Compression molding

In compression molding, a polymer charge is placed into an open heated cavity. The cavity is then closed and compressed to force the material to fill the entire cavity.

Compression molding can be used with both thermoplastic and thermoset materials.

Considerations

In compression molding there are several considerations to keep in mind:

• the proper amount of material to ensure the cavity is filled
• optimizing the cooling circuit to ensure rapid cooling of the part
• the force required to compress the initial charge of material
• the minimum time and energy required to ensure the plastic melts

Polymer charge

For 3D compression molding analyses, you must create a block of mesh to act as the initial polymer charge. The charge mesh is used to define which nodes in the part mesh will already be filled, at the start of the analysis. Make sure the charge mesh is a realistic representation of the actual charge geometry; only charge elements that are inside the XY lateral extent of the part mesh will be considered for the simulation.

The charge mesh can be created in an offset position relative to the part. This is for convenience, to ensure the part mesh and the charge mesh are not overlayed before the start of the analysis. At the start of the analysis, the solver automatically moves the initial charge mesh into the starting position above the part mesh. As the simulation progresses, the solver continues to move the charge mesh as far as possible into the mold cavity, in the appropriate, user-defined Z-direction (positive or negative) towards the fixed side of the cavity mesh so that it overlays the part mesh correctly.

Best practices

For best simulation results,

• use plenty of layers of elements in the Z-direction in the part mesh, especially in the XY region of part mesh that will be compressed. This region of the part mesh region will get stretched by the press open distance at the start of analysis so more layers helps accuracy.
• The accuracy of the outer surface of the initial charge is more important than the number of mesh layers.

Warnings

Initial charge is too large

If the initial charge is more than enough to fill the part, the solver gives a warning before the timesteps start, such as:

WARNING 307018 ** The volume obtained from the initially filled compression element is too big compared to the volume needed to fill the cavity. It may lead to an overfill.

Initial charge is too small

If the initial charge is not enough to fill the part, the solver gives a warning before the timesteps start, such as:

WARNING 307016 ** The volume obtained from the initially filled compression element is too small compared to the volume needed to fill the cavity. It may lead to a short shot.