4. CAD geometry preparation for center airbag CAE

Center airbag CAE begins long before the first explicit run is submitted. The quality of the CAD geometry that enters the pre-processor determines whether the cushion can fold, whether the housing can deploy, and whether the occupant contact forces are physically meaningful. In practice, a center airbag model is assembled from several CAD sources: the folded cushion surface, the inflator and its mounting bracket, the housing (also called the module can or retainer), the cover or tear seam, and the surrounding seat and console trim. Each source carries its own tolerance, topology, and level of detail. This chapter explains how to clean, simplify, and mesh those geometries so that the resulting CAE model is both robust and computationally affordable.

Why CAD Clean-Up Is Not Optional

Production CAD is built for manufacturing, not for simulation. It contains fillets, chamfers, logos, draft angles, and small features that are irrelevant to the structural response but expensive to mesh. More critically, it contains gaps, sliver faces, and non-manifold edges that cause meshing failures or, worse, produce a mesh that runs but gives wrong results. A center airbag cushion that is modeled with a 0.5 mm gap along a seam will leak gas in the simulation and under-predict internal pressure. A housing with an unmerged flange will separate under inflator thrust and show a false failure.

The clean-up workflow therefore has three goals: geometric validity (watertight, manifold, no duplicate entities), topological consistency (shared edges between adjacent surfaces), and appropriate simplification (removing detail that does not affect the physics).

Cushion Geometry: From Panel Surfaces to a Foldable Mesh

The cushion is the most demanding component. It is typically delivered as a set of sewn panels — front panel, rear panel, side gussets, and tether attachment patches. The first task is to identify the seam lines and confirm that adjacent panels share coincident edges. In most pre-processors this is done with a geometry cleanup tool that merges nodes within a tolerance, commonly 0.01–0.05 mm for cushion panels. A larger tolerance risks collapsing legitimate narrow features; a smaller tolerance leaves gaps that will leak.

After merging, the cushion is simplified. Small radii at panel corners, decorative stitching, and label embossing are removed. The tether and seam reinforcements are retained because they control the deployed shape. The cushion is then meshed with membrane or shell elements. For a typical driver-side center airbag, a target element size of 4–6 mm on the main panels and 2–3 mm near the seam and tether attachment gives a good balance. The mesh must be quad-dominant and as uniform as possible, because highly distorted elements cause negative volume errors during inflation.

Housing Geometry: Shell Idealization and Thickness Assignment

The housing is usually a thin stamped or molded shell. It should be idealized as a mid-surface shell rather than a solid, because solid elements through a 1.5–2.5 mm wall would require an impractically fine mesh. The mid-surface extraction step removes the original solid, offsets the surface to the wall centerline, and assigns a shell thickness equal to the measured wall thickness. For a typical center airbag housing, the wall is 1.8 mm in the base and 2.2 mm at the mounting flanges.

Simplification of the housing focuses on removing features that do not carry load: small drainage holes, clip slots that are not engaged during deployment, and cosmetic ribs. However, the inflator mounting studs, the tear-seam notch, and the hinge line must be preserved, because they define the deployment kinematics. The housing mesh is typically 3–5 mm shell elements, with local refinement to 1.5–2 mm at the tear seam and stud holes.

Meshing Strategy and Quality Criteria

The table below summarizes the recommended meshing targets for the main center airbag components. These values are representative starting points for a passenger-car center airbag and should be adjusted to the specific program.

Quality criteria matter because explicit solvers are sensitive to element distortion. A common failure mode is the negative volume error, which occurs when an element inverts during cushion inflation. This is almost always caused by a small, badly shaped element near a seam or fold line. Checking the minimum Jacobian and the maximum aspect ratio before submission catches most of these cases.

Practical Workflow Example

Consider a center airbag cushion delivered as a STEP file with 14 panels. The engineer imports the geometry, runs a topology check, and finds 6 free edges. After merging with a 0.02 mm tolerance, 4 edges close; the remaining 2 are genuine vent openings. The cushion is then simplified by removing 3 mm corner radii and a logo emboss. The mesh is generated at 5 mm global size with 2.5 mm refinement along the seam. A free-edge check confirms only the vent openings remain. The housing is mid-surfaced, thickness 1.8 mm assigned, and meshed at 4 mm with 1.8 mm at the tear seam. The inflator is imported as a rigid body. The assembly is then ready for folding and inflation simulation.

Visual Overview of the Preparation Chain

The SVG below maps the geometry preparation chain for the cushion and housing, showing the input CAD, the clean-up and simplification operations, and the resulting mesh targets. It is intended as a quick reference for the sequence and the key decisions at each stage.

Common Pitfalls and How to Avoid Them

• Over-simplification of the housing: removing the tear-seam notch eliminates the stress concentration that controls cover opening. Keep the notch and refine locally.
• Under-simplification of the cushion: retaining every stitch line creates thousands of tiny elements that slow the explicit run without improving accuracy.
• Ignoring thickness variation: a housing with a 1.8 mm base and 2.2 mm flange modeled as a single 2.0 mm shell will mispredict flange stiffness. Assign per-region thickness.
• Mismatched seam topology: if the cushion seam nodes do not coincide with the tether nodes, the tether will detach during inflation. Merge and equivalence before meshing.

In summary, CAD preparation for center airbag CAE is a deliberate trade-off between fidelity and cost. The cushion needs a clean, watertight, foldable mesh with controlled element quality; the housing needs a mid-surface shell with correct thickness and preserved deployment features; the inflator can be treated as a rigid body. A disciplined clean-up and simplification sequence, followed by a quality gate before submission, prevents the most common explicit-solver failures and ensures that the simulation predicts the physical deployment with confidence.