Design Principles and Restrictions for Continuous Fiber-Reinforced Additive Manufacturing

The outcome of product development greatly hinges on the available design knowledge [1]. Requisite knowledge for component development can stem from personal experience or systematic provision [2,3]. However, depending solely on personal experience constrains the solution space and hampers the creation of innovative solutions [1]. Frequently, the absence of comprehensive knowledge regarding inherent process potentials leads to suboptimal design choices [3]. As a result, considerable efforts have been invested over the years in devising methods to extract and organize process- and product-specific knowledge. There is a notably high demand for design knowledge in emerging manufacturing processes like additive manufacturing (AM), as the novelty of these processes has not yet facilitated the rapid accumulation of knowledge among users. Generally, additive manufacturing processes are defined as manufacturing processes that are characterized by a layer-by-layer construction principle [4]. Additive manufacturing processes have rapidly evolved, transitioning from processes focused on rapid and cost-effective prototype and visual aid production to becoming a viable alternative for manufacturing end products and small series [4,5]. However, this rapid development leads to untapped potential in the use of additive manufacturing techniques [6]. Simultaneously, additive manufacturing processes hold significant potential for integrating functionality, creating lightweight solutions, and consolidating components. Nonetheless, these possibilities frequently go unrealized due to the absence of experience and foundational knowledge, resulting in challenges for the broader adoption of additive manufacturing processes [7,8]. To counteract these shortcomings, the research field of Design for Additive Manufacturing (DfAM) has emerged, with the aim of developing and providing tools, methods, and design knowledge for additive manufacturing [9]. While a range of methods, development processes, tools, and design knowledge has been devised for established additive manufacturing processes with a diverse user base, continuous fiber-reinforced material extrusion (CFR-MEX) has only recently gained attention within this context. However, CFR-MEX has great potential for lightweight solutions and functional integrations, but a broad knowledge base must be available to fully realize its potential.

Accordingly, Sec. 1.1 offers an overview of the process principles and potentials of continuous fiber-reinforced MEX, followed by a review of recent research findings within the DfAM domain. This review specifically examines opportunistic and restrictive design knowledge in Sec. 1.2.

CFR-MEX builds upon the conventional MEX process, which is also commonly referred to as fused deposition modeling (FDM) or fused filament fabrication (FFF). MEX stands as one of the most extensively employed additive manufacturing processes, recognized for fabricating components through the continual, layer-by-layer deposition of molten thermoplastic strands. The initial thermoplastic material is guided into a heated nozzle in filament form, where it undergoes melting and extrusion. Following the completion of a layer, the build platform descends by the specified layer thickness, initiating the repetition of the process (Fig. 1) [4,5].

Prior to the printing process, the component undergoes a design phase followed by a process termed slicing. During slicing, the component is partitioned into layers, and crucial process parameters like printing temperature, infill pattern, and layer thickness are established [4,5,11,12]. The MEX process boasts significant benefits, encompassing a broad array of materials, minimal material usage, and cost-efficient equipment [13,14]. However, relatively modest mechanical properties frequently render the process unsuitable for crafting heavily stressed components. Yet, this drawback can be effectively mitigated by the integration of continuous fibers into the MEX process [13,15]. Integrating continuous fibers, typically composed of carbon, glass, or aramid fibers [16,17], leads to a significant increase in mechanical properties [15]. However, the underlying process principle remains the same. Typically, a printer is expanded by incorporating an extra print head, facilitating the extrusion and cutting of a continuous fiber-reinforced strand. These continuous fibers can either be pre-embedded within a thermoplastic matrix and directly extruded (Fig. 1) [18,19], or they can be merged with the molten thermoplastic in the print head and jointly extruded [18,20,21].

The primary motivation for integrating continuous fibers into the MEX process is to improve mechanical properties. In this context, a variety of principles have already been developed for the efficient use of continuous fibers and for optimizing mechanical properties or component weight. Principles showcased in literature for enhancing mechanical properties or diminishing component weight encompass continuous fiber-reinforced lattice structures [22,23], fiber orientations based on principal stresses [24,25], and hybrid composites [26–30]. Nonetheless, the incorporation of continuous fibers also presents a plethora of opportunities for functional integration. Integrating additional functions can aid in minimizing the required components and the related manufacturing and assembly processes of a product, thereby resulting in weight and cost savings. The inherent physical properties of continuous fibers have also been harnessed to showcase a wide array of functional integrations, including piezoresistive structures [10,31–33], electromagnetic shielding [34], or electrical circuits [35]. Nonetheless, in the absence of a systematic compilation and user-friendly exposition of these abundant possibilities, these potentials frequently go untapped. This gap in utilization stands as one of the principal driving forces within the DfAM research domain and will be elaborated upon below.

