Cooling channels

Complex geometry in plastic molds can create areas that are difficult to cool.

For example, parts of the mold that project into the cavity, such as bosses and ribs, carry high heat loads because they are surrounded by plastic. They also restrict the area of metal through which heat can escape.

To channel coolant into high heat-load areas, it may be necessary to design intricate cooling systems.

This means linking cooling circuits together to form a network of cooling channels. Networking requires the inclusion of bends and devices such as baffles and bubblers. It is important to consider how these devices affect the operation of the cooling system.

Cooling system design considerations

The aim of the mold designer is to design a cooling system with the following characteristic:

• Uniformly cool the part
• Achieve the desired target mold temperature for the start of the next cycle
• Minimize cycle time

The mold designer must also consider the following factors, which affect the performance of the cooling system:

• The physical layout of channels and the mold material into which they are cut
• The coolant parameters such as coolant type, temperature, flow rate and pressure drop

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 may not provide adequate cooling.

The physical design of the cooling system is normally restricted by the mold geometry, positioning of split lines, moving cores, and ejector pins.

Bends in cooling circuits

The inclusion of a bend in a cooling channel increases turbulence, which results in a large pressure drop and an increase in heat transferability through the bend.

In a Cool analysis, bends are handled rather like an extra section of cooling channel that has unique resistance and heat transfer characteristics. These sections are assigned apparent lengths for resistance and heat transfer that are much greater than the actual flow length through the bend.

For example, a bend can have a resistance equivalent to a flow length 50 times the diameter of the cooling channel. Heat transfer capacity equates to a channel 10 times the diameter.

The apparent lengths are used to calculate the pressure drop and heat transfer capability. These characteristics are then applied to a single point in the flow channel that represents the bend.

Turbulence also occurs when there is a change in the diameter of the cooling channel.

Position of cooling channel inlet/outlets

The inlets and outlets should ideally be positioned on the bottom of the mold. This eliminates the risk of the coolant dripping onto the mold.