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Electrically conductive thermoplastic/metal hybrid materials for direct manufacturing of electronic components.

INTRODUCTION

Electric and electronic products become increasingly important in numerous industries and fields of every-day life. Over the last years, global competition has increasingly intensified. Moreover, product life cycles are becoming shorter, which contributes to the growing significance of complex, highly integrated electric/electronic assembly groups. In addition to the integration of multiple functionalities in one component, the increasing miniaturization and modularization are fundamental aspects in future product development and a major challenge for enterprises of various industries, for example, in automotive, information, automation, aerospace, and medical engineering (1), (2).

Typical electric/electronic assembly parts, which increasingly require new solutions, are highly electrically conductive components in complex assembly groups (such as plug-in connections, elements in transmission and engine control systems, etc.), but also components with defined conductivity ranges, for example, for control units, sensors, enclosures. Lower conductivity values are demanded for applications that have to prevent electrostatic charge. Other electronic components such as enclosures for electronic devices of all kinds require a defined electromagnetic shielding. Finally not only the electrical, but also the thermal conductivity can be of particular interest, for example, for assembly parts like engine covers, heat sinks, and so forth.

Here, often it is not any longer possible for conventional materials alone to meet the demands on the functionality of such components and their profitable, resource-efficient manufacturing (3), (4). Therefore, for a long time, different materials like metals, ceramics, and plastics have been used in combination to benefit from the advantageous properties of the source materials. Innovative polymer/filler combinations, however, open up the opportunity to combine the polymers' excellent properties regarding their processability, density, and so forth with the advantages of metallic fillers and their outstanding electric and thermal characteristics. The development of thermally and electrically conducting polymers over the past years has led to engineering materials integrating various properties. The most common fillers include carbon blacks, graphite, metallic fibers, flakes or powder, carbon fibers, metallized glass fibers or spheres, and increasingly nano-fillers like carbon nano-tubes (5-21). Because of their processability in the conventional injection molding process, these compounds have numerous advantages over many other manufacturing methods such as forming or machining methods: The compounds can be processed directly and resource-efficiently to the finished part, as the injection molding technique can be fully automated, allows for short cycle times, and requires no or hardly any reworking of the molded parts. Especially, the multi-component injection molding process makes the efficient manufacturing of highly integrated, complex components with high reproducibility possible (e.g., electric conductors on insulating carriers without separate steps of handling and assembly) (22), (23).

The compounds form a two- or multicomponent system, in which the filler accounts for the creation of a conductive network (5), (13), (24). The more pronounced the network structure is, the higher is the conductivity. Therefore, high electrical conductivities generally require high filling degrees. A compromise has to be found for each filler system about the amount of filler inserted as high contents ensure low resistance in the molded part, but have negative effects on the mechanical properties and processability due to melt viscosity increasing considerably. As a result, a complete filling of the cavity could be difficult to realize, especially for thin-walled parts. Moreover, higher wear on machinery and molds is the consequence. Thus, the properties and processability of the materials used so far are not sufficient to meet the increasing demands on component functionality (13), (25), (26). Therefore, typical functional components such as electromagnetic shields, plug-in connections, and strip conductors have to be often produced in several complex processing and assembly steps (e.g., insert molding, hot stamping, or metallizing) using a high personnel expenditure, entailing high labor costs, or using capital-intensive automation solutions (5), (13), (27), (28).

Here, metal alloys with low melting points can be used as a novel filler material to further increase the electrical conductivity of conventional compounds and ensure good processability despite high filling degrees (29-31). These metal alloys are liquid during the processing phase and do not start solidifying until the cooling phase. Therefore, many of the disadvantages of highly filled molding materials can be reduced or even eliminated.

