When it comes to Injection Molding, optimal 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 circuit with the higher pressure drop will experience the lowest flow rate causing it to be less efficient than its counterparts. 
 
Here are some reasons why circuits would not be balanced:

 
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.

 

 
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.

 

Notice how the smaller ¼" circuit (on the top of the image) has the greater pressure drop resulting in the highest pressure requirement and the larger ½"circuit (bottom of the image) has a very low pressure drop.
 
The resulting flow rates are as follows: 

 

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 through the larger 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.  The image below shows the corresponding Reynolds numbers for the circuits.

 

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!  We need all circuits 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.
 
Cooling circuit design has many criteria and rules of thumb but very rarely is the combination of pressure drop and circuit balance considered in that design.  If circuit pressure drop and balance are considered in the design phase, molds can be designed to have very efficient cooling resulting in shorter cycle times which translates into tremendous cost savings.
 
Bozilla Corporation considers both pressure drop and circuit balance when analyzing cooling circuit designs in Injection Molds and even 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 for your FEA and injection molding troubleshooting needs and please visit our website at www.BozillaCorporation.com.