1 Introduction

Filament-based material extrusion (MEX) is a popular additive manufacturing technology, which has seen major market growth over the recent decade [1]. The process involves the extrusion of molten polymer material through a nozzle to subsequently create three-dimensional objects using a filament material as a feedstock. The major advantages of this technology besides the high geometrical freedom and flexibility, which are seen in almost all additive manufacturing techniques, are the low material and machine costs that accompany the usage of filament-based MEX. In addition to PLA and ABS, which are the most commonly used materials within MEX [1], the technology is also employed to process fiber or particle-filled filament materials to improve mechanical properties [2] or integrate additional functionalities [3] for certain applications. These possibilities can be used within rapid tooling for the creation of close-to-contour mold cooling channels [4] for example or the production of fast available spare parts.

A variant of MEX, which is also based on the processing of a highly filled filament, enables the production of metal parts by filament-based material extrusion in a multi-step process (MEX-MSt/M, as defined by DIN EN ISO ASTM 52900 [5]). This process combines the advantages of a cost and time-efficient production of geometrically complex parts through the polymer shaping process with the advantages and all the characteristics of a certain metal material [6]. The follow-up process for MEX-MSt/M after the original shaping of the part is similar to the powder (PIM) or metal injection molding (MIM), which usually implicates a two-step debinding consisting of a solvent debinding and a thermal debinding step to remove the polymer [7]. Afterwards, the part is sintered at higher temperatures to obtain a compact, almost completely dense [8] metal part with mechanical properties comparable to conventionally produced parts [6, 9].

Although the integration of filler particles or fibers grants new opportunities, several difficulties go along with the processing of filled or even highly filled materials. In this regard, a general challenge in the processing of highly filled polymers is the increased viscosity. This parameter strongly depends on the filler characteristics such as the type, geometry, size, and amount of filler incorporated into the matrix [10, 11]. Especially the filler amount is not only crucial for the processing properties in the molten state but also for the mechanical properties of the final part, which usually manifests in a decrease in ductility by a reduction of elongation at break as well as a rise in the tensile modulus [12, 13]. As a result, for MEX a high filler content leads to drawbacks in the handleability and flexibility of the filament for both particle [14] and fiber-filled [15] feedstocks. Furthermore, high filler loadings also often lead to nozzle clogging [14] due to an arch formation of particles [16] by successive accumulation of particle agglomerates at the die entry. One of the determining factors for nozzle clogging, besides the amount of filler in the compound, is the ratio of nozzle diameter to filler diameter [14]. Another aspect that is crucial for avoiding nozzle clogging, guaranteeing a homogeneous flow, and preventing particle segregation, is the choice of a suitable binder system [17]. In this regard, the matrix needs to compensate for the difficulties that arise when processing highly filled filaments for MEX to successfully produce parts. The choice of the binder composition also affects the tendency for powder binder segregation in MIM and multi-step MEX (MEX-MSt) [18,19,20,21]. This effect is particularly evident for higher shear rates [18] and is ,therefore ,especially crucial for the formulation of binders for MIM, where the highly filled polymer experiences higher shear rates, but could also pose a problem within MEX by enhancing nozzle clogging [14]. In addition to the filament flexibility and sufficient melt flowability, the binder must be suitable for the follow-up processes such as the thermal debinding, where the material should degrade in a stepwise and slow process to avoid a sudden release of gas which may lead to cracks in the final part [7] or cause blistering [6]. Binder systems for MEX-MSt are often derived from feedstock systems that are used in MIM [8] and in many cases contain waxes like paraffin wax [17, 19, 22, 23] to reduce their viscosity.

Although the use of waxes within the feedstocks for MEX-MSt is common practice, there is little work done on the evaluation of waxes besides paraffin [12] and their effect within a highly filled filament as a feedstock for MEX-MSt. Therefore, this work aims at the creation of a better understanding of how different types and contents of waxes affect the processing properties of filament feedstocks for MEX-MSt, concentrating on their thermal, mechanical and rheological properties.

