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CAD Mode

SimScale offers a dedicated environment to interact with your CAD model and perform CAD-related operations, after uploading it, as a part of preprocessing, referred to as ‘Edit in CAD mode’ or simply ‘CAD mode’.

Having a separate environment to delete, extrude, or scale CAD parts helps users to zone out from the simulation setup and only focus on optimizing their CAD model. Once done, users can exit to the Workbench with a copy of the new model while still retaining active access to the original CAD model.

To access the CAD mode from the Workbench click on the imported geometry and go to ‘Edit in CAD mode’ in the panel:

access cad mode from workbench
Figure 1: Access CAD mode from the SimScale Workbench by clicking on the imported geometry and then on Edit in CAD mode icon as shown.

Or just click on the icon to open the context menu as shown below:

access cad mode option
Figure 2: Access CAD mode by clicking on icon and then ‘Edit in CAD mode’ in the context menu.

CAD Mode Interface

The interface looks as shown in Figure 3:

cad mode interface simcale
Figure 3: CAD mode interface has four important sections viz., 1. Operations Toolbar, 2. History, 3. Scene Tree (for geometry), and 4. Finish/Cancel to conclude operation.
  1. Operations Toolbar: All the supported CAD operations are listed here.
  2. History: All the performed operations will be listed here in order.
  3. Scene Tree: The original geometry with its solid bodies is enlisted here. Toggle Show faces to see and keep a track of all the faces of each solid body present in your geometry.
  4. Finish/Cancel: Click ‘Finish’ to create a copy of your modified geometry. The new geometry will be listed as a copy of the original under the Geometry tree. Click ‘Cancel’ to exit. Both will take you to the SimScale Workbench.

CAD Mode Operations

The following operations are supported with more coming soon. The operations are categorized based on their application.



The Delete operation helps you to delete a face from the CAD model by selecting ‘Delete’ from the operations toolbar and then assigning one or multiple faces to be deleted.

face delete simscale
Figure 4: Delete face operation in CAD mode with healing methods: Shrink and Cap. More than one face can be assigned.

The settings panel also allows choosing between the available healing methods, viz. shrink and cap while the face delete operation is being performed. Choose No healing to opt-out.

In the following section all the healing methods are described using simple geometry as follows:

reference geometry to demonstrate healing methods
Figure 5: Reference geometry used to demonstrate the healing methods for the face delete operation is a cylinder with an extra curved face.

The reference geometry is a cylinder with one of its edges rounded to produce 1 curved and 3 planar faces.


The Shrink healing method will try to fill up the surface lost to the delete operation by trying to either expand or contract the adjacent faces until they intersect.

shrink healing method in face delete cad mode
Figure 6: The Shrink healing method. Deleting the top face should return the exact same result as if the operation wasn’t performed. Deleting the curved face should raise the side face and expand the top face until they intersect.


The Cap healing method will try to fill up the surface lost to the delete operation by connecting the adjacent faces without changing them.

cap healing method in face delete cad mode
Figure 7: The Cap healing method. Deleting the top face should return the exact same result as if the operation wasn’t performed. Deleting the curved face should create a face that connects the previously connecting faces without changing them.

No Healing

This healing method will leave a void as a result of the face delete operation and no additional efforts will be taken to heal the connecting faces.

no healing method in face delete cad mode
Figure 8: The No healing method. Deleting the top face and the curved face should just remove it, creating a sheet body. In case there survive faces that are not in contact with any of the remaining faces they will still be part of the same sheet body.


With the Move operation, you can extrude a face in the direction of its normal. Select ‘Move’ from the operations toolbar to open the settings panel for this operation. There are two methods to move a face or faces. For illustration let’s consider the highlighted heat sink face:

cad mode move operation on heat sink
Figure 9: Selecting a face for performing the Move operation. More than one face can be selected.

First, select the face and then choose one of the following methods:


Specify the distance up to which the selected faces need to be moved/extruded.

cad mode move operation with distance method
Figure 10: Selecting the Move method to Distance will extrude that face up to the desired value.

Up to Entity

Specify the entity face up to which the selected faces need to be moved/extruded.

cad mode move operation with up to entity method
Figure 11: Selecting the Move method to Up to entity will extrude that face up to the plane of the entity face.



The Delete operation will delete the solid bodies present in the CAD model. Click on ‘Delete’ under Body in the operations toolbar and assign one or more volumes from your CAD model before hitting ‘Apply’.

body delete cad mode
Figure 12: Assign bodies to be deleted. More than one body can be assigned at once.

Close Sheet

There are instances where the CAD is missing one or more faces (these CAD models are called sheet bodies) and you are unable to proceed in the simulation process. In such cases the Close Sheet operation for bodies (volumes) is helpful. In the settings panel, select the bodies with missing faces to close and choose a closing method.

settings panel close sheet cad mode simscale
Figure 13: Settings panel for the Close Sheet operation requires you to select sheet bodies and a closing method to apply.

