Average volumetric shrinkage result

The average volumetric shrinkage result shows the average value of volumetric shrinkage over the half-gap thickness for 3D models.

Volumetric shrinkage is the percentage increase in local density from the end of the packing phase to when the part has cooled to the ambient reference temperature (the default value is 25°C/77°F).

Note: The packing phase includes both packing time and cooling (holding) time.

Volumetric shrinkage calculations begin once the cavity is filled, based on the difference between the current pvT state and the reference state:

Where:

• VS is the volumetric shrinkage
• AD is the average density
• D is the density
• the pressure p is zero and temperature T is the specified ambient temperature

As the mass of an element changes (for example, with polymer flow during packing), shrinkage continues to change with each change in the element's pvT state. Once the mass stops changing, the element's current pvT state is fixed in the shrinkage calculation as the reference state.

The mass of an element stops changing when the cavity pressure has decayed to zero. After this, the volumetric shrinkage becomes a constant. However, if the holding pressure is removed before the material is frozen or while the pressure in the cavity is still non-zero, the volumetric shrinkage may rebound due to possible backflow into the nozzle or other warmer areas of the part.

Using this result

For unambiguous interpretation of the Volumetric shrinkage result, deselect the Nodal averaged display option. Right-click the result name and select Properties, then select the the Optional Settings tab, then deselect Nodal averaged.

The average volumetric shrinkage for 3D is the average value of volumetric shrinkage over the half-gap thickness, and is plotted on the surface. This result can be used to detect sink marks on your model. High shrinkage values could indicate sink marks or voids inside the part.

Volumetric shrinkage should be uniform across the whole part to reduce warpage.

Note: For thermoplastic materials, volumetric shrinkage should be less than the observed maximum value for the material. You can find material shrinkage information in the Thermoplastic material database. Right-click the material in the Study Tasks pane and select Details. The Thermoplastics material dialog is displayed. Select the Shrinkage Properties tab. The observed maximum shrinkage values are in the Observed shrinkage section.

Material shrinkage information is not provided in the Thermoset material database.

Volumetric shrinkage can be controlled by the use of packing profiles.

Things to look for

• Localized areas of high shrinkage can result in internal voids or sink marks when the part cools.
• Shrinkage values should be uniform throughout the part. This is important for good packing of the material, ensuring good structural and visual integrity of the part. Use a packing profile to make the shrinkage more uniform.
• Negative volumetric values indicate expansion rather than shrinkage. Avoid negative shrinkage on ribs as this can cause ejection problems.
• Are the values in the expected range for the material?
  • Materials that shrink isotropically have a linear shrinkage which is approximately one third of the volumetric shrinkage.

    For molded materials, the linear shrinkages in the thickness, flow and transverse directions depend on the effects of relaxation and orientation.

  • For shell-like geometries, it is expected that the shrinkage in the thickness direction should be higher than the shrinkage in the plane of the part. Shrinkage in the thickness direction is likely to be greater than one third of the volumetric shrinkage, while in-plane shrinkage should be less than one third of the volumetric shrinkage. Many mold features act as constraints to in-plane shrinkage. If you are using a fiber-filled material, the orientation of the fibers in the plane of the part will limit shrinkage in this direction. Shrinkage in the thickness direction is relatively unconstrained.