Forming technology is main group 2 of DIN 8580: a solid body is given a new shape by plastic deformation while mass and material cohesion are preserved, nothing is removed and nothing is added. DIN 8582 sorts the processes by the stress state in the workpiece into five groups, covered by DIN 8583 to DIN 8587. Insulating materials spring back far more than sheet metal, because the ratio of yield strength to modulus is around 20 to 30 times higher for PET film than for steel. That is why insulating film is creased, folded or thermoformed rather than press-braked. This article explains the standard's system, the physics of springback and the processes GOBA uses to produce stamped and bent parts from insulating materials: fold lines pre-embossed in the same punching stroke, insulating parts from 0.023 to 3.0 mm thickness in formats up to 1,000 x 2,000 mm, complex geometries formed under heat.
Bulk forming or sheet forming: where insulating materials sit in the classification
In the order of DIN 8580, forming has two neighbours that differ in what they do to the cohesion of the material: primary shaping creates it in the first place, as in casting or extrusion, while separating removes it locally, as in punching. The complete order of the six main groups is set out in the article on DIN 8580.
Within main group 2, the field distinguishes two families. Bulk forming works on workpieces of similar size in all three directions, meaning forgings, extruded profiles and rolled stock. Sheet forming works on flat material of constant wall thickness that is bent, drawn or embossed. Everything we form at GOBA belongs to the second family: films, papers and laminates from 0.023 to 3.0 mm thick that arrive as web or sheet before forming.
The five groups of forming processes according to DIN 8582
DIN 8582 is the framework standard of main group 2. It sorts forming processes by the stress that predominantly causes the deformation. This yields five groups, each with its own subsequent standard. There is no sixth group; open-die forging, which is sometimes listed as one, is a process within compressive forming.
| No. | Group | Standard | Predominant stress | Example processes | With insulating materials |
|---|---|---|---|---|---|
| 2.1 | Compressive forming | DIN 8583 | Compression | Rolling, closed-die forging, extrusion, indenting (embossing) | Creasing and embossing of beads, pre-embossing of fold lines |
| 2.2 | Combined tensile and compressive forming | DIN 8584 | Tension and compression from different directions | Deep drawing, ironing, spinning, collar drawing | Deep drawing of film caps under heat |
| 2.3 | Tensile forming | DIN 8585 | Tension | Lengthening, widening, deepening (stretch forming) | Pre-stretching in thermoforming |
| 2.4 | Bending | DIN 8586 | Bending moment | Air bending, die bending, swing bending, roll bending, flanging | Folding, bending, flanging the cuff of slot liners |
| 2.5 | Shear forming | DIN 8587 | Shear | Displacing, twisting | practically absent with flat material |
DIN 8586 further divides bending by the tool motion: linear as in air bending, rotary as in swing bending. For insulating parts made from flat material this group is the most important of the five, group 2.1 comes in as creasing and embossing, groups 2.2 and 2.3 only in the thermoforming of three-dimensional parts.
How forming works physically: yield point, elongation and springback
Every solid material first deforms elastically under load. Remove the load and it returns to its original shape. Only above the yield point does plastic deformation begin, the kind that stays. Forming therefore means taking the material deliberately past its yield point without reaching its elongation at break. The margin between those two values is the formability: a material with a low yield point and high elongation at break forms easily, a brittle material with a small gap between the two tears first.
In bending, the outside of the bend is stretched and the inside is compressed. Between them lies the neutral axis, which keeps its length. When the tool opens, the plastic share of the deformation stays and the elastic share returns. That return is called springback, and it shifts the angle of every bent part. How much depends on a single ratio: yield strength divided by the modulus of elasticity.
| Material | Modulus | Yield strength | Ratio of yield strength to modulus | Calculated springback at 90°, inside radius equal to thickness |
|---|---|---|---|---|
| Structural steel S235 | around 210,000 MPa | 235 MPa | around 0.001 | around 0.5° |
| PET film, biaxially oriented | around 3,500 MPa | around 100 MPa | around 0.03 | around 10° |
The calculation uses the springback factor from sheet metal forming, k equals 1 divided by 1 plus 3 times (yield strength over modulus) times (inside radius over thickness plus 0.5), as given in forming handbooks. For polymers it delivers only an order of magnitude, because polymers also creep over time. That order of magnitude is enough to see the core problem: a 90° angle in steel almost holds after air bending, the same angle in a PET film opens by a double-digit number of degrees. The film modulus comes from the datasheet of Mylar A in 0.25 mm (3.5 GPa according to DuPont Teijin Films); the yield strength is a range value.
Cold, warm and hot forming
For metals the standard draws the line at the recrystallisation temperature. Cold forming runs below it, usually at room temperature, and work-hardens the material. Hot forming runs above it, for steel up to around 1,200 °C, and the microstructure rebuilds itself during forming. Warm forming sits between 500 and 900 °C for steel and combines the dimensional accuracy of cold forming with lower forming forces.
