Calculating Mould Cooling Power

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The below article can be used for machine setters and machine operators of injection moulding machines.

View our other articles form our setter and operators guide for injection moulders. Taken from our moulders catalogue (Toolbox Edition)

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From our article, “Mould Cooling Power Might Be More Important Than You Think” we will examine the expression: CP = ΔT x Q x C

  1. C is the heat capacity of the coolant.

For water, this value is 1 Btu/lb-°F. We don’t have a good way to make this value higher, but we can try to do no harm.

For example, moulders sometimes use glycol-based antifreeze in solution with water to allow lower chiller setpoints and a colder mould. Adding glycol to water decreases the heat capacity and increases the viscosity of the coolant. The greater the percentage of glycol, the greater the effect. These effects result in increased pumping cost compared with water. Additionally, higher flow rates are required to achieve turbulence, possibly taxing available pumping capacity.

In some cases, a colder mould works against part quality requirements such as surface finish, warpage control, and development of crystalline structures.

Moulders should carefully examine the economics of using glycol-water solutions and very low mould temperatures, as is discussed further below.

  1. ΔT is the change in coolant temperature as it flows through a circuit.

The temperature increase is influenced by several factors.

Heat input from the shot, diameter and length of the circuit, circuit placement and condition, mould materials, and coolant flow rate all play a part.

It is generally true that turbulent coolant flow results in relatively low values of ΔT in mould cooling circuits.

Figure 1 shows values of ΔT per inch of circuit length vs. flow rate at three different heat input values. Heat input corresponds to shot size.

In Fig. 1, ΔT is shown for a 7/16-in. (0.44-in.)diam. x 22-in.- long circuit with flow of 1 gpm and three relatively small shot sizes (such as for small parts in an eight-cavity mould).

The values of ΔT for these shot sizes would be only 1.1-4.4° F. If we could make the circuit longer we could squeeze more Cooling Power from every Lpm of coolant flow. But how can we stretch a cooling circuit?

We can nearly double the circuit length by looping a pair of similar cooling circuits together. In fact, you might loop more than two as long as it doesn’t cause excess moldtemperature variation or too much pressure drop and flow reduction.

Avoid looping restrictive circuits with free-flowing ones, because the restrictive circuit will control the flow rate.

The takeaway: Don’t be afraid to experiment with circuit looping. Carefully applied, it can multiply your Cooling Power.

  1. Q is the coolant flow rate in a circuit.

It is well known that turbulent flow increases the effectiveness of heat transfer in a cooling circuit and is an important factor in achieving efficient cooling.

Turbulent flow depends on circuit diameter, coolant flow rate, and coolant temperature (viscosity). Turbulence can be predicted using available flow-measuring tools and published charts.

As flow is increased beyond the turbulent transition, the potential heat-removal rate continues to increase but there is a point of diminishing return.

Our studies show that if cooling circuits are properly sized, increasing coolant flow to more than 1½-2 times the turbulent rate yields very little additional reduction in steel temperatures,

(Fig. 2). Excess flow is costly since flow-related pressure losses increase with the square of flow rate.

These facts can help you develop an efficient cooling setup and avoid excessive flow rates and wasted Cooling Power.

Is Colder Coolant Really The Answer? Read More