Stack and Structure: Ultrafast Lasers for Additive Manufacturing of Thin Polymer Films for Medical Applications

Abstract

Overcoming the limitations of powder-based additive manufacturing processes is a crucial aspect for the manufacturing of patient-specific sophisticated implants with tailored properties. Within this work, a novel manufacturing process for the fabrication of polymer-based implants is proposed. This manufacturing process is inspired by the laminated object manufacturing technology and is based on using thin films as raw material, which are processed using an ultrafast laser source. Utilizing thin films as a starting material helps to avoid powder contamination during additive manufacturing, thus supporting the generation of internal cavities that can be filled with secondary phases. Additionally, the use of medical materials mitigates the burden of a later certification of potential implants. Furthermore, the ultrafast laser supports the generation of highly resolved structures smaller than the average layer thickness (from 50 to 100 µm) through material ablation. These structures can be helpful to obtain progressive part properties or a targeted stress flow, as well as a specified release of secondary phases (e.g., hydrogels) upon load. Within this work, first investigations on the joining, cutting, and structuring of thin polymer films with layer thickness of between 50 and 100 µm using a ps-pulsed laser are reported. It is shown that thin film sizes of around 50 µm could be structured, joined, and cut successfully using ultrafast lasers emitting in the NIR spectral range.

1. Introduction

Laser-based additive manufacturing (AM) is continuously gaining importance in industry due to its potential regarding sustainability [1] and its high freedom of design [2]. This design freedom allows for the fabrication of sophisticated parts with complex internal structures which cannot be manufactured using conventional manufacturing technologies [3]. In this instance, powder-based AM processes, like the laser powder bed fusion of polymers (PBF-LB/P), have proven promising for the manufacturing of high-strength components [4].

However, the use of powder as feedstock material is associated with several disadvantages. On the one hand, excess material needs to be removed from the part and recycled after the build job [5]. This can be challenging especially when internal cavities are hard to access, meaning that closed contours cannot be freed from loose powder. The residual powder will therefore affect the final part properties [6]. One negative influence could be an undesired softening of the part due to insufficient bonding of the particles to the polymer matrix. Furthermore, the powder material could be released upon product failure. This powder release can be critical depending on the final use case of the part. On the other hand, the fabrication of multi-material parts consisting of two different phases is difficult to realize. Even when internal cavities are intended for a secondary phase or different material, these cavities are typically contaminated when depositing a fresh powder layer.

One promising alternative to powder-based AM are processes that utilize plates or films. These raw materials were once already used in laminated object manufacturing (LOM) [7]. Using thin films opens, in theory, the potential for internal cavities [8] that can be either maintained without powder contamination or filled with secondary phases such as liquids or hydrogels. These secondary phases could potentially be applied by, e.g., using a dispensing unit for the local deposition of fluids or bulk solids [9]. Correspondingly, it becomes possible to manufacture multi-material parts without cross-contamination. The initially established LOM processes became more and more irrelevant due to several disadvantages such as low build rates, high material waste, and high process but low part complexity [10]. However, further developments lead to the applications of LOM for, e.g., ceramic-based additive manufacturing [11].

The conventional process chain of LOM is that the raw materials are cut [12] and joined in an iterative process. This process sequence demands a pre-cutting of the films either in advance or in situ during processing [13]. If the latter is the case, then an energy source is needed that can perform both operations welding and cutting [14]. Processing the materials using laser radiation is a promising approach to both cut and join films [15]. In a continuous wave (cw) process, the laser source needs to irradiate at a wavelength that is absorbed well by the polymer. Suitable systems are, for example, CO₂ [16] or ultraviolet (UV) lasers [17]. However, the laser energy is primarily absorbed at the surface of the workpiece [18]. This near-surface absorption makes it difficult to weld thicker materials that are non-transparent for the chosen wavelength. Furthermore, the energy absorption at the surface will lead to an overheating of the structure, resulting in a potential thermal degradation of the polymer [19].