Thermoplastics do not recrystallise. That role is taken by the glass transition, above which the molecular chains become mobile, and the crystalline melting point, above which the material flows. For PET the glass transition is around 70 °C and the melting point 250 to 260 °C. Between them lies the rubber-elastic window in which thermoforming works; processing guides quote 140 to 170 °C for PETG sheet. Whoever forms a semi-crystalline film too cold freezes in residual stresses that release at the next heating in the motor: the part relaxes back. An oriented PET film also brings its own shrinkage, around 1.5 % for Mylar A after 30 minutes at 150 °C. Thermosets have no such window and cannot be formed at all; the structural difference is explained in the article on thermoplastics.
Forming technology for insulating materials: why film and paper behave differently from sheet metal
Sheet metal flows plastically at the bend and stays there. An insulating material does so only under conditions, and which conditions depends on its material family. Thermoplastic films store the deformation elastically and spring back. Fibre-based materials such as aramid paper and pressboard deform plastically but break along the fibre when the radius gets too tight. Thermoset laminates do not deform plastically at all. These three behaviours decide which process from DIN 8582 is even an option.
| Insulating material | Behaviour in forming | Suitable process | Limit |
|---|---|---|---|
| Polyester film (Hostaphan, Mylar) | strong springback, shrinkage around 1.5 % at 150 °C | creasing, folding with overbend, thermoforming | edges stay in shape only with a crease or heat |
| Polyimide film (Kapton) | tough, high elongation at break, no melting point | folding and creasing, tight radii included | no thermoforming, the film does not soften |
| Aramid paper (Nomex 410) | anisotropic, elongation 16 to 22 % along the fibre, 13 to 18 % across | folding and flanging along the fibre, radius from five times the thickness | breaks across the fibre at too tight a radius |
| Pressboard | stiff, becomes more pliable with moisture | folding, embossing, creasing | no deep draws, breaks at the edge when dry |
| Laminates DMD, NMN, NKN | follow the weakest partner in the composite | creasing, folding | outer layer delaminates at too tight a radius |
| Thermoset laminates (laminated paper, laminated fabric, GRP), mica, glass fabric | no plastic elongation | separating only: punching, cutting | cannot be formed |
Bending insulating film: crease and fold instead of press braking
Because a film springs back when bent freely, we prepare the edge instead of forcing it. A crease compresses the cross-section along the bend line, which is compressive forming per DIN 8583. The part then kinks exactly there, the radius is fixed, and springback drops because the cross-section at the crease is plastically densified. Four further measures reduce springback, and in series production they are combined:
- overbending, meaning the tool angle is set tighter than the drawing requires
- coining in the bend zone, which plastically embosses the edge
- dwell time in the tool, because polymers relax under load
- heat, which makes the molecular chains mobile and freezes the shape
The crease has a price. It thins the material at the edge, and dielectric strength drops there accordingly. A creased edge therefore belongs at the margin of the part or in a zone where the insulation path is multi-layered, never in the main insulation path. Which radius each material needs at minimum is covered in the article on the minimum bending radius, and how the finished bent parts are measured in the article on those. One note on terminology: in manufacturing, the bend line is the marked line along which the part is folded; in engineering mechanics the deflection curve describes the bending of a loaded beam.
With papers, the grain direction decides whether the edge holds
Aramid paper and pressboard are made on a paper machine, and their fibres lie predominantly in the machine direction. The Nomex 410 datasheet shows how large the difference is: at 0.25 mm the paper carries 296 N/cm along the machine direction and 161 N/cm across it, elongation is 22 % along and 18 % across, and the tear propagation force across the fibre is roughly twice that along it. The manufacturer itself notes that the orientation of the paper should be chosen for each application.
For folding this means: an edge parallel to the fibre holds, the same edge across the fibre tears open at a tight radius. As soon as a part has more than one fold, the grain direction belongs on the drawing. If it is missing, we set it at cutting so that the main fold sits favourably, and document that in the sample. Moisture helps fibre materials: Nomex 410 gains elongation and tear resistance with rising humidity, pressboard becomes more pliable. A paper stored bone dry breaks at the edge where the same paper holds in normal climate.
Embossing, deep drawing and thermoforming of films
Embossing is compressive forming: a punch presses beads, ribs or locating features into the surface, and the part gains stiffness without gaining thickness. Deep drawing and thermoforming belong to combined tensile and compressive forming and to tensile forming, and with films they run under heat. The heated film is laid against the mould by vacuum at around 1 bar or by compressed air at 7 to 10 bar. Three effects govern the result, and all three rarely appear on the drawing:
- The wall gets thinner, because the material spreads over the larger surface of the formed part. Whoever needs a certain insulation thickness at the deepest point chooses a correspondingly thicker starting material.
- The part shrinks on cooling, for thermoplastic semi-finished products by 1 to 3 %, mainly in the orientation direction. The tool dimension is therefore never the part dimension.
- Tight edge radii tear. For warm bending of sheet, roughly twice the wall thickness is the lower limit, and for corners one to three times depending on the material.
Oriented PET film adds one particularity. Its strength comes from biaxial orientation that was heat-set at 200 to 230 °C. Thermoforming must stay above the glass transition but below the range in which this orientation relaxes. What comes out of it are caps, trays and shells, described in the article on moulded parts; how much thickness must remain at the draw is covered in the article on material thickness.
