Approaches for automated wiring harness manufacturing: function integration with additive manufacturing

With the electrification of powertrains and the progressive implementation of assisted and automated driving functions, the vehicle wiring harness is becoming increasingly important in the automotive industry. The design of the wiring harness is gaining considerable variation and is becoming more and more complex. In order to master this complexity in the manufacturing processes in a reliable manner, new approaches are required for the progressive automation of the wiring harness production. Additive manufacturing processes have not yet been used in the production of vehicle wiring harnesses. The development of additive processing of conductive materials therefore creates a new basis for the development of automation solutions to produce vehicle wiring harnesses. With the approach of function-integrated multi-material application, the possibility of using electrically conductive polymers in the vehicle wiring harness is specified in detail. A fundamental study was carried out to determine the values of electrical conductivity that can be achieved in the field of plastics. Based on these findings, the research question being addressed is whether polymers can be made electrically conductive to an extent that is suitable for use in a vehicle’s wiring harness. The materials of electrically conductive components from conventional vehicle electrical systems serve as a reference. A specially developed test set-up for measuring the electrical conductivity of polymers provided the required measured values. The quantitative evaluation of the measurements clearly shows that the use of conductive polymers as a conductive material in the vehicle wiring harness is only possible to a limited extent. The major benefit of the study identified the use of electrically conductive polymers for the automatable production of electrical connections.


The role of the wiring harness in vehicle construction
The wiring harness, which until now has mostly played a hidden role in the world of vehicles, has involuntarily gained fame in the year 2022.The war in Ukraine has hit a region that is home to numerous large manufacturing facilities.About 80% of the major suppliers of wiring harnesses in Europe also produce in Ukraine [1].Delivery failures from these plants had a direct impact on vehicle production, which came to a partial standstill at several car manufacturers in March 2022 [2].This dramatic effect clearly demonstrates the great importance of the wiring harness in the overall vehicle and the relevance of the production processes of vehicle wiring harnesses in the context of the production as a whole.The wiring harness of a vehicle is usually manufactured as a so-called customer-specific cable harness 1 3 (Fig. 1).This means that each vehicle produced has its own individual wiring system, which is manufactured to match the equipment of the vehicle in question.The wiring system is tailored to the customer's vehicle configuration and corresponds exactly to the electrical network required for the desired vehicle equipment in each individual vehicle.
No electrical lines are installed in the vehicle that are not required.This results in an exceptional degree of individuality in the vehicle's electrical system.The use of identical wiring harnesses in several vehicles is, therefore, generally not possible.

The supply chain of the wiring harness
During vehicle assembly, the wiring harness is installed in the vehicle at an early stage.For vehicle production, this results in a direct dependency on the on-schedule and vehicle-synchronized supply of the vehicle wiring systems.The demands of vehicle manufacturing on the supply of wiring harnesses result in a high degree of complexity in the supply chain.If we focus on vehicle production in Germany and Europe, the supply of wiring harnesses usually takes place via two routes: overland from Eastern Europe and by sea from North Africa.The long transport routes, the use of different means of transport, their loading and unloading as well as the interim storage in depots generate considerable logistical costs (Fig. 2).The need to reduce emissions along the supply chains [3] leads to the question of how long transport routes influence the emissions that arise in connection with vehicle wiring systems.The greenhouse gas emissions for the logistics of a vehicle wiring harness with a mass of 37 kg is 22 kg CO 2 eq [4].
The reason for the complex logistics is the largely manual production of vehicle wiring harnesses in conjunction with the cost advantages that result from manufacturing in so-called low-wage countries.A manufacturing country is classified as a low-wage country if the wage level is at least 50% below that of a high-wage country such as Germany [5].The high share of labour costs in the unit costs of the wiring harnesses, which are elaborately manufactured by hand, is considered an argument for producing in low-wage countries with low investments and a low degree of automation, which predestines the wiring harnesses for international production [6].

