Sensors in individualized form are interesting for tasks in automation technology, as they can be used in many different ways. Inductive proximity sensors in cylindrical metal housings, in which a coil, a circuit board and a connector are installed in a rigid constellation - a standard component with a fixed geometry - are available and used in large numbers. However, to date there have been no inductive proximity sensors with a housing shape that fits into a specific environment, such as a robot arm gripper finger. This is now set to change thanks to the collaboration between a research team from the Center for Additive Manufacturing at the Fraunhofer Institute for Manufacturing Engineering and Automation IPA, Arburg, a system manufacturer for plastics processing, and sensor and automation supplier Balluff.

The basic idea was to print the housing of a sensor out of plastic so that it could be produced in any shape. A plastic with high dielectric strength and flame-retardant properties was required for this housing. The experts chose the semi-crystalline plastic polybutylene terephthalate (PBT), which is used as a standard injection molding material for the production of electronic housings. However, this type of material had not previously been used for 3D printing.

The plastic was fed as granulate into the so-called 'freeformer', Arburg's industrial additive manufacturing system. This has a material preparation unit with a special plasticizing screw. Once the standard granulate had been melted, tool-free freeforming followed: A high-frequency pulsed nozzle closure discharged tiny plastic droplets, which could be precisely positioned with the help of a movable component carrier. In this way, three-dimensional components with cavities were created layer by layer in the freeformer, into which components could be inserted during the printing process. To make this possible, the freeformer automatically interrupted the construction process in the respective layers so that it was possible to integrate the coil, circuit board and connector with a precise fit. A dispenser was then used to produce the silver conductor tracks inside the housing in a separate system. Finally, it was necessary to overprint the cavities with the freeformer and cast them with polyurethane.

In this way, the team produced more than 30 demonstrators of the individualized sensors in order to then test them extensively: for example, the components had to withstand temperature changes and vibrations, be waterproof and pass an electrical insulation test. By optimizing the design and manufacturing process, the tests were successfully completed in the end.

The research project 'Electronic function integration in additively manufactured components' ran for one and a half years. Stefan Pfeffer, who was responsible for the project at Fraunhofer IPA, is currently researching in cooperation with Arburg how conductive plastics can also be used in the future to open up further fields of application.