Abstract
Selective laser sintering (SLS) of polymers is on the edge from a pure prototyping technique to a small-scale production. For this transition, characteristic values such as long-term properties, and thus the degradation mechanism, are crucial factors for enabling a series application. Due to the specific characteristics of SLS parts like porosity and rough surfaces, a direct transfer of known mechanisms and models for injection molded (IM) parts is not or just to a limited extent possible. This leads to the aim of this paper, which is to investigate and compare the degradation behavior of polyamide 12 parts produced by SLS and IM.
1 Introduction
Additive manufacturing technologies, like selective laser sintering (SLS), are on the edge from a pure prototyping technique to a small-scale production of highly individual products. Therefore, their fields of application range from models in architecture, functional prototypes in the automotive section, up to the use in series production, as in handling systems [1, 2]. Due to the possibility of producing parts without the need for tools and molds, a high potential lies in the precise individualization of parts independent from their complexity [3]. As knowledge about the long-term properties and especially the aging behavior is strongly limited, the possibility of this production technology is restricted [4, 5]. Therefore, a prediction depending on the lifetime and the changing of the part properties over time is not yet possible [6]. Approaches exist for the aging behavior of injection molded (IM) parts [7] as well as polyamide 12 (PA12) powder during the SLS [8, 9]. Due to the specific processing steps of SLS like the pressureless, layerwise production, a direct transfer of the existing aging behavior is not or just to a limited extent possible and therefore the aim of this paper.
2 Fundamentals
2.1 SLS of polymers
SLS is divided into three steps. In the first step, a roller or blade homogeneously distributes a layer of powder with a thickness of 100 μm equally across the building platform. Afterwards, the powder is heated up to the specific building temperature, which is between the crystallization and melting temperature of the used semi-crystalline polymer. Following the theory of “quasi-isothermal” process control, this enables the parallel presence of solid powder and the molten part. In addition, due to the high temperature, shrinkage and warpage may be inhibited, but ageing effects can occur [7, 10]. In the third step, a CO2-laser melts the particular cross-section and the surrounding powder serves as a support structure. Then, the building platform is lowered and the three steps repeat until the part is finished [11, 12].
As the process is pressureless and the surrounding powder holds the melt, the resulting parts show a rough surface [13, 14] as well as a porous inner structure [14, 15] with a porosity between 3% and 5%, and a brittle breaking behavior under tension testing [6]. The porosity may result from an incomplete coalescence between particles [6, 16]. Resulting from the layerwise production, SLS parts show an anisotropic behavior. The highest mechanical properties are enabled along the layer, which shows an increase for the tensile strength and elongation at break in comparison to a load parallel to the layers [6]. Due to the process-specific part properties, like high surface roughness and porosity, changes of the mechanical properties result from thermal storage up to 4 weeks. The resulting properties are caused by the specific process steps and parameters [4]. The used energy density during processing is a main influence for part behavior. With a higher density, the mechanical properties increase until the density is too high and enables degradation behavior, which leads to decreased material properties and to a higher porosity [15]. In comparison to IM parts, SLS parts tend to have less internal stress resulting from a slow cool-down step during the process, which also influences the crystalline structure. Due to the layerwise production, ageing effects can occur differently in the layers because of the different heating duration [7, 15, 17].
2.2 Ageing of polyamide 12
During the lifetime external influences, like temperature, various mediums and mechanical loads can influence the long-term properties of polymer parts due to chemical or physical ageing effects [18, 19]. Physical ageing effects mainly occur under thermal load and result in reversible morphological changes, like the relaxation of orientations as well as post-crystallization processes. These effects reduce the operating lifetime but they are reversible by re-melting [7, 18].
Post-crystallization processes mainly occur at higher temperatures and lead to an increase of the degree of crystallization and lamellae thickness. Additionally, these processes lead to a completion of the crystal structure, which enables shrinkage and warpage effects [7, 20] as well as the reduction of mechanical properties like elongation at break or tensile strength [20]. In comparison, chemical ageing effects are irreversible because they influence the chemical structure of the polymer due to chain scission, branching, cross-linking and post-condensation effects [7, 18]. This can lead to a degradation of the polymer, for example, due to mechanical load, energetic emission, and thermo-oxidative processes [18, 19].