The wide-ranging capabilities of additive manufacturing, including innovative design freedoms and the potential for functional integration, coupled with the rapid expansion of the AM industry, have given rise to the extensive realm of research in Design for Additive Manufacturing. DfAM includes an array of tools, methods, knowledge systems, and resources that aid designers in the development of components for additive manufacturing processes [6,36]. Concerning process-specific design knowledge, a differentiation has been made between two distinct approaches [36,37]. Opportunistic approaches primarily seek to harness the capabilities of AM to achieve functionalities or geometries that would pose challenges or even be unattainable through traditional manufacturing methods [36–38]. This encompasses lattice structures, internal cooling channels, and the consolidation of multiple components. The second category encompasses the so-called restrictive approaches [36–38]. Opportunistic AM potentials (as well as restriction) are also utilized in DfAM tools for assessing additive manufacturing methods in the production of specific components [39]. While additive manufacturing processes provide substantial design freedoms, designers must navigate various restrictions. These encompass process-related constraints like achievable wall thicknesses, openings for support structure removal, and considerations of limited build volumes.

In the context of opportunistic approaches for DfAM, the collection and preparation of design principles have particularly established themselves. According to Fu et al. [40], design principles are strategies or laws derived from experience and/or empirical investigations that support the design process in order to increase the likelihood of success for a design solution. In our comprehension, design principles for additive manufacturing processes embody strategies, heuristics, and methodological approaches that aid in the design process, fostering the development of innovative solutions while fully exploiting the unique potentials of the respective manufacturing method. Well-known examples of AM-specific design principles include the use of lattice structures in low-stress areas of components, undercuts, internal cooling channels, or the integration of electrically conductive materials. Drawing from existing AM components, Blösch-Paidosh and Shea [41,42] formulated 29 general heuristics that facilitate the design process by harnessing design freedoms. Watschke et al. [43] systematically gathered solution principles tailored for multi-material AM and presented them as principle cards, encompassing crucial information about these principles. Access to the provided principles is based on associated partial functions to support access during the conceptual phase. Weiss et al. [44] compiled general AM-specific design principles in the form of a design catalog, where access to the principles is made through linked general functions. Yang et al. [45] identified AM-specific principles based on a literature review and prepared them in the form of principle cards, which include a description, advantages, and applicability of each principle. Perez et al. [46,47] developed 23 AM design principles based on existing AM components and also utilized the preparation in the form of principle cards. The cards consist of three main elements and include a brief textual description of the principle, a simplified principle sketch, and an exemplary sample component.

For the systematic highlighting of potential conflicts among AM principles, Fuchs et al. [48] introduce a potential systematics where conflicting principles are interconnected, facilitating users in their identification. In the case of CFR-MEX, there have been no formulated design principles for the design process yet. Initial design liberties stemming from the incorporation of continuous fibers were articulated by Prüß and Vietor [18]. Potentials are shown which relate to geometric or mechanical properties of additively manufactured components. This includes, for example, the utilization of continuous fiber-reinforced strands for support-free bridging of overhangs and lattice structures or the use of fibers with favorable thermal expansion coefficients to reduce temperature distortion.

Despite the substantial design flexibility provided by additive manufacturing processes, it is crucial to account for process-specific limitations during component design. According to Ponn and Lindemann [49], design guidelines are to be understood as collections of specifications that serve the purpose of requirement-compliant component design. These guidelines are often associated with specific objectives, such as load-appropriate design or design for manufacturability [49,50]. The guidelines comprise a multitude of specific quantitative or qualitative design rules. Design rules are often based on quantitative experiments or analyses of case studies and are used during the detailed design phase to optimize geometric features or shapes [49,50]. Design guidelines or specific design rules are commonly provided in the form of design catalogs, where the rules offer recommendations or instructions on certain geometric parameters, such as minimum wall thicknesses, overhangs, hole diameters, etc. [51,52]. These guidelines are often illustrated with schematic representations of positive and negative instances, accompanied by quantifiable values and concise explanations. They serve as the foundation for design in alignment with additive manufacturing.