Closing Method

The Closing method defines how to close the sheet body using either the Cap or the Grow method. These methods are described below using an example of Crystal Towers:

crystal towers
Figure 14: A CAD model of Crystal Towers used as a reference for the Close Sheet operation. The middle tower is missing a face on the top.

The tower in the middle has a missing face on the top. Hence, the close sheet operation is perfect for this CAD model.


The Cap method looks for the minimum possible surface path to close the sheet body and is the default closing method.

cap closing sheet method
Figure 15: The missing top face for the middle tower is now closed with a face with a minimum possible surface area

Applying this method to the middle tower closes the top face with minimum possible surface area.


The Grow method will try to extend the connecting faces to the missing face until they meet at a vertex.

grow closing sheet method
Figure 16: The middle tower is extended up to a point where the connecting faces meet.

Applying this method by picking up the middle tower extends the connecting faces of the missing face elongating the middle tower until they all meet at a point.


You might run into situations where your CAD model has multiple overlapping parts. This might pose as a hindrance to the simulation physics causing the simulation setup to fail. If this is the case try our set of Boolean operations. These operations allow you to perform union, subtraction, and intersection operations between different parts. These are described below:


To perform the boolean operations the user must select two or more bodies that share overlapping volumes. If there are assigned bodies that don’t overlap they will be excluded from the operation. Even if no bodies overlap, the operation will still show as successful but no changes will be shown in the viewer and in the scene tree.


The Union operation is used to merge two overlapping entities into a single entity. The resulting body encloses the volume of both the overlapping entities. This is shown below:

union operation in cad mode
Figure 17: Union boolean operation is used to merge two or more bodies into a single body.

The CAD geometry consists of two overlapping bodies, Part 1 (cube) and Part 2 (sphere). Their different colors represent two separate entities. These two bodies are separately identified in the scene tree. To perform the Union operation select the two bodies and hit ‘Apply’.

result of union operation simscale
Figure 18: A successful union operation of two bodies resulting into a single body, shown in the scene tree by listing only one part.

On success, the CAD geometry shows a single body identified with the name Part 2 in the scene tree and the same color. This signifies that the two bodies united into one single body.


The Intersect operation is used to create a new entity out of the common volume between the overlapping entities. You just need to select the involved entities. Using the same example as in Figure 15 the intersection operation results in the following CAD model:

intersect boolean operation
Figure 19: Intersect boolean operation between overlapping bodies results into a new body whose surfaces are enclosed by their common volume.

The result is a quarter of a sphere.


The Subtract operation can be used for the following operations:

  • To remove the common volume from one of the entities while keeping it intact in the other
  • To remove the common volume along with one of the entities

The settings panel for this operation is shown below:

subtract operation cad mode
Figure 20: Subtract operation can remove the common volume and the tool body or the common volume from the target body only.

The settings panel asks the user to select target bodies and tool bodies. A target body is a body to subtract from while a tool body is a body to be subtracted. In this example, we choose to subtract the sphere (in pink) from the cube (in blue).

One more important step is to choose whether to keep the tool bodies or not. If Keep tools is selected then the common volume gets subtracted only from the target body. The final result still has two separate bodies.

If Discard tools is selected then both the common volume and the tool body gets subtracted. The final result now has a single body. Both the results are shown in Figure 19:

keep tools versus discard tools in subtract boolean cad mode
Figure 21: Difference between Keep tools and Discard tools feature for the subtract boolean operation. The former results in the same number of bodies while the latter removes the tool bodies.


Under this section, there are operations that allow users to move the part volumes within their geometry by either translating them whole up to a certain distance or rotating them by a specified angle.


As the name suggests, the Translate feature aids in translating the body or bodies up to a certain specified distance in the specified directions. Similar to the Move operation, there are two methods to proceed:


Using this translate method you can specify the distance up to which the selected volume needs to be translated. A negative value would translate in the opposite direction. Kindly follow the orientation cube at the bottom right to avoid confusion.

Following the same example from Figure 9, this time instead of moving the face of the heat sink we will move the whole heat sink body in the positive y-axis by a distance of 0.025 \(m\).

translate xyz cad mode
Figure 22: Selecting the Translate method to X,Y,Z will translate that body to the desired coordinates.

Up to Entity

In this method, you just have to assign the face of the entity up to which the assigned volume needs to be translated. This time we assigned a face of the chip for the algorithm to translate the heat sink such that its foremost face is coplanar with that of the chip.

translate up to entity cad mode
Figure 23: Selecting the Up to entity method will translate that body up to the plane of the entity face.

The following schematic perfectly represents the possibilities of a successful up to entity translate operation:

translate operation schematic to represent possibilities in cad mode
Figure 24: The possibilities of success and failure using the up to entity translate operation


As the name suggests, the Rotate feature aids in rotating a body or multiple bodies about a specified axis. This axis of rotation passes through the center of an imaginary bounding box whose dimensions represent the minimum and maximum coordinates of that body (or bodies combined) in the x-, y-, and z-direction.

Let’s look at the following example:

rotate operation settings
Figure 25: Rotate feature allows rotation of one or more bodies about a specified axis.