The degradation behavior of polyamides is mainly influenced by thermo-oxidative effects, which lead to degradation due to radical chain reactions [7, 21]. According to Schnabel [21], this mechanism consists of three steps: initiation, chain growth and termination reaction. Initiation occurs due to thermally induced chain scission at higher temperatures and mechanical load like shear stress [19, 21]. These radicals develop peroxide radicals due to the reaction with oxygen, which can interact with other macromolecules to form hydroperoxide connections. This leads to a scission of molecules, development of new radicals and a brown discoloration. The shortening of main chains can decrease the mechanical properties, but is not directly correlated with the discoloration [7, 18, 21]. Free radicals can recombine, which reduces the reaction-speed, and can lead to cross-linking [21]. During the application time, the molecular mass decreases due to oxygen and leads to a decrease of the impact-, bending- and tensile-strength as well as the part flexibility [18].
As shown, oxygen plays a major role in the thermo-oxidative degradation behavior. The oxygen diffusion takes places faster in the amorphous regions due to the higher amount of free volume, as in crystalline areas [22, 23]. Additionally, a higher degree of crystallization as well as orientations can slow down the thermo-oxidative degradation effects [18]. The mechanisms have different influences, which can result in a change of the average molar mass and its distribution and the melt viscosity through shorter chains. Furthermore, the crystallization temperature can increase because of nucleation effects [7, 24, 25, 26, 27]. According to Ferrer-Balas et al. [28], temperature load may lead to temper effects which reduce inner tensions and can result in an embrittlement, and therefore, a reduction of the tensile strength and elongation at break [7, 21, 29, 30].
The degradation behavior of polymers depends on the type of load as well as the part properties, which for SLS, is mainly influenced by the powder and the processing parameters. The powder is affected by the flowing behavior and the thermal history by previous build jobs. Therefore, the physical and chemical part properties as well as the macroscopic characteristics such as surface roughness can be influenced [9, 31].
2.3 Motivation
The specifics of SLS parts vary widely from conventional parts, such as IM, due to their characteristics like the layer wise production and therefore an anisotropical mechanical behavior. Furthermore, these parts show differences in the inner structure like porosity and morphology, and on the outer structure in the form of a high surface roughness and powder attachments. A main limitation for this technique is the missing knowledge about the long-term properties and its degradation behavior. Therefore, knowing about the degradation behavior through thermal and thermo-oxidative load is missing. As SLS parts have different part characteristics, a known degradation mechanism from IM cannot or restricted be transferred.
Therefore, the aim of this paper is to outline the difference in the degradation behavior between IM and SLS. For this, tensile bars are manufactured by both manufacturing techniques and stored in convection ovens at different temperatures for varying time periods. Afterwards, the density, surface roughness, and the mechanical properties and changes in morphology and material are analyzed in order to characterize the differences in degradation behavior.
3 Materials and methods
3.1 Material
For the following studies, PA12 powder from the type PA2200 (EOS GmbH, Krailing Germany) is used with a powder refreshing rate (used/new powder) of 50 weight % according to industrial standard. As an indicator of material quality, the melt volume-flow rate (MVR) is measured according to DIN EN ISO 1133-1:2012-03, which is 38.2±2.2 cm3/10 min in the experiment. This value is determined at the chair of manufacturing technology at the University Duisburg-Essen with an MVR-testing instrument of type Karg Industrietechnik (Krailing, Germany) MeltFlow @on at 235°C with a weight of 5 kg. Before the measurement, the powder was dried in a double-stage drying process with a moisture analyzer of type Kern DBS 60-3. The first drying stage, which takes 15 min at 105°C, is followed by a second one, which takes 2 min at 140°C. The aim of the second stage is to reduce the water absorption from the ambient air during the filling of the specimens in the measurement device. During the process of aging, the MVR decreases and thus is an indicator for the viscosity of the melt [7, 8]. The MVR of pure new powder of type PA2200 ranges from 60 cm3/10 min to 80 cm3/10 min depending on the batch used [32].
3.2 Process strategies and specimens
For further analysis, tensile bars are manufactured in SLS as well as IM according to DIN EN ISO 3167 (Typ A) [33] at a scale of 1:1. In order to store the produced specimens in the convection oven, a hole was drilled in one of the shoulders. The specimens are hung up in the oven to provide a homogeneous influence of temperature and oxygen.
