If your original RFQ to the mold builder said, "Design and build a mold to produce part XYZ to print specifications on a 25-second cycle," hold his feet to the fire until that's what you've got! Don't fall for the trick: “We'll process the dimensions in with mold temps and cycle times.” If you raise the mold temp, you extend the cycle. If you lengthen the cycle time, you're using the mold as a shrink fixture. While a second on an eight-cavity mold in a machine running a 20-second cycle at $100/hour machine rate might not seem like much, figure out the profit and machine time you lost in a half million parts! When the part dimensions are out, fix the mold! It is always more profitable than “processing it in.”
Since the mold is supposed to give the part its shape, how cool does it have to be? The common practice is “touchable,” meaning you can pick it up with your hands. But usually the mold has taken way too long to get to this temperature. The scientific answer is 80% of the heat distortion temperature (HDT). Any cooler is a waste of time.
At 80% of the HDT, there will still be thermal shrinkage. There will also be post-mold stress relief shrinkage/distortion. But, it's better to see this distortion sooner than later. I worked on a grille project once. It was twisting, so production left it in the mold longer. Sure enough, when it came out of the mold, it was dead flat but warm. They packed them immediately in boxes and shipped them off. Months later the customer was complaining about warped parts. It turned out the internal heat in the warm parts and the number of grilles packed into the box was enough heat to stress relieve them. Hence, they came out looking like a potato chip. They remolded the parts but this time they had a Lazy Susan shrink fixture allowing a ten-minute air cool while clamped in the proper geometry. Problem solved.
If you set up a simple spreadsheet and look at the cost of shrink fixtures compared with the cost of extended cycle times and capacity you can't use for future jobs, you'll usually see the payoff is very fast and profitable.
Keep in mind you are cooling the melt into a solid state—you are not quenching it! When initially injected into the mold, the plastic molecules are not in their preferred molecular geometry. It takes only microseconds for them to reform into their preferred state while liquid. But if they become solid too quickly, you now have a part with a large amount of internal stress and it will definitely warp.
Even cooling starts with good mold design (as discussed in an earlier article). It requires turbulent flow in the water lines for the best efficiency. This is a matter of consistent setups and temperature control.
But sometimes there are overly thick parts that will “blow out'” when not cooled long enough because the liquid center is still under packing pressure and will soften