Here, different parts of the airplane wing are being rotated about the positive y-axis. This means, all these parts will rotate, in the anti-clockwise direction, at the same time about the central axis of the bounding box pointing in the positive y-direction. It is advised to refer to the orientation cube while assigning the axis of rotation (orientation). Clicking on the inverse button inverse button will result in the reversal of the axis assigned, negative y-axis in this example.

result of rotate operation
Figure 26: Result of the Rotate operation above


Often there are instances where the CAD model has too much detailing that is insignificant from the simulation point of view. This may include threads on a bolt or pattern imprint on a tire. These fine details can cause simulation overhead by requiring a fine mesh and subsequently a high consumption of core hours.

With the Simplify feature, these fine details can be replaced with primitive shapes like cylinder or box that occupy minimum bounding dimensions.

simplify settings panel cad mode
Figure 27: Simplify feature allows replacement of a detailed body with a box or cylinder primitive
replacement with box and cylinder test
Figure 28: Examples showing the difference between replacement with a box against a cylinder

Multiple bodies can be replaced in a single operation either with cylinder or box. By default each body is replaced separately however this can be toggled off to generate a single resultant body.

different replacement options in simplify
Figure 29: Multiple bodies can be selected at once for replacement and can be replaced individually or as a group.



In order to detect interfaces between two solids or a solid and a fluid, we need to perform an imprint. This action basically cuts the interface between two bodies in physical contact into recognizable surfaces. Please refer to our article where a detailed explanation of what an imprint does is well mentioned.

In the CAD mode, click on ‘Imprint’ and hit ‘Apply’.


In case the uploaded model does not match the original dimensions or the dimensions of your choice, the Scaling operation can be used to adapt the model size accordingly. With scaling, the simulation setup parameters might also change and care should be taken to avoid unreliable results.

scaling cad mode
Figure 30: Scaling operation can be used to adapt the model size accordingly using a scaling factor.

The settings panel contains a slider to set a scaling factor while also allowing direct entry of the value.

Facet Split

Facet Split allows users to split the CAD model into multiple faces based on the defined maximum split angle. This operation comes in handy when dealing with .stl files as they often consist of only a single face.

facet split cad mode
Figure 31: Use the Facet Split operation to split your CAD model into multiple faces based on the maximum split angle.

The angle can be added in degrees between 0 and 180 and is by default 30.


The Split operation allows users to cut the CAD model into two parts based on the position and the orientation of the plane defined.

split operation in CAD mode
Figure 32: Split operation in CAD mode where the model can be split into two parts

The orientation of the splitting plane is defined by specifying the normal to the plane. The part lying on that side of the plane where the normal vector faces is retained while the other part is removed. The user has the option to keep parts from both sides of the plane as well.

Fix Interferences

Fix interferences should be your option when trying to fix the interfering bodies in your CAD model automatically. Just click on the icon and hit ‘Apply’. It should get rid of all the interfering parts. Users should note that, in this automatic operation, the smaller body is subtracted from the larger body.

Don’t forget to run an interference check again just to be certain (see Interferences below). You should see the “No Interferences found in your CAD Model” message as displayed in Figure 30.

Create Flow Volume

A CFD simulation is performed on the actual fluid volume. For an internal flow problem, this means that the fluid volume needs to be extracted from the CAD geometry or, be created around the CAD geometry for an external flow problem.

This can be performed in the CAD mode using the Flow volume operation. More information on flow volume extraction can be found here.

External Flow Volume

To create an external flow volume, you need to enter the minimum and maximum dimensions for the volume in x-, y-, and z-direction. Optionally, select a seed face that corresponds to any face on the CAD geometry.

external flow volume cad mode
Figure 33: External flow volume extraction requires minimum and maximum dimensions, and an optional seed face.

By default, this operation retains all the CAD parts. You can specifically exclude them by selecting them under Excluded parts. If unsure, the parts can also be deleted later using the delete operation for the body (discussed above).

Internal Flow Volume

To create an internal flow volume you need to specify a seed face and one or more boundary faces ( lies between the external environment and the internal).

internal flow volume cad mode
Figure 34: Internal flow volume extraction requires a seed face and one or more boundary faces.

Similar to external FVE internal FVE also retains all the CAD parts. You can exclude them under Exclude parts option.


User should note that faces selected for operation in a flow volume extraction process, such as seed face and boundary face, cannot belong to the excluded parts (volumes) of the CAD model.



Your CAD model is not ready for the simulation setup if it has interfering solid parts. Interfering/Overlapping solid parts can result in failure of geometry operations performed in CAD mode and more importantly cause the meshing algorithm to fail. Hence, it should be avoided at all costs.

To check for interfering parts click on the ‘Interference’ icon and wait for a message while the operation runs in the background. A valid CAD model with no interfering parts should display the message on the left as shown in Figure 30:

cad mode interference check
Figure 35: If you have a CAD model with multiple solid parts don’t forget to run an interference check.

To fix interferences manually use boolean or delete-body operations described above. Additionally, for automatic fixing of interferences use the Fix interferences feature discussed under Models.

Last updated: December 6th, 2021

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