Another alternative is provided by the use of ultrafast or ultra-short pulse (USP) laser sources. These lasers allow for an energy coupling that is based on non-linear effects rather than a continuous energy input at the surface [20]. However, the main challenge is to process the materials adequately at a specific wavelength to trigger the thermal mechanisms responsible for the processes cutting [21] and joining [22]. Another advantage of ultrafast laser processing are the high peak powers, which allow for a precise ablation of material from the surface of the workpiece [23]. Different review papers are available on the potentials and limitations of ultrafast laser processing. These works include reviews on the processing of different materials [24], their application in manufacturing processes in general [25], and also their application in additive manufacturing processes [26].

Lasers emitting in the far-infrared range, such as CO₂- or CO-lasers, as well as shorter wavelengths in the UV range, are only suitable to a limited extent for performing these different process operations due to the surface-near absorption of the energy. The focus of this study is to investigate the possibility of processing thin polymer materials using ultrafast lasers that emit in the near-infrared range. Furthermore, the different processing strategies for performing the operations cutting, structuring, and joining are elaborated.

Welding Polymers

Voisiat et al. [27] studied the welding of different polymers using a USP laser emitting at a wavelength of around 2 µm. It was possible to weld two PMMA polymer substrates in a butt-welding approach due to the energy absorption at the surface. In another work, Roth et al. [28] investigated the welding of polymers when using a femtosecond-pulsed laser. For these experiments, fs-pulses coupled with a high repetition rate were used. The work shows that it was possible to join two PMMA sheets with a USP laser source emitting at 1028 nm. A linear rise in weld track width was determined when increasing the applied laser power during processing. The shear strength of the specimens increased when using higher laser powers. Furthermore, compared to welding experiments that were performed using a thulium laser (λ ≈ 2 µm), a smaller heat-affected zone was observed. Another work by Roth et al. [29] reveals that media-dense joints can be achieved when processing PC substrates using a USP laser emitting at a wavelength of 1028 nm.

Cutting Polymers

The second key part is the cutting of polymers using a USP laser. A first work by Heberle et al. [30] shows that it is possible to process polymeric intraocular lenses at a wavelength of 1064 nm. Different influencing factors were identified that affect the quality of the final cut. It was found that short laser pulses with a moderate repetition rate are suited best for achieving a good cut without risking a melting or brittle fracture of the material. Another work by Heberle et al. [31] compares the suitability of fs- and ps-pulsed lasers for the USP-laser-based cutting of intraocular lenses. It was found that an increased repetition rate results in heat accumulation, especially for longer pulses. This heat accumulation affects the quality of the final cut. By using ps-pulses, a higher cut rate can be achieved compared to fs-pulses. The use of fs-pulses, however, allows for an improved quality of the cut, indicating a trade-off when choosing the applied pulse length.

Structuring Polymers

Early investigations on the ablation with ultrafast lasers date back to 1987. Srinivasan, Sutcliffe, and Braren [32] studied the influence of the pulse width on the ablation behavior of polymers. In this work, it was shown that multiphoton excitation could help to induce photochemical etching. Another work by Küper and Stuke [33] studied the femtosecond-based ablation using an ultraviolet laser. A model for ablation was proposed in this work, which was validated by the experiments performed in the study. Daskalova et al. [34] performed experiments on the USP-laser-based structuring of poly-lactic acid (PLA). A wavelength of 800 nm coupled with a low repetition rate was used in the presented investigations. It was possible to generate hierarchical structures which also affect the biocompatibility of the final part. The altered biocompatibility was explained by the structured surface and the modified wettability. Ortiz et al. [35] investigated the USP processing of medical polymers for potential cell engineering applications. A ps-pulsed USP laser emitting at 1064 nm was used for the experiments. Micro-patterns that improve the cell confinement could be generated successfully using the USP laser.