In co-operation with Siemens AG, Erlangen, Germany, the IKV developed a hybrid material having particularly high electrical conductivity. This material consists of 15 wt% (56 vol%) polyamide 6 (type 6NV12 by A. Schulman), 33 wt% of a low-melting tin/zinc alloy with a melting point of 199[degrees]C, and 52 wt% of fine copper fibers (average length: 0.65 mm, diameter: 35 [micro]m). With this electrically conducting polymer/metal hybrid material passage, conductivities of [10.sup.5]-7 x [10.sup.6] S/m and conductivities over the wall thickness of [10.sup.3]-[10.sup.5] S/m are achieved (31). These values are comparable with conductivities of pure metals (see Fig. 1). By this means, the material allows the production of conducting structures and junctions for plug-in connections and/or cables in a single processing step, whereby a flexible adjustment of the conductor path's geometry to the particular product is ensured by the injection molding process. In many cases costly, time-consuming joining and soldering processes afterward can be eliminated. The newly developed polymer/metal materials do not only facilitate flexible manufacturing of electrically conducting elements by directly integrating contact pins, but also the realization of electromagnetic shieldings and modules with electrical function for an efficient cooling of electrical devices and motors (see Fig. 2).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

OBJECTIVES OF INVESTIGATIONS AND PROCEDURE

The thermoplastic/metal compounds can be processed in conventional single- or multicomponent injection molding. The injection molding trials show that the filling and freezing behavior of the material changed considerably due to the material's high metal content. Aim of the research work at the IKV is the characterization of the material-dependent injection molding process for an optimized production of electronic parts. The tests--carried out with an injection molding machine of the type Allrounder 320 S 500-150 from Arburg GmbH + Co, Lossburg, Germany--are mainly conducted for test specimen of simple geometries. This allows for various possible characterizations of the properties. The focus lies on plate-type samples with an area of 80 x 80 [mm.sup.2] and a varying wall thickness. Flow spirals and practice-oriented conducting structures with a high length/diameter ratio are also produced and analyzed.

Molded parts of highly filled polymers also show a dependence of the distribution and direction of the filler material on the local position along the flow path and over the part's thickness. The local differences are caused by the injection molding process and are a consequence of the flow conditions during cavity filling. They can already be detected in low filled polymer melts (26), (31), (32). The part morphology can be systematically influenced by the metal/thermoplastic material during the compounding and injection molding phases. Since the filler distribution correlates directly with local electrical conductivity, the knowledge of the influence of the process parameters is essential for a reliable design of the molded part. Therefore, the IKV aims at thoroughly analyzing the dependence of the processing conditions and the resulting part properties. This knowledge allows for the adjustment of the process to the peculiarities of the ultra-highly filled metal/polymer hybrid material.

For the analysis, it is necessary to assess the filler distribution in the molded part by using microscope images that are evaluated by graphic software. First, the structures essential for the analysis are determined and automatically recognized by using threshold values (see Fig. 3). In the next step, metal content, number, size, and orientation of the individual particles and their two-dimensional distribution can be calculated. A spreadsheet program is used for the final evaluation. It must be pointed out that the measurement is done in an area rather than in a three-dimensional space. To gain a three-dimensional impression of the filler distribution in the molded part, it is, therefore, necessary to take several microscope pictures of different positions and from different angles.

[FIGURE 3 OMITTED]

The following sections will first provide basic information on the local filler distribution and electrical conductivity of the hybrid material, and will then discuss the processing properties of the metal/thermoplastic compound.

CHARACTERIZATION OF LOCAL FILLER DISTRIBUTION AND CONDUCTIVITY

Dependence of Conductivity on Filler Content

The combination of copper fibers and metal alloys with low melting points forms a dense, three-dimensional metal network within the molded part, as can be seen in Fig. 4 showing a typical close-up view of a part's cross-section. It is this special part morphology that enables such high conductivities, as if the distance between metal particles becomes more than 10 nm, resistance of the polymer matrix will impede good conductivity (5). In contrast to purely fiber-filled polymers, these distances can be sufficiently reduced due to the fine dispersion of the alloy and good affinity of copper fibers and alloy. Furthermore, the alloy covers the fibers like a solder and forms out numerous connections between them. Thus, passage conductivities may range up to [10.sup.6] S/m (see Fig. 5).