Due to process similarities, design rules relevant to conventional MEX are particularly applicable to CFR-MEX. In this context, the works of Adam and Zimmer [53,54] deserve special mention, as they also address the LPBF (laser powder bed fusion) process and laser sintering in addition to MEX. Guideline 3405 [55] of the VDI (Association of German Engineers) provides design rules for MEX, and manufacturers of conventional printers, such as Stratasys [56], as well as manufacturers of printers for CFR-MEX, such as Markforged [57] and Anisoprint [58], offer design guidelines as well. However, comprehensive design guidelines for CFR-MEX have only been compiled in the work of Prüß [59] so far. The custom-built 3D printer used for the experimental investigations did not have a cutting unit and a second print head for the extrusion of unreinforced thermoplastic, limiting the applicability of the rules to commercially available printer systems.

The use of CFR-MEX offers a promising approach for cost-effective and rapid manufacturing of highly loaded components and for additional functional integration. In order to fully harness these process-specific potentials and overcome design constraints, it is crucial to prepare and provide process-specific design knowledge. To the best of the authors' knowledge, no design knowledge has been developed and made available specifically for CFR-MEX, encompassing both opportunistic potentials and process restrictions. Furthermore, access to opportunistic design knowledge is largely grounded in functional connections that hold greater relevance during the initial phases of product development. As a result, they do not adequately facilitate the extensive utilization of process-specific potentials across the entirety of the product development process.

The aim of this paper is to systematically derive and provide both opportunistic and restrictive design knowledge specific to the process of CFR-MEX. This aims to facilitate inexperienced designers in developing and constructing components to be manufactured with CFR-MEX, thereby fully leveraging the specific potentials of the process. Simultaneously, it aims to ease the incorporation of design constraints inherent in the process principle by providing restrictive design knowledge.

To bridge the recognized research gap concerning opportunistic design knowledge for CFR-MEX, Sec. 3 introduces the development and methodical provision of design knowledge in the form of design principles. This includes the formulation of the principles themselves, the development of a goal- and function-oriented access system, as well as the user-oriented preparation of the principles. In Sec. 4, the initial necessity for the compilation of restrictive constraints is highlighted, followed by the collection of restrictive design guidelines and their preparation in the form of a design catalog. Section 5 demonstrates the application of the formalized design knowledge through a weight-optimized example component.

Opportunistic design knowledge for additive manufacturing processes is usually conveyed through design principles that describe unique AM potentials. This paper proposes user-friendly design principle cards as a medium for providing knowledge. The following section illustrates how the required knowledge is gathered and transformed into user-friendly principle cards.

The literature provides various methods for deriving design knowledge. Based on a literature review, Fu et al. [40,60] highlight that one of the most common approaches for deriving knowledge is the analysis of existing products. Additionally, aside from transferring and adapting existing principles, the concept of acquiring expert knowledge or conducting focused experimental investigations for derivation has been suggested. Perez [61] also proposes the derivation of AM design knowledge based on existing components. In the realm of additive manufacturing, online databases like Thingiverse.com [62] prove useful, allowing users to share their AM-compatible designs. A general procedure for identifying design principles and heuristics was presented by Yilmaz and Seifert [63], which was adapted for additive manufacturing by Blösch-Paidosh and Shea [42]. The general approach involves the development of heuristics and principles through an analysis and selection process of existing components and products.

In conclusion, the main emphasis in deriving design principles for additive manufacturing processes lies in analyzing existing products to distill expert knowledge and design solutions for wider accessibility. Nonetheless, this approach is only partially applicable to novel processes like CFR-MEX due to a restricted pool of available products. As a result, a combined approach is employed to gain process-specific design knowledge, which is described with practical examples in Fig. 2.

