STOP doing this to Cooling Circuits!

STOP ignoring the importance of proper cooling circuit design for injection molding. Cooling circuit design is typically not heavily weighted with regards to importance in the injection molding industry. Cooling circuits need to be designed properly and the importance of the design must not be overlooked. A common practice is to simply ‘place cooling in the mold’ and not much more. Sometimes it is taken a step further where the practice is ‘to place as much cooling in the mold as possible’. This is a better practice but is it the best? We are going to discuss the importance of proper cooling circuit design and the implications of overlooking this important practice.

Cooling Circuit Diameter

Is the diameter of the cooling circuit important? It certainly is. In fact, the diameter of the cooling circuit must be a part of a larger consideration such as circuit spacing, pressure drop and flow rates. A larger diameter may be thought to cool better since it is larger, which is somewhat true, however it will take up more space due to its larger diameter so it may be difficult to route in tighter locations.

The spacing of larger cooling lines can be increased based on the diameter which can result in fewer cooling lines hence less gun—drilling, i.e. a cost savings but this is not always the better condition if the mold design is complex or small.  Larger cooling lines will also have a lower pressure loss resulting in less power required to pump the water through the circuit. This is a big plus but the difference in pumping efficiency may be negligible. It is also important to understand that varying cooling circuit diameters within the same mold will only be as efficient as the smallest diameter of that circuit due to the higher pressure loss within the section containing the smaller diameter. The flow may be turbulent in the portion with the smaller diameter yet may take flow away from the larger diameter portion resulting in a laminar flow condition in the section with that larger diameter.

The Importance of Balanced Cooling Circuits

Optimal designed cooling circuits are the driving force behind productivity and cycle times.

If your cooling circuits are not as working as efficiently as they can be, they will be costing you precious time and money. Balancing cooling circuits plays a tremendous role in cooling efficiency.

So why do the cooling circuits need to be balanced and what exactly does that mean?

In short, all circuits are not created equal. If cooling circuits are not equally balanced, then the flow rate will be different in each circuit resulting in differential heat removal from each circuit and a non-uniformly cooled mold.

The cooling circuits within the mold are rarely all the same. In fact, they are typically different in total length, diameter and may also contain various amounts of bubblers or baffles.

When circuits are balanced, that means that they have the same pressure drop across them. If they have different pressure drops, and are hooked up to the same manifold, the flow rate will be different for each circuit. The circuits with the higher pressure drop will experience the lowest flow rate causing them to be less efficient than their counterparts.

Reasons Cooling Circuits would not be Balanced:

  • Flow lengths are not equal
  • Diameters are not equal
  • The addition of bubblers or baffles
  • Number of turns in a circuit (circuits hooked up in series)

A typical cooling system has a supply manifold with hoses running to the mold and hoses running out of the mold to a return manifold.

Below is a simplified illustration of a cooling system with a series of 5 cooling circuits, each with a different diameter, all hooked up to the same two manifolds with short hoses.

Cooling Circuit 1 (pic manifold)

cooling circuit manifold

This example is not far from what can actually occur. Because the circuits are hooked up to the same manifold, the manifold will deliver the coolant according to the pressure drop as seen in the next image. The coolant will always favor the circuit with the lowest pressure drop or path of least resistance.

Cooling Circuit 2(pic-circuit pressure drop)

Cooling circuit pressure drop

Notice how the smaller diameter circuit (on the top of the image) has the greater pressure drop resulting in the highest pressure requirement and the larger ―”circuit (bottom of the image) has a very low pressure drop.  The varying diameters will have a tremendous effect on flow rates.

The resulting flow rates are as follows:

Cooling Circuit 3 (pic circuit flow rate)

cooling circuit flow rate

With a total inlet flow rate (into the manifold) of 4 gallons per minute, observe how the largest circuit (bottom of image) has the highest flow rate at 1.33 gallons per minute and the smallest circuit (top of the image) has a flow rate of 0.319 gallons per minute. That’s more than 3 times more flow rate than the smaller circuit.

Above all, when it comes to heat transfer, we need to have a Reynolds number above 5000 in order to have turbulent flow (0-2000 is laminar flow, 2000-5000 is transition flow and 5000+ is turbulent flow).

When the flow rates vary through circuits, some circuits may no longer have turbulent flow causing the circuit to be very inefficient and sometimes useless. Circuits with low flow rates are also prone to fouling, another hidden cost.  The image below shows the corresponding Reynolds numbers for the circuits.

Cooling Circuit 4 (pic-circuit Reynolds number)

cooling circuit reynolds number

The smallest cooling circuit (top of image) only has a Reynolds number of 4379 while the largest cooling circuit has a Reynolds number of 9118! All circuits need to have a Reynolds number higher than 5000, preferably 8000 and higher. Imagine how inefficient a mold would be with these cooling circuits.

Unfortunately, imbalanced cooling circuits exist in many molds today especially with the incorporation of conformal cooling designs and the increasing complexity of injection molds.

In summary, cooling circuit design is extremely important and needs to be given more attention. Properly designed cooling circuits will:

  • Be less prone to fouling
  • Require less frequent maintenance due to less fouling.
  • Have greater efficiency resulting in shorter cooling times which will decrease cycle times dramatically.
  • Provide more uniform cooling (fewer hot spots) which can decrease issues such as part sticking.

STOP ignoring the importance of proper cooling circuit design. 

Bozilla Corporation considers both pressure drop and circuit balance when analyzing cooling circuit designs in Injection Molds and in Thermoforming Molds. We believe in making every effort to help our customers save money. The next time you design your mold, call Bozilla Corporation and we’ll work with you to make your mold more efficient.

Contact Bozilla Corporation today and let’s discuss how we can successfully contribute to your project. Bozilla Corporation’s Injection molding Team has over 21 years of experience analytically and on the floor. We specialize in optimization, consulting, engineering, troubleshooting and Autodesk Moldflow software training. Additionally, our plastics engineers have a full understanding of polymers and how they influence an injection molded part. Your success is our success. Our skilled Team is focused upon meeting the goals and timelines of our customers., 800-942-0742,

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About the Author:

Chris Czeczuga is a Plastics Engineer, Injection molding expert, Military Veteran and the President of Bozilla Corporation. He has proven success with many Fortune 500 companies throughout the injection molding industry. A graduate from UMass Lowell, he is Expert Certified with Autodesk, has 20+ years of field experience, intimate knowledge of injection molding part, tool and feed system design. Bozilla Corporation’s success is built on providing the highest level of injection molding simulation and consulting advice to businesses who have short lead times, require an efficient, cost-effective molding process, and desire to produce a correct part the first time.