Core shift simulation

Core shift is the spatial deviation in the position of the core from its original position in the mold before plastic is injected into the cavity.

It is a frequent problem with long, slender, and not necessarily thin-walled products, such as vials, test tubes, and pen barrels. It is also experienced often in molds for thin-walled containers.

Core shift can result in undesirable variations in wall thickness which will affect the final shape and mechanical performance of the part. The Core shift simulation provides detailed information about the movement of the mold core and its interaction with the polymer flow process as the plastic is being injected. Designers can use this information to correct for the core shift phenomenon, for example, by modifying the design of the part, or adjusting process conditions such as the gate location or core/mold temperatures.

There are three main causes of core shift:

• Inaccuracies in the machining or setting of the mold which leads to alignment problems when the mold is closed.
• Deflection of the platens or mold plates due to insufficient strength under the high injection pressures experienced during molding.
• Deflection of the core as a result of pressure differentials on opposing mold walls. These differentials arise as a result of the gate location or variations in part thickness.

The Core shift simulation in Fill+Pack accounts for the third cause described above; the other two causes are difficult to model with a Fill+Pack analysis product such as Autodesk Moldflow Insight.

How the Core shift analysis works

The Core shift simulation uses the Stress analysis capabilities of the warpage product to predict the deflection of the core under the loads (pressures) experienced in the molding process. The analysis scheme can be outlined as follows:

• A portion of the cavity is filled (the %volume increment is user-defined in the solver parameters).
• The Fill+Pack analysis provides the pressure load distribution on the core.
• The pressure load data is used to perform a structural analysis of the core.
• The structural analysis determines the core deflection.
• The core deflection is used to calculate the change in part thickness.
• The part mesh is adjusted to account for the change in thickness.
• The above steps are repeated until the cavity is filled.
• If the packing analysis is also being simulated, the structural analysis is also repeated at regular time increments during the packing phase.

For a detailed description of how core shift is simulated, see the following published article:

Bakharev, A., Fan, Z., Costa, F., Han, S., Jin, X. and Kennedy, P., Prediction of Core Shift Effects using Mold Filling Simulation, Soc. Plastics Engineers, ANTEC 04.