The approach employed for the systematic derivation of design principles is founded upon three pillars, as schematically depicted in Fig. 2. First, products from databases [62] and published “use cases” or customer applications from manufacturers of CFR-MEX printer systems, such as Markforged [66,67] and Anisoprint [68], were selectively analyzed (Fig. 2(a)). These products were examined for innovative design solutions that could benefit inexperienced designers during development and construction. In addition to analyzing existing products, published scientific studies that explore the novel potentials of CFR-MEX were also consulted (Fig. 2(b)). This analysis was underpinned by a comprehensive literature review, considering only those principles whose applicability had been successfully validated in published studies and led to reproducible outcomes, such as in terms of sensory behavior or the enhancement of mechanical properties. As the third pillar for the targeted derivation of design principles, our own experimental investigations were conducted [25,27,30,69] (Fig. 2(c)). The approaches and principles investigated are based on both existing methods for conventional composites and those specific to CFR-MEX, as well as our own theoretical considerations. Notably, the basic principles introduced in Sec. 3.2, which can serve as a starting point for the development of innovative principles, have been employed. Our investigations aimed to enhance the applicability of selected design principles by validating the principle and exploring previously uninvestigated factors influencing their effectiveness.

The assessment of the suitability and applicability of all principles was always conducted by several of the contributing authors, who possess several years of experience in designing and constructing components for both conventional MEX and specifically for CFR-MEX. In the evaluation process, the general applicability and benefits were evaluated based on our own experiences with conventional MEX and CFR-MEX in particular.

Applying design principles is consistently tied to associated value propositions or objectives that users anticipate when utilizing the particular principle. Thus, we suggest allocating the derived design principles to value propositions that users connect with CFR-MEX. To this end, the existing literature, which was also used for the derivation and collection of design principles, was analyzed for described value propositions that are linked by the authors to the continuous fiber integration into the MEX process and the resulting process-specific potentials/principles (Table 1).

The value propositions were categorized into five categories: warpage minimization, weight reduction, improvement of mechanical properties, cost reduction, and functional integration/component consolidation. Levers or basic principles that underlie each value proposition can also be identified based on the derived value propositions. Through an analysis of the value propositions presented in Table 1, a total of four overarching basic principles were identified:

1. Utilization of variable fiber orientation

2. Utilization of material combinations

3. Utilization of physical properties of fibers

4. Utilization of biological properties of fibers

A significant number of the identified design principles are founded on the combination of two or more basic principles. This is particularly relevant for the principle of “Integrated fiber-based piezoresistive sensors.” The practical implementation of this principle exploits two basic principles. It utilizes both the variable orientation of fibers and their physical properties, specifically the electrical conductivity and piezoresistive characteristics of carbon fibers, for example. Figure 3 illustrates an exemplary linkage of design principles with basic principles and value propositions. The identification of the underlying basic principles can subsequently be used for systematic knowledge acquisition by specifically identifying unexplored potentials of the process based on the determined basic principles and examining them experimentally [27,30].

The comprehensive utilization of the technology's potential can be achieved through a systematic approach to accessing the accumulated design knowledge. An access system allows users to quickly and specifically identify relevant information in a knowledge repository. A key advantage of an access system is the decoupling of subjective experiences and assessments. Instead, it encourages the selection of design solutions based on criteria or objectives. Furthermore, an access system facilitates handling and clarity as the size of the knowledge repository increases.

Opportunistic design principles are particularly important in the early stages of the product development process, as this is where the foundation for the product architecture is laid. This is especially true for new developments [74]. Methodical product development frequently entails the establishment of a functional product structure during the concept phase [75]. In this context, function-oriented access plays an important role [76]. This allows the identification of principles associated with the established sub-functions, thereby comprehensively considering potentials for functional integration in the early stages and supporting the development of innovative solution concepts [44,45]. Moreover, Watschke [77] emphasizes the importance of function-oriented access in order to avoid pre-fixations and support the context-independent application of principles. Therefore, the identified design principles have been assigned to general functions (Table 2), enabling a direct assignment of the principles to specific functions within the functional structure.

One drawback of function-oriented access is that not all principles can be straightforwardly linked to a specific function. This is evident in cases like principles aimed at decreasing component weight or streamlining manufacturing processes. To enable systematic access to such principles, a dual access system is proposed, which includes both function-oriented access and access based on the value propositions presented in Sec. 3.2. Linking principles to value propositions facilitates the utilization of process-specific potentials in subsequent stages of product development, allowing, for instance, the reduction of component weight, enhancement of mechanical properties, or reduction of manufacturing time and consequent component costs. The access system is illustrated in Fig. 4.

