Determining the cooling time

Because thick sections take the longest to cool the runner usually determines the cooling time.  If you have a hot runner system this idea is moot.  So when do you want to open the mold?  Thermodynamic equations can calculate the amount of heat that must be removed for the resin to be reasonably solid enough to eject. Some of these calculations are already embedded in FEA programs.  However their major fault is that they live in the world of ‘Perfect’.  Perfect being a melt temperature that is consistent and predictable, a mold temperature that doesn’t vary, and a mold whose heat exchange properties work perfectly (i.e. no air gaps between two pieces of metal, perfect conductivity, turbulent flow in the cooling lines etc.)  BUT the world isn’t perfect. Bummer.

Many of us use squinty-eyed common sense by pulling a cooling time out of our pocket then experimentally shorten or lengthen it until we find the minimum time by making a dimensionally bad part or the part warps.  After this Kentucky Windage approach to engineering, we are satisfied we’ve found the minimum cycle time.  But there is a better way. Ideally you have an infrared pyrometer.  If not, a fast response contact pyrometer will usually work.  If you don’t have either, try to purchase an infrared one. With some extensive research into the technical literature data you can find a temperature at which you should eject.  This is nifty because it looks at the practical end of your theoretical thermo dynamic equation without having to go through all those nerdy calculus equations.  This table/research was provided by my friend and associate in the quest to making molding easier, Steve Erickson, President of the Rocky Mountain Chapter of SPE, and molding engineer extraordinaire.

Find your material and you have three ranges to eject your part.  Each will determine a different level of shrinkage. It is also interesting to look at different cavities.  In many cases you’ll find one is coming out at a different temperature than the others.  Different cooling rates will give you different dimensions.  Think about it.
 

Contact me with questions put in the subject line QUESTIONS.
Bill Tobin, WJT Associates, E-mail: bill4012@hotmail.com