Let’s start with some engineering basics. Most of you have heard something about turbulent flow and that it is good for cooling.

But just what is turbulent flow? How does it help? What flow rates are needed to achieve turbulent flow?

Turbulent flow begins when the velocity of fluid in a channel increases to a critical level. Above this critical velocity, vigorous internal mixing of the fluid occurs as it flows. This improves heat transfer by mixing warmer fluid near the wall of the cooling passage with the relatively cooler interior fluid. the precise velocity for turbulent flow depends on several variables, including the cooling passage geometry, fluid viscosity, and roughness of the pipe walls. The formula for a ratio known as Reynold’s number includes these variables. A Reynold’s number greater than 3000 denotes turbulent flow.

Having said this, I can tell you that in some cases turbulent flow doesn’t matter too much, and in other cases it matters a lot. In one example, the cycle time for a coffee mug with a 0.200-inch thick wall was very poor. The molder wanted to improve the cooling in the mold cores with the goal of achieving a substantial cycle improvement and spent a significant sum making cooling “improvements”. When the mold was sampled, the molder was surprised to learn that the cycle was about the same as before. What was going on there?

The best cooling system in the world won’t take away heat any faster than the molded part will give it up. Most unfilled resins transfer heat at a rate 1/10 to 1/25 that of steel. The outer walls of a thick part insulate the mold from the heat trapped in the center of the part. The message here is that for very thick part, the cooling system will have relatively little effect on cycle time. the other hand, let’s say you are running a very thin polyethylene lid. This part can give up its internal heat quickly because of its thin walls and typically runs on a fast cycle.

These factors combine to greatly increase the demands on the cooling system, so good cooling performance requires well-placed passages in the mold as well as greater flow rates to carry away the heat. Thus, it is generally true that if the molded parts will give up their heat, it is worthwhile to use higher cooling flow rates. And it is true that the faster the flow rate, the more total heat you remove — even though the change in the temperature of the water flowing through the mold is very slight. Intuition may suggest that the water would pick up more heat at a slower flow rate, but it won’t. Although the temperature of the water increases more at a slower flow rate, total heat removed does not.