Which forming processes GOBA uses for insulating parts
We have been forming insulating materials since 1959, and the processes for it are a small selection from DIN 8582. The tool that punches the contour pre-embosses the fold lines in the same stroke; that applies to the economical steel rule die as much as to the solid steel tool we use for tight tolerances and high volumes. The details on tools and workflow are set out under stamping and forming.
| Process at GOBA | Group per DIN 8582 | Typical materials | How it runs in production |
|---|---|---|---|
| Creasing and pre-embossing of fold lines | 2.1 compressive forming | polyester film, laminates, pressboard | in the same punching stroke as the contour, steel rule die or solid steel tool |
| Folding and bending | 2.4 bending | pressboard, aramid paper, laminates | angle set on the sample via springback, delivered flat or pre-bent |
| Flanging | 2.4 bending | aramid paper, laminates, polyester film | cuffs on slot liners that hold the part in the slot |
| Embossing of beads and structures | 2.1 compressive forming | polyester film, pressboard | stiffening and locating aid, uniform embossing over the service life |
| Thermoforming of complex geometries | 2.2 and 2.3 under heat | polyester film, laminates | fitted to the customer's lamination stack, surface and structure remain undamaged |
The limits of our production are 0.023 to 3.0 mm material thickness and a 1,000 x 2,000 mm format. We process Hostaphan and Mylar, Nomex, Kapton, pressboard, the laminates DMD and NMN as well as glass fibre and mica products; the overview is under insulation materials. Parts ship flat, pre-bent, self-adhesive or as kiss-cut on a carrier liner, on request stacked or packaged, and in individual projects with marker lines by which the sensors in the customer's assembly recognise the part. The finished stamped and bent parts go into electric motors, battery modules, appliances and ventilation equipment.
The limits are just as clear. We offer neither bulk forming nor sheet metal forming; whoever needs press-braked steel parts needs a sheet metal shop. Thermoset laminates such as laminated paper and laminated fabric as well as mica and glass fabric products are punched or cut at GOBA but never bent, because their crosslinked structure allows no plastic elongation and breaks at the edge. Polyimide film we fold cold; thermoforming is not an option because the material has no softening range.
Can insulating film be bent to dimension at all?
The objection from design engineering runs: a material that springs back by ten degrees is no good for a toleranced angle. The answer is yes, on two conditions. First, the angle is established in a sampling loop in which the tool overbends until the part sits within tolerance after springback; this setting is found on the sample, not calculated. Second, a soft part needs an agreed measuring method, because it lies differently under contact pressure than free. We measure angles without contact pressure and check series parts in a gauge the part has to fit. For comparison: suppliers of formed slot liners quote width tolerances of plus minus 0.2 to 0.5 mm and length tolerances of plus minus 1 to 2 mm in their datasheets. That is the realistic order of magnitude for folded parts from 0.2 to 0.6 mm material; tighter is only possible with a crease, an embossing or heat.
The second objection comes from purchasing: why not deliver flat and fold at the customer. Our recommendation is unambiguous. The edge belongs in the tool, because only there do crease, radius and angle come out reproducibly. A film folded by hand kinks wherever the fitter starts, the radius is undefined, and right there the dielectric strength drops without control. Add the assembly time per part. One exception we plan for: if a pre-bent part is so bulky that it takes damage in transit, we deliver it flat with a pre-embossed crease. Folding it up at the customer is then one motion along a defined line, not free bending. In all other cases we deliver insulation and moulded parts fully formed and fitted to the lamination stack.
Forming or separating: where punching ends and forming begins
In enquiries, punching and forming are often named in one breath; in the standard they sit two main groups apart. Punching is separating, more precisely shear cutting with ordinal number 3.1.1: cohesion is removed along the contour. The crease in the same tool is forming per 2.1, the later fold forming per 2.4. A stamped and bent part touches two main groups with one stroke, which is why the drawing describes both separately: the contour with its tolerance, the edge with radius, angle and grain direction. What belongs on the drawing of a flat part is covered in the article on stamped parts.
One special case shows how closely the main groups sit together. If a cuff is flanged over to hold the part in the slot, that is bending. If the same flanging joins two parts to each other, it counts as joining by forming per DIN 8593-5, which the article on joining technology classifies. The standard asks about the result for the cohesion, not about the motion.
GOBA Takeaway
In the standard, forming technology is a matter of stress state; in production it is a matter of material. The five groups of DIN 8582 apply to film just as they do to sheet metal, but film and paper bring three peculiarities sheet metal does not have: springback 20 to 30 times higher, a grain direction that decides between holding and tearing, and a narrow temperature window in thermoforming. Whoever knows this creases instead of press braking, specifies the grain direction and plans the sampling loop for the angle.
We form insulating parts from polyester film, aramid paper, pressboard and laminates from 0.023 to 3.0 mm thick and tell you before design release which edge holds in which material. Send us the drawing with radius, angle and grain direction, or let us propose those three values. From that come our creased, folded and thermoformed stamped and bent parts from insulating material.
