Heater Thermal Oil
From the known heat dissipation required per cycle, we can easily calculate the required volumetric flow rate of coolant, and then derive the necessary cooling capacity. Most mold temperature controller manufacturers provide formulas for calculating the minimum pump flow rate. Table 4.1 is very useful when selecting a pump, as it accurately lists the heat dissipation capacity of different plastics.
The following rule of thumb determines the minimum flow rate that the pump needs to provide:
If the temperature difference across the mold cavity surface is 5°C,
0.75 gal/min/kW @5°C temperature difference or
3.415 l/min/kW @5°C temperature difference
If the temperature difference across the mold cavity surface is 1°C, the required minimum flow rate needs to be multiplied by five, i.e., 3.75 gal/min/kW or 17.03 l/min/kW. To achieve product quality stability, many injection molding companies should control the temperature difference on the mold cavity surface within 1-2°C. However, in practice, many injection molding manufacturers may not be aware of the importance of this temperature difference or may think the optimal range is 5-8°C.
To calculate the required volumetric flow rate of coolant, the following procedure should be used:
1. First calculate the heat to be removed from a plastic/mold combination: If
Taking the aforementioned PC cup mold as an example, the actual heat to be dissipated is:
Gross weight per part (g) / cooling time (s) = 208/12 = 17.333 g/s
The heat dissipation rate of PC is = 368 J/g or 368 kJ/kg
So the heat to be dissipated per cycle = 368 × 17.33 / 1,000 = 6.377 kW
2. Then calculate the required volumetric flow rate for cooling:
According to the above rule of thumb, if the temperature difference on the mold cavity surface is 5°C, flow rate = 6.377 × 0.75 = 4.78 gal/min or = 6.377 × 3.41 = 21.75 l/min. If the temperature difference on the mold cavity surface is 1°C, then flow rate = 4.78 × 5 = 23.9 gal/min or = 21.75 × 5 = 108.73 l/min.
3. Pump flow rate specification
To achieve good heat dissipation, the pump's flow capacity should be at least 10% greater than the calculated result, so a pump of 27 gal/min or 120 l/min is required.
4. Pump pressure specification;
Generally, the operating pressure of mold temperature controllers is 2-5 bar (29-72.5 psi). Since insufficient pressure can affect the volumetric flow rate of coolant (flow resistance causes pressure loss), the higher the pump pressure, the more stable the flow rate.
For molds with very small cooling channels (e.g., channel diameter 6mm/0.236 in), the pump pressure needs to be 10 bar (145 psi) to provide sufficient heat dissipation rate (i.e., coolant velocity).
Generally, the higher the required volumetric flow rate of coolant, and the smaller the channel diameter, the greater the required pump output pressure. Therefore, in general applications, the pressure of the mold temperature controller should exceed 3 bar (43.5 psi). B. Heating capacity
Figure 4.8 is a typical heating calculation table, providing the heating capacity required for the mold weight. The calculation usage of Figure 4.8 is as follows:
1. The vertical axis represents the mold weight.
2. The horizontal axis represents the heat required to raise the mold to the desired temperature, in kW/hr.
3. The temperature diagonal lines from 37°C to 121°C provide the relationship between mold weight and the heating capacity of the mold temperature controller at corresponding temperatures.
For example, we can find from the figure:
1. The heating capacity required to raise a mold weighing 500 kg to 50°C is 3.3 kW/hr.
