1. Introduction
Industry increasingly adopts additive manufacturing (AM) as a way to produce prototypes and small series of end-use parts in a fast, reliable, and cost-effective manner. Multiple studies [
1,
2,
3] indicate that amongst the AM technologies that use polymers as a feedstock, laser sintering offers high levels of design freedom and productivity. The limitations of the laser sintering process are the high equipment investment costs, and the limited number of usable materials.
Laser sintering is a powder-based process wherein parts are created layer-by-layer. A roller or blade first deposits a thin powder layer of typically 100–150 µm thickness. The cross-sections of parts present in this layer are molten by scanning the surface with a CO2 laser. As a result, the powder particles fuse together with the underlying layer and with each other. Once scanning has finished, the build platform lowers by a single layer thickness, and the process is repeated. In this way, three-dimensional objects are created.
A powder material must fulfill a number of requirements in order to be suitable for laser sintering, as summarized by Schmid [
4]. Materials must be carefully designed, both on an intrinsic polymer level, and on an extrinsic powder level. The most important requirements are summarized in
Figure 1.
The intrinsic polymer properties in the bottom section of the diagram in
Figure 1 can be established by choice of the material grade, or better, by specific design of the polymer chains and architecture via chemistry. The extrinsic properties in the top part of the diagram are those associated with the shape, size, and mechanics of the powder particles the material consists of. Tuning and optimization of these properties is done by selection of an appropriate powder production technology and subsequent classification methods.
Because of these many requirements, there are only a handful materials on the market that can be used with the laser sintering process. Market studies [
6] show that the largest portion of parts built with laser sintering is based on polyamide (PA), but that there is a strong desire for parts made from different materials. One of these materials is polybutylene terephthalate (PBT). PBT has a good chemical resistance and is characterized by a high insulation resistance and dielectric strength. This makes it the material of choice for housings for electrical appliances and various under-the-hood car parts. The ability to produce such parts with laser sintering, thereby making use of the full design freedom the process offers, opens up many possibilities. Examples are weight reduction, downsizing, and consolidation of many parts into a single laser sintered one.
Various authors described the production and use of PBT powders for laser sintering. Investigating five different PBT grades, Wegner et al. [
7] reported the production of powders by cryogenic grinding, and their subsequent processing on an SLS machine. The parts exhibited a relatively high Young’s modulus. However, tensile strength and elongation at break compared to injection-molded parts were limited, signaling a need for further improvement.
Schmidt et al. [
8,
9,
10,
11] produced powders by wet grinding PBT granules, then running the milled powder through a so-called downer reactor to obtain spherical particles. Parts consisting only of a single layer could be produced with laser sintering, but no mechanical properties were tested. The performance of dry blends of PA12 mixed with up to 30% PBT was investigated by Salmoria et al. [
12]. The authors reported a minimal increase of flexural modulus upon blending PA12 with 10% PBT. The blends were otherwise featured by phase separation and inferior mechanical properties compared to the PA12 base component.
The mechanical properties and crystalline structure of laser-sintered parts made from a cryogenically milled PBT copolymer powder were reported by Arai et al. [
13]. The copolymer, which contained 10 mol % isophthalic acid as a comonomer, was processed on a laser sintering machine at 190 °C. The produced parts exhibited only a slightly reduced strength compared to their injection-molded counterparts, but a drastically reduced elongation at break. In a subsequent study [
14], the same authors investigated the behavior of PBT powder dry blended with short glass fibers.
Most of the work up to this point was carried out on PBT produced by grinding granules into powder, typically resulting in irregularly shaped particles that give a powder suboptimal flowability and packing density. In this article, melt emulsification is used as an alternative method to directly create PBT powder consisting of spherical particles.