It can be concluded that it should be possible to join, cut, and structure polymers using ultrafast lasers emitting in the near-infrared wavelength spectrum. For welding the polymer material, high repetition rates are preferred to generate a sufficient energy input. When aiming to cut or structure the material, lower repetition rates are favorable to avoid undesired melt formation. Ultimately, the prerequisite is that the laser emits in the near-infrared range and provides a high flexibility regarding the repetition rate.

2. Proposed Process Idea

The possibility to cut, join, and structure the layers allows for the generation of parts or structures that are smaller than the average layer size (<50 µm). Potential structures include, for example, (i) pockets that can be filled with secondary phases, such as liquids or drugs, or (ii) geometries for tailored loading behavior, e.g., to reproduce a progressive material behavior. Especially the latter part, properties are difficult to achieve using conventional PBF-LB/P processes. An exemplary schematic of the proposed process is illustrated in Figure 1. This manufacturing process supports the fabrication of highly precise parts with unique properties. Thus, this technology has the potential to complement existing laser processes due to the vast possibilities in the field of implant manufacturing.

In this proposed multi-layer process, the same laser source is used for performing all three key process mechanisms (welding, cutting, structuring). A thin film is applied to a build plate. This film can be either supplied continuously through coiled foil (similar to a plastic wrap) or by using single layers with specific edge lengths. The mounting can be performed by joining (welding, gluing) the film to the build plate at specified positions.

In the next step, the second (or n-th) layer is applied to the previous film or part. The average layer thickness can be varied by selecting the thickness of the films. In this way, either the build rate (thick films) or the dimensional accuracy (thin films) of the manufacturing process can be improved. The ultrafast laser is then used for joining the two films with each other at specific positions. By using an ultrafast laser, the heat input into the material during welding can be minimized. This minimal heat input, often referred to as “cold” processing, opens the possibility of maintaining the properties, e.g., anisotropic properties, of the base material. When aiming at generating media-dense parts, a complete welding around the edges of the final geometry is necessary. In this instance, the energy input via the USP laser needs to be controlled to generate a molten state for a connection between the two materials.

The second step focusses on the cutting of two films. In this case, it is important to consider several things. First, the applied layer needs to be joined with the lower layer. Then, the contour can be cut completely or only partially. In a continuous manufacturing process where further layers will follow, a complete cutting might be undesirable since the cut material most likely will need to act as support material for the following layers. Typically, the cutting will be performed around previously joined regions. Consequently, the energy input should be controlled in a way that the melting state is surpassed and the material is evaporated directly to avoid material warpage. The goal is to generate a clear and precise cut through the thickness of the underlying film.

The final step covers the local structuring of the thin films. By using the USP laser, it is possible to ablate material selectively from the surface without penetrating the entire layer or workpiece. This procedure facilitates the generation of structures within one layer. These cavities (e.g., pockets) can be used for, e.g., preserving a secondary phase (liquid, bulk solid). It is possible to either generate large pockets (thickness > film size) within the part by cutting and removing the material or small pockets (thickness < film size) by ablating the desired geometry. Correspondingly, a precise process that allows for ablating the material while not fully penetrating the entire layer is required for structuring. The structuring and filling is finally followed by the deposition and joining of a new layer on top.

The generated pockets could be filled with secondary phases like liquid drugs. These drugs could potentially be released upon pressure to support healing or provide antibacterial substances [36]. In doing so, two contrary key mechanisms (slow and fast drug release) could be targeted when considering an artificial soft tissue such as meniscus (see Figure 2). On the one hand (see Figure 2i), the secondary phase is released more-or-less constantly after the surgery. The goal could be to boost wound healing or increase the biocompatibility and thus the acceptance rate of the implant. In this case, the artificial meniscus needs to be designed in such a way that the secondary phase is released either constantly over time or under moderate load conditions. Therefore, the load conditions need to be considered during the design of the implant to assure a constant drug release. On the other hand (see Figure 2ii), the implant could act as an advanced first aid support. Rather than releasing the secondary phase constantly over time, the drug is kept inside the pocket until a critical load is exceeded. The threshold for the critical load is associated with the loads that occur when specified traumas, e.g., a meniscus tear, occur. Consequently, the drug is released over a short time period to reduce pain and hinder inflammation. Hypothetically, hydrogels or other materials could be added as a secondary phase to improve the mechanical properties. The addition of elements with varying stiffness could be beneficial to generate parts with, for example, progressive material properties.

