Sponsored By

Technical molders know that turbulent cooling flow can improve cycle times and help the bottom line. But what factors can inhibit good cooling, and how can you improve your mold cooling results?

Philip M. Burger

April 14, 2010

6 Min Read
When it comes to mold cooling, 
viscosity matters

Technical molders know that turbulent cooling flow can improve cycle times and help the bottom line. But what factors can inhibit good cooling, and how can you improve your mold cooling results?

Even if you think you already know this, let’s start with Turbulent Flow 101. As flow velocity increases to a critical speed in cooling channels, the flow begins to swirl and mix vigorously, the condition known as turbulent flow. Turbulence increases heat transfer by the mixing and faster flow at the boundaries of the coolant and steel. Turbulent flow may be predicted using a simple calculation to determine a Reynolds number:

Reynolds number = 
(velocity x diameter) / viscosity

According to the Standard Handbook for Mechanical Engineers by Baumeister & Marks, “Above a Reynolds number of 4000, the flow is generally turbulent.” I have rearranged the expression to solve for the flow rate needed for turbulence based on a Reynolds number of 4000 and the use of convenient units. The new equation is:

gpm = 117,495 x d (inches) x kv (ft²/sec)

This simply says that the larger the pipe and the higher the kinematic viscosity, the more flow you need for turbulence. Kinematic viscosity is the English unit of viscosity (ft²/sec). Using this expression, Burger & Brown Engineering developed the Turbulent Flow Reference Chart (below), which has been provided to the industry free of charge for many years.

moldcool_chart.jpg

It is important to realize that kinematic viscosity of the coolant increases significantly as the temperature decreases. Add antifreeze and the viscosity increase becomes dramatic. Figure 1 shows the effects of both temperature and ethylene glycol (a common antifreeze compound) on kinematic viscosity.

MF_MoldC1.jpg

Figure 1. Temperature vs. kinematic viscosity of ethylene glycol and water solutions.

MF_MoldC2.jpg

Figure 2. Turbulent flow requirements for ethylene-glycol solutions vs. water.

For a 0.500-inch-diameter flow passage in a mold with water at 60°F, it takes about 0.5 gpm to get turbulent flow. But what if you are using a coolant of 30% ethylene glycol in water at 40°F? Figure 2 shows that you now need a flow rate approximately 2.4 times greater or about 1.2 gpm to get turbulent flow in a single cooling circuit. Now, imagine that you have a dozen cooling connections on an average mold and your plant has 24 molding machines. You can begin to see the implications.

What does coolant 
really do?
The specific heat capacity is the amount of heat required to change a unit mass of a substance by one degree in temperature. Think of it as the capacity of a substance to hold or carry heat. Ethylene and propylene glycol have slightly less than 60% of the heat capacity of water. A solution of 30% ethylene glycol in water will have about 90% of the heat-carrying capacity of water.

It is easy to assume that colder is better for mold cooling fluid. But to run coolant at 45°F or lower, you must add glycol to the water to prevent evaporator coil freezing in the chiller. That means you will need more than double the flow to get turbulent conditions,which can tax the available coolant pumping capacity. If pumping capacity is limited, are you better off with 100% water at a higher temperature, requiring less than half the flow to be turbulent?

Low-temperature coolant (with antifreeze) costs extra. First is the cost of the antifreeze and the ongoing cost to replenish it as makeup coolant is added. Next is the additional operating cost of refrigerating to lower temperatures. There are capital and operating costs associated with the additional pumping capacity needed for more viscous coolant. Other costs can result from degraded part quality associated with very cold, sweaty molds. Ethylene glycol is toxic and also poses a health and an environmental risk in the event of a serious spill.

To see if we could demonstrate measurable effects of antifreeze solution vs. water coolant, we used a small eight-cavity mold that makes a PP nut and installed a thermocouple in the cavity plate to sense steel temperature near the cavities. We started the mold with 100% water circulating at 60°F at a flow rate sufficient for turbulence. After an hour of running, the steel temperature stabilized at 78°F. We stopped the machine and added glycol to the chiller to a concentration of about 40%. We restarted the mold at the same coolant flow rate and temperature. As the process stabilized, the steel temperature rose to 83°F. Molded part temperatures were running around 8 deg F warmer than with water cooling, clearly indicating reduced cooling capacity caused by the combination of increased viscosity and reduced heat capacity of the coolant.

You need clean H2O
Water quality is an important factor in the performance of coolant circulating systems, and can affect cooling in four ways:
• Microbiological influenced corrosion (MIC) is caused by the corrosive effects of microbiological growth on metal components. Fouling of heat transfer surfaces can occur. The results are restriction of coolant flow and dramatically reduced heat transfer coefficient.
• Minerals can precipitate out of water and onto heat transfer surfaces, restricting coolant flow and reducing heat transfer.
• Undesirable water chemistry can attack metal components of the system. MIC, mentioned earlier, is one type of corrosion. Galvanic corrosion can occur from electrical interaction of different metals with the coolant.
• The use of ethylene or propylene glycol complicates the water treatment process. Glycols are known to make the water more corrosive. Some glycols have corrosion-inhibitor packages added to them. The question is, “Are they right for the specific metal components of your system?” Low levels of glycol can encourage microbiological growth in the system water.

Water treatment issues are unique to each system due to the chemistry of the local water supply and individual plant conditions. The Assn. of Water Technologies (Rockville, MD) provides training and technical information and can assist you in reaching a local water treatment professional.

Improving mold cooling is the low-hanging fruit in improving profits. Know how much flow you need for turbulence and measure flow in each cooling circuit. Establish minimum flow rates for all circuits and put this information into setup sheets. Simple, affordable tools for turbulent flow measurement and flow regulation are available from suppliers like Industrial Molding Supplies and DME Molding Supplies. Also, our website has technical documents, articles, and free downloads to assist your quest for process improvement.

Philip M. Burger is the founder and director of product development of Burger & Brown Engineering Inc. (Grandview, MO). He wrote this article in collaboration with Doug Thorpe, director of process training, Nypro University (Clinton, MA) and Rick Lorfing, Water Improvement Services (Kansas City, KS).

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like