Electrical and electronic products have gained tremendous importance, including in the construction industry, where they are used in highly integrated assembly groups. Furthermore, there is a growing trend towards miniaturization, and the conductors of electrical and electronic products must meet different requirements depending on their field of application. An increasingly common solution appears to be hybrid super-conductive polymer materials, which we refer to here, quoting from the study "Highly Conductive Plastics: Custom and formulated Functional Materials for Electric and Electronic Applications," published by Walter Michaeli, Tobias Pfefferkorn, and Jan Fragner from the University of Dover.

Different Tasks, Specific Solutions

Like metallic conductors, plastic or composite conductors must meet specific requirements. For instance, a very high electrical conductivity is necessary for connecting transmission systems and controlling a motor. However, for control units, sensors, and casings, different conductivity ranges are required. Additionally, electromagnetic screening is crucial.

In other applications, lower values of electrical conductivity are needed for applications that must prevent electrostatic charging. Contrary to appearances, polymers, which are typical insulation materials, can represent a solution. Interest in using polymers for other electrical applications has increased due to advantageous properties such as weight, processability, and resistance to chemicals.

Over the years, thermally and electrically conductive polymers have been developed by adding common filler materials such as carbon black, graphite, metal fibers, flakes, or carbon fibers, and increasingly, nanofillers like carbon nanostructures. These compounds have already been successfully implemented in a range of anti-static and electromagnetic shielding applications.

Therefore, to ensure a high level of electrical conductivity, a high content of conductive fillers that forms a tight percolation network is necessary. The solution can come from the manufacture of components made by injection molding.

The Necessary Compromise

Hence, depending on the specific application, a compromise must be found for each material composition regarding the amount of filler. Higher filler content usually has a negative impact on mechanical properties and processability due to a considerable increase in melt viscosity.

Simultaneously, machine wear is higher than for unused polymers. Material compositions used so far are unable to meet the future functionality requirements of components, especially for thin-walled and miniaturized elements. Therefore, function components that require high electrical conductivity are still conventionally produced in intensive processing and assembly stages.

Filler content and thus electrical conductivity can be significantly increased without reducing processability by using metal alloys with a low melting point. These metal alloys are liquid in the processing phase and do not solidify before the cooling phase. This allows for the production of complex cast parts with precise electrical and thermal properties.

As a result, disadvantages related to highly conductive plastic materials are reduced compared to high filler content casting compounds. Thus, it becomes possible to produce conductive structures and junctions for connection and/or cables in a single processing step through an injection molding process, avoiding time-consuming joining and bonding processes.

Casting characteristics

Cavity filling to make thermoplastic/metallic compounds can be done in conventional single or multi-component injection molding processes. Due to the high metal content, the filling and freezing behavior of the materials differs from that of unused thermoplastics. This is caused by solid and liquid filling. The tests were performed using an Allrounder 320 S 500-150 die casting machine (Arburg GmbH + Co KG, Lossburg, Germany) with a screw diameter of 30 mm (L / D = 20).

In these tests, the effects on the flow properties of the new material were considered, followed by the characterization of the local distribution of the filler. Unfilled thermoplastics and small fillers showed typical frontal flow during mold filling. This is specific to viscoelastic fluids, whereby a parabolic velocity profile is formed across the thickness of the parts. With increasing filler content, the velocity profile decreased.

The thermoplastic composition of the heavily filled material showed significantly altered filling behavior due to the high filler content of 85% by weight. In addition to the significant melt elasticity of the compound, rapid heat flow, increased thermal conductivity and specific phase transition were found.

Perspectives

Investigations demonstrate the high potential of the new compounds for use in castings with high electrical conductivity. The range of materials ranges from carbon black compounds to metals and forms the transition between semiconductors and conductors.

The described thermoplastic/metallic hybrid materials have a high potential to be used in the production of complex electrical and electronic components, with very high requirements regarding electrical conductivity.

Due to the injection molding process, the molded parts exhibit a dependence of the distribution and direction of the filler material on the local position along the flow path. Therefore, in the design of the cast part it is important to take into account not only the global characteristic values, but also the influence of the material composition, process parameters, as well as the geometry of the yielding system and the cavity.

This allows the morphology and properties of parts to be influenced during the molding and injection molding of polymer/metal hybrid materials. So, it can be shown that the matrix polymer can be varied widely, to adjust the properties of the compounds for the specific requirements of the applications.

Thus, a freezing of the polymer at the same time, or later, than the metal alloy, and a low viscosity of the material, can greatly improve the level and homogeneity of the conductivity.

Furthermore, the conductivities of the hybrid material are only slightly influenced by high temperatures. The conductivity is not significantly reduced until the softening temperature of about 200 C is reached.

As long as the cross-section is greater than 5 mm², at a relatively low current load of about 10 A, the hybrid material does not heat up critically and can be used for conductor paths.

In addition to excellent electrical conductivity, shielding effectiveness is ensured by the hybrid material due to the pronounced metal mesh. This allows the material to also be used in electromagnetic shielding applications.