2 Experimental

2.1 Materials

In this study, a commercially available low-density polyethylene (PE-LD) with a processing range between 120 and 230 °C and a melt flow rate (MFR) of 65 g/10 min was used as the base material for all compounds and for reference purposes. Three different wax-types were used as modifiers for the feedstock in concentrations of 12.5%, 25% and 50% related to the binder fraction within the compound. Therefore, the PE-Wax “E25K”, the PP-Wax “P36G” and the PE/PA-Wax “H92G” by Deurex (Elsteraue, Germany) were employed. The filler content of the compounds was kept constant at 60 vol.-%, which corresponds to 92.5–92.8 wt.-% depending on the wax density and complies with regular filler contents used for MEX-MSt/M [8, 9] or MIM [7]. For filler, a 1.4404 (316 l) steel powder by m4p (Magdeburg, Germany) was used. The particles can be described as spherical as Fig. 1a demonstrates. Furthermore, the particle size distribution is depicted in Fig. 1b and shows a D10 of 6.3 µm, a D50 of 13.5 µm and a D90 of 23.2 µm.

Fig. 1
figure 1

Particle characteristics of the 316 l steel powder: a SEM image of the spherical particles; b volumetric particle size distribution determined by dynamic picture analysis on a Camsizer X2 by Microtrac Retsch (Haan, Germany)

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2.2 Processing

The wax fractions were dry blended with the PE-LD in a tumbling mixer and dried at 60 °C for 24 h prior to compounding. For the incorporation and homogenizing of the filler, a co-rotating twin screw extruder ZSE HP17 from Leistritz (Nuremberg, Germany) applying temperatures from 140 °C at feeding to 180 °C at the die with a constant screw speed of 100 rpm was used. The strand was cooled down on a cooling plate and granulated afterwards for further processing.

For the mechanical characterization of the compounds, dog-bone tensile bars were produced using the MIM-technique [7] on an Ergotech 25–80 injection molding machine by Sumitomo Demag (Schwaig, Germany). The specimens were manufactured using a dual-cavity mold with a film gate at the end of the tensile bar shoulder at a mold temperature of 45 °C, a mass temperature of 200 °C and an injecting speed of 30 mm/s, resulting in a total cycle time of 50 s.

Filaments with a diameter of 1.75 mm were produced for further processing via MEX and for the determination of the filament flexibility using a Composer 350 by 3devo (Utrecht, The Netherlands). The filaments were extruded at a temperature of 200 °C and spooled onto a standard spool for filament-based MEX with an inner diameter of 120 mm, if possible.

The printability trials via MEX were carried out using a Funmat HT by Intamsys (Shanghai, China) equipped with a 0.8 mm nozzle and a dual direct drive extruder by Bondtech (Värnamo, Sweden). For the evaluation of the general extrudability, filament segments of 50 cm length were fed from the top of the open building chamber through the feeder mechanism to extrude material through the nozzle at temperatures of 200 °C, 220 °C and 240 °C. Tensile bar specimens were then printed at 20 mm/s printing speed and nozzle temperatures of 200 °C, 220 °C and 240 °C. The building plate temperature was set at 80 °C. The cooling fan as well as the filament retract were turned off.

2.3 Methods

The thermal processing and decomposition behavior of the compounded feedstocks were evaluated by differential scanning calorimetry (DSC) as well as by thermogravimetric analysis (TGA) for the compounded feedstocks without wax and with 50% wax within the binder fraction. The samples for DSC weighed in at around 5 mg and at 10 mg for TGA. Both samples were taken from the filament material after filament extrusion to ensure a similar thermal exposure close to the MEX process. For the DSC-measurements, a Discovery DSC 2500 by TA-Instruments (New Castle, USA) was used with nitrogen as a purging gas. The measurement comprised two heating cycles from − 20 °C to 200 °C with one cooling cycle in between. Heating and cooling rates were set at 10 K/min. The TGA-measurements were performed on a TGA Q5000 from TA-Instruments (New Castle, USA) at a constant heating rate of 10 K/min up to a maximum temperature of 700 °C under nitrogen atmosphere.