Motivations for automating the production of wiring harnesses
In order to meet the demands that will be placed on vehicle wiring harnesses in the future, extensive automation of the production is necessary.Increased requirements arise in particular with regard to: -Product quality -Installation space requirements -Conservation of resources -Resilience of the supply chain One of the direct effects of automation is the reduction in the general error potential of manual production, which is referred to as the "human factor".Humans are considered a possible cause of errors in manual manufacturing processes, for example through counting, fatigue or carelessness [7].In combination with complete traceability of the manufactured wiring harnesses, the automation of manufacturing processes serves to achieve higher manufacturing qualities than are achieved today.This corresponds in particular to the requirements that arise with the use of highly automated driving functions.Automating the production of wiring systems ensures the required, consistently high product quality [8].
In addition, automation enables the reduction of component dimensions and promises cost advantages due to the reduced material requirements and positive effects in terms of sustainability through resource conservation.In the vehicle, miniaturization results in advantages regarding the package.The previous design of the wiring harness also considers measures to reduce the error potential of manual handling, for example using additional color coding.Automated handling allows the elimination of these color variants down to a single-color wiring harness.Advantages in manufacturing costs and logistics are the positive consequence of the reduced need for variants, with a lasting effect on the automation of the manufacturing processes.The potential of automated wiring system production is greatest when it is implemented in conjunction with a change of location to production close to the OEM. 1 Small geographical distances serve to shorten delivery times and eliminate the risk of disruptions along the long logistics routes, which has a positive effect on the resilience of the supply chain [9].
In addition, shortened transport routes reduce logistics emissions, which improves the sustainability of the entire process.

An economic view of automated wiring system production
The automation of manufacturing processes is always accompanied by investments in equipment and manufacturing technology.It must, therefore, be clarified from the start which economic advantage would be gained from automation [10].This question has not yet been clarified for the automation of wiring harness production.In a generic analysis, it was, therefore, initially necessary to clarify which degree of automation in wiring harness production can be economically attractive.
One reason for the low level of automation in wiring harness production to date is the economic operation of manual production in low-wage countries.A comparison of gross national income per capita (PPP)2 based on the World Development Indicator [11] shows that incomes in Western European countries are around five times higher than in low-wage countries in Eastern Europe and North Africa.
The development of the cost shares for labour costs, machine costs and logistics was determined considering the location dependency and the mutual interaction.This consideration follows the evaluation criteria from development and production of an OEM and enables a prediction of the unit costs to be expected as a function of the automation costs for the respective degree of automation.The result is shown in Fig. 3.The diagram shows two economically optimal degrees of automation (cf.[12]).According to this, the optimal degree of automation in the low-wage country is 30%.Compared to purely manual production, a cost advantage of 6% can be expected.In the high-wage country, the optimal value determined is a degree of automation of 70%.The expected component costs are 65% higher than those of purely manual production in the lowwage country.The study assumes that manual production and automated production take place at the same speed.In addition, it is assumed that machine costs rise significantly and non-linearly with increasing levels of automation.This is justified, for example, by the insufficient utilization of special machines, which are only used for a few wiring system variants.The study, which is exclusively economically motivated and focuses on the unit costs of the wiring system, does not provide any arguments for a degree of automation of more than 30% or for more automated production at a location close to the OEM.

