Cooling Channel Spacing

Most of the heat transfer is between the part and the cooling lines. Therefore, it is imperative that the cooling channels be appropriately spaced based on their diameter.

The diameter of a channel should be roughly 3 times the thickness of the part. So if the part is 2mm, the channel diameter should be approximately 6mm. This is only a guideline but is very important as channels that are incorrectly sized can create gaps in cooling efficiency across the part surface if not spaced correctly. The channels must then be spaced from the part surface and from one-another. 

These spacing guidelines will ensure the most effective cooling across your part.

Reynolds Number

 Reynolds Number is a unitless number which is used to determine turbulent flow in coolant channels. Below is the equation for calculation the Reynolds Number:

A Reynolds number of 0-2300 is laminar flow, 2300-5000 is the onset of turbulent flow and 5000 and above is considered turbulent. As turbulence increases, there will be a continued improvement in cooling efficiency. A Reynolds Number of 10,000 or better is preferred to ensure efficient cooling.(Injection Molding Handbook, pg. 302, section 6.7).

It is imperative to calculate the correct flow rate through each circuit ensuring there is turbulent flow.

Cooling Channel Pressure Balancing

This is probably the most overlooked of all aspects of cooling design. If the pressure drop across each circuit is not equal then the flow rate through each circuit will not be equal. Therefore, the Reynolds number will not be equal and some circuits may have laminar flow creating hot spots within the tool.

As I reflect on some recent jobs, flood cooling design principles were applied to some molds where balancing was not considered and actual FIRE HOSES were hooked up to the molds! These hoses are capable of flow rates of 50-100 gallons per minute! The same system, if designed properly, would probably only need about 5 gallons per minute.

Pushing an excessive amount of water into a mold is a method often used to ensure efficient and effective cooling at the expense of tremendous cost with regards to how much energy it takes to pump the water through the system.

Because many tools utilize cooling manifolds, it will be necessary to ensure the coolant running to each hose off of that manifold has the same flow rate. In order to accomplish this, the pressure drop across each circuit must be very close. Balancing cooling lines is simply a matter of designing cooling lines to ensure the pressure drop through each ENTIRE circuit has the same pressure drop. This calculation must include the anticipated routing of the hoses accounting for the entire circuit. Hand calculating the pressure drop through each circuit can be tedious.

A good method of checking the balance and efficiency of cooling circuits is to have a cooling simulation analysis performed using software such as Autodesk Moldflow Insight.

We have discussed three major cooling design techniques that will allow you to have the most effective and efficient cooling circuits in your mold.

Technique 1.    Appropriately designed cooling channels will allow the most heat to be extracted from the part as efficiently as possible.

Technique 2.    Calculating a flow rate where a Reynolds Number is at least 10,000 will ensure turbulent flow which will take advantage of the efficient coolant channel design.

Technique 3.    Balancing cooling circuits to each have an equivalent pressure drop ensures uniform flow rates through each circuit allowing turbulent flow to extract heat from the part as efficiently as possible.

Bozilla Corporation makes it a standard of practice to review cooling circuit designs informing the customer if they are designed according to the aforementioned standards, pressure balanced and have turbulent flow.

Properly designed cooling circuits will minimize cycle time which translates into cost savings!

Bozilla Corportion is an Expert Certified Consulting Partner and Trainer with Autodesk, Inc.

Contact Bozilla Corporation for your Moldflow (FEA) and Injection Molding troubleshooting needs at www.BozillaCorp.com