Numerous research studies have introduced the concept of “principle cards” as a means of delivering and formalizing knowledge [46,52,76]. Therefore, in this section, the requirements for formalized design knowledge specifically tailored for AM will be identified first. Subsequently, the chosen approach for the design and content of the principle cards will be presented to fulfill the established requirements.

Various approaches for providing formalized design knowledge have been discussed in the literature, and requirements have been established. For fostering creativity in the design principles, it is crucial to incorporate both textual and graphical representations of the principles [61,76,79]. Valjak and Bojčetić [76] suggest formalizing the principle cards with a functional classification, a text-based and graphical description, virtual and physical 3D models, as well as manufacturing data and case studies. However, a sufficient level of abstraction of the principles must be maintained to avoid overly restricting the solution space and preventing fixation [80]. Perez et al. [46,61] developed AM-specific principle cards, which, in addition to a brief textual description, also include a simplified principle sketch and an exemplary example component. However, the functional classification of the cards is omitted. Perez [61] emphasizes that the formalization of principles should be done in a way that allows for easy implementation in a database. Watschke [77] also formalizes design principles for multi-material processing via MEX in the form of principle cards. These cards include explanations of the principle's functioning, illustrated through sketches and application examples. The applicability of the principle cards was validated in a workshop [80]. Blösch-Paidosh and Shea [81] also validated the applicability of design heuristic cards in a workshop conducted within an industrial setting.

Drawing from the discussed methods and suggestions, along with the research gaps outlined in Sec. 2, the subsequent criteria are established for the developed principle cards:

• Design principles need to be presented coherently and comprehensibly, employing a combination of graphical and textual elements.

• Principle descriptions should encourage creativity and prevent fixation.

• For easy and quick access in the early phases of product development, function-oriented access options should be implemented.

• The presentation of the design principles should allow for easy implementation in digital knowledge systems/storages.

• To optimally support the use of all relevant principles and to make conflicts and synergies visible early on, the principles should be logically linked to one another.

The structure of the developed principle cards can be seen in Fig. 5. In total, the principle cards comprise 10 information fields. Each principle is assigned a number to streamline the cross-referencing of specific synergistic or conflicting principles across other cards and to simplify integration into databases and matrices. Additionally, the principles are given concise designations. The cards are also provided with a textual description, which is further illustrated by schematic principle sketches. In this context, the principle to be conveyed is explained in more detail, and, for example, relevant process parameters, positive and negative properties, and the underlying physical effect are described. Besides the schematic principle sketches, one or more illustrative example components are depicted, when applicable, to demonstrate the application of the principle. Using more than one example can help prevent fixation by the user. In another information field, linked synergistic and conflicting design principles are noted (Sec. 3.5). Furthermore, the principle cards are equipped with two access fields, which contain information on functional classification and value propositions relevant to the developed access logic. Beyond the value propositions, additional positive and negative impacts tied to the principle's implementation are presented in a different information field. These impacts may concern, for example, component properties, effort, or cost. Moreover, references to the derived principles are appended to the underlying literature, aiming to facilitate obtaining more comprehensive insights.

In Fig. 6, an exemplary principle card featuring the principle “Principal stress-based fiber orientation” is shown. The principle was derived based on our own experimental investigations and additional scientific studies and was prepared in accordance with the structure and content shown in Fig. 5.

The approach to collecting design principles resulted in the identification of a total of 21 design principles for CFR-MEX ( Appendix). Owing to technological advancements in manufacturing processes and the broader range of materials, a substantial growth in available design principles can be anticipated in the near future. In addition, in the future, design principles for general AM or conventional MEX can also be incorporated into the knowledge repository. To ensure comprehensive consideration of all appropriate principles and early identification of conflicting ones, even with an increasing number of design principles, a connection of these principles based on synergies and conflicts is suggested.