To ensure the high flexibility and applicability of the parts, adjusted processing strategies need to be developed in the future. One approach is the manufacturing of multi-material parts using different film polymers. However, adjusted processing strategies are required that consider the varying thermo-physical properties of different polymer materials. These properties (e.g., different melting and crystallization temperatures) significantly affect aspects like the thermal degradation of the final part since different energy inputs are required for melting and joining different films. The second important topic is the investigation of application-specific welding strategies. It can be expected that the properties of the final part depend strongly on the joined cross-section. Increasing the welding cross-section will most likely result in a higher tensile strength of the final part. However, larger cross-sections will negatively affect the process time since the welding process using ultrafast lasers is rather slow compared to high scanning speeds in other laser-based additive manufacturing processes. Consequently, future investigations need to focus on the impact of the scanning and joining strategy on endurable tensile and shear stresses. The high flexibility of laser processes allows for the generation of complex structures, as shown exemplarily in Figure 3.

The aim of these tailored weld geometries is to affect the behavior of the workpiece under load. By modifying the orientation of the weld beads, the mechanical stresses can be directed more specifically. However, an in-depth understanding on the influence of the welding strategy starts with a fundamental understanding of (a) the bonding strength of the weld tracks as well as (b) an adequate numerical modeling of these bonding properties. The latter is essential for the a priori design of the welding strategy to obtain parts with customized properties.

From these presentations, the following key potentials of the stack and structure manufacturing process can be identified as follows:

-

Multi-material processing by applying foils from different materials;

-

Processing of hard-to-weld materials (e.g., sensitive to thermal degradation) by employing a “cold” joining process;

-

Generation of internal cavities that can be filled with secondary phases (e.g., hydrogels, drugs, etc.) for the generation of, e.g., progressive part properties;

-

Load-adapted joining strategies to ensure (an)isotropic part properties;

-

Fabrication of parts with sub-layer-sized structures through local material ablation.

The goal of this work is to experimentally investigate the correlations between the applied strategy in the USP laser processing of thin polymer films and the process result. Key parameters, such as polymer film thickness and USP parameters, will be varied. The main research hypothesis is that it is possible obtain the different processes, structuring, cutting, and joining, using an ultrafast laser emitting in the near-infrared range. In summary, this manuscript presents preliminary investigations and first findings for a novel USP-based AM process for the prospective generation of medical implants or similar products.

3. Materials and Methods

An ultrafast laser (Fuego, Time-Band-Width Products) was used for performing the experiments. The laser emits at a wavelength of 1064 nm and has a repetition rate of up to 4 MHz, as well as a pulse duration of 10 ps. A laser spot diameter of 80 µm was used for performing the experiments. Tracks of 10 mm length were produced for the investigation of welding, cutting, and structuring.

Table 1. Process parameters applied for processing thin polymer films.

Parameter	Investigated Values
Laser power in W	3.33–5.05–6.90–8.56–9.70–10.90
Scanning speed in mm/s	4–6–8–10–12
Repetition rate in MHz	1–2–4
Film thickness in µm	50–100

lists the process parameter combinations that were studied in this work.

PET polymer films (RS Pro, Polyester Film—Mylar A, Germany) with thickness of 50 and 100 µm were used for the investigations. This material was selected since it is already approved for medical applications, making it suitable for the proposed manufacturing process. Two foils were joined in all experiments. The first foil with a thickness of 100 µm was manually placed on a glass substrate. The next foil, which was either 50 or 100 µm, was applied on top of the previous foil. An aluminum block was used as a weight for fixing the films along welding direction. The laser was focused on the interface between the two foils.