Automation in conventional wiring harness production-state of the art
Wiring harness production is divided into several steps of value generation.These include, for example, cutting the individual-colored wires, pre-assembling the wires, assembling the complete wiring harness, electrical tests and packaging the wiring harness for transport.The degree of automation of these sub-steps varies greatly.The cutting of the electrical cables is almost completely automated throughout the industry, with only a few manual operations.The final assembly of the wiring harness is almost exclusively manual [13].
Current degrees of automation in wiring harness production can only be researched to a very limited extent.In view of the high motivation to automate wiring harness production, the OEMs are anxious to carry out their own analyses of the progress of automation.According to a market analysis by an OEM in 2021, modern series production for vehicle wiring harnesses achieves automation levels of approximately 16% when evaluated according to Eq. 1.More conventionally set-up production facilities are significantly lower at around 5%.The suppliers of wiring harnesses' own figures are slightly more optimistic at around 20% [13].A comparative look at the degrees of automation in the production of vehicle components shows that the automation of wiring harness production to date is comparatively low.In modern car body construction, for example, automation levels of over 90% are achieved [15].
Despite numerous research projects with the goal of automation, the wiring harness is still primarily manufactured manually [16].Previous research projects aimed, for example, to automate the laying of electrical cables to a large extent using robots [17] or to create electrically conductive metallization on the surface of polymer structures using a laser beam [18,19].The wide variety of manufacturing processes and components to be processed means that the potential of manual production has so far been predominant.

The challenges of automated wiring harness production
Despite the high incentive, the large-scale production of vehicle wiring harnesses quickly reaches its limits in terms of automation.In particular, the high number of components from which a vehicle wiring harness is made, the large variance of these individual components and the flexibility of the shape of the cables are obstacles for today's machine technology [20].The possibility of automating the production of complex wiring harnesses with many different cables, contacts and other components has always been rated as very poor [21].For the respective connection, contact and wire must be matched to each other, and a crimping tool defined for this purpose with specific crimping parameters must be used [22].
In November 2021, a draft of E DIN 72036:2021-11 was published for the first time.The new standard is an industrywide attempt to counteract component variance in the wiring harness by standardizing wiring components.For example, the design guideline classifies the conventional through connector (Fig. 4) as "not acceptable" due to the lack of an automation solution [23].This form of cable connection, known as a splice, is often used in vehicle wiring harnesses in a sealed design to reduce the use of individual cables.
The reason for this advance is the lack of flexibility in existing manufacturing systems.Up to now, the focus of automation has been on using existing wiring system components and replacing manual production processes.The result is automation solutions from classic mechanical engineering, such as the use of robotics and other special machines.If these machines must handle frequently changing wiring system components due to component variance, this leads Fig. 3 Optimal level of automation (own graphic, based on [12]) to frequent changes of tool inserts.Time utilization and output of the machine deteriorate as a result.One cause of this problem is the very high number of different contact systems [21].Viewed as a whole, the current approaches to automating wiring harness production are automated replicas of manual production processes.

Lack of research on increased automation of wiring harness production
In the future, higher degrees of automation can only be achieved with the development of alternative production forms that supplement the limiting design guidelines and allow more flexibility in the production of wiring harnesses.The basic idea that electrical conductivity is exclusively assigned to metals such as aluminum and copper further reduces the possibilities of wiring system design and process engineering.Additive manufacturing processes of selfsupporting conductor structures and electrical connection points have not yet appeared in automated series production of vehicle wiring harnesses.In particular, the automation potential of additively processable electrically conductive polymers thus remains unexploited.This is because no information is available on the usability of conductive polymers in the area of vehicle wiring harnesses.Existing studies of new manufacturing methods have so far concentrated on the additive production of plastic components [24] or on the additive application of electrical conductors to a carrier material [18].
For the advancing automation of wiring harness production, new approaches and solutions are required, with the focus on: -Reducing the combinatorial variance of components and processes -Evaluation of additively processable electrical conductors regarding their suitability for wiring system applications -Improving flexible processes to produce electrical connectors 3 Additive manufacturing for the automation of wiring harness production