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

By contrast, polyamide exclusively filled with copper fibers will just achieve conductivity values that are approximately three powers of ten lower even if the filling degree is the same. Here, the single filled compound shows a much coarser, less dense metal network than the multiple filled polymer as it becomes obvious comparing test plate's cross-sections (see Fig. 6).

[FIGURE 6 OMITTED]

Figure 5 also shows that there is no linear relationship between the quantity of filler inserted and conductivity. At a low filler content, the filling materials are still surrounded by an insulating polymer layer and only have punctual contact with each other. In this range, conductivity of the matrix polymer is dominant. With increasing fiber content, number and intensity of the contacts between fibers also increases and the specific conductivity escalates in a small window. The critical concentrations of volume required for this escalation--called percolation threshold--range between 20 and 25 vol% for both the metal/thermoplastic hybrid material and the polymer exclusively filled with copper. Above this concentration of filler, the metal network is densified to an extent where conductivity can be increased only marginally (24), (26), (33), (34). With the used volume percentage of 44 vol% the parts, therefore, have relatively stable conductivity values.

Examination of the Part's Outer Layers and Surfaces

If conductivity is not measured through the part's interior but over the thickness regarding the part's surfaces, it will be approximately two powers of ten lower compared to passage conductivity. The reason for this is the surface layer effect, which is known to occur in solid-filled polymers. It means that the parts' surfaces as layers of minimal thickness contain hardly any filler material (13), (26). Especially, fiber-filled polymers also show a tendency to separate during the injection molding process. In laminar shear flows, for example, during filling of the mold, particles dislocate from areas of high shearing action in the outer areas to areas of lower shear stresses in the middle zone of the material (25). Examining the cross-section of the molded part, the structure apparently tends to distribute the major percentage of the fibers closer to the core section leaving the outer layers low in fibers (26), (35). Despite the fact that the low-melting metal alloy, in contrast to metal fibers, also establishes a connection to the surface of the molded part, thus reducing the outer and surface layer effects, the influence of the shear zones near the surface cannot be eliminated entirely.

These effects are often difficult to observe with the microscope due to the layers' small thickness and the often minor differences in filler amount between core and outer zone. However, by employing extreme part geometries--for instance, sharp direction changes, great differences in wall thicknesses and very thin part areas where very high shear velocities may occur--it is possible to make significant differences in filler distribution through out the part's thickness visible. Figure 7 illustrates the filler distribution in an area close to the sprue of a flow spiral in dependence on the injection molding parameters.

[FIGURE 7 OMITTED]

The sprue cone with a diameter of 8 mm redirects the melt in the molding part, which is 1.5-mm thick. Injection velocity, melt and mold temperatures were varied on a low, a medium, and a high parameter level. Significant differences in the fineness of the metal structure as well as the shape of the filler-poor outer layers can be seen in the molding part area displayed just behind the point where the melt changes direction and the diameter of the sprue passing into the narrow part. With decreasing melt and mold temperatures, the zone of highest shear stress, that is, the filler-poor outer layer, moves closer to the center of the part. In contrast, in an area further away from the sprue an even distribution of metal throughout the part's thickness is found.

Dependence of Filler Distribution on Flow Distance

The density of filled polymer parts often rises almost linear with an increasing flow path due to the flow conditions in the gating system and in the molded part. The increasing filler content and above all the reduced outer layer effect and increased incorporation of metal alloy at the part's surface cause the electrical surface conductivity to rise toward the end of the flow path. The rate of increase varies depending on the composition of the material, geometry, and process conditions. For instance, the conductivity (measured over the thickness) of the tested metal/thermoplastic hybrid material on average shows better values and less dependence on the part's position as the thickness of the plate increases (see Fig. 8).