Ex situ analyses of the joined foils were performed using a laser scanning microscope (LSM) of type Lexmark3000 (Zeiss AG, Germany) and an optical light microscope of type Olympus BX53 M (Evident Europe GmbH, Germany). The LSM was used for determining the width and depth of the generated structures. All LSM samples were analyzed in the as-joined state. For optical light microscopy, the specimens were cut perpendicular to the weld bead. Both top-view (continuity of weld track) and side-view images (melt diffusion of the films) were generated to analyze the quality of the weld tracks.

Throughout this work, the three main laser–material interaction mechanisms of the proposed process are described. The nomenclature is therefore specified in the following:

-

Welding describes all operations that result in a firmly bonded or positive-locking connection of two foils.

-

Cutting describes all process operations where a cut through the top layer is achieved.

-

Structuring describes all operations where only the upper film is modified without cutting through.

4. Results and Discussion

This chapter is divided into three key sub-sections about joining, cutting, and structuring of thin polymer films. Joining polymer films without the risk of cutting through the material is considered the most difficult mode of operation. Therefore, the focus of these investigations was to analyze the joining of thin polymers, whereas structuring and cutting are only of secondary importance in this work.

4.1. Joining of Thin Polymer Films

In the first step, different repetition rates were investigated. Three different frequencies of 1, 2, and 4 MHz were applied for welding the foils, as shown in Figure 4. The goal was to identify suitable repetition rates for the generation of the welds with a high-quality contour. The welding process is based on local heat accumulation effects, even though ultrafast laser processing is typically known as a “cold” process. Depending on the repetition rate and other key parameters, the energy is accumulated in the weld seam, resulting in a local melting of the polymers and joining a bond with the lower layer.

For repetition rates of 1 MHz and 2 MHz, a direct correlation between the quality of the weld bead and the scanning speed can be observed. Lower frequencies result in a continuous weld track, even at higher scanning speeds (top row), while higher frequencies are characterized by incomplete weld beads at the corresponding scanning speed (middle and bottom row). The weld track width increases when reducing the scanning speed during processing. A frequency of 4 MHz appeared to be too high to achieve a reproducible welding process. First signs of film cutting were identified in the high-frequency case at low scanning speeds (bottom row, far right). Accordingly, the subsequent investigations are focused on repetition rates of 1 and 2 MHz. Figure 5 portrays the key results of these studies.

In general, a low repetition rate leads to smaller weld beads. This effect can even be reflected in discontinuous weld beads, as can be seen for the weld tracks generated using a scanning speed of 12 mm/s. Both repetition rates promote the formation of continuous weld beads. Furthermore, the average width of both the weld track and the heat-affected zone increases with decreasing scanning speed for a constant laser power. A similar trend can be observed for different laser powers. Higher laser powers result in a wider area of thermally affected material. Furthermore, it can be concluded that higher repetition rates are favorable when aiming a thin weld bead. Thin weld beads could be desirable when a high resolution is demanded. The result is a widely tunable range of weld bead widths by adjusting the USP processing parameters. Figure 6 presents the correlation between scanning speed, laser power, repetition rate, and weld bead width.

The LSM measurements reveal that the width can be reduced from almost 140 µm to around 60 µm by increasing the repetition rate (from 1 to 2 MHz) in combination with a reduced laser power (from 6.45 to 3.06 W). Consequently, there is the possibility to manufacture structures with a high contour accuracy, which is pivotal for the fabrication of highly resolved components. Applying higher laser powers and lower repetition rates support the fabrication of wider weld beads. Wider weld beads are of interest when larger areas of the foils need to be joined, similar to the hatching principle of a main body in powder- or liquid-based additive manufacturing processes.