For manufacturing the specimens, an SLS-machine of type Formiga P100 from the company EOS GmbH, Krailing is used in order to manufacture 125 specimens. The specimens are oriented in the z-direction (building height) with the powder being applied in the x-direction. For the process parameters, the laser power is set to 21 W, hatch distance to 0.25 mm, scan velocity to 2500 mm/s and the layer width to 0.1 mm. It is known from the literature that the resulting characteristics of SLS parts can vary [14, 15] due to the inhomogeneous temperature distribution across the powder surface. Especially at edges and corners of the building platform, reduced mechanical characteristics can be expected. In order to cope with this behavior, the construction space is divided into four corner and one center zone. For every storage combination, one specimen out of every zone was used, therefore the influence of the building position was considered. Here, two build-jobs are executed for the sample selection in order to reduce the fluctuations introduced during the build process.
In order to characterize the process of the aging characteristics of SLS, IM specimens are prepared. Therefore, a part of the PA2200 powder is compounded by a twin-screw extruder of type ZSE25/GL-400 from the company Leistritz AG with a rotational speed of 100/min and a mass temperature of 210°C. Following this, the material is granulated and prepared for the IM process. After the granulate is dried at 70°C for 24 h, the tension bars are produced with an IM machine of the type Allrounder 370 V/ 800-315 from Arburg GmbH+Co KG. A mass temperature of 240°C and a mold temperature of 80°C were used at an injection pressure of 600 bar with an injection speed of 300 mm/s. The holding pressure is 500 bar.
3.3 Preparation and conditioning of the test specimens
The test pieces are stored at different temperatures and varying durations in convection ovens to set up defined states of conditioning. For this, the tensile bars are stored hanging, using a hole through the shoulder of the tensile bars with adequate distance to each other to eliminate mutual influence. For the conditioning 60°C, 80°C, 100°C, 120°C, 140°C were used for a storage time of 72 h, 168 h, 336 h and 672 h.
3.4 Analysis methods
Specimens with a length of 10 mm are prepared from the middle (Figure 1) and the density is determined through the buoyancy method according to DIN EN ISO 1183-1 [34]. In addition, the porosity in the middle part of the tensile bars was determined using a subμ-CT from Fraunhofer.
Overview of measuring points and sections for microscopy, density and viscosity number (Vn) measurement.
In order to be able to characterize the process impacts, the specimen’s width and thickness are measured at the shoulders and middle section of the tensile bars (Figure 1). Furthermore, the surface roughness of the SLS specimens was measured by a Hommeltester Waveline 20 (radius of the tip 2 μm and contact force 0.8 mN) at three parallel lines, to detect external changes. As the translatory space is kept constant, the covered distance is measured for SLS and IM. The ratio of the covered distance approximately reflects the differences in the surfaces.
In order to determine the mechanical characteristics of the specimens depending on the different times and temperatures of storage, tensile tests according to DIN EN ISO 527 [35] are conducted at room temperature (23°C) and a humidity of 50%. For every storage combination, five SLS and three IM specimens are tested and an equal amount of unconditioned specimens are stored to reference the storage conditions. Determining the Young’s modulus is done with a test speed of 1 mm/min whereas tensile strength and elongation at break are determined with a speed of 50 mm/min due to the expected ductile fracture behavior of IM [35].
In order to quantify the influence of temperature at different storage times, the viscosity number (Vn) is measured for the different storage combinations. The Vn is determined by the average molecular weight as well as its distributions [36]. This is done by solution viscometry with concentrated sulfuric acid at 25°C according to DIN EN ISO 307 [37]. In order to produce comparable results independent of the extraction positions along the transverse section, the material to be dissolved is extracted through a thin-cut of the sample’s cross-section close to the fracture zone (see Figure 1). The SLS tensile bars are only extracted from the central withdrawal area of the constructions space, as reproducible component characteristics can be obtained here [14]. In addition, material from the core and edge areas are extracted from the SLS and IM specimens at 120°C and 140°C with a storage time of 672 h, in order to determine the diffusion of the aging mechanisms in the core area. For the microscopic characterization, thin sections with a width of 10 μm are extracted from the middle and the morphological structure is examined by transmission light microscopy using polarized light. Pictures were taken of the cross-section for SLS and IM specimens aged at a 120°C and 140°C. In addition, differential scanning calorimetry (DSC) measurements with a heating rate of 10 K/min are conducted on separately extracted cross-sections according to DIN EN ISO 11357-1 [38]. Therefore, the melting enthalpy and the crystallization temperature were determined according to Figure 2.