New approach: function-integrated multi-material deployment
The approach of using as few materials and semi-finished products as possible to model the functions of the wiring harness is referred to as function-integrated multi-material use. 3 The underlying concept of this approach revolves around the capability of an electrically conductive polymer to fulfill numerous functions.This material can be manufactured in a single production process.This results in a lean manufacturing with a high automation potential.It is expected that the achievable electrical conductivity of the polymers that can be used is below that of common conductor materials.However, in addition to electrical conductivity, the FIM approach also considers the mechanical and additive properties of the polymers and their potential for automating wiring harness production.
The main research topic is the additive production of electrical connections between conventional wiring system components.Compared to previous mechanical methods for manufacturing electrical connections, the additive process promises geometric independence of the contact components to be connected and the elimination of specific joining tools.The FIM approach includes a new processing procedure for this purpose.This is based on: -Use of new materials -Redesign of the electrical connection The focus is on the use of new materials whose structure allows functional integration.Many required functions, such as electrical conductivity, mechanical strength and medium sealing, are integrated into a material that can be processed in a single additive manufacturing step.Polymers serve as a matrix material, which enable additive processing and provide suitable mechanical properties.The electrical conductivity is created by introducing conductive fillers.The combination of several materials creates a multi-material.There are also new approaches for the design of electrical connections.In vehicle wiring harnesses, crimp connections are the most commonly used [22].In through-connections, ultrasonically welded splices are primarily used.As described in 2.2, both forms are problematic in terms of their automation capability.The new approach provides for form fitting, tightness and electrical connection to be established via the polymer material, which is applied to the components to be joined.
It had to be clarified whether polymers can be equipped in such a way that they meet the requirements regarding electrical conductivity for use in a vehicle wiring harness.Research on this had to be carried out in order to evaluate the basic usability of electrically conductive polymers in the wiring harness.The testing of the electrical conductivity is the initial focus of the project and serves as a gradual preselection compared to conductive copper as a reference material.

Electrically conductive finishing of polymers: the percolation model
Functional integration is achieved by combining several basic materials.Polymers serve as matrix materials that enable additive processing and provide suitable mechanical properties.The electrical conductivity is created by the additional introduction of conductive fillers.The combination of several materials creates a multi-material.Plastics are usually excellent insulators [25].The percolation model describes how a polymer becomes an electrical conductor through the introduction of a suitable filler [26].The physical interfaces between the filler particles form micro-contacts within the polymer.These micro-contacts cause the emergence of continuous current-carrying load paths if they are sufficiently interlinked [27].A model of the current-carrying paths is shown in Fig. 5.The electrical conductivity of the multi-material is influenced by (cf.[27]): -Filler concentration in the polymer -Shape of the filler particles -Particle size -Electrical properties of the filler The curve of the electrical resistance as a function of the filler concentration is shown in Fig. 6.In the case of electrically conductive polymers, an inhomogeneous filler concentration must be assumed [28].This can be caused by processing or the influence of density differences, which create the effect of sedimentation.Therefore, conductive polymers with a filler concentration in the over percolated range are mostly used in practice.In this way, the electrical conductivity is reliably maintained at a high level [26].The filler content in the polymer is usually 60-80% [29].Disadvantageous effects arise with the high filler concentration in terms of economy and mechanical properties [28].The electrical conductivity of the multi-material is also affected by the shape of the filler particles [27].
Long metal fibers show suitable electrical properties.Additive processing methods such as extrusion, show susceptibility to faults with fiber-filled semi-finished products, for example due to clogging of nozzles [30].A more suitable material design is to use the filler in grain form or in flake form [29].For our own modelling, the spherical form is assumed, which generates the smallest number of flowcarrying paths and enables particularly precise material application [28].
The grain size of the filler influences the electrical conductivity via the numerical formation of micro-contacts per  [29].A smaller grain size produces a larger number of micro-contacts in the same material volume.In total, this results in a higher electrical resistance.On average, grain sizes between 10 and 50 µm are used [29].The electrical properties of metals are why they are preferably used as fillers.High electrical conductivity and good oxidation properties speak in favor of gold, silver and silver-coated particles as fillers [29].Due to high material prices and speculative influences, the sparing use of these precious metals is recommended in the field of technology [31].Graphite, aluminum and copper are therefore also considered suitable fillers [26].For example, the additive manufacturing of circuits and printed electronics based on thermoplastic filaments with carbon black, graphene and copper as fillers has already been investigated [32].An overview of current developments in 3D printing of electrically conductive polymers shows various additive manufacturing processes that can be used [33].