[FIGURE 8 OMITTED]

Correlation of Morphology and Conductivity in Comparison With Other Materials

As knowledge about the local conductivity is essential for a reliable design of the molded part, this section will discuss its level and homogeneity and correlate it with the filler distribution. For this purpose the local density, passage conductivity in flow direction and crosswise as well as the conductivity over the part's wall thickness is determined using sample plates with dimensions of 80 x 80 x 2 [mm.sup.3]. Density is a direct measure of the filling degree due to the great difference between the density of the filling material and the polymer. For a better assessment of material behavior, the values are compared with the conductivity of plates solely filled with copper fibers (at the same volume filling degree) and plates filled with long steel fibers (PA-SF, filling degree: 20 wt%, length: 11 mm, diameter: 8 [micro]m, state of the art of polymers obtainable on the market with the highest conductivity values to date). The results are shown in Figs. 9-11 and Table 1.
TABLE 1. Density and electrical conductivity depending on measurement
direction.

                                  Hybrid material       PA + copper

                                 Average  Deviation  Average  Deviation

Conductivity in flow direction    5.59      0.13       2.02      0.12
[lg(S/m)]

Conductivity transverse to flow   5.42      0.14       1.96      0.15
direction [lg(S/m)]

Conductivity over part's          3.28      0.52      -1.34      0.62
thickness [lg(S/m)

Density [g/cm]                    4.15      0.15       4.37      0.17

                                                     PA + steel fibers

                                                     Average  Deviation

Conductivity in flow direction [lg(S/m)]              3.31      0.15

Conductivity transverse to flow direction [lg(S/m)]   3.21      0.23

Conductivity over part's thickness [lg(S/m)           1.01      0.25

Density [g/cm]                                        1.30      0.08


[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

The synergy effect of the binary filler system in form of a significant increase of contacts between individual particles becomes apparent in the level of conductivity--as was already shown with the percolation curves (see Fig. 5). Achievable passage conductivities of the hybrid material range between [10.sup.5] and [10.sup.6] S/m. and allowing for the outer and surface layer effects, conductivity over the part's thickness still ranges between [10.sup.3] and [10.sup.4] S/m. Polyamide exclusively filled with copper fibers, however, only achieves significantly lower conductivity values. Polyamide filled with steel also has a conductivity that is always ~2 decades lower. The difference between passage conductivity and conductivity over the thickness is greatest for the copper compound and smallest for the hybrid material due to the high amount of alloy contained in the surface layer (~30%).

All materials show a good correlation of local density and conductivity depending on the direction. The variation of the local density ranges from 3.5% (hybrid material) to 6% (PA + steel). With comparable deviations, the logarithmized passage conductivity is very stable. Because of the outer and surface layer effects, it is only the conductivity over the wall thickness that shows considerable local differences (15-45%). But the metal alloy at the surface improves homogeneity of the hybrid material. Although short copper fibers are easily carried with the flow moving closer to the core, long steel fibers partially form such tight clusters that their behavior in the melt is more static. Thus, in case of steel fiber-filled compound, density and conductivity are highest in the center of the plate (remote from the sprue), whereas the values of materials filled with copper fibers are highest toward the outer edge (remote from the sprue).

Both copper fibers and fine steel fibers are very ductile. This results not only in highest conductivity values for all materials in flow direction, but also in relatively small differences for the cross direction. Compounds containing rigid fibers like short carbon fibers, however, show much higher differences (5).

The advantage of combining copper fibers and metal alloy is not only seen in electrical conductivity, but also in electromagnetic shielding (see Fig. 12). To show this effect, the plates with a thickness of 2 mm were measured using transmission-line measuring equipment. Although the values of the hybrid material reached the scale span limits of the used apparatus and are comparable to metal, the shielding capability of the plate exclusively filled with copper was significantly lower. The material filled with steel fibers could also not compare with the values of the hybrid material.

[FIGURE 12 OMITTED]

Adjusting Material Properties by Changing the Matrix Polymer

It is possible to purposefully adjust the filler distribution, thus the electrical properties, by varying the matrix polymer. Here, the ratios of the melt temperatures and viscosities of polymer and metal alloy as well as the non-Newtonian behavior of the polymer viscosity play an important role. The material concept allows for choosing almost any polymer matrix. This enables the hybrid material to also meet special requirements, for instance, its dimensional stability under heat or its strength. As an example, the influence of the polymer on compounds based on polyethylene (PE, type MA9230 by Borealis) and polyphenylene sulfide (PPS, type Fortron 0203 by Ticona) is examined. The chosen materials differ greatly from PA6 in their melt temperatures, but not in their viscosities. Furthermore, a second polyamide 6 (type Ultra-mid B50L01 by BASF) with a significantly higher viscosity was employed.