Differential scanning calorimetry (DSC) scheme.
4 Results and discussion
As a result of the different manufacturing techniques, differences in the specimens occur. Figure 3 shows the dimensions of IM and SLS before and after conditioning. At the beginning, IM specimens show a larger deviation of the width (Table 1). In comparison, SLS exhibits an undersize as well as a larger standard deviation. This is the result of powder attaching to the surface (Table 2) and therefore a high and uneven texture. These behaviors are also reflected in the specimen’s thickness (Figure 3). These differences also occur from the differences in the process control. As IM melts the polymer, fills a mold with high pressure and counteracts shrinkage with a holding-pressure phase, a high accuracy can be achieved. In comparison, SLS operates pressureless and forms the specimens layerwise in a powder bed by melting the polymer via a laser. Therefore, the standard deviation in SLS is higher. Both dimensions show no significant influence of the storage conditions on the dimensions.
Influence of storage time and temperature for selective laser sintering (SLS) and injection molding (IM) on the width (left) and thickness (right).
Comparison of the discoloration across selective laser sintering (SLS) and injection molding (IM) after storage for 672 h at 120°C and 140°C.
| 120°C | 140°C | |
|---|---|---|
| SLS |  |  |
| IM |  |  |
IM, Injection molding; SLS, selective laser sintering.
Morphological comparison for selective laser sintering (SLS) and injection molding (IM) before and after the storage of 140°C for 672 h.
| Reference specimen | 140°C, 672 h | |
|---|---|---|
| SLS |  |  |
| IM |  |  |
IM, Injection molding; SLS, selective laser sintering.
In addition to the differences in the dimensions, the manufacturing techniques lead to varying morphologies. SLS shows a homogeneous structure for the cross-section (Table 2) which occurs due to the high temperature of the building chamber above the crystallization temperature and the slow cooling down stage. Due to the pressureless process, a porosity of 1.7% occurs in the resulting specimens. In comparison, IM shows a crystalline core-section which merges into an amorphous peripheral area, which is dependent on the mold temperature used and therefore a fast cooling of the melt during the injection step and contact with the mold surface. Additionally, no pores occur due to the used holding pressure.
Table 2 shows the specific cross-sections of pictures of the attaching powder on the surface due to the pressure- and modeless manufacturing process and therefore explains the higher surface roughness in comparison to IM (Figure 4). The resulting roughness of the surface of SLS is about 39% bigger than IM. Thereby, the limitation of detecting undercuts with the tactile measurement device has to be taken into consideration [13], which leads to a much bigger surface in reality.
Influence of the storage time and temperature for selective laser sintering (SLS) and injection molding (IM) on the surface roughness (left) and density (right).
The shown differences of the specimens after processing may result in three main specifics for the degradation processes. First, SLS possesses a larger surface due to the higher surface roughness, which therefore may enable the diffusion of thermo-oxidative degradation processes. The second factor is the amorphous outer section of IM, which also enables faster diffusion processes due to a higher free volume, but may lead to a post-condensation mechanism [7, 30]. A third factor might be the porosity, which can influence the diffusion mechanism depending on the gas inside and therefore the degradation mechanism.
Therefore, Figure 4 shows the density development for SLS and IM at 120°C and 140°C storage temperature. In general, the density of SLS is beneath IM, which is a result of the pressureless manufacturing process and therefore resulting porosity, which decreases the density. According to Schmid [6], incomplete coalescence during processing may lead to pore development. Additionally the heating rate, deposition parameters and mechanism mainly influence the density development [39]. In addition to the different densities, an increase develops over the storage time, which is somewhat stronger with a higher storage temperature. This results due to post-crystallization processes which lead to a denser crystalpacking therefore a higher density [7]. This effect is somewhat stronger for IM due to the higher amount of free volume in the peripheral amorphous section, which enables a denser packing due to the post-crystallization processes.
