

This may sound very very stupid, but I am trying to justify the need of anything on my workfloor. Working close to China, I got a whole batch of "shifu" that would ask me why we are not adding chillers to all machines. Why are we running our molds comparatively "hot" (I am using 30 degrees tank outlet temperature) when they are used to run with chilling 15/16 degrees water. I have been looking into it for weeks but could not justify the need for extra chiller capacity. I am running 99% PP.

All formulas I have found over the web on mold heat exchange do not take in consideration the water temperature in itself, but the water temperature difference between the inlet and outlet of the lubricant. So from my point of view, if I reach turbulent flow, it does not matter if I have 30, or 40 degrees water for my application, right? That's based on the assumption that I am always running the same material and type of resin, no special engineering resin.

(all temps are in C)

Then... why do we need extra chillers? Unless we run specific material, why would anyone run tools at a chilling temperature? I have been through a lot of shops locally and always found tools sweating like crazy (humidity is high here, but why running at below ambient temperature if the heat exchange won't be any better at a few degrees higher?)

Let me know if I am wrong, just needed sanity check.

[Dinngus]

Hi Your set up is fine with the material you are using so its all good by using chillers you may be able to reduce cycle time as plastic will set quicker or if you having problem with warpage this will help on the negative side it might effect flow rate into your mould.

[JayDub]

A few considerations:

Coolant temperature from the mold no more than 2C higher than coolant temperature to the mold indicates you have efficient heat transfer.
Lower coolant temperature means faster cooling, so faster molding cycle … but:

Below about 20C the viscosity of water increases rapidly, so it’s harder to get turbulent flow and cooling may be less efficient – there is a point of diminishing returns.

Also, faster cooling means parts may not achieve full crystallinity. This affects dimensions, and more importantly, for PP it means dimensions are likely to be unstable (part dimensions will change as parts sit on the shelf).

So in general, dimensions will be more consistent and physical properties will be better at 30 – 40C, at the cost of a longer cooling time.

I had to look up the term ‘shifu’, but if I understand you correctly, we have plenty of those in the US too.

[Suhas]

Mold temps are important in crystalline resins to get the crystallinity you need for the part and to achieve the required properties. In both, amorphous and crystalline materials, mold temps are important to get the required cosmetics on the parts. The higher the mold temps are (of course to a limit) the better the surface finish gets. Lower mold temps can also result in higher post mold shrinkage, warpage and in-build stresses.
Keeping the above in mind, if you are molding acceptable parts with a chiller, with no other associated mold issues such as corrosion because of sweating and so on, you are good to use a chiller. If not, a thermolator is a must.
Hope this helps.
Suhas