In a recent e-mail, I received this question regarding crystallinity…
Can you explain Crystallinity in layman’s terms? Also how the process affects it.
In essence… crystallinity is a phenomenon where segments within the polymer chain either (1) align themselves in an orderly, structured fashion or (2) align themselves with portions of adjacent polymer chains in a similar fashion. These semi-crystalline structures tend to improve properties such as strength, barrier, and impact.
These semi-crystalline structures do not exist when the polymer melt is above the melt temperature. As the polymer temperature drops below the melt temperature, these semi-crystalline structures begin to form. The faster the polymer cools, the fewer semi-crystalline structures appear. Likewise, the slower a polymer cools, the higher the degree of crystallinity within the polymer.
One if the biggest factors regarding crystallinity is dimensional stability… Since semi-crystalline regions are more dense than the polymer in the amorphous state, higher degrees of crystallinity create higher shrinkage. The more even the cooling, the more even the shrinkage, and therefor the more dimensionally stable the part will ultimately become.
Keep in mind, the same principles apply to both extrusion and blow molding. Ultimately, any process change that increases melt, mold, or die temperature will decrease the rate of cooling… resulting in an increase in crystallinity. This is one reason why it is always important to think twice before making a temperature change in your process.