Design rules

Mold cooling design considerations

The design rules presented here provide some guidelines for attaining proper and efficient mold cooling. Cooling channels should be of standard sizes in order to use standard machine tools, standard fittings, and quick disconnects. Based on the part thickness and volume, the mold designer needs to determine the following design variables when designing a cooling system:

Location and size of channels

Part thickness
To maintain an economically acceptable cooling time, excessive part wall thickness should be avoided. Required cooling time increases rapidly with wall thickness. This calculation is shown in Cooling system equations. Part thickness should be as uniform as possible, as shown in Figure 16 below.


FIGURE 16. An alternative design can be used to maintain uniform part thickness.

Cooling-channel location and size
The best location for cooling channels is in the blocks that contain the mold cavity and core. Placing the cooling channels outside the cavity or core block will cool the mold less adequately.

Generally, the surface of the cooling channels (i.e., depth) should be one to two channel diameters from the cavity or core. The rule of thumb is that the depth should be 1 diameter for steel, 1.5 diameters for beryllium copper, and 2 for aluminum. The pitch (distance between cooling channels' centers) should be three to five times the channel diameter. A typical cooling channel diameter ranges from 10 to 14 mm (7/16 to 9/16 inches), as shown in Figure 17 below.


FIGURE 17. Typical dimensions for cooling channel diameter (d), depth (D), and pitch (P).

Flow rate and heat transfer

Temperature difference
Keep the temperature difference on opposite sides of the part to a minimum; it should not exceed 10ºC for parts that require tight tolerance.

Heat transfer of coolant flow
The effect of heat transfer increases as the flow of coolant changes from laminar flow to turbulent flow. For laminar flow, heat can be transferred only by means of heat conduction from layer to layer. However, in turbulent flow, the mass transfer in the radial direction enables the heat to be transferred by both conduction and convection. As a result, the efficiency increases dramatically. The diagram below illustrates this concept.


FIGURE 18. Laminar flow and turbulent flow.

Since the increase of heat transfer will diminish as the coolant flow becomes turbulent, there is no need to increase the coolant flow rate when the Reynolds number exceeds 10,000. Otherwise, the small, marginal improvement in heat transfer will be offset by the higher pressure drop across the cooling channels, along with more pumping expense.

Figure 19 below illustrates that once the flow becomes turbulent, a higher coolant flow rate brings diminishing returns in improving the heat flow rate or cooling time, while the pressure drop and pumping expenses are drastically increasing.


FIGURE 19. The relationship of heat flow rate and coolant flow rate. (Click Replay)

NOTE: It's important to make sure that the coolant reaches turbulent flow everywhere in the cooling system. C-MOLD Coolant Flow analysis can help you identify and correct such problems as stagnated cooling channels, by-passed cooling channels, and high pressure drops in some cooling circuits.

Restrictive flow plugs
Coolant will take the path of least resistance to flow. You should use a restrictive flow plug in certain cooling channels to re-direct the flow of coolant to other cooling channels that have a high heat load.

Air gaps
A layer of air can impair the transfer of heat effectively. Therefore, you should take steps to eliminate any air gaps between the mold insert and molding plates, and any air pockets in the cooling channels.