Figure 5 shows the changing Young’s modulus with time and temperature for SLS and IM. For both processing techniques, the modulus increases from the beginning of the conditioning. This behavior increases with higher temperatures for both techniques. In general, SLS shows a higher modulus and a minor increase independent from the conditioning parameter. Both show the highest modulus after conditioning at 140°C for 672 h. As Drummer et al. [4] showed, this behavior may be the result of physical ageing effects, by which morphological changes occur in dependence of the temperature.
Influence of the storage time and temperature on the Young’s modulus for selective laser sintering (SLS) (left) and injection molding (IM) (right).
The increase of the Young’s modulus for SLS and IM results in post-crystallization effects. Table 2 shows the different morphologic structures before and after temperature treatment. The difference in the reference specimens of SLS and IM may result from the morphological structure of the cross-section. A higher storage temperature enables faster post-crystallization and therefore increases the Young’s modulus due to a raised stiffness of the specimens. This effect is seen after a storage time of 672 h where for IM, a higher Young’s modulus for higher storage temperatures occurs. This behavior is reflected in the changes of the morphology in the amorphous outer sections of the IM samples, which enables faster post-crystallization processes [30]. As SLS shows a homogenous morphology across the specimen, IM has an amorphous peripheral area in which crystallization processes can be seen after storage at 140°C for 672 h.
The different crystallization behavior is shown in Figure 6, which reflects the higher degree of crystallization for SLS in a higher melting enthalpy during the first heating of the DSC measurement. Due to the higher degree of crystallization, more energy is needed to transfer the material into a molten state. Therefore, the post-crystallization shows a higher influence on IM due to the amorphous outer area, which encourages these effects and therefore increases the enthalpy for the stored specimens [22, 30]. In comparison, the IM specimens show a higher crystallization temperature which according to Ehrenstein et al. [26] may result from shorter chains resulting in thermo-oxidative effects. This is also shown in Figure 9 for the Vn. The chain shortening for IM may therefore result from the processing and the shear stresses during compounding and molding.
Influence of the processing technique and the storage at 140°C for 672 h on the crystallization temperature (left) and the melting enthalpy (right).
These physical changes may occur in an embrittlement, which is reflected in a decrease of the elongation at break (Figure 7). As shown, SLS specimens have a lower elongation at break, which results in addition to the porosity and surface roughness from the layerwise production. The connection area between layers enables the development of pores, which lead to failure under mechanical loads [6, 16]. Above a storage temperature of 100°C, the decrease of the elongation at break increases up to 140°C. In comparison, the elongation decreases for IM already at a storage temperature of 60°C and decreases with higher temperatures and longer storage times. The highest decrease results at 140°C and 672 h. The decrease in the elongation at break is reflected in a higher Young’s modulus. In general, it is shown that the elongation at break is very sensitive to temperature loads. The decrease is somewhat stronger than the decrease of the Young’s modulus and mainly results in thermo-oxidative degradation processes [7].
Influence of the storage time and temperature on the elongation at break for selective laser sintering (SLS) (left) and injection molding (IM) (right).
A further effect to reduce the elongation at break is the decrease of the Vn over time and temperature (Figure 9, left) which reflects the chain shortening through degradation processes. The high standard deviations for the elongation may result from the high surface roughness (Figure 4), which also can be influenced by the positioning during the building stage. Due to the inhomogeneity, different notch-effects can result during the mechanical testing load, which results in varying failures. This effect is influenced by the porosity, which reduces the bearing cross-section and therefore has a great influence on the resulting mechanical properties.
The higher decrease at enhanced temperatures shows due to a faster thermo-oxidative degradation process, which leads to higher diffusion speeds of oxygen and therefore an increased reaction speed and chain shortening. As known for IM [30], this leads to an embrittlement of the outer surface and therefore a reduction of the elongation at break.
The elongation reduction is also reflected in the tensile strength (Figure 8). At the beginning of the storage as well as at storage temperatures between 60°C and 100°C, no difference between the IM and SLS exists. At higher storage temperatures (120°C and 140°C), the tensile strength for SLS is strongly reduced. In comparison, a reduction for IM only occurs at a 140°C and storage duration of 672 h.
Influence of the storage time and temperature on the tensile strength for selective laser sintering (SLS) (left) and injection molding (IM) (right).
