A bent part is a component brought into its final shape along defined lines after cutting. At GOBA, bent parts are made from electrical insulating materials: polyester films, aramid papers, technical laminates and pressboard in thicknesses from 0.023 to 3.0 mm. Shaping is done by creasing, folding or hot forming. This page explains the three processes and their limits, why spring-back is the real issue with insulating materials, how the minimum bending radius is determined and what belongs on the drawing of a bent part.
Insulating materials are creased and folded, press braking belongs to sheet metal
When sheet metal is press braked, the material flows plastically. It takes the new shape and keeps it. Why film and paper react differently from sheet metal at exactly this point is explained in the article on forming technology. Insulating materials behave differently. A polyester film stores the deformation elastically and returns part of the way to its original position, an aramid paper breaks along the fibre, a laminate splits on the outside of the bend. The bend is therefore prepared rather than forced: a crease weakens the cross-section exactly where the material is meant to fold.
For purchasing this means the same split as with stamped parts. Anyone needing sheet metal bent parts in steel looks for a sheet metal shop. Anyone needing slot closures, phase separators, cover angles or cell insulation with defined edges will find our stamped and bent parts from insulating material.
Three ways to shape an insulating part
Which process fits is decided by the material and the geometry, not by the required angle.
| Process | Principle | Suited to | Limit |
|---|---|---|---|
| Creasing | The cross-section is weakened along the bend line so the part folds under control | Films, laminates and papers in thin to medium gauges | The crease lowers dielectric strength at the edge, it does not belong in the main insulation path |
| Folding | The part is laid around an edge and pressed until it holds the shape | Pressboard, aramid papers and stiffer laminates | Spring-back remains material-dependent and has to be allowed for |
| Hot forming | Shaping under heat in the tool, the material takes the shape permanently | Complex geometries and tight radii that would crack when cold | Cooling has to be controlled, otherwise the part distorts |
Our recommendation follows the order in the table. We crease wherever possible, because the process is fast, reproducible and works without heat input. We fold where the stiffness of the material makes a crease unnecessary. We move to hot forming when the geometry cannot be reached cold, for instance on deep draws or parts with several bends.
Spring-back shifts the finished dimension
Once the tool opens, every insulating part springs back a little. The angle set in the tool and the angle measured on the finished part are therefore never identical. How large the difference turns out depends on the material, the thickness and the dwell time in the tool. Polyester films spring back considerably more than pressboard, thin layers more than thick ones.
- Material: polyester films spring back more than pressboard and aramid papers
- Thickness: thin layers return further than thick ones
- Bend radius: the tighter the radius, the smaller the return
- Dwell time in the tool: holding longer reduces spring-back
- Temperature: hot formed parts hold their shape better than cold bent ones
In series production the tool is set to overbend until the part lands on dimension after springing back. That setting comes from the sample, not from paper. Anyone tolerancing an angle tightly should mention it in the enquiry, because an additional sampling loop is then scheduled. For parts that engage a lamination stack, we check the fit in the actual component, because spring-back behaves differently there than on the measuring bench.
The minimum bending radius depends on material and thickness
If the radius is too small, the outside of the bend tears open. The rule of thumb is: minimum bending radius equals factor k times material thickness, with k coming from the material. For aramid papers in winding processes, radii from five times the material thickness upwards are usual so that the edge does not crack. Stiff and brittle materials need larger factors, tough films manage with smaller ones. The derivation and further practical values are in the article on the minimum bending radius.
A frequent drawing error is the sharp edge without a radius. A sharp edge cannot be bent into an insulating material without losing the insulating effect at exactly that point. So state a radius, or let us propose one. How much dielectric strength remains at the bend edge ultimately decides whether the part works.
Grain direction decides whether the edge holds
Papers and laminates are anisotropic. Their fibres lie predominantly in machine direction, and across it the material breaks more easily. A bend parallel to the fibre holds, the same bend across it can tear at the edge. With pressboard and aramid papers the difference is clearly measurable, with polyester films it is smaller.
In practice this means the grain direction belongs on the drawing as soon as a part has more than one bend edge or the edge carries mechanical load. Where the specification is missing, we set it during cutting in the way that best suits the main bend and document it with the sample.
What belongs on the drawing of a bent part
Bent parts add four details to what a stamped part needs. Without them, sampling runs an extra loop.
- Bend angle with tolerance, stated per edge where the requirements differ
- Inside radius per bend, or a note that we should propose one
- Bend direction, meaning whether the printed or coated face sits inside or outside
- Grain direction for papers and laminates
For all dimensions without an individual tolerance, the general tolerance from the title block applies, on insulating parts usually to DIN ISO 2768. The maximum size of our stamped and bent parts is 1,000 x 2,000 mm.
Measuring bent parts: the angle is the critical dimension
A bent part from insulating material cannot be measured like a sheet metal part. It is compliant, it rests differently under its own weight than when installed, and the contact face deforms under measuring pressure. The angle is therefore recorded without contact pressure, ideally optically. For series inspection, a gauge the part simply has to fit into works better than measuring the angle on every piece.
Agree the measuring method as well when you tolerance a bend angle tightly. Without that agreement, supplier and customer measure the same part differently and arrive at different results. This matters more with soft materials than in metalworking.
Bent part, stamped part or formed part: the distinction
The three terms are often used interchangeably in enquiries, but they describe different depths of manufacture. A stamped part is flat, it leaves the tool in one plane. A bent part is a stamped part with at least one edge, it is processed a second time after cutting. A formed part has a three-dimensional geometry that is not made up of straight edges, such as a cap or a shell.
The distinction has consequences for price. Every edge is an additional processing step, every three-dimensional geometry needs a forming tool. Anyone designing a contour saves most by avoiding edges that serve no function. One note in passing: the German term Biegelinie carries two meanings. In manufacturing it is the marked line along which the part is folded. In engineering mechanics the deflection curve describes how a loaded beam bends.
GOBA conclusion: the edge is made in the tool, not on the drawing
With bent insulating parts the bend angle sits on the drawing, but the finished dimension only emerges through spring-back in the tool. So schedule a sampling loop as soon as an angle is tightly toleranced, and state the inside radius. For aramid papers in winding processes a radius from five times the material thickness applies, otherwise the edges crack. Where the geometry cannot be reached cold, we form under heat. Which material suits which edge is best settled before design release, when a change costs only drawing time: our material range covers films, papers, laminates and pressboard in the common thermal classes.