The appearance alone shows significant differences in the distribution and orientation of both filler materials (see Fig. 13). In case of the PE matrix, the metal alloy solidifies first and then freezes in the still molten polymer. This leads to a very even and fine distribution of the metal on the surface. In the PPS compound, however, the polymer freezes first. Transmission of pressure is increasingly realized in the alloy, which is consequently pushed closer to the cavity wall. Therefore, the greatest amount of the metallic phase in the material can be found at the plate's surface. Substantial metal particles will form in the part's center. Moreover, the big difference between the melt temperature of the polymer and mold temperature leads to a rapid decrease of the free flow cross-section, resulting in an increased orientation of the filler material in areas near the surface. More pronounced shear zones with a high degree of orientation are only observed with employment of highly viscous polyamide.

[FIGURE 13 OMITTED]

The freezing behavior is reflected in the conductivity values. Although passage conductivity is at a similarly high level of > 4 x [10.sup.5] S/m for all compounds--with the highly viscous, high-orientated polyamide compound having the best values--conductivity over the part's thickness shows significant differences along the flow path (see Fig. 14). As for this measurement, the part's interior was not directly contacted the different occurrence of the outer layers and the contact resistance of the untreated surfaces were taken into account. All materials show a high conductivity over the thickness, which, remote from the sprue, ranges between approximately 4 and 7 x [10.sup.4] S/m. But the PA and PPS compounds only reach this conductivity at the end of the flow path. It becomes evident that a freezing of the polymer at the same time or later than the metal alloy and a low viscosity of the material can considerably improve the level and homogeneity of conductivity.

[FIGURE 14 OMITTED]

Influence of Process Control on Electrical Conductivity

Material and processing properties are not only influenced by the composition of the material, but also by the injection molding process. To illustrate this, basic parameters of the dosing and injection phases were systematically varied on different levels, using design of experiments. It was shown that electrical passage conductivity is nearly constant in a broad process window due to the dense metallic network. However, the parameters injection velocity, melt and mold temperatures allow significant changes in the level and homogeneity of conductivity over the part's thickness, because they determine the flow and freezing behavior in the cavity. As Fig. 15 illustrates, a longer filling phase can reduce the formation of shear zones and hence improve the conductivity over the part's thickness. Moreover, by optimizing the dosing parameters dynamic pressure and dosing speed it is possible to ensure a homogeneous melt in front of the screw while maintaining a fine dispersion of the metal melt. By this, the conductivity values can be improved (see Fig. 16).

[FIGURE 15 OMITTED]

[FIGURE 16 OMITTED]

Following this characterization of filler distribution and electrical conductivity, the next sections will discuss the processing of the hybrid material more in detail.

CHARACTERIZATION OF MOLD FILLING

The thermoplastic/metal hybrid materials are a material system consisting of three phases. Besides the polymer matrix there is also copper, the fibrous filler material, and the initially molten metal alloy with a low melting point that influence the filling and freezing phases. To evaluate the influence of the solid filler (copper fibers) on the one hand and of the liquid metal phase on the other hand, material viscosity is measured using an online rheometer (36) and compared with different filled and unfilled polymers. The results show that even though the viscosity is higher than that of the unfilled matrix polymer, polyamide, it is still in the margin of unfilled polymers due to the effect of the low-viscous metal melt (see Fig. 17). Similarly highly filled molding materials, for example, powder injection molding feedstocks, which contain only solid fillers like ceramic powder, show clearly higher viscosities.