Like the elongation at break, the tensile strength also may be influenced by the increasing embrittlement resulting from the growing degradation. The decrease of Vn (Figure 9 left) between 72 h and 672 h correlates with the decreased lower tensile strength for 140°C. The strong decrease of SLS parts from around 50 N/mm2 to 25 N/mm2 is a critical parameter, as this is a main factor for the design and long-term usability. In comparison, the IM specimens only show a reduction for 140°C after 672 h.
Influence of the storage time and temperature for selective laser sintering (SLS) and injection molding (IM) on the viscosity number (Vn) across the specimens (left) and in the core and peripheral section (right).
The different mechanical behavior may occur from encouraged oxygen diffusion for SLS and therefore an increased material degradation. In Table 1, the discoloration after 672 h for 120°C and 140°C is shown along the cross-section of the specimens. In comparison, the SLS specimens show a more intense brown coloring in the outer sections, which indicates a progressed degradation mechanism also in the inner sections and partially correlates with the decrease of the mechanical properties. As the inner section shows a stronger degradation in comparison to IM, this can be a reference for a stronger thermo-oxidative degradation in the inner section and therefore an enabled diffusion of degradation effects. This can be enabled by the porosity as well as the differences in the degree of crystallinity.
The comparison of the Vn (Figure 9 left) across the cross-section shows decreased chain lengths for IM which may result from the shear stresses and high temperatures in the compounding and IM step during processing, which leads to chain shortening [7]. Correlating with the increase of the Young’s modulus (Figure 5), the Vn increases for SLS as well as IM up to 168 h, which is an indicator for post-crystallization processes and therefore leads to an embrittlement and a higher Young’s modulus. For increasing storage times, the thermal load predominates and the decrease of the Vn therefore shortens chains and strongly reduces the mechanical properties. Figure 9 right shows the Vn in the outer and inner section of SLS and IM before and after the storage at a 140°C for 672 h. As shown in [31], for polyamide 66 parts produced by IM, the Vn in the surface increases while the core of the specimens stays nearly constant, which results in limited diffusion of oxygen. Therefore, no difference is found after storage between SLS and IM for the outer and inner section. As the degradation processes are intruding from the surface, the outer sections show a lower chain length than the inner sections. For SLS, the Vn decreases on the outer and inner section. In comparison, IM contrary to [31], only shows a decrease in the inner section, which correlates with the development of Young’s modulus and tensile strength. The decrease of the Vn for SLS correlates with the reduction of the mechanical properties and therefore the stronger degradation in the inner sections of the test specimens. In comparison to IM, nearly the same Vn develops over time as SLS in the inner section and therefore leads to the conclusion that the degradation for SLS exhibits a faster diffusion.
5 Conclusion
The presented results exhibit differences in degradation behavior for SLS and IM. It is shown that due to the specific characteristics like the slow degradation processes, the degradation processes are somewhat stronger for SLS than for IM over the investigated storage temperatures and times.
As shown, a typical aging mechanism is the increase of the density, which is somewhat stronger for IM due to an amorphous peripheral area. This also leads to more post-crystallizations processes and therefore a more temperature-dependent behavior as higher temperature increase in the Young’s modulus for IM specimens. This is also reflected in the microscopy analysis where nearly no change is seen for SLS but a crystalline structure for IM after temperature treatment. These differences were also shown in the DSC measurements, which reflected the higher degree of crystallinity for SLS due a deeper crystallization temperature and a higher melting enthalpy in comparison to IM. These differences, resulting from the specific process management, lead to an earlier decrease of the tensile strength of the SLS specimens, which correlates with the decrease of the Vn. This leads to a stronger reduction of the chain length and a somewhat stronger discoloration across the specimens.
These differences lead to an earlier decrease of the tensile strength for SLS which correlated with the stronger decrease of the Vn and therefore a stronger reduction of the chain length. The stronger degradation was also shown in the discoloration effects, which leads to a stronger discoloration in the core and on the surface of the SLS specimens.
Acknowldegments
The IGF project “Resource Saving Small Series Production by Polymer Laser Sintering – Stabilization of the Long-Term Properties of Laser Sintered Parts (LZE-LS)” (17945 N) of the Institute of Energy and Environmental Technology e.V. (iUTA) has been supported by the German Federation of Industrial Research Associations (AiF) within the program to support the industrial collective research (IGF) of the Federal Government Department for Economics and Energy as a reason of the resolution of the Bundestag.
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