Criteria for assessing cooling performance
Cooling circuit design is always a compromise between achieving reasonably uniform cooling and the shortest possible cycle time.
The most acceptable compromise will vary from part to part. In some cases, quality requirements mean that uniform cooling is most important, whereas in other cases cost requirements mean that minimizing the cycle time is the priority.
Cooling performance parameters
A number of parameters are listed below which enable you to draw some conclusions about the efficiency of the cooling system. General values to be used in the design of new molds are shown below.
| Parameters | General Guideline |
|---|---|
| Max. variation between temperature across bottom face of part and target mold temperature | 10°C |
| Max. variation between temp across top face of part and target mold temperature | 10°C |
| Max. variation in temperature across thickness of part | The acceptable variation in temperature between the inside and outside of a plastic part at the time of ejection will depend on the local stiffness of the part and the percentage of the thickness frozen. Large flat areas with big temperature variations and which are not completely frozen are more prone to warping than structures that are stiffer. This is due either to their shape or their temperature at ejection. |
| Min. % of wall thickness frozen on both top and bottom faces of part | The amount of the wall thickness which must be frozen before ejection depends on the part's stiffness (as above), the degree to which the part is resisting ejection (due, for example, to mold finish or overpacking) and the design and position of the ejector system components. |
| Max. difference between the average temperature, part result and target mold temperature | The Automatic analysis attempts to reduce the average cavity temperature to within 1°C of the target mold temperature used in the original Fill+Pack analysis sequence. In some cases, this requirement creates excessively long cooling times with the part being totally frozen and cooled to well below its ejection. This problem is normally caused by the coolant inlet temperature being too close to the target mold temperature specified in the process setting wizard. To rectify this problem, lower the coolant temperature or raise the target emperature. A typical difference between the coolant and average part surface will be between 10°C and 30°C for molds made from P20 steels. |
| Max. difference between coolant inlet temp and circuit metal temp in any circuit | 5°C |
| Max. coolant temperature rise from start to finish of any circuit | 2°C |
| Pressure required to circulate coolant | The pressure required to circulate the coolant must be within the available system pressure. This value will depend on whether the coolant is being supplied from a heater/circulator unit, from a cooling tower system, a chiller or from the mains. In the case of heater/circulator units, the manufacturers normally supply performance curves showing the pressures available at various flow rates. |
| Max. cycle time | Minimum possible |