[FIGURE 17 OMITTED]

Nevertheless, the injection molding tests conducted so far show that the high metal content significantly influences the rheological and thermodynamical properties of the metal/thermoplastic melt. Unfilled and low filled polymers show a typical source flow during filling of the mold, while the velocity profile over the part's thickness is parabolic with the highest velocity in the center of the part and wall adhesion at the surface. As the filling degree increases, this profile often becomes rather block-shaped (37). Such flow shapes result in a typical flow pattern, which was determined by a mold filling study for unfilled polyamide (Fig. 18 on the left) and polyamide exclusively filled with copper fibers (Fig. 18 in the center). The test plate is filled using an unbalanced runner system with a film gate of 1.5. mm in height. Since melt elasticity decreases as the filling content increases, the filler distribution--compared to unfilled polyamide--also becomes less even due to the copper fibers. This is seen in the change of the flow front's parallelism to the film gate.

[FIGURE 18 OMITTED]

The highly filled metal/thermoplastic hybrid material, however, shows a notably different filling behavior when the filling degree is particularly high (Fig. 18 on the right). This behavior suggests high wall shear stress and a reduced source flow. The reason for this is not only the considerably reduced melt elasticity of the entire compound, but also the flow and freezing behavior of the low-melting metal alloy as well as the high thermal conductivity of the material ([much greater than]2 W/mK) as such.

INFLUENCE OF MOLD TEMPERATURE CONTROL DURING PROCESSING

High thermal conductivity does not only change flow behavior, but also leads to a premature freezing of the material. As a result, the maximum filling length is reduced compared with the unfilled matrix polymers. For the production of shielding housings and two-dimensional components with short and medium flow paths, there is often hardly any need to take this behavior into consideration. However, it plays an important role for the realization of high flow path/wall thickness ratios. They may be necessary for the direct manufacturing of multifunctional electronic components like electrically conducting structures on electrically insulating polymer carriers, thus requiring an adjustment of the process.

The mold temperature control can be used to significantly influence the flow and freezing behavior of polymer melts in the mold cavity. Increasing mold temperature during the entire cycle is a simple method to slow down the freezing of the material (38). First tests on flow spirals show that their filling length can be considerably increased, if they are produced in a notably extended process window with mold temperatures from 80[degrees]C to more than 180[degrees]C (see Fig. 19). To increase the reproducibility of the filling lengths of partially filled spirals, an injection pressure of 1800 bar was applied and kept constant for 10 s. A pressure sensor of the company Kistler AG, Winterthur, Switzerland, positioned close to the sprue also recorded the pressure in the cavity of the mold. The pressure significantly increases when the mold temperature rises (see Fig. 20). That means that the pressure loss strongly decreases through the gating system and the inlet area of the cavity, because the narrowing of the cross-section of the molded part takes longer due to the slowed-down freezing of the material.

[FIGURE 19 OMITTED]

[FIGURE 20 OMITTED]

Figure 21 shows typical pressure curves inside the mold. A certain mold opening near the sprue could not be prevented due to the high injection pressure and the low rigidity of the employed mold. Comparing the pressure curves, one finds the curve for a mold temperature of 200[degrees]C particularly striking. It can be clearly seen that, in contrast to lower mold temperatures, the material does not freeze throughout the whole cross-section during the entire phase of injection and pressure holding phase for 10 s. Instead, high pressure is kept up in the molten core and reduces only after the injection pressure is reduced and pressure on the melt core is relieved.

[FIGURE 21 OMITTED]

Therefore, it will be of particular interest to bring the mold to a temperature at which polyamide and metal alloy are molten to avoid high injection pressures and realize especially long flow paths. Since higher temperatures entail a significant decrease in dimensional stability of the matrix polymer and an increase in installation and energy costs, an economical alternative is the application of vario-thermal temperature control, for example, by inductive heating, because of its high power transmission possible (30,000 [W/cm.sup.2]) (39). This procedure allows to briefly generate mold temperatures that are considerably higher than the melting points of metal and polymer, thus preventing a premature freezing of the material. As a result, a disproportionately high increase of the maximum flow path length can be expected. Furthermore, short cycle times can be achieved due to short handling times of the induction head as well as the systematic heating of only those mold layers close to the surface. The IKV already made first examinations with such a heating system, which already look promising.

