Highly crystalline polymers exhibit relatively high shrinkage when cooled from the molten state. Almost all molten polymers have a completely amorphous structure and lack molecular orientation. However, highly crystalline polymers develop a partially crystalline structure after cooling, which is their natural or relaxed state. Crystallization causes the polymer to increase in density as it forms a crystalline structure. The molecules arrange into a more ordered and more tightly packed structure than in the amorphous state. For example, when high-density polyethylene (HDPE) is cooled from the molten state to its semi-crystalline natural state, its density increases from approximately 49 lb/ft³ to 59 lb/ft³. This represents a 20% increase in density, or an overall shrinkage of 20%. Other crystalline polymers include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), polypropylene (PP), nylon 66, polyethylene terephthalate (PET), polylactic acid (PLA), and polytetrafluoroethylene (PTFE).
At intermediate temperatures between the polymer's melting point and its glass transition temperature, crystallization proceeds very rapidly. When the temperature drops below this upper limit, the crystallization rate decreases rapidly because the molecular mobility required to reach the relaxed state is reduced. In most processes, 60% to 80% of crystallization occurs during the cooling stage of processing, and as much as 90% occurs within one week after processing. The remaining crystallization may take several months to complete, depending on the temperature. Nevertheless, crystallization continues until a stable crystalline structure is formed.
Why does this matter? An amorphous structure may be temporarily "frozen in" by rapid cooling. However, because polymers are poor conductors of heat, one surface of an extruded product may be cooled while the opposite surface may still be hot when it leaves the production line. This causes one side to develop a largely amorphous structure, while the other side develops a partially crystalline structure. Since the crystalline portion is denser and shrinks more than the amorphous portion, considerable internal stress is generated within the part when one side shrinks more than the other.
In extruded sheet, rapid cooling on one side can cause warpage. In pipe, this effect creates high stress in the pipe wall, reducing its physical properties, especially impact resistance and stress-crack resistance. In blow-molded products, it can cause wall distortion and uneven stress between thinner and thicker sections. This negatively affects stress-crack resistance and drop impact strength. In complex profiles, it can lead to edge warpage and even sink marks.
Control of crystallization or shrinkage, and the resulting stresses, depends on controlling the cooling rate across the entire part. This can be achieved by reducing the overall cooling rate or by interrupting cooling so that the outer surface of the extruded part cools more slowly and heat from the opposite side is allowed to diffuse back into the cooled surface. Since extruded parts often need to rapidly freeze their external dimensions to conform to a specific shape, the initial cooling usually must be sufficient to set the shape. If rapid cooling continues, internal stress may develop within the part.
How can overcooling be identified? A simple and quick method is to cut small specimens from the extrudate and heat them in an oven at approximately 200 to 250°F for a few minutes, then cool them and measure the shrinkage of the cooled surface compared with the opposite surface. The specific heating conditions and time will vary depending on the polymer and the thickness of the specimen.
Curved samples can be measured using tape. Complex surfaces that are difficult to measure can be evaluated by comparing their deformation with a fully annealed control sample.
These measurements should not be confused with directionality caused by drawing or post-extrusion stretching. Because of drawing and die forming, almost all thick-walled extruded parts show greater longitudinal shrinkage than transverse shrinkage. To evaluate cooling shrinkage separately, only the differences in relative surface dimensions should be compared, rather than the overall shrinkage of the workpiece.
Uneven cooling can lead to several types of part failure. For example, as the amorphous layer transforms to its stable crystalline structure, the layer can be subjected to significant stress, which may lead to post-extrusion cracking. In cases of severe uneven cooling, these failures may even appear as layer separation and be misdiagnosed as lamination problems caused by poor mixing.
If part failures occur at low stress levels, the cause may simply be excessively rapid cooling. This is particularly common when cooling occurs from only one side. To test, simply shrink a few specimens; if the surface differences are significant, reduce the coolant temperature or interrupt cooling to anneal the part.
