Generative Design > Generative Design Workflow > Define a Generative Study > Design Criteria
Design Criteria
Design criteria for a study includes specifying the following:
• Design goal or objective for the study
• Manufacturing and geometric constraints
• Materials to use for manufacturing the model
You can define multiple design criteria for a study, but only one can be active at a time. During topology optimization and generating designs, only the active design criteria are considered.
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You might notice that the mass of the generated body is slightly different than the set target. For more accurate results, it is recommended to specify a smaller mesh element size.
The bodies designated as preserved, excluded, and starting geometries may have materials assigned to them. During topology optimization and generating designs, the following materials are considered:
• Designated bodies—The materials assigned to individual bodies are ignored and the active material defined in the active design criteria is considered.
• Undesignated bodies—The materials assigned to individual bodies are considered.
To Define a Design Criteria for the Study
1. Click Add Design Criteria. The Design Criteria dialog box opens.
2. Select the Design Goals based on your study type.
Study Type
Design Goal Selection
Structural Study
Select one of the following:
• Select Maximize stiffness and specify the target volume percentage, or specify the target mass and select a unit from the list.
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By default, target volume (in %) is selected in the design goal. Target volume is the percentage of the starting geometry volume.
If you designate a body as the preserved geometry without defining the starting geometry, the default design goal is limit mass.
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• When target volume is the design goal, the properties of the active material may have minimal effect on the optimization result.
• When you want different optimization results for different materials, you should select target mass as the design goal.
• Select Minimize mass and specify the safety factor.
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To use safety factor, the material must have yield strength value. Define the yield stress value for the material.
The material assigned to the undesignated bodies must also have yield strength value.
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A combination of safety factor design goal and Displacement constraint is not recommended. Use only one of them.
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Run initial simulation to get an idea about the success of optimization with the desired safety factor. An optimization with a large difference between the lowest safety factor on the starting geometry and the desired safety factor is difficult to achieve.
Avoid stress concentrations on the starting geometry to get the most the optimized resultant geometry.
Modal Study
Select one of the following:
• Select Maximize Fundamental Frequency and specify the target volume percentage, or specify the target mass and select a unit from the list.
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By default, target volume (in %) is selected in the design goal. Target volume is the percentage of the starting geometry volume.
To use target volume (in %) as the design goal, you must designate a body as the starting geometry.
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• When target volume is the design goal, the properties of the active material may have minimal effect on the optimization result.
• When you want different optimization results for different materials, you should select target mass as the design goal.
• Select Minimize mass and specify the lowest value of the fundamental frequency.
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The fundamental frequency will be higher than the specified frequency for all load cases.
3. Under Design Constraints, click Add Constraints and select the relevant manufacturing constraints and geometric constraints.
4. To add materials to the design criteria, click Add Materials. The Materials dialog box opens. The master material of the part appears in the Materials in Model list.
a. To add more materials, double-click the materials from the Materials in Library list. The selected materials are added to the Materials in Model list.
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You can add up to 10 materials to the design criteria.
b. Click Select. The Materials dialog box closes. The selected materials appear under the Materials section of the Design Criteria dialog box.
5. To set a material as the active material, move the pointer over the material and click .
6. Click OK. The design criteria are listed under Design Criteria node in the Generative Tree.
Manufacturing and Geometric Constraints
You can add different manufacturing and geometric constraints to the design criteria. The following table describes the available constraints and the steps to add them to the design criteria:
Constraint
Steps to Add the Constraint
Build Direction—This manufacturing constraint helps in reducing the amount of support needed at the time of 3D printing.
You specify the direction of 3D printing and the value of the critical angle. Critical angle is the maximum angle value with respect to the print direction at which supports are not needed.
1. Click Add Constraints, and then select Build Direction. The Design Criteria dialog box expands.
2. Click in the Build direction box.
3. In the graphics window, select a surface, a coordinate system, the Csys axis, an edge, or a datum plane as the reference. An arrow appears that shows the build direction.
4. To change the build direction, do one of the following:
◦ In the graphics window, click the arrow.
◦ In the Design Criteria dialog box, click .
5. In the Critical angle box, specify the value.
Parting Line—This manufacturing constraint can be used in casting and forging methods.
You specify the type of the parting line, 2D parting line or 3D parting line. A parting line is a line on the part that indicates the contact between the base plate and the top plate. A 2D parting line lies on a datum plane while a 3D parting line is not restricted to any plane.
You also specify the pull direction and the draft angle, the angle between the walls of the mold plates.
1. Click Add Constraints, and then select Parting Line. The Design Criteria dialog box expands.
2. Click in the Pull direction box.
3. In the graphics window, select a surface or a datum plane as the reference.
4. In the Design Criteria dialog box, specify the Draft angle value.
5. To define the Draft line, do one of the following:
◦ Click 2D and select a plane in the graphics window.
◦ Click 3D.
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After selecting the Parting Line constraint, you cannot select the Linear Extrude constraint. They are opposing manufacturing constraints.