Determining the electrical conductivity of multi-materials
A specially developed test set-up was used for the investigation and evaluation of the electrically conductive polymers and fillers (Fig. 7).with electrically conductive polymer (Fig. 8), the maximum layer thickness (l) of the material to be expected is in the range of a few millimeters.A value of 1.0 mm was selected for determining the electrical conductivity.The construction of prototypes later confirmed the assumption regarding the layer thickness.The selection of materials and dimensions of the test set-up follow the approach to using the materials in the electrical connections of the wiring harness.In this way, the electrical interactions between the different materials to be expected in the application, such as the influence of contact resistance, are already taken into account in the results.A four-conductor circuit is used to determine the electrical resistance [34].
The electrical resistance (R) of the material sample is measured with a milliohmmeter, which is connected to the test profiles.The measured ohmic resistance (R) allows for a determination of the specific resistance (ρ) of the material sample, taking into account the known dimensions of the measuring point: The selection of the investigated fillers is based on materials which are successfully used in the conventional wiring harness for the manufacture and surface coating of currentcarrying cables and connections.Such materials are copper, aluminum, tin and silver.The choice of materials largely corresponds to the materials from 3.2.The measurement results are shown in Table 1.The measurements were carried out 15 times for each material, always using new material samples, with the fluctuation in the values determined being less than ± 3.8%.It has been shown that the dosage of the amount of material has the greatest influence on the tolerance of the measured values.The previous determination of the material densities enables precise dosing of the sample with the help of a precision balance.For better comparability, Table 1 lists the values that deviate from the required amount of material for a layer thickness of 1 mm with a maximum mass deviation of 0.001 g.Cu-ETP (CW003A) is often used as the conductor material.The electrical conductivity of this copper alloy is 58,580,000 S/m according to DIN EN 1977:2013-04 [35].A frequently used contact material in vehicles is tin, with an electrical conductivity of 9,090,000 S/m [36].These values are used as a reference in the discussion of the measurement results.

Discussion of the study results
The investigation of different fillers yields electrical conductivities in a broad spectrum.The comparison with pure conductor materials shows a significantly lower electrical conductivity, which is usually several orders of magnitude lower than that of the pure reference material (Table 1).
The investigation shows that with graphite as a filler in polymers, an electrical conductivity below 1500 S/m can be produced.Graphite is, therefore, rather unsuitable for use in vehicle wiring harnesses.With copper as a filler, an electrical conductivity close to 4500 S/m was achieved in the study.Copper with a grain size of 45 µm and a filler content of 50% was used.
The use of silver as a filler produces higher values of electrical conductivity.A filler content of 50% in combination with a grain size of 63 µm achieved an electrical conductivity of about 315,000 S/m in the study.With a higher filler content of 87%, an electrical conductivity close to 10 MS/m is achieved.It should be noted that such a high filler content requires very small filler particles with a size of 10 µm.According to 2.3, these test results must be used to answer the question of whether polymers can be equipped in such a way that they meet the electrical conductivity requirements of the vehicle electrical system.The investigation concludes that this is technically possible.Furthermore, the study concludes that primarily silver as a polymer filler achieves suitable values of electrical conductivity.
As a limitation, it must be considered that conductive polymers preferably show potential as a material for electrical connections or as a conductor material on short distances.Replacing meter-long copper or aluminum cables in the vehicle is not realistic.The reason is the required conductor cross-section corresponding to the ohmic resistance.According to Eq. 3, conductors made of conductive polymer would have to have significantly larger cross-sections than currently used conductors.
The replacement of a 1.0 mm 2 copper cable with a length of one meter would have a cross-section of at least 5.85 mm 2 .Installation space conditions do not usually allow for this.The polymer to be used for this with a silver content of 87% is also expensive.Per meter of cable length, the use of silver is valued at 38.97 €4 compared to the use of copper at a value of 0.07 €. 5 In an electrical connection, silver is used on an area of 63.6 mm 2 and a layer thickness of 1.0 mm with a value of 0.23 €.In addition to the electrically better ratio of length to area, the lower silver requirement also argues for using electrically conductive polymers in electrical connections.

