3D printing for the production of sheet metal bending dies

Bending is a popular processing method that induces plastic deformation on a sheet metal by bending it permanently. The shapes obtainable from this process can range from extremely simple geometries to more complex ones. The materials compatible with this type of processing must be characterised by plasticity such that it does not lead to breakage during bending. The process is usually carried out at room temperature, but in the case of very small fillet radii or large cross-sections, a hot process may be required to allow for greater deformability of the material as well as reduced forces involved.

In practice, the processing is carried out on a machine called a bending press which has a punch and a die. The sheet metal is placed on the die and is deformed by the punch exerting a bending force.

Bending techniques fall into two categories:

• free bending which consists of deforming the sheet metal resting directly on supports through the application of force by a punch: in this case by varying the depth of the punch stroke it is possible to obtain bends of various angles without changing the die;
• in-die bending in which the die represents a mould with known geometries and the punch pushes the sheet to the end of its stroke in order to produce a well-defined shape while also giving greater rigidity to the profile.

Bending dies must meet certain requirements both geometrically and structurally as the resulting shapes must be precise and must take into consideration elastic shrinkage phenomena in the material. Furthermore, the forces involved, which depend on the type of material and the thickness of the sheet metal to be processed, are not negligible.

The materials commonly used for bending dies are:

• Steel (X210Cr12): this steel is commonly used to make punches and dies for cold machining on sheet metal. It is known for its strength and durability.
• Steel (X155CrVMo12): another steel used for bending dies. It offers good wear resistance.
• Steel (X45NiCrMo4): this steel is suitable for bending operations and offers a good combination of strength and toughness.

3D printing applied to the bending process

Given the not entirely negligible costs of steel moulds, it was realised that 3D printing could be a viable alternative to the purchase of traditional moulds. In fact, thanks to the increasing development of materials suitable for 3D printing with excellent mechanical strength such as PPS CF, it was possible to achieve excellent bending results through the use of dies made exclusively by the 3D printing process.

The collaboration between Emiliano Corrieri from Accademia Della Piegatura and Crea3D has resulted in a V-bending die entirely 3D printed, which takes its cue from a version already on the market but made in steel using traditional techniques.

This type of die differs from the others in the presence of two oscillating rotors that support the sheet before and during the bending phase. The peculiarity of these inserts is that as they rotate, they support the sheet during bending, significantly reducing friction and making it possible to significantly shorten the length of the minimum edge. It is possible to make very small edges and bend close to holes and slots, including profiles on slant-cut sheet metal, avoiding deformation.

In addition, the use of this type of die can significantly reduce the costly polishing operations that follow bending, as there are no straight lines mirroring the bend line that usually appear when bending with traditional dies, and other scratches or bend marks are minimised. Due to the elimination of additional processing steps, production costs decrease and, more importantly, delivery times are shortened.

The advantages listed above apply to both traditional steel tools and those made by 3D printing. The big difference between the two, however, is when evaluating costs, lead times and the possibility of customise the mould to your preference.

From design to testing

The design phase of the sheet metal bending die was inspired by a similar product already on the market. Upgrades were made to allow the correct positioning of the die on the machine by inserting magnet inserts.

For the production of the assembly, a specific material such as PPS CF was used, which has excellent compressive strength, making it the ideal candidate for this type of application. This type of material is composed of carbon-fibre-reinforced polyphenylene sulphide and has properties of high mechanical strength that remains even at temperatures in the 230 °C range. These characteristics make it an excellent competitor to far more expensive and difficult-to-print materials. For printing PPS CF, it is of great importance to use high-performance printers that have the ability to reach extrusion temperatures around 320 °C and that have a heated chamber so that the natural shrinkage of the material during printing can be controlled.

In our case, we made use of UltiMaker latest release, the Factor 4 which, thanks to its print core HT suitable for printing high-temperature materials and its print profiles, enabled the die to be produced without difficulty. Thanks to the collaboration with ADP, it was possible to carry out tests that made it possible to highlight the critical aspects of the project and then make iterative improvements to the matrix design. The end result determined that the developed die could be used to perform V-bending on both 1 mm thick AISI 304 and 2 mm thick aluminium sheets.

This case study demonstrates how 3D printing is undergoing a transition from rapid prototyping to a full-fledged production process for functional components. This phenomenon is leading more and more industrial companies to implement 3D printers in their production chain. Indeed, there are advantages in terms of cost, flexibility and reduced warehouse space as parts can be produced as and when they are needed.