Various researchers showed the advantages of using powders with spherical particles in laser sintering. Berretta et al. [
15,
16] characterized different powders with respect to size distribution, and particularly particle shape. The flowability as expressed by measurement of the angle of repose was found to correlate with the circularity and roundness of powder particles. Verbelen [
17] provided screening methodologies for laser sintering powders, which particularly included assessment of the powder flowability. Van den Eynde [
18], following this methodology, extensively characterized the relations between size, shape, flowability, and performance of a wide range of materials. It was found that in general, a smooth and spherical particle shape contributes to the laser sintering material performance. In a comparison of two laser-sinterable PA12 powders, Schmid et al. [
19] found increased tensile strength and elongation at break for the powder with the higher sphericity. This was attributed at least partially to the improved packing density achieved with spherical particles. Quantitative results relating a more spherical particle shape to improved flowability and laser sintering performance were presented by Amado [
20] and Vetterli et al. [
21].
The term melt emulsification refers to the process of melt blending two incompatible materials to create a structure consisting of droplets of one component embedded in the matrix of the other component. The melt emulsification method is well-established and reviewed [
22] in the pharmaceutical domain to produce dispersions of active components with spherical morphology in a polymer matrix. For the production of laser sintering powders, the method is less well documented. Drummer et al. [
23] reported the production of spherical PA12 particles by melt blending with polyvinyl alcohol (PVA). The authors also showed some preliminary results on blends of PBT and polyethylene glycol (PEG). The production of spherical polypropylene particles via melt blending with hexadecane was described by Fanselow et al. [
24,
25], however no parts were produced with the obtained powder.
The current work describes the production of a PBT powder consisting of spherical particles by melt emulsification with PEG as the matrix phase. The advantage of this process route is the absence of organic solvents and the requirement of only elementary polymer processing equipment; as will be demonstrated, it is already possible to produce reasonable amounts of powder that allow for laser sintering trials using a conventional laboratory-scale single-screw extruder.
In an emulsion, the equilibrium between break-up and coalescence governs the mean droplet size. For a system in shear, the droplet break-up depends on the shear rate and relative viscosity of the two phases. The coalescence is mainly governed by interfacial tension, which determines the probability of recombination of two colliding droplets. A system consisting of non-Newtonian materials flowing in a non-uniform shear field across a wide temperature range, as is the case in a typical extruder, cannot be described by simple models. Attempts have been made to describe the droplet size as a function of material and process parameters, as has been reviewed by multiple authors [
26,
27]. These models however typically have an empirical basis and hold only for the particular investigated system.
As a first step in the current investigation, the extruder rotational speed and nozzle were varied, and the resulting particle size was evaluated. The influence of blend ratio PBT/PEG on the particle size was investigated as well. An ideal and aimed for size range for typical laser sintering powders lies between 10–100 µm [
19]. After optimization of extruder settings and blend ratio, the powder was extracted from the extrudate through a series of washing, drying, and classification steps. These steps are summarized visually in
Figure 2. The particle size and shape distribution, thermal properties, and flowability properties of the resulting powder were analyzed. The material was then tested on a commercial laser sintering machine with specifically designed inserts to allow for processing with small amounts of powder. Finally, the mechanical properties of the built parts were evaluated and compared to those of compression and injection molded samples made from the same PBT grade.
2. Materials and Methods
Materials: PEG Polyglykol 35000 S (Clariant, Muttenz, Switzerland) flakes were ordered and used without further purification or drying. PBT TORAYCON 1200M (TORAY, Tokyo, Japan) was purchased in granulate form. The material did not contain any stabilizers or other additives so as to prevent these from negatively influencing the laser sintering process. The material was dried before use. A second reference PBT CCP PBT 1100-S600 (Chang Chung Plastics, Taipei, Taiwan) granulate was acquired as well. This material contained stabilizers and process aids. The material was dried before use. Fumed silica flow aid Aerosil R812 (Evonik, Essen, Germany) used without further alteration.
Extrusion: the materials were mixed in a laboratory scale Extrusiograph (Brabender Technologie, Duisburg, Germany) single-screw extruder with a barrel temperature profile of 230-240-250-250 °C going from inlet to nozzle. The screw had a 19 mm base diameter and L/D ratio of 25.7. A wide-slit nozzle was used to produce the bulk of the material, which was collected and passively cooled down against ambient air on a steel sheet.
Powder extraction: the slabs obtained from extrusion were broken into small pieces and divided into batches of 2.5 kg each. To each batch, 10 L water was added and stirred in a container inside a concrete mixer for two hours. The mixture was then allowed to settle for 12 hours, after which the watery phase was removed. Another 10 L fresh water was added to the remaining slurry, and the process was repeated. The complete washing cycle was carried out three times for each batch. Removal of the PEG phase was confirmed by DSC measurement.