In the next step, the quality of the joint was investigated for a film thickness of 100 µm and 50 µm. The rated results are illustrated in Figure 7.

The analysis was performed from a subjective perspective to assess the quality of the generated weld tracks. It needs to be mentioned, at this point, that a quantitative characterization of the bonding strength was not yet performed and is not part of this work. When comparing different film thicknesses, the process window is significantly enlarged (0 and + region) when joining a 50 µm film and a 100 µm film compared to when bonding a 100 µm film with another 100 µm film.

In general, it was possible to join polymer films of different thickness using the USP laser. However, warping of the films was observed at the edges of the weld beads when joining two 100 µm films. This effect could be reduced by joining a 50 µm film with a 100 µm film. The result was a smoother surface of the part, which is desirable when aiming for a close-contact layer-by-layer manufacturing process. One potential explanation for the increased bulging when joining thicker films is the larger energy input required during USP-processing. At high repetition rates, which are typically used for welding, more energy is introduced into the polymer. Correspondingly, the process dynamics, in combination with a reduced viscosity of the heated polymer films, increases the bulging tendency. Using thinner films reduced the energy necessary for joining the two films in a heat-conduction-like welding process, thus leading to a smoother surface that is almost free of bulging (see Figure 8d). This observation is also linked to the heat-affected zone of the welded regions. While the weld bead width appears similar (cf. Figure 8a,c) for the different film sizes, the heat-affected zone is more promoted when joining thicker films.

Associated with the enlarged heat-affected zone could be undesired effects like thermal degradation of the polymer. Investigations regarding the decomposition of the material will be part of future studies. Another point that needs to be considered is the diffusion between the two films. A low diffusion might affect the mechanical properties, such as the maximum shear stress that can be withstood, while a high dilution could promote undesired bulging of the material.

Expanding on these findings, the next step was to assess the joint quality for different parameter combinations when bonding 50 µm films to 100 µm films. Figure 9 reveals the different joints that were generated when processing the material using the USP laser. It can be seen that the quality of the joint improves with increasing energy input. Similar results regarding the joining of the two foils were obtained when adjusting the energy input, as shown in (b) and (c). Further increasing the energy input promotes a through-welding process (Figure 9d). Consequently, the joint will be increasingly susceptible to tearing during load.

Furthermore, good bonding can already be achieved at lower laser powers and potentially also at higher scanning speeds. This observation provides the possibility to improve the process speed when using USP lasers with higher output powers. However, thinner films would result in reduced build rates, since more layers would be required for manufacturing the same thickness.

4.2. Structuring and Cutting of Thin Polymer Films

The two processes, structuring and cutting, are to some extent inevitably linked to each other. By ablating material from the surface, it is possible to generate a specific structure on the surface. Increasing the energy input, e.g., via the number of trespasses, allows us to cut the material. Figure 10 presents first findings on the structuring and cutting of thin polymer films with a thickness of 100 µm. In this case, two 100 µm foils were placed on top of each other.

High repetition rates in combination with high laser powers result in the largest ablation depths. The increasing ablation depth can be attributed to two key mechanisms: On the one hand, the continuous energy input leads to material displacement. The lower viscosity of the polymer film at elevated temperatures results in a flow of the material away from the interaction zone. On the other hand, the USP laser causes a local material evaporation. The induced pressure and vaporized material further support the displacement of the lower-viscous polymer film. Consequently, ablation depths that range from 10 µm for low laser powers and low repetition rates to almost 45 µm for high laser powers and high repetition rates can be achieved. The flexible ablation depths generated within this work reveal the potential of structuring pockets into each film layer and thus throughout the entire workpiece. The USP laser-generated pockets can be used as design elements to manipulate the part properties, for example, to achieve progressive properties or to introduce secondary phases. In the latter case, the previously ablated and filled pocket can be closed by applying and joining the next foil on top. Future steps will include the ablation of larger cross-sections, e.g., rectangular-shaped geometries, and the subsequent investigation on the surface properties of the ablated surface.

