Definition: Rate of Cooling, 1: rate - a certain value or quantity, 2. charge with reference to a calculation, 3. cooling -moderately free of heat 4- combined: how fast an object is cooled, or how fast heat is allow to be taken away in a controlled manner.
The points of concern with temperature have been address previously in articles, but the one omission is why we care. The aspect of temperature control within the plastic community is that it is best to have control over it within the part when molded or extruded, and or by what ever process being used. This control in the cooling phase does lead to proper crystallization of the material. One most famous uttered phrase has to be "it measured to print when I made it." When discussing cooling it is best to describe it as cooling rate. Cooling rate is the time it takes to cool the object so as to be handled properly or ejected from our mold.
In the abstract the mold in principle is a radiator similar to that of a car, except that it is cooling the plastic at hopefully the correct rate. In the extrusion process it is either air or water which is performing this function against the plastic. The rate at which the part is cooled does influence the physical properties, size and shape of the product. This effect is most notable in semi crystalline parts, because of the crystalline nature that they have. Amorphous parts, due to their relative low shrinkage and softening characteristics are less of an issue.
In analytical testing there is an instrument / test referred to as a DSC or differential scanning calorimeter. It basically measures the energy adsorption within our sample and can show graphically the melting point and may also show incomplete crystallization within our sample. (please a very basic functionality) The key to this testing is not to normalize the sample beforehand. The test goes through a heat up and a cool down at a preprogrammed rate.
What this means is that documentation of the first testing (as molded) would yield what was produced by the process. If the test were reran it would produce what the DSC has been programmed to produced. Any difference may be attributed to the process conditions, similarities are materials characteristics.
In semi-crystalline parts the cooling rate is what forms the degree or percentage of crystalline growth. Thus if a very hot mold were to be used with a material such as HDPE the result would be to create a very large crystal site, and thus achieve a very high shrinkage rate. In the same case if a cold mold/ water drop were used to quench the part rapidly the result would be many small crystal sites and have minimal shrinkage which results in a larger part. Please note that semi- crystalline means just that, we have both amorphous areas and crystalline areas within the polymer, a fully crystallized nylon 6 for example may only be 35% crystal (example use only).
The consequences of incomplete crystallization are the following:
- Poor dimensional stability
- Reduced fatigue resistance
- Reduced creep resistance
- Reduced chemical resistance
- Warpage
- Reduced modulus and strength at elevated temperatures
The issue that comes to mind the most is size. Within many material suppliers trouble shooting guides they well comment on part size and relate / suggest that the mold temperature be adjusted to influence size. But the issue that is not brought up is that if the part is going to be placed in an environment that is above the mold temperature and the mold temperature is at a point on the chart where it does not achieve the maximum crystallization growth the part will change size in its new environment.
Another issue with part size is that it changes shot to shot when molders have their steel temperature set on the steep part of the crystallization curve. In nylon this is between the temperatures of 95ºF and 185ºF.
Another example is PPS where (in various supplier data) in manuals a statement is made to keep the mold temperature either above 135ºC or below 82ºC, and that if you operate between those temperatures you will see varying results for your dimensions.
Further the graph below from a supplier illustrates the effects on heat distortion temperature dependant on mold temperature.
The point is that there needs to be awareness of what our steel temperature is and further what the part/material requires in the end use environment. Ideally someone would have specified or stated what the end use environment was to be so that all could be sure of the rate of cooling necessary, and degree of crystallinity needed for that application. If not than the molder and plant may produce product which meets a size specification yet when transported to the field and in use fails.