The mechanical characterization for the injection molded specimens was carried out according to DIN EN ISO 527–1 [24] on a 5948 MicroTester by Instron (Norwood, USA) using an optical extensometer. The testing speed was set at 1 mm/min and at 0.33 mm/min for the determination of the Young’s modulus. Further, the specific filament flexibility was determined by the testing of the minimum bending radius according to VDI 3405 [25]. Therefore, two different 1 m long strand sections of the extruded filaments were fed through a bending template with stepped radii.

For the evaluation of powder binder adhesion and breaking morphology after tensile testing, scanning electron microscope (SEM) was performed on a Gemini Ultra-Plus by Carl Zeiss (Oberkochen, Germany). The gold-sputtered samples were observed using a secondary electron detector at a magnification of 2.500 × and an acceleration voltage of 10 kV. For this study, breaking surfaces are depicted for the sample without wax as well as for samples with 25% wax within the binder.

The melt volume rate (MVR) was determined according to DIN EN ISO 1133-1 [26], and was used to evaluate the rheological behavior regarding the processability of the feedstocks for MEX. The testing temperature was set at 200 °C and the additional weight was 2.16 kg.

3 Results and discussion

3.1 Thermal characterization

DSC was conducted for the evaluation of melting and crystallization behavior. Cooling and heating thermograms are depicted in Fig. 2 for the highly filled compounds with and without wax addition. The heating for the reference without wax, illustrated in the lower graph, shows a distinct melting peak at 103 °C. The addition of the PE-wax leads to a broadening of the melting peak towards lower temperatures. This can be deduced to the mobilizing of crystalline areas of the short-chained wax component by the heat input, which leads to a lowering of the melting range [27]. The presence of the low molecular mass PE-wax also leads to a rise in the degree of crystallinity due to better chain mobility, and therefore, an enhanced ability to organize themselves even within the branched PE-LD areas. The addition of PP-wax results in a significantly smaller PE-LD-peak due to the lowered PE-LD content and a second melting range of the PP-wax component between 143 and 158 °C with a clear separation between the two melting areas. An addition of PE/PA-wax has a similar influence on the melting as the pure PE-wax, which leads to a broadening of the first melting peak due to the PE-chain segments within the wax. The second melting peak at 141 °C is a result of remobilizing of the PA-chain segments within the hybrid-wax. Overall, all waxes with the exception of the PP-wax lead to a significant increase in the degree of enthalpy of melting. The crystallization behavior for the compounds can be seen in the upper thermograms in Fig. 2. The reference material herein shows a sharp crystallization peak at 93 °C, while the PE-wax addition leads to a 4 °C shift of the peak towards higher temperatures and to a broadening of the crystallization range. An addition of PP-wax and PE/PA-wax shows two separate crystallization peaks due to the different chemistry of the wax compared to the PE-LD. Herein, the PA-fraction experiences the highest single crystallization peak at 137 °C.

Fig. 2
figure 2

DSC-measurements for highly filled feedstock materials without wax and with 50% PE-, PP- and PE/PA-wax addition within the binder for the evaluation of melting and crystallization behavior

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The TGA is depicted in Fig. 3 and displays the degradation behavior for the different feedstock compositions, while purging with nitrogen. The nitrogen atmosphere is used to simulate processing conditions as the debinding process for 316 l in MEX-MSt/M is usually carried out under exclusion of oxygen either in a protective gas atmosphere [28] or in vacuum [6] in order to prevent the metal from oxidizing [29]. Within the measurement, this might also lead to a gain of weight and would falsify the result in regard to the real processing conditions. The neat PE-LD binder compound shows a one-step degradation peak in the deviation of the binder weight between 395 and 500 °C. An addition of 50% PE-wax within the binder leads to a slightly broadened course of the curve with an earlier, slow weight loss of around 10% until the onset of the main degradation at around 400 °C. The PP-wax and the PE/PA-wax cause an earlier, more continuous degradation, beginning at around 250 °C, and an overall slower progress in decomposition up to the final temperature of 500 °C. The slow degradation progress of the PE/PA-wax and the PP-wax is favorable for a sole thermal debinding, but requires several holding phases, because the gas which is produced during the decomposition of the material can slowly come out of the part. This prevents the formation of gas bubbles due to accumulation of trapped air within the part, which can lead to blistering or crack formation during the debinding process [30].

