Volumetric shrinkage result

The Volumetric shrinkage result shows the volumetric shrinkage for each node, as a percentage of the original volume.

Mesh type:

• Midplane
• Dual Domain
• 3D

Analysis sequences that include:

• Pack

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 the pressure p is zero and temperature T is the specified ambient temperature):

(1)

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.

Once the part has filled, or a short-shot has occurred, the software checks to determine whether cooling can start at the injection location. Cooling at a specific injection location starts when the flow rate at that injection node drops below a defined value for two time steps in a row. Until that time, the node is considered at melt temperature. Thus, different injection locations can start cooling at different times. Once a location has started cooling, it will continue cooling regardless of the flow rate. See Injection location temperature during packing and cooling for more information.

Using this result

This result can be used to detect sink marks on your model. High shrinkage values could indicate sink marks or voids inside the part.

To minimize warpage, the variation in volumetric shrinkage throughout the cavity should be minimized. If the material has been characterized for shrinkage (thermoplastic materials only), the magnitude of the volumetric shrinkage can also be compared to the range of volumetric shrinkage values found in the Shrinkage Molding Summary table, which will provide a guide to typical average volumetric shrinkage values for the material.

To access the Shrinkage Molding Summary table, right-click the material in the Study Tasks pane and select Details. Select the Shrinkage Properties tab from the Thermoplastics material dialog.

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

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

Note: Use the Cutting Plane tool to view 3D Volumetric shrinkage results. A cutting plane shows volumetric shrinkage values through the solid model. This will tell you whether your solid part is likely to shrink on the inside.

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?
  • For isotropic 3D solid materials, linear shrinkage is approximately one third of the volumetric shrinkage, where volumetric shrinkage is evenly distributed in all directions. This value can be considered an upper bound.
  • For transversely isotropic materials in thick parts, volumetric shrinkage is approximately equal to the shrinkage in the flow direction plus two times the shrinkage in the transverse direction.
  • 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.