INJECTION MOLDING OF CONDUCTOR PATHS

The feasibility of the production of fine conducting structures with increased flow paths on polymer carriers by applying multicomponent injection molding does not only depend on processing conditions, but also on the type of filling material. For instance, the size of copper fibers used limits the minimum cross sectional area of the conducting structures produced with the metal/polymer hybrid material because at a certain point, they start clogging the flow channel. For this reason, further tests were carried out and conductor paths of various cross-sections were produced by insert molding on polymer base plates with a maximum flow length of 78 mm. Their flow lengths, morphologies, and conductivities were determined in dependence on the cross-section of the conductor. The melt temperature was kept constant at 265[degrees]C, while the mold temperature was varied at three stages (80, 130, and 180[degrees]C). The maximum injection pressure was limited to 1500 bar to avoid an overpacking of the fine structures.

As expected, the longest flow paths can be realized by applying the highest mold temperature (Fig. 22 on the left). A temperature of 180[degrees]C is required for filling the entire length of the conductor path with a cross sectional area of 1 x 1 [mm.sup.2]. At a cross sectional area of 0.5 x 0.5 [mm.sup.2], only considerably reduced filling lengths can be realized without the application of vario-thermal temperature control. Microscope images show that there is still no extensive clogging of the flow cross-section (Fig. 22 below). However, differences in the filler content along the flow path (as described in "Dependence of Filler Distribution on Flow Distance" section) increase due to higher shear velocities. Because of the lower filler ratio near the gate, this material behavior leads to a reduction of passage conductivity (measured over the entire flow path) when the cross sectional area is decreased (Fig. 22 on the right). Further tests, which are currently conducted at the IKV, will show to what extent temperature control by inductive heating may also have positive effects on the local filler distribution.

[FIGURE 22 OMITTED]

CONCLUSION AND PROSPECTS

The described thermoplastic/metal hybrid materials have a high potential for being employed in the production of complex electronic components with very high demands on the electrical conductivity.

By using the conducting hybrid materials, several manufacturing steps can be integrated into the efficient (multi-component) injection molding process. As the material allows the production of conducting structures and junctions for plug-in connections and/or cables in a single processing step, costly, time consuming joining and soldering processes afterward can be eliminated and new design solutions can be established. The injection molding process ensures a flexible adjustment of the geometry of conducting structures and junctions to the particular product.

High filling degrees and high thermal conductivity change the filling and freezing behavior of the material. This is particularly important for the production of conducting structures with high flow path/wall thickness ratios and may require an adjustment of the mold temperature control. Current research projects at the IKV are aimed at developing mold and process solutions for an optimized material processing to complex multicomponent parts.

Moreover, due to the injection molding process molded parts of highly filled material show a dependence of the distribution and direction of the filler material on the local position along the flow path and over the part's thickness. For the design of the molded part it is, therefore, important to consider not only global characteristic values, but also the influence of the material composition, the process parameters, as well as the geometries of gating system and cavity. This also makes it possible to systematically influence morphology and part properties during the compounding and injection molding of the polymer/metal hybrid materials. To assess the extensive possibilities and also the limits of the new compound, the IKV currently carries out further investigations for the optimization and characterization of the material and its processing properties.

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Walter Michaeli, Tobias G. Pfefferkorn

Institute of Plastics Processing at RWTH Aachen University (IKV), Pontstr. 49, D-52062 Aachen, Germany

Correspondence to: Tobias G. Pfefferkorn: e-mail: pfefferkorn@ikv.rwth-aachen.de

Contract grant sponsor: German Federal Ministry of Economics and Technology (BMWi) [("Arbeitsgemeinschaft industrieller Forschungsvereinigungen e.V.") "Industrieller Gemeinschaftsforschung" (IGF)]; contract grant number: AiF No. 15258 N.

DOI 10.1002/pen.21374

Published online in Wiley InterScience (www.interscience.wiley.com).

[c] 2009 Society of Plastics Engineers
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Author:Michaeli, Walter; Pfefferkorn, Tobias G.
Publication:Polymer Engineering and Science
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Date:Aug 1, 2009
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