Fig. 3
figure 3

TGA-measurements for the determination of the thermal decomposition behavior for a feedstock without wax addition and for compounds with 50% wax within the binder

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3.2 Mechanical characterization

Figure 4 shows the mechanical characteristics determined via tensile testing for the addition of different amounts of the various waxes. As described earlier, the integration of high filler amounts strongly affects the mechanical behavior of polymer compounds [12, 13]. The mechanical characteristics for MEX are particularly crucial given the requirement for filament form processing in this technique. In addition to being able to be spooled for storage, the filament needs to be flexible enough to be fed via narrow tubes towards the hot end and build up the necessary extrusion pressure. The most important characteristic for the evaluation of filament flexibility is the elongation at break. Here, as a result of the high amounts of fillers incorporated, the reference without wax already only shows a maximum elongation at break of 3.29%. The addition of wax causes a significant, absolute reduction in the elongation at break between 1.39% and 3.0% depending on the type and amount of wax used. In general, a higher amount of wax leads to a stronger decrease of ductility for all types of waxes. In this regard, the addition of PE/PA-wax has the most dominant influence on the elongation at break when adding the same amounts of waxes with a minimum value of 0.29% elongation.

Fig. 4
figure 4

Mechanical values of the highly filled compounds fabricated via MIM for varying wax contents and types within the binder fraction

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The PE-wax shows the least decrease among the waxes, but only has slightly higher values than the PP-wax when adding 25% or more into the binder. The tensile strength of the compound shows no clear or general trend for the addition of wax. Therefore, while the addition of PE-wax leads to a rise in strength, both PP-wax and PE/PA-wax contribute to a decrease in strength with rising wax contents. However, the addition of wax amounts higher than 50% cause a decline in the tensile strength for all waxes. The Young’s modulus is a factor that is also strongly dependent on the added wax content. Here, a higher wax amount in the binder leads to a significant increase of the Young’s modulus value, and PE-wax reaches the highest values among the investigated materials. In this regard, a sufficient stiffness of the filament is important in order to prevent the filament from buckling inside the feeding tube towards the hot end, which guarantees a proper force transmission for the extrusion.

The minimum bending radius, which determines the flexibility of the filament, is an indication for the processability of the filament up to the entry of the feeding mechanism of the MEX-printer and is represented by Fig. 5. The values resemble the insights that were given by the tensile testing and show a rise in the minimum bending radius when increasing the wax contents, which corresponds to a more brittle material behavior. In order to spool a filament onto standard spools, a maximum bending radius of 60 mm must be given. This value is exceeded by all waxes when their share within the binder corresponds to 50% or more. While PE-wax addition demonstrates the best filament flexibility, an addition of PE/PA-wax leads to a notably brittle filament, even at low amounts within the binder.

Fig. 5
figure 5

Minimum bending radius determined according to VDI 3405 for the 60 vol.-% metal filled filaments of 1.75 mm with 0%, 12.5%, 25% and 50% of PE-, PP- and PE/PA-wax addition within the binder fraction

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A reason for the mechanical behavior can deduced from the fracture morphology after tensile testing, which can be seen in Fig. 6. While the binders with PE-wax (Fig. 6b) and PP-wax (Fig. 6c) show an enhanced wetting behavior of the particles by the matrix, this behavior is not evident for the PE/PA-wax (Fig. 6d). This indicates a bad particle matrix interaction, and therefore, a bad interfacial adhesion between the particles and the matrix. The addition of the polyolefinic waxes into the matrix polymer PE-LD leads to a better compatibility here and as a result, to better mechanical properties and a higher flexibility compared to the PE/PA-wax. An improved powder binder adhesion also leads to a more uniform particle distribution and reduces segregation effects [20]. A similar effect can be achieved by the employment of stearic acid, which is common practice for highly filled feedstock systems to achieve a better dispersion of particles [31, 32], especially at higher shear rates as they appear in MIM [18]. However, a good particle–matrix interaction also contributes to a good filament homogeneity, which results in a more uniform final part and processing behavior.