Linear Extrude—This manufacturing constraint can be used in the 2-axis and 3-axis milling methods.
This constraint creates a linear pull direction extrude, which is the direction of the tool used for milling.
You can have a unidirectional or bidirectional linear extrude. A unidirectional extrude is flat on one side while on the other side, it is in free form for a 3-axis milling machine. A bidirectional extrude is flat on both the sides, which is for 2-axis cutting.
1. Click Add Constraints, and then select Linear Extrude. The Design Criteria dialog box expands.
2. Click in the Extrude direction box.
3. In the graphics window, select a surface, an edge, a datum plane, or the Csys axis as the reference. An arrow that shows the extrude direction appears.
4. To change the extrude direction, do one of the following:
◦ In the graphics window, click the arrow.
◦ In the Design Criteria dialog box, click .
5. In the Extrude angle box, specify the value.
6. To have a bi-directional extrude, select the Bi-directional check box.
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After selecting the Linear Extrude constraint, you cannot select the Parting Line constraint. Both are opposing manufacturing constraints.
Minimum Feature Size—This geometric constraint controls the feature size in the optimized shape.
This constraint makes sure that the thickness of the resulting model, at any cross section, is always greater than the specified value.
1. Click Add Constraints, and then select Minimum Feature Size. The Design Criteria dialog box expands.
2. In the Minimum Feature Size box, specify the value and select a unit from the list.
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Make sure that the specified value is higher than three times the element size.
Symmetry—This geometric constraint builds planar, rotational, or both types of symmetry.
The planar and rotational symmetry constraints enforce shape symmetry regardless of asymmetric loading in the study.
1. Click Add Constraints, and then select Symmetry. The Design Criteria dialog box expands.
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You can add a planar constraint, a rotational constraint, or both types of constraint simultaneously.
2. To add a planar constraint, do the following:
a. Click .
b. In the graphics window, select planes as the Symmetry planes. You can select up to three symmetry planes.
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Press Ctrl to select multiple symmetry planes.
3. To add a rotational constraint, do the following:
a. Click .
b. In the graphics window, select an axis as the Symmetry axis.
c. Specify the number of rotational repetitions as the Instances.
4. To add both, a planar and a rotational constraint, do the following:
a. Click .
b. In the graphics window, select Symmetry planes.
c. In the graphics window, select Symmetry axis.
d. Specify the number of Instances.
Minimum Crease Radius—This geometric constraint can be used to smooth out the solved geometry and reduce the webs in optimization.
This constraint makes sure that all the surfaces keep a curvature about a minimum radius.
1. Click Add Constraints, and then select Minimum Crease Radius. The Design Criteria dialog box expands.
2. In the Minimum Crease Radius box, specify the value and select a unit from the list.
Material Spreading—This geometric constraint controls the spreading of material.
The material spreading value ranges from 0 to 100. Increasing this value will result in fewer thick and solid regions, and more thin walls and struts.
1. Click Add Constraints, and then select Material Spreading. The Design Criteria dialog box expands.
2. To define Material spreading, adjust the slider or specify the value in the box.
Design Criteria and Materials Status
The glyphs on the Generative Tree indicate the status of the design criteria and materials:
• —Design criteria is defined but not active.
• —Design criteria is active. Only the active design criteria are considered for topology optimization and generating designs.
• —Design criteria is undefined.
• —Design criteria is defined but has issues.
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Move the pointer over the icon to see the tooltip that helps you resolve the issues.
• —The item that is the design criteria or the material is tied to the Generative Design Feature (GDF).
• —Material is active.
Operations on Design Criteria from the Generative Tree
You can perform the following operations on the design criteria from the Generative Tree:
Operation
Steps to Perform the Operation
Activate a design criteria.
Select a design criteria node, and on the mini toolbar click Activate.
Modify a design criteria.
Select a design criteria node, and on the mini toolbar click Edit Definition. The Design Criteria dialog box opens. Edit the design parameters and click OK to save to changes.
Duplicate a design criteria.
Right-click a design criteria node, and click Duplicate. A copy of the selected design criteria is created and added to the study.
Alternatively, right-click a design criteria node, and click Copy. Right-click the study node, and click Paste.
Create new design criteria.
Select a design criteria node, and on the mini toolbar click New. The Design Criteria dialog box opens.
Rename a design criteria.
Right-click a design criteria node, and click Rename.
Delete a design criteria.
Right-click a design criteria node, and click Delete.
Operations on Materials from the Generative Tree
You can perform the following operations on materials from the Generative Tree:
Operation
Steps to Perform the Operation
Change the material.
Select the material, and on the mini toolbar click Edit Definition. The Material Definition dialog box opens. Select a new material and click OK to save the change.
Activate a material.
Select the material, and on the mini toolbar click Activate. If you activate a material, the manufacturing method associated with it also gets activated.
Remove a material.
Right-click the material, and click Remove.
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