Heat transfer potential using water cooled channels in a mold is affected by several factors:
- Part thickness – Cycle time increases with the square of wall thickness. Excessive part thickness is the single biggest factor in cooling time and poor cycle time.
- Coolant temperature – Affects mold temperature & Reynolds Number (Due to change in water viscosity)
- Coolant flow rate – Affects cooling capacity, Reynolds Number, and ability to control mold temperature
- Cooling Channel Area (p x *diameter x length) – Affects cooling capacity
- Cooling Channel Condition – Scale and biological buildup affects heat transfer capability, cooling capacity, steel temperature, and cycle time
- Coolant Characteristics – Ethylene Glycol in coolant increases viscosity and pumping energy requirements. It decreases coolant heat capacity, Reynolds number, and impedes turbulent flow.
- Mold materials – High performance alloys can help when it is difficult to get cooling close to a molding surface, but the other factors generally have a greater influence on mold cooling.
*Use Hydraulic Diameter if cooling circuit is not round
Select Material and Enter Molding Variables
Select from a list of 21 common polymers and its properties will automatically display. Manually enter cycle time, part or shot weight, and molding room temperature and the calculator determines and displays heating and cooling energy values. The user may also override the default processing and safe ejection temperatures.
Enter Cooling Variables and Display Calculation Results
Enter desired Reynolds number, water temperature, *coolant ΔT/inch, and cooling circuit diameter. The calculator will determine and display the coolant flow rate associated with desired Reynolds number and coolant temperature selections. The required cooling length is also displayed. These values are useful for designing cooling circuits and evaluating the adequacy of cooling designs in existing tooling.
What is Reynolds Number?
Reynolds number is a dimensionless quantity that predicts turbulent flow of a fluid in a pipe or passage, according to Baumeister & Marks "Standard Handbook for Mechanical Engineers." Reynolds number depends on flow velocity, passage diameter, and water kinematic viscosity. Reynolds numbers of 2000 to 4000 are transitional, meaning flow may be laminar or turbulent. A Reynolds number above 4000 generally results in turbulent flow. Water viscosity decreases with increasing temperature, thus yielding a higher Reynolds number. In mold cooling Turbulent Flow is associated with more effective and stable cooling conditions. Our studies show that as Reynolds number increases well above 4000 the cooling benefit increases at a declining rate – in other words, less bang for the buck.
*Hydraulic Diameter
Not all cooling circuits are round. In these cases one should determine the "hydraulic diameter" and use this value in the "Enter Cooling Parameters" section. For your convenience the following link is provided to our Smartflow online Hydraulic Diameter Calculator.
Limiting Factors
Your cooling system may not be capable of cooling at the rate your calculations suggest. Factors such as scale or biological deposits inside the cooling channels can decrease the heat transfer rate, increase pressure drop, and prevent reaching the full cooling potential. The size of cooling circuits in a mold may not be adequate. These conditions will of course result in longer than optimum cycle times.
*What is ΔT/inch and how can I know what value to use?
ΔT/inch is the increase in coolant temperature per inch of flow length in the cooling channel. If ΔT/inch = .15 and a circuit is 10 inches long, the total ΔT in that circuit would be 1.5 °F. In a mold cooling circuit the amount of heat flowing into a cooling circuit determines the value of ΔT/inch. We have defined the term Energy Density as the amount of heat flowing into the circuit divided by the total area of the circuit. The higher the Energy Density, the higher the value of ΔT/inch. The circuit area is simply Diameter x π (3.1416) x length. Using data from our laboratory studies we have developed a graph showing the relationship between Energy Density and ΔT/inch at four different coolant flow rates. This graph gives users a science-based method for estimating ΔT/inch values.
Energy Density and Mold Temperature
Energy Density also influences mold temperature and is useful in predicting the temperature. In our experiments mold temperatures responded linearly to Energy Density values, but the mold geometry makes a difference in the temperature response. The "Energy Density vs. Steel Temperature" graph illustrates this difference and clearly shows the importance of managing Energy Density in cooling circuit design. This is to say, one should design a cooling circuit with adequate area to achieve an Energy Density value that produces the desired mold temperature.
Footnote: We offer this Mold Cooling Calculator tool as a free service to the injection molding industry. While some molds or inserts have simple, straightforward cooling circuits, many have multiple circuits of various sizes and configurations. In these cases each circuit might remove a different percentage of the heat input. Users must therefore employ this tool with awareness and judgment. Trying out different molding and cooling variables is simple and quick. In complex cooling schemes one can easily analyze each cooling circuit separately and combine the results. We are eager to learn how you have used the calculator and to hear your constructive feedback so we can enhance and improve the utility of the Smartflow Scientific Cooling Calculator.
