In the casting process, cavities that are not open and lined up with the sliding direction of the die are not feasible. Designs obtained by topology optimization often contain cavities that are not viable for casting. Transformation of such a design proposal to a manufacturable design could be extremely difficult, if not impossible.
OptiStruct allows you to impose draw direction constraints so that the topology determined will allow the die to slide in a given direction. These constraints are defined using the DTPL card. Different constraints can be applied to different structural parts, specified by PSOLID IDs. There are two DRAW options available. The option 'SINGLE' assumes that a single die will be used and it slides in the given drawing direction. The bottom surface of the considered casting part is the predefined contra part for the die. The option 'SPLIT' implies that two dies splitting apart in the given draw direction will be used to cast the part described in this DTPL card. The splitting surface of the two dies is optimized during the optimization process.
A casting may contain a non-designable region in addition to a designable region. These non-designable regions must be defined as obstacles for the casting process on the same DTPL card. This preserves the casting feasibility of the final structure.
Only the density method can be used for topology optimization with draw direction constraints. Therefore, it is possible that you will find the responses at the initial design differ from those obtained with the same input deck without using draw direction constraints. This is because the method used, when draw direction constraints are not present, could be the homogenization method.
Also note that there is a default minimum member size for use with draw direction constraints. This is determined internally to be three times the average mesh size of the relevant components. Therefore, the mesh density of the model and the target volume fraction should be chosen so that enough material is available to fill members of the default minimum size. The user can specify a desired minimum member size for each design part defined by a DTPL card. This value must be bigger than the default value or else it will be replaced by the default value.
Example 1: Beam under Torsion
The considered beam is clamped on one end and loaded with a pair of twisting forces on the free end. The finite element model is shown in Figure 1.1. The design problem is to minimize the compliance with a volume fraction constraint of 0.3. The final design without draw direction constraints is shown in Figure 1.2. The chosen draw direction is along the Z-axis. The designs with the options 'SINGLE’ and 'SPLIT' for draw direction constraint are shown in figures.1.3 and 1.4, respectively.
Figure 1.1: Finite element model of the beam under torsion
Figure 1.2: Design without draw direction manufacturing constraint
Figure 1.3: Design with draw direction Z and die option 'SINGLE'
Figure 1.4: Design with draw direction Z and die option 'SPLIT'
As expected, the result without manufacturing constraints is a tube-like structure that is indeed the optimal topology for torsion load. However, this design does not permit the sliding of the die in the Z direction. The result that allows the sliding of the die for casting is not very intuitive, it forms a periodical X pattern to cross the pair of twist loads until they reach the supported end. Significantly more material at the crossing point reflects the doubled shear force at this point. Compared to an upward facing C channel solution, the cross pattern has the advantage that the stress is periodical in every X cell, thus eliminating higher order influence of the span of the beam for any solution that has system level bending action.
Example 2: Engine Bracket
The example shown below is an engine bracket model of a car. The finite element model of the design domain is shown in Figure 2.1, in which 9046 elements are used and the design domain is shown in blue color. Six load cases were considered, which reflect the following driving and service status: 1) start; 2) backwards; 3) into a pothole; 4) out of a pothole; and 5) loads from an attaching part and 6) loads during engine transport. The final topology that allows a single die sliding upwards is shown in Figure 2.2. The design that allows two dies to slide up and down, respectively, is shown in Figure 2.3.
Figure 2.1: Finite element model of a engine bracket
Figure 2.2: Design with draw direction Z and die option 'SINGLE'
Figure 2.3: Design with draw direction Z and die option 'SPLIT'
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Design Variables for Topology Optimization