Building Lighter, Less Expensive Products with Conductive Plastics

TE has developed injection moldable grades of electrically conductive plastics and demonstrated their use in various applications. A variety of thermoplastics can be used with the proprietary electrically conductive filler package, while the conductivity can be tuned according to the target application requirements by adjusting the filler concentrations. The ability to be processed by injection molding opens up a wide range of application possibilities, including automotive, where environmental stability, weight savings, and mechanical performance are critical, as well as data and devices, where small and intricate form factors are often required in addition to high electrical performance.

An exemplary application of injection molded electrically conductive plastics is a connector housing with built-in EMI shielding functionality. Traditional methods for achieving EMI shielding functionality would be to employ metal shielding components by deposition, plating, stamping, etc., which can be costly and/or pose significant design constraints. Conductive plastics, on the other hand, offer a potentially cost-effective solution along with great design freedom. The housings can be molded separately, or over-molded directly onto current carrier structures. Alternatively, two-shot molding procedures can be used to incorporate the conductive plastic in specific areas of an otherwise electrically insulating plastic housing.

Functional testing of injection molded conductive plastic housings for a representative connector has demonstrated comparable performance to a metal shielded plastic housing. Figure 1 shows the insertion loss and cross-talk data for injection molded conductive plastic housings made from two different conductive plastics (red and blue curves), in comparison with a reference part made with a traditional insulating plastic housing and metal shielding. The conductive plastic housings provide nearly equivalent performance to the reference part.

TE has also developed electrically insulating and thermally conductive plastics, which can be injection molded. They have found potential applications such as connector housings and heat sinks, where new generation product designs often involve complex form factors and demand greater design freedom than what traditional metal components can offer.

While conductive plastics can be injection molded like traditional thermoplastics, it is important to keep in mind that the injection molding parameters can have a significant impact on the final conductive properties of the part. For example, the shear forces experienced during molding affect how the conductive particles are aligned and distributed in the material. This can lead to the conductivity varying throughout the part depending on the flow pattern, and also to the conductivity varying depending on the direction in which it is measured. Therefore, it is important to consider the specific functionality requirements in conjunction with the flow patterns that will be experienced during processing when designing the geometry of the part. And it is equally important to optimize the processing conditions to achieve the desired properties.

While it is well-known that the high conductivity of conductive plastics is often achieved with the drawbacks of high viscosity, rigidity and brittleness of the materials, there are relatively few discussions on the particular challenges that such drawbacks pose for extrusion related applications, such as conductive tubing and tapes. To make useful thin wall products by these continuous melt processes, it is necessary for the conductive plastics to have high tensile elongation and high flexibility, in addition to high conductivity, which is a big challenge for the formulation and the processing. In fact most commercially available conductive plastics are only recommended for injection molding applications and are unsuitable for extrusion related applications. Those available in the market for extrusion are mainly carbon black containing plastics, whose conductivity levels are often orders of magnitude lower than TE extrusion grade conductive plastics.

TE electrically conductive plastics for extrusion are homogeneous synergistic blends of selected thermoplastics, a proprietary conductive filler package, and other additives, accomplished by the combination of material science, polymer processing science and art. The resistivity and tensile elongation of the conductive plastic compounds can be tuned. For example, at a resistivity of 1E-3 ohm.cm, the typical tensile elongation at break value is around 300%, while lower resistivity or higher conductivity may be achieved at the expense of lower tensile elongation. The excellent electrical and mechanical properties of these conductive plastics make them great candidates for new generation product development in many applications, such as tubing, wire and cable, consumer devices, and so on.

As examples, the TE conductive plastic compounds have been extruded into tubing and tapes, which can be used as EMI shielding materials, alternative or complementary to the conventional metal braids and tapes in wire and cable products, with projected weight and cost savings. The shielding effectiveness data of TE conductive plastic tubing and tape are shown in Figure 2, in comparison with a commercial coax cable, double shielded by metal braid and aluminized Mylar polyester tape, as well as a commercial Mylar polyester tape alone. The results demonstrate excellent EMI shielding performance of the conductive plastic tubing and tape in the microwave/GHz frequency range, which is very desirable for today’s high speed data transmission devices.

Besides being made into tubing and tapes, the TE conductive plastics may also be co-extruded or tandem extruded directly onto wire and cable, taking advantage of the existing cable manufacturing lines and providing the opportunity to remove or reduce the braiding of conventional cabling process.

Another exemplary application of TE conductive plastics could be the solder-free joining of electronic components, to provide electrical continuity with a simple, inexpensive and residual-free solution.

As commonly understood by the industry, conductive plastics are prone to lose conductivity upon high shear, high drawdown ratio or expansion during the extrusion processes, due to damage of the filler morphology and/or the breakdown of the conductive network under such conditions. Besides the formulation science, mastery of the polymer processing art for highly filled systems is also required to achieve the desired properties.

3D printing has become popular for rapid prototyping and even low volume manufacturing. Fused deposition modeling (FDM) is one of the most commonly used additive manufacturing processes and involves extruding thermoplastic filaments through a heated head and depositing the material into the desired structure. FDM-type 3D printing of conductive plastics, including electrically conductive plastics and electrically insulating/thermally conductive plastics, has generated a lot of interest for rapid prototyping of functional prototypes.

There are several different conductive plastic filaments for 3D printing on the market today. All of them contain carbon-based fillers, limiting their conductivities significantly. The most conductive filaments we have seen on the market have conductivities 5-6 orders of magnitude below IACS. The most conductive of these utilize expensive specialty fillers such as carbon nanotubes and graphene. The TE Enterprise Technical Center Materials Group has developed electrically conductive plastics for 3D printing based on a proprietary platform with conductivities up to 0.1% of IACS, a significant improvement over the currently available materials. TE has also developed 3D printable electrically insulating/thermally conductive plastics.

FDM printing poses several unique challenges for conductive plastics. The presence of the conductive particles may impede flowability through the nozzle in the heated head, leading to a tradeoff between flowability (larger nozzle is better) and part resolution (smaller nozzle is better). Furthermore, the inherent presence of boundaries between the layers of a printed part may decrease the conductivity of the part as well as other important properties such as mechanical strength. The printing parameters should be optimized to minimize these effects.