The final aspect is the cutting of the polymer films. The goal was to cut the top polymer films using the USP laser without joining the upper film with the lower film. This separation between the two materials is essential to assure that the cut foil can be removed during processing.

The results indicate that it was possible to structure (S) the materials at low energy inputs. By increasing the laser power, the material was partially cut (PC). At laser powers in the range of 10.9 W, at least partial cutting of the material was possible at different scanning speeds (see top right in Figure 10). This partial cutting was indicated by a non-continuous cut along the scanning direction. At scanning speeds of 6 mm/s and a laser power of 10.9 W, it was possible to generate a cut (C) through the entire top foil. Furthermore, no excessive thermal energy input was observed into the lower foil. This energy input is described as the heat-affected zone (HAZ).

At higher laser powers or lower scanning speeds, an increasing energy input into the lower film was observed (C or PC + HAZ). This energy input is reflected by excess material (“negatives”) from the scan tracks of the upper foil on the surface of the lower foil. Consequently, two key risks arise in this instance. The excessive energy input could lead to a thermal degradation of the lower film or result in the joining of the two films, thus triggering the exact opposite process mechanism. The ability to cut the material using the USP laser system lays the groundwork for creating three-dimensional structures by locally removing the material from two-dimensional foils.

4.3. Classification of Future Tasks

The general feasibility of the proposed manufacturing process is shown in this work. However, future investigations need to be more specific to address the different challenges that might arise during the proposed stack and structure process.

One important topic is the optimization of process parameters. It was shown, and is already known, that the manufacturing parameters play an important role in material processing with ultrafast lasers. Since this manufacturing process aims to exploit all generally feasible processes (structuring, cutting, and welding) of ultrafast lasers, a distinct understanding on the mechanisms is necessary. Thermal effects, as shown in [37] and in [38], could play an important role during welding of the films and during build-up of larger parts. Numerical and analytical models are powerful tools that can accelerate the development times [39]. These tools should therefore be used in the future.

Another key issue that needs to be addressed are local weld seam imperfections, for example, excessive material at the surface of the weld (e.g., see Figure 8b). These weld reinforcements are detrimental regarding the final accuracy, since they could accumulate during the build-up of larger parts. It is shown that these imperfections could be minimized by reducing the overall energy input (see Figure 9). Consequently, the first solution should be an adjustment of welding parameters to minimize elevations at the surface of the processed layer. If the adjusted welding parameters do not result in a satisfying result, the process part “structuring” could be applied to remove excessive material from the surface. However, this solution is not favored, since it would result in an increased processing time.

Furthermore, future work is needed on the welding pattern. Depending on the amount and orientation of the weld seams, the mechanical properties will vary. In-depth investigations on the correlation between material, joined cross-section, and mechanical properties are essential.

5. Conclusions

A novel ultrafast-laser-based additive manufacturing using thin foils as feedstock material is proposed. It is shown that it is possible to trigger the process mechanisms joining, cutting, and structuring using the same laser source by solely adjusting the processing parameters.

-

The joining of polymer films is achieved by increasing the pulse repetition rate. By adjusting the process parameters, it is possible to modify the thickness of the weld butts.

-

Best process results for joining two films were obtained when using thinner foils (50 µm). Thick films lead to the bulging of the material, while thin films were not as sensitive to this effect.

-

Increasing the laser power and lowering the pulse repetition rate supported the structuring and cutting of the material.

-

It was possible to achieve cuts of the upper film without excessive thermal energy input into the lower film at specific USP process parameters.

The high flexibility of this process paves the way for an innovative manufacturing approach that can help to avoid powder contamination, which is typically present in powder-based processes. Future investigations will focus on in-depth investigations regarding the joinability, cuttability, and structurability of thin films when using a high-power USP laser.