Tutorial: Analyzing Skirt Surface Options: ShutOff Extension

Analyzing Skirt Surface Options: ShutOff Extension

Analyzing Skirt Surface Options: ShutOff Extension

By default, the outer loops of a specified silhouette curve are extended to the boundaries of the workpiece. This may cause an undesirable shape in the resulting core and cavity mold components.

You can specify a shutoff extension to stop the skirt surface from extending all the way out to the workpiece boundaries by using the ShutOff Ext option in the Skirt Surface dialog box. The ShutOff Ext option enables you to specify the amount of extension toward the workpiece boundaries that the skirt surface will undergo before stopping, extending in the pull direction, and finally stopping at the workpiece boundaries. There are two methods available for specifying the shutoff extension location:

- ShutOff Dist – Specifies a uniform offset value around the silhouette curve perimeter that the skirt surface will extend. In the upper-right figure, a shutoff distance of 2 was specified.

- Boundary – Enables you to specify your own boundary that the skirt surface will extend out to. You can use either of the following two methods to specify the boundary:

  • Select – Enables you to select an existing sketch as the boundary. In the lower-left figure, the sketch created on the top surface of the workpiece was selected as the boundary.
  • Sketch – Enables you to sketch the boundary on-the-fly by specifying the sketch plane, reference plane, and reference direction. In the lower-right figure, the boundary was sketched on the top surface of the workpiece.

Regardless of which boundary method is used, the boundary must form a closed loop. It is not necessary for the boundary to be located on a specific sketching plane or even a sketching plane that is perpendicular to the pull direction. However, the boundary is ultimately extended in the pull direction, so if the boundary is created on a plane that is not normal to the pull direction, you may not get the expected result.

When the skirt surface extends outward from the silhouette curve, it stops either at the workpiece boundaries or a shutoff extension, whichever it encounters first. If the selected boundary falls outside of the workpiece boundaries, the skirt surface will stop at the workpiece boundaries. In the lower-right figure, only one of the four sides of the sketched square boundary falls within the workpiece boundaries. Hence, the skirt surface stops extending at the workpiece boundaries on three sides, and at the sketched boundary on the fourth side.

Specifying the ShutOff Plane

The ShutOff Plane option is used in conjunction with the ShutOff Extension option. The shutoff plane is the planar reference that the shutoff extension extends to. The shutoff extension extends in the positive or negative Z-axis (pull direction) up to the shutoff plane and finally extends outward again to the workpiece boundaries. The shutoff plane specified in all three figures is datum plane SHUTOFF.

While the ShutOff Plane is optional in the Skirt Surface dialog box, it is a required reference if a shutoff extension is defined. Without the shutoff plane defined, the shutoff extension does not extend at all in the Z-axis, and therefore is not used.

The following rules apply to the shutoff plane reference specified:

  • The reference you specify must be planar.
  • There can be only one shutoff plane specified.
  • The shutoff plane must be normal to the pull direction.
  • The shutoff plane may be above the skirt surface. However, the resulting extension may not create valid geometry for a mold parting line.

Specifying the Draft Angle

The Draft Angle option is also used in conjunction with the ShutOff Extension option, although it is not a requirement. The draft angle option drafts the Z-direction surfaces of the shutoff extension by the specified draft angle value. You might want to apply a draft angle to these surfaces to help with wear in the resulting mold core and cavity. Note that you can only specify a positive draft angle. The draft angle specified in all three figures is 6.

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