When combining different principles, conflicts may arise in which the principles negatively affect or exclude each other. In this context, early verification of potential conflicts is beneficial in order to prevent misdevelopments and avoid extensive design changes in the advanced stages of the product development process. The identification of conflicts and synergies is predominantly founded on our theoretical contemplations and subjective evaluations. Accordingly, the appraisal was conducted by several contributors to ensure a broad perspective. Despite these efforts, we acknowledge that our analysis does not claim to be exhaustive. For appraising potential conflicts and synergies, we also considered specific example components associated with the principles to facilitate a comparative and compatibility analysis with the remaining principles. An exemplary case of possible conflicts within the identified principles is, for example, the use of lattice structures to minimize weight and the use of dense fiber-reinforced structures for electromagnetic shielding. To ensure the most effective shielding performance, it is necessary to place electrically conductive fibers as close to each other as possible, which precludes the use of lattice structures [34]. In addition to the possible occurrence of conflicts, synergies between principles can also arise. For lightweight applications, for instance, the principles “Principal stress-based fiber alignment” and “Fiber-reinforced lattice structures” can be combined effectively to ensure efficient fiber utilization while minimizing material usage. An overview of the identified synergies and conflicts between the design principles for CFR-MEX is presented in Fig. 7.

Based on the synergy and conflict connections shown, it becomes evident that predominantly synergies (32) could be identified. Only six potential conflicts were found, which do not necessarily imply that the corresponding principles fundamentally exclude each other but rather that special consideration is required when applying both principles. Figure 8 depicts a schematic process flow for identifying synergies and conflicts.

In this process, starting from the developed access system, suitable design principles are initially identified and checked for potential conflicts using the synergy and conflict matrix. If conflicts occur, an intermediate step involves weighing and provisionally selecting the design principles. Subsequently, potential synergies are identified, and a re-examination of possible conflicts is carried out. The process concludes with a final evaluation and selection of the identified design principles.

Despite the extensive design freedoms offered by additive manufacturing processes, restrictive limitations must be considered for all processes. In the context of CFR-MEX, in addition to the fundamental restrictions that also apply to conventional MEX, some fiber-related peculiarities need to be taken into account. To address these restrictions, the development of a design catalog with restrictive design rules is presented in the following. First, relevant existing design guidelines are identified and examined for transferability. Subsequently, the processing of the rules in the form of a design catalog is outlined.

To compile the available restrictive design knowledge for CFR-MEX, relevant design guidelines must first be identified. For the derivation of CFR-MEX-specific design guidelines, several sources are at hand, which are briefly introduced below and subsequently discussed in detail.

In addition to experiential knowledge gained from our own designs and investigations, access to pre-existing knowledge is feasible within this context. This includes extant collections of CFR-MEX design guidelines provided by manufacturers of CFR-MEX printing systems that can be adopted directly [57,58]. Nevertheless, the extent of these sets of rules remains limited. Given the similarities between CFR-MEX and conventional MEX, it is also possible to leverage MEX-specific design rules. Moreover, alongside regulations derived solely from the MEX domain, design principles for conventional fiber-reinforced plastics can be adapted and applied [82]. An overview of the design guidelines used to create the design catalog is presented in Table 3.

The CFR-MEX is based on conventional MEX and typically features a print head for extruding unreinforced thermoplastics and a print head for extruding continuous fiber-reinforced plastics (Sec. 1.1). Therefore, the numerous MEX design rules already collected are particularly relevant for unreinforced areas [53–55,83,84]. At the same time, additional novel design rules must be considered for fiber-reinforced component areas, or specific quantitative values of existing MEX rules are influenced by fiber integration. This includes, for example, the minimum wall thicknesses (Fig. 9(a)) or the minimum pin diameters in the build direction (Fig. 9(b)) due to the minimum deposition length of the fiber strands (Fig. 9(c)). To take these peculiarities into account, existing MEX rules for geometric features are examined for their transferability to fiber-reinforced component areas in Table 4.

In addition to restrictive design rules directly pertaining to additive manufacturing processes, considerations also encompass guidelines and recommendations derived from the design of conventional fiber-reinforced plastic (FRP) structures. However, as these rules are intended for the configuration of laminates or explicitly address specific manufacturing processes of FRP, they are not immediately transferrable and necessitate significant adaptation. Nevertheless, rules regarding aspects such as layering, fiber orientation, and the design of load introductions can be leveraged and, in an adjusted format, prove valuable to designers of CFR-MEX components. The transference and adaptation of existing recommendations for conventional FRP are elucidated herein through the depiction of two derived design rules, showcased in Fig. 10.

