Sharp Corner Mullions for Steel-Glass Façades
Frontal mullions in steel have become a common standard in curtain wall design, and they are generally solved with T sections or rectangular hollow sections. Corners are a different matter. A corner mullion has to receive glass panels on two separate planes, and the angle between those planes follows the geometry of the building rather than a convenient 90 degrees. For this project a custom corner section of 300×100×6 mm was required, with the corner leg designed at 90 degrees plus or minus 15 degrees so that the same profile family could follow the real layout of the façade. Montanstahl produced the sections by three dimensional laser welding, a process carried out without filler material that combines the characteristics of laser welding with the flexibility of a five axis robot, giving access to weld seams that do not lie on a single plane.
Why Façade Design Has Moved Beyond Standard Section Shapes
Aesthetic façades are built around structural profiles that have progressively moved away from catalogue shapes and towards geometries defined by the architect. The driver is technological, because current processing methods make it possible to design, machine and assemble sections for almost any requirement, aesthetic first of all but structural above everything else. Projects such as the Hayward Library show how far a section can be adapted to a specific design intent, and they explain why architecture, building and construction remains one of the most demanding sectors for special profiles. In a steel-glass envelope the mullion is never only a load bearing element. It is visible, it sets the rhythm of the glazing and it is assessed from a short distance, which is why architectural profiles for curtain walls are specified with the same attention as any other architecturally exposed component. Surface condition, edge sharpness and weld appearance become part of the design brief rather than production details.
Two Connecting Sides, One Angle That Is Never Exactly 90 Degrees
A corner mullion carries a requirement that a frontal mullion does not. It must offer two separate faces for connecting the glass panels, each one lying on its own plane, and the angle between them is dictated by the building rather than by the section catalogue. On this project the reference geometry measures 300 mm across the main face with a depth of 100 mm and 6 mm walls, and the corner leg is drawn at 90 degrees with a variation of plus or minus 15 degrees, so that a single section concept can follow acute and obtuse corners across the same envelope. Conventional fabrication reaches its limit here, since fillet welding a non-orthogonal joint on a thin walled section introduces heat input, distortion and a visible seam exactly where the profile is most exposed to view. The alternative adopted by Montanstahl is three dimensional laser welding without filler material, where a five axis robot moves the welding head along seams that do not lie on a single plane. The section is therefore joined at the exact angle the design requires, with sharp corners, a narrow and regular seam and controlled penetration, in the same way described for the rectangular hollow sections produced for an exquisite façade.
Application Context
Custom corner mullions of this type belong to Montanstahl’s architectural profiles range, supplied for steel-glass façades where the curtain wall changes direction and the junction stays visible from inside and outside. The same production approach covers acute and obtuse corners, transitions between façade planes and sections whose legs meet at angles fixed by the building geometry rather than by a standard shape. Because laser welding works without filler material, the surface condition remains consistent along the full length of the bar and the finishing operations required before installation stay limited, which matters when the profile is architecturally exposed. Sections are produced in carbon steel and stainless steel, with dimensions, wall thicknesses and angles defined case by case together with the design team. Questions on tolerances, materials and section design are addressed in the Montanstahl technical Q&A.