Fig. 6
figure 6

Morphology of the fracture surfaces at 2.500 × magnification after tensile testing; a without wax; and with 25% of b PE-wax, c PP-wax, d PE/PA-wax addition

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3.3 Melt flow behavior

The melt flow behavior is illustrated in Fig. 7 by the graphs for the MVR-values at 200 °C.

Fig. 7
figure 7

MVR-values of the highly filled compounds for varying wax contents and types within the binder fraction at 200 °C

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The melt flowability demonstrates a significant increase when adding wax into the binder. The short-chained wax fraction already increases the flowability at 12.5% addition by more than 100%. The highest MVR-values can be observed for the PE/PA-wax, where a 23.2-times increase is recorded, followed by the PP-wax. Compared to the other waxes, the PE-wax has the lowest MVR, yet it still exhibits a 13-fold increase in MVR-value. Overall, a rising wax content, independent of the wax-type, causes an increase in the melt flow. As a result, when increasing the wax content within the feedstock, less pressure is needed in order to achieve the same extrusion volume in MEX. This enables faster processing and leads to a reduced probability for nozzle clogging [14]. The decrease of the necessary extrusion pressure also reduces the chance of processing difficulties due to buckling or a grinding of the filaments [33].

3.4 Printability in MEX

The printing trials demonstrated the general extrusion capability at temperatures between 200 and 240 °C in MEX for all feedstock materials, except for the filaments with PE/PA-wax. In this regard, the added PE/PA-wax allowed no conveying by the feeding mechanism as it was too brittle and would break before sufficient pressure for material extrusion was applied.

However, the other materials were printable as pictured in Fig. 8 for the compounds with additions of 50% PE-wax (Fig. 8a) and 50% PP-wax (Fig. 8b) at temperatures between 200 and 240 °C. Although the materials were also printable at 240 °C, it has to be noted that the low viscosity, especially for the PP-wax compound, led to a blurring of the extruded strands due to over extrusion. This is already slightly indicated at 220 °C in the top right hand corner of Fig. 8b, where an accumulation of material is present. Within the study, the addition of PE-wax displayed the best processability, resulting in a homogeneous extrusion behavior and layer structure.

Fig. 8
figure 8

MEX-fabricated samples printed at 220 °C for a 50%.PE-.wax addition and b 50% PP-wax addition

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4 Conclusions

The present study systematically investigated the effects of different types and amounts of waxes on the material behavior of filament feedstocks for the processing via MEX. Thermal behavior was found to be strongly affected by the type of wax added, as the crystallization is affected by the short-chained waxes, leading to a shift of crystallization peak temperatures and melting areas. The waxes also had an impact on the thermal decomposition behavior, where the onset for degradation shifted towards lower temperatures leading to a gradually decomposition instead of a rapid one. Thereby, depending on the wax-type used, the decomposition range for thermal debinding can be tailored. The mechanical characterization found that a rising wax content led to a less ductile material behavior, independent of the wax-type used. This contributes to a less flexible and also more brittle filament, which makes spooling and processing more challenging. This behavior needs to be compensated for by the choice of wax used, further ductility-enhancing components or adhesion promoters in the future. The addition of PP- and PE-wax also helped creating a better particle–matrix-adhesion in combination with the PE-LD matrix, which favors a homogeneous particle distribution. Furthermore, the wax addition led to a strong improvement in melt flow behavior, even at low amounts added. This enabled the successful processing of the PP- and PE-wax compounds in MEX with adapted parameters. Future research will focus on the addition of a ductility-enhancing component within the filament feedstocks to fully use the advantageous properties that come along with the use of waxes, which can widen the variety of materials for the indirect additive manufacturing of ceramics and metals.