Figure 10(a) illustrates a design recommendation for creating a “pin-loaded strap,” particularly suitable for transmitting high-point loads. In this arrangement, screws or bolts are encircled by fiber strands aligned parallel to the prevailing tensile stresses. From a structural standpoint, it is imperative to ensure that the fiber strands converge at the greatest possible distance to prevent “pull-apart stresses” and stresses transverse to the fiber direction [82]. The adapted version of this recommendation tailored to the process-specific conditions of CFR-MEX is depicted in Fig. 10(b), showcasing both a favorable and an unfavorable design configuration (see Sec. 4.2).

As a second example, “adaptation to the local stress state” is addressed through a technique known as “thickening,” which can be applied in conventional FRP structures existing in laminate form. Thickening involves introducing additional laminate layers in regions subjected to higher loads (Fig. 10(c)) [82]. This rule can be implemented through additive manufacturing methods in diverse ways. The mechanical properties of printed composites can be enhanced by employing localized thickening in the build direction (z-direction) as well as by adjusting the fiber volume content within the layer plane. Sketches illustrating the derived rule are presented in Fig. 10(d).

In scientific and industrial practice, the collection and provision of design guidelines in the form of design catalogs have proven to be effective. They enable quick and goal-oriented access to design knowledge with the most comprehensive solution spectrum possible [50,78]. To collect and adapt relevant design guidelines, the knowledge repositories identified in the previous section are utilized. In addition, our own experiences with the process are incorporated into the creation of additional design rules. The developed catalog follows the guidelines from VDI-guideline 2222 (Part 2) [88]. The design catalog is divided into a structure, main, relevance, and data section. The structure section serves to systematically, and as conflict free as possible, organize the individual design rules [78]. For this purpose, the developed rules are divided into manufacturing-appropriate, load-appropriate, and cost-appropriate rules. In the second, more specific organizational level, the rules are divided according to characteristics such as resolution or dimensioning. The third organizational level assigns the rules with concise names that describe the core of the rules (e.g., “wall thicknesses” or “perpendicular hole diameter”). In the fourth organizational level, numbers are assigned to the rules to facilitate later reference (Table 5). An excerpt from the developed catalog is shown in Fig. 11. The entire design catalog can be found in the  Appendix.

The main section of the catalog describes the design rules. The rules are provided with their respective orientation in relation to the build space. Furthermore, illustrations are included to depict the rules. The use of positive and negative examples has proven to be effective [89]. Furthermore, the rules are explained with a brief description that succinctly describes the most important aspects. In the relevance section of the catalog, it is indicated whether the respective rule applies to fiber-reinforced or unreinforced component areas. In addition, the rules are assigned to specific phases of development. This is particularly important for design rules that should already be considered in the concept phase (e.g., maximum build space). Quantitative values for the rules are provided in the data section whenever possible. The specification of these quantitative values is also accompanied by a reference to the source, as the determined values mostly apply considering the system- and/or material-specific parameters.

The following describes the exemplary application of the developed knowledge system. The application is demonstrated using a lightweight-optimized crankset for racing bicycles. In competitive sports, there is a constant effort to reduce the weight of sports equipment. Additive manufacturing processes can be used not only to manufacture weight-optimized components and sports equipment but also to adapt them to the respective athlete if necessary. To demonstrate the application and potential of the developed knowledge, the optimization of a crankset for a racing bike is discussed below. For the construction of the crankset, various design principles were identified and found suitable based on the benefits promised by “improve mechanical properties,” “weight reduction,” and “cost reduction.” Suitable design principles could also be identified based on the sub-function (channel mechanical energy) that the exemplary component performs in the system. The exemplary application of the access system using the demonstrator is shown in Fig. 12.

