Some of the heat supplied to the material during the plasticising andinjection phase must be removed from the tool before ejection of the parts can take place. However, not only is it unnecessary it is also wasteful to continue cooling the tool until the part has reached
ambient temperature, as many materials may be safely ejected at temperatures up to 50 °C or more.
The cooling phase can be up to 80% of the overall cycle and often the most expensive cost component of the moulding. Clearly it is highly desirable to minimise the cooling cycle, and in order to do this it is essential to pay sufficient attention to the design and
efficiency of all cooling systems.
The overall requirement is to cool the moulding as quickly as possible while preserving the physical properties of the material and the required quality of the moulding. When molten material is injected into the cavities of a mould it has to be allowed to solidify
before the resulting mouldings can be ejected. As the ejection temperature may be 200–300 °C lower than that of the molten material, heat must be removed from the mould to reach ejection temperature as soon as possible.
A mould tool, is in effect, a highly stressed heat exchanger and the cooling phase of the injection moulding cycle is extremely important. The overall objective is to cool the polymer as quickly as possible to a temperature at which the moulding can be safely
ejected. In general terms, amorphous materials that have a random structure may be cooled quickly in most cases without any ill effect on the properties of the material.
Crystalline materials are different. They have a preferred almost linear molecular chain structure and in this state they are stable and have their full physical properties. However, when the molten polymer is first injected into the cavity these molecular chains have a random orientation similar to amorphous materials. If this material is prematurely frozen while it is still in this state, the result will be far from satisfactory. The properties will be impaired and the moulding will strive to achieve its preferred structure after it has been ejected. Over a period of time the moulding will warp and distort and suffer dimensional changes.
As crystalline materials are frequently used for technical applications, this situation is clearly undesirable as the mouldings will be in an unsuitable state for use. To avoid this problem, heat must be supplied to the mould tool rather than cold water.
While all mouldings require cooling it is the rate of cooling that is most important. With the supply of heat to the mould tool instead of cold water, the crystalline polymers do not suffer such a high degree of thermal shock. The material is given more time for its preferred structure to form, which results in less internal stress being set up.
The temperature of the mould tool should be:
- High enough to allow the cavities to fill without premature freezing of the material.
- As uniform as possible to ensure the moulding is cooled equally in all areas.
- High enough with crystalline materials to avoid an unsatisfactory structure.
Walsh's Plastic consulting. 