Demoldable Surface (SURF_DEMOLD)

Controller-Based Optimization

Defining a demolding or forging restriction involves specifying the manufacturing direction. The area to restrict is defined using a node group of surface nodes. Demolding in the specified direction or forging against this direction is maintained during optimization and undercuts are avoided. The link condition for the demold restriction is defined as follows:

CLIENT      = SURF_DEMOLD
DEMOLD_DIR  = <x_1>,<x_2>,<x_3>
CS          = name_of_coord_system
CHECK_GROUP = name_of_node_group
TOL         = <tol_1>, <tol_2>, <tol_3>

The direction defined by the DEMOLD_DIR specifies the demolding direction respectively the negative forging direction. All nodes in the node group are checked for undercuts and for demolding against the specified CHECK_GROUP. This group of surface nodes should qualify the cast sufficiently. In the simplest case, the group of all surface nodes can be chosen. In the following figure, node group 1 describes the demolding area. If the check group contains only node group 1 the undercut is not detected because the check group describes no restricting surface. Only node group 1 and 2 together qualify a surface to detect undercuts reliably.

In addition, an angle for the surface inclination might be specified using the ANGLE parameter:

ANGLE = <real>        

The undercut tolerance specifies the maximum valid undercut (default = 0):

 UNDERCUT_TOL = <real>

Important: It is possible to define demolding restrictions for initial components that are not demoldable at the beginning of the optimization procedure. However, this is not recommended as the conditions for demolding are met in the first design cycle. This might lead to large node displacements and a distorted finite element mesh. No single main nodes need to be determined for demolding restrictions. The main criterion in this case is used to determine a priority rule between nodes in the undercut and overlapping nodes. All SURF_xxx link commands have a parameter called SURF_PARAM = <main_areas>, <net_points> that are used to create a spline that defines the surface in the SURF_xxx LINK_SHAPE commands. The values <main_areas>, <net_points> are integer values and automatically determined by Tosca Structure.

• In the event of an error like

  
  
  

  it might help setting SURF_PARAM. Important is that <main_areas> must be larger than 4 and at least double the size of <net_points>. Good values are, for example, SURF_PARAM = 12, 4.

Important: Another reason for the above error message is that there is not enough nodes in the design area to create the surface for the SURF_xxx condition. Default tolerances for all SURF_xxx LINK_SHAPE commands are TOL = 0.01, 0.01, 0.01. If you have a mesh where these values are either very large or very small consider setting them to about 0.25 * element edge length in design area.

Sensitivity-Based Optimization

The commands used to set up the demold manufacturing constraint for sensitivity-based shape optimization does not differ much from how to set it up for the controller-based shape optimization, but the enforcement happens in another way. For the sensitivity approach, a face is enforced to be demoldable by adding a constraint to the optimization problem stating that the angle between the face normal vector and the demold direction must be less (or equal) to 90°.

Since too many optimization constraints slow down the optimization process too much, the constraints for one demold command are aggregated into one single constraint by summing up the single constraints:

$c_{\text{dem}}={\left(\frac{1}{m}\sum _{i=1}^m{\left(g_k\right(x)-g_{\text{shift}})}^p\right)}^{\frac{1}{p}}+g_{\text{shift}}$

Two parameters that influence the behavior of the aggregation are

AGGREGATION_PARAMETER=<real>

and

AGGREGATION_NUM_CONSTRAINTS=<int>

. AGGREGATION_PARAMETER is used as the power $p$ in the aggregation method and AGGREGATION_NUM_CONSTRAINTS is used to switch back to the "one constraint per face" strategy. To do so, select AGGREGATION_NUM_CONSTRAINTS=-1

A command example might look like the following:

LINK_SHAPE
  ID_NAME        = demold
  CLIENT         = SURF_DEMOLD
  DEMOLD_DIR     = 0.,0.,1.
  CS             = CS_0
  ANGLE          = 0.0
END_
 
DVCON_SHAPE
  ID_NAME        = dvcon_demold
  ND_GROUP       = demold_nodes
  CHECK_LINK     = demold
END_

The faces covered by the node group demold_nodes will be demoldable in z-direction.