Heated molds and temps

[moldingppm]

What’s the reasoning for running tools at a higher temperature than just say tower temp? I’ve always believed that you run higher temperatures for high gloss finishes, and when you have surface issues such as weld lines and such. We generally run PP/PE/PS and are starting to run more PA66, and I’ve been told running hotter molds for engineering resins is overall better. Generally we just throttle the water valves, but if I can justify buying a thermolator and get better quality of parts than I’m getting (even though the parts we’re running have no issues).

I’m just wondering if someone can enlighten me on when hotter tools are better, or when colder is better. This is one thing I’ve never actually fully known, although never had an issue to find the need to ask.

[iautry1973]

I read a good article the other day about cooling. A consultant went into a company that was trying to lower the water temps to decrease cycle time. Turned out the opposite worked. As they raised the water temp they were able to lower the cooling time. Mainly because the weren't getting turbulent flow at the lower water temps. I will try to find the article again and attach it.

That being said my normal experience with running molds hotter is due to a cosmetic need (PC lenses etc). I know morphology can play into mold temps as well. I will have to break out Suha's book and read that before I can give a coherent answer. I kinda forgot some of that now that I am not shooting plastic anymore.

[iautry1973]

Found it

https://www.plasticstoday.com/injection-molding/colder-injection-mold-doesn-t-always-result-colder-part-temperatures/141199397959414

[chrisprocess]

Running warmer mold temp generally helps reduce stress in the molded part and cooler mold temps generally will give you faster cycle times, assuming turbulent flow is equal.
Imagine phases changes in metallurgy - quenching hot steel quickly in water vs letting air cool you will get very different end material properties. The same goes for many different plastics specifically more crystalline materials than amorphous

[SeanC]

Great article!

[MTUHusky]

What’s the reasoning for running tools at a higher temperature than just say tower temp? I’ve always believed that you run higher temperatures for high gloss finishes, and when you have surface issues such as weld lines and such. We generally run PP/PE/PS and are starting to run more PA66, and I’ve been told running hotter molds for engineering resins is overall better.

Most engineering resins are semi-crystalline and these long strands of the polymer have to curl back up into little crystals during cooling. These little crystals only form at certain temps (glass transition temperature), and if you don't reach that temperature, you will see a "grainy" finish as well as some loss in properties. One big issue you may see from not reaching the correct mold temperature is built in stresses, as Iautry has pointed out, and you will have a real issue if the part sees temperatures close to the glass transition temp out in the field as the part will warp as the crystals start to form again.

The correct mold temperature is a HUGE deal with engineering resins. When I start with a new process, I always start with a low melt temp and a high mold temp as a rule of thumb.

[JayDub]

For PP & PE especially, molding a slower cycle in a hotter tool will give the material more time to crystallize and give more stable dimensions. That’s because the Tg for olefins is well below room temperature, so if you mold them cold and they are largely amorphous out of the press, they will re-crystallize and warp on a warehouse shelf. The tighter your tolerances, the more of an issue this becomes.

As far as your general question of when hotter tools are better, it’s going to vary by what you need from the part and what resin it’s made from. Nylon and other crystalline resins should be cooled with a mold temp higher than their Tg to develop optimal physical properties. As Chris says, running parts in a cold mold increases stresses. In an amorphous resin this can increase susceptibility to stress corrosion. So, for example, a PC part molded in a (relatively) hot mold might be dishwasher safe, while the same part out of a cold mold might craze in warm soapy water.

Incidentally, raising mold temp by choking coolant flow just means you have poorer (and most likely uneven) heat transfer, which is likely to produce internal stresses and warpage.

[Joel JS]

Hello,

Great info above, and here's my .02 cents....... With PP, many times I see a warmer mold release better (ie pin push) and therefore sometimes you can get faster cycle times. And generally speaking, warm molds will give better cosmetic appearance and have lower gloss.
Joel