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SOLIDWORKS Plastics: Calculation of closing pressure


Calculation of closing pressure


When a Flow and Pack calculation is performed on a Shell Mesh in SOLIDWORKS Plastics, are values ​​from closing pressure calculations missing from the Pack information? (See picture 1.)


This is how the software is designed - it requires a "Solid Mesh" to bring out these values.


Let us review a brief explanation of the packaging phase of the injection molding process.

When the plastic material melts, it expands, but the mass remains the same, resulting in a reduction in density.

When the expanded plastic material begins to shrink back to its solid density inside the mold cavity, it shrinks backwards away from the walls of the cavity.

In the Packing phase, more material is packed into the cavity to push the material waste out again for contact with the cavity walls (the walls of the mold cavity), so that the desired design achieves the right geometry.


A Shell Mesh consists of triangular 2D elements on each side of the part thickness. Between these elements the part is hollow, and the elements know only the distance between them.

This is usually referred to as "two and a half" (2.5D) dimensioned Mesh. A Shell Mesh does not represent the true volume above the part thickness.

Plastic materials are compressible, and a Shell Mesh does not fully represent the full volume of the part.

Therefore, we do not know exactly how much material may be in each element at a given temperature and pressure in a Pack analysis.

For the same reason, it is generally not recommended to perform a Pack analysis on items with a Shell Mesh.

Image 1: Displayed information "Clamp Force" is inaccurate "Shell Mesh"

Image 2: Surface Mesh, where the part is hollow without mesh elements.

Image 3: Closing pressure example, necessary for filling process.


It is strongly recommended to use a Solid Mesh for Pack analyses, just as the same recommendation applies for cooling and Warp / (Deformation) analyses.


Image 4: Solid Mesh bounded by Surface Mesh and solid elements throughout the thickness of the workpiece.


Same item as before, but with required closing pressure for Pack.

A difference of 150 tons. This information is only possible by using a Solid Mesh

I recommend only using Surface Mesh for initial party design to quickly get a general Flow result.

If you need help, you can call us at support.


Jens-Ole is an application engineer at Denmark's largest SOLIDWORKS dealer PLM Group. 

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