Shrinkage

Shrinkage is inherent in the injection molding process. Shrinkage occurs because the density of polymer varies from the processing temperature to the ambient temperature. The shrinkage of ed plastic parts can be as much as 20 percent by volume, when measured at the processing temperature and the ambient temperature.

Semi-crystalline materials are particularly prone to thermal shrinkage; amorphous materials tend to shrink less. When crystalline materials are cooled below their transition temperature, the molecules arrange themselves in a more orderly way,  forming crystallites. On the other hand, the microstructure of amorphous materials does not change with the phase change. This difference leads to semi-crystalline materials having a greater difference in specific volume between the processing state (point A) and the state at room temperature and atmospheric pressure (point B) than amorphous materials (see figure below).

pvT curves for amorphous and crystalline polymers.

During injection molding, the variation in shrinkage both globally and through the cross section of a part creates internal stresses, (residual stresses). If the residual stresses are high enough to overcome the structural integrity of the part, the part will warp upon ejection from the mold or crack with external service load.  

Uncompensated volumetric contraction leads to either sink marks or voids in the interior of the part. Shrinkage that leads to sink marks or voids can be reduced or eliminated by packing the cavity after filling. Controlling part shrinkage is particularly important in applications requiring tight tolerances.

Excessive shrinkage can be caused by a number of factors:

• Low effective holding pressure
• Short pack-hold time or cooling time
• Fast freezing off of gate
• High melt temperature
• High temperature
  

The relationship of shrinkage to several processing parameters and part thickness is shown schematically in the following figure.

Processing and design parameters that affect part shrinkage.

For both unfilled amorphous and mineral-reinforced thermoplastics shrinkage is largely isotropic; shrinkage in flow direction is about equal to the shrinkage across flow. The glass fiber reinforced grades, on the other hand, show anisotropic properties. Due to fiber orientation in the direction of the melt flow, shrinkage values in flow direction often are substantially smaller than across flow direction (see figure below).

The assumption a material has isotropic properties is often a good starting point but if anisotopy is totally ignored significant errors can occur in the design of thermoplastic parts.

The designer must be aware that as the degree of anistopy increases so does the number of moduli required to describe the material, up to a maximum of 21. Therefore especially for glass-reinforced materials the use of simple analysis techniques has limited value and extensive FEA (finite element analysis) methods are often required to analyze anistropic materials in critical applications.

Not only fiber orientation but also molecular orientation can lead to anistropic shrinkage. An unfilled molded part containing high levels of molecular orientation, due to high shear stresses, can show anistropic shrinkage because aligned chains shrink more in the direction of orientation.

Relation between the shrinkage of glass fiber reinforced plastics and the orientation of the glass fibers (in thickness direction).