Designing an electrical connection with additive manufacturing
With the values of electrical conductivity that have been achieved, new possibilities arise for the design of electrical connections.A new design approach envisages making full use of the potential of multi-materials to automate wiring harness production.These include: -Possibility of additive processing -Electrical conductivity -Additive properties -Flexible form fit between molded parts -Medium tightness Material extrusion processes (MEX-TRB/P/multi-material) or contactless micro-dispensing systems can be used as additive processes [39].These are also referred to as material jetting processes (MJT-MSt/P/multi-material).Systems are already available that use jetting processes and are suitable for multi-materials [40].In general, the use of dispensing technology is also considered suitable for processing electrically conductive polymers [41].
The electrical connection between the contact partners is established by the electrically conductive fillers within the polymer.The contact partner of an electrical line can be, for example, another line, an electrical plug-in contact or the connection of an electrical component (Fig. 8).In addition to the electrical connection, the polymer also creates the mechanical connection between conductor, sheath and connected component.At the same time, the complete sheathing of the open conductor ends results in increased longitudinal water tightness.The polymer layer is able to compensate for geometric differences between the molded parts.This interrupts geometric dependencies, for example between contact and cable cross-section.This reduces the combinatorial variance in wiring harness production and increases the flexibility of production.
The complex and multi-variant processes for manufacturing electrical connections in conventional wiring harness production are replaced by an additive process according to the FIM approach.This replaces many sub-steps in production and components that were previously provided separately, such as heat-shrink tubing and seals.With reference to the problems described in 2.3, the FIM approach as a flexible process with low combinatorial variance (Fig. 9) offers significant advantages regarding the automation of vehicle wiring harnesses.

Summary and outlook
Enhancing manufacturing automation is deemed essential for maintaining manageable complexity in wiring harnesses and enabling their production in close proximity to the OEM.Additive manufacturing can support the automation of wiring harness production.The focus of the study was the use of electrically conductive polymers.The aim was to clarify whether polymers can be prepared in such a way that their electrical conductivity meets the requirements for use in vehicle wiring harnesses.A test set-up specially developed for this investigation provided the desired results.The achievable electrical conductivity of the materials is in the range of 10,000,000 S/m.After thorough investigation, it was determined that electrically conductive polymers are suitable for application in electrical connections in low-voltage systems.However, their viability as a material for longer cables was dismissed.Based on these results, an approach to redesign electrical connections for the use with additive manufacturing was developed.The approach of functionintegrated multi-material use promises a reduction in the number of variants of the individual parts from which the wiring harness is manufactured.In addition to the electrical properties and the possibility of additive processing, conductive polymers can provide high mechanical strength in the connection.Additional advantages of the polymers are medium impermeability, favorable corrosion behavior and tolerance to vibrational stresses.For use in vehicles, further investigations are required to verify these properties.Suitable methods for this are, for example, the slow-motion bending test and the measurement of the conductor pull-out force in a tensile test.The findings of this basic investigation into the electrical conductivity of the polymers are regarded as an initial parameter for further research.

Fig. 1 Fig. 2
Fig. 1 Customer specific wiring harness of a VW Golf

Fig. 7 Fig. 8
Fig. 7 Test set-up for determining the electrical conductivity of fillers and multi-materials

Fig. 9
Fig. 9 Cross-sections of three prototypes of additively manufactured connections of wire and contact

Table 1
Overview of measured values and reference values of electrical conductors