Using the access system through general functions and value propositions as well as the synergy and conflict matrix, a total of four design principles were identified. The principle of “partial fiber reinforcement” involves adapting the fiber content depending on the component load. This is particularly effective when the costs of the relatively expensive fiber filament are to be reduced. In addition, “principal stress-based fiber orientation” was identified as suitable. In this case, the fibers are aligned along the first and, if necessary, second principal stress direction to take advantage of the longitudinal strength and stiffness properties [25]. This principle can be used synergistically with the “fiber-reinforced lattice structures” principle. This principle involves omitting the filling of the areas between the fiber strands in order to save weight, manufacturing time, and, therefore, costs. To counteract the problem of abrasive wear on heavily loaded plastic functional surfaces, the “metallic inserts” principle was also identified as suitable. This involves integrating metallic components such as sleeves or threads into the printed components to increase the service life of the components. These can be pressed into the component afterward or inserted when printing is interrupted. The exemplary implementation using the demonstrator is shown in Fig. 13.

Even in the construction and design of comparatively simple components intended for fabrication using CFR-MEX, a multitude of restrictions must be taken into account. This is exemplified using the demonstrator in Fig. 14, where two applied design rules are highlighted. In the less stressed central region of the pedal crank, where fibers are introduced in the form of lattice structures, minimum wall thicknesses must be adhered to, a factor integrated into the 3D model design (design rule “Reinforced minimum wall thicknesses”). To ensure a manufacturable design, wall thicknesses should be conceived such that the walls can be reinforced by two internal fiber strands and two externally positioned unreinforced thermoplastic strands (Fig. 14(b)).

From the perspective of load-appropriate design, the positions of the start and end points of the fiber strands must be accounted for (design rule “fiber start and end point”). Due to the necessary cutting of fiber strands (see Sec. 1.1) and the associated local weakening, it is essential that the cutting positions vary from layer to layer, as depicted in Fig. 14(c). This is particularly important in mechanically heavily loaded component regions or in areas reinforced by a single strand. For fiber strands that cross both highly stressed and lightly stressed areas, placing the start and end points within the lightly stressed component region is also conceivable.

This article describes the development and user-friendly preparation of opportunistic and restrictive design knowledge for the CFR-MEX. The CFR-MEX is a novel additive manufacturing process that has received little attention in the context of DfAM. Therefore, there is currently no process-specific, user-friendly design knowledge available in the form of opportunistic design principles and restrictive design guidelines. Existing DfAM approaches for providing process-specific design knowledge were analyzed and further developed. In doing so, the common access system based on functional-oriented assignments of principles was also adopted and expanded to include access based on value propositions. This promotes the applicability of the principles throughout the product development process. The value propositions were derived based on motivations described in the literature for using the CFR-MEX. The design principles were then assigned to one or more value propositions. The process-specific design principles were derived from scientific studies, existing components, and our own experimental investigations and prepared in the form of design principle cards. In order to ensure enhanced utilization of the process-specific potentials, a linking of the principles via synergies and conflicts was also proposed. Thus, a suggestion of possible conflict and synergy partners based on an identified principle is possible with the help of a linkage matrix. In addition, restrictive design guidelines with concrete design rules were developed by collecting and adapting existing rules and provided in the form of a construction catalog. The application of the knowledge system was demonstrated using a weight-optimized bicycle crankset.

A large number of the design principles collected in this work are based on literature. To facilitate the implementation of these principles, the degrees of freedom of the manufacturer-specific slicing software need to be expanded. In addition, continuous monitoring, updating, and supplementing of the collected design knowledge is essential due to ongoing advances in additive manufacturing machinery, slicing software, and available materials. In addition, it is useful to expand the knowledge system to include the principles of conventional or multi-material MEX, as these principles can also be used in the context of CFR-MEX. To facilitate the selection and identification of relevant principles and design rules, the knowledge system should also be made available in the form of a digital database.

• The Ministry for Science and Culture of Lower Saxony (MWK), School for Additive Manufacturing SAM (78904-63-3/19).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.

The appended design catalog represents a compilation of existing and extended design rules, both in terms of material extrusion in general and continuous fiber-reinforced material extrusion in particular (Table 6). Unless a source is explicitly cited, the design rule is based on our own investigations. The quantitative value specifications apply exclusively to the printer systems used in the respective source (see Table 7). The product development phases (PDP) refer to a procedural model currently under development, which encompasses the following three stages: (I) Planning, (II) Conceptualizing, and (III) Elaboration & Detailing (Table 8).