Powder classification and drying: the fine fraction of the powder, consisting of particles smaller than 10 µm, was removed using a specially designed settling tank. Depending on the flow rate of a dilute dispersion of powder in water through the settling tank, particles of certain size settle to its bottom. Small enough particles with reduced settling velocity do not reach the bottom of the tank before the water reaches the outlet, and are washed out in this way. In a subsequent step, the slurry at the bottom of the tank was collected, and the majority of water was removed by vacuum filtration over a Büchner funnel. The final traces of water were removed by drying the material for 24 hours at 80 °C in a vacuum drying oven. The powder was then sieved over a sieve with a 150 µm mesh width. The powder was dry blended with 0.05 wt. % fumed silica as flowing aid.
Scanning electron microscopy: scanning electron microscopy images were obtained on a JSM 7100F scanning electron microscope (JEOL, Tokyo, Japan). Samples were deposited on a piece of carbon tape on an aluminium SEM stub and sputter coated with a 10 nm layer of Pt/Pd (80/20).
Optical microscopy: to assess the morphology and microstructure of built parts, a sample was fractured and embedded in resin. The sample was sanded, polished, and observed using a DM6 optical microscope (Leica Microsystems, Wetzlar, Germany) with incident light and differential interference contrast.
Particle size distribution: the particle size distribution was measured by dynamic light scattering on an LS230 (Beckman Coulter, Brea, CA, United States) instrument. Approximately 0.1 g of powder was added to approximately 20 ml demineralized water and thoroughly stirred. Before measurement, the dispersion was treated with ultrasound to break apart any agglomerates. The particle size was calculated based on the Fraunhofer model, for the range between 0.4–2000 µm.
Particle size and shape distribution: the size and shape of particles was recorded optically with a DM6 optical microscope (Leica Microsystems, Wetzlar, Germany) at 100x magnification in transmission. On the basis of 216 images, 51862 particles were counted and evaluated using an in-house developed imageJ script and MATLAB evaluation procedure, described in [
28].
Powder flowability: characterization of the powder flowability was carried out on a Revolution Powder Analyzer (RPA) (Mercury Scientific, Newtown, CT, United States). An exact measure of 25 mL powder at tapped density was added to a rotating drum with a diameter of 50 mm. The drum was rotated with a speed of 0.6 rpm, while a camera recorded 384 avalanche events. Each time directly following an avalanche, the avalanche angle and roughness of the powder surface were evaluated.
Thermal analysis: differential Scanning Calorimetry (DSC) measurements were carried out on a DSC 25 (TA Instruments, New Castle, DE, United States). All measurements were conducted under a nitrogen atmosphere, from 25 °C to 250 °C, with heating and cooling rates of 10 °C/min.
Rheology: a RG20 high pressure capillary rheometer (Göttfert, Buchen, Germany) was used to assess rheological properties. Measurements were carried out at 250 °C with a nozzle with a 1 mm diameter and 20 mm length. Both channels were used simultaneously for measurement, only relative results are provided.
Mechanical tests: mechanical tests were performed on a Z100 universal testing machine (Zwick Roell, Ulm, Germany). The tests were performed according to the ISO 527-1 standard, with testing geometries of type ISO 527-2A-1BA-25.
Laser Sintering: laser sintering trials were conducted on a Sinterstation 2000 commercial laser sintering machine (DTM, Austin, TX, United States) equipped with inserts on both sides of the central powder bed to allow for the processing of reduced amount of material. The machine disposes of a 50 W CO
2 laser operating at a wavelength of 10.6 µm. New layers were deposited on the powder bed by means of a counter-rotating roller. The process parameters are listed in
Table 1 below.
4. Conclusions
The melt emulsification method is exceptionally suited to make powders with spherical geometry in the size range that is required for laser sintering. Compared to conventional laser sintering powders, the powder that was fabricated in this study showed outstanding flowability and packing density properties. The measured properties could be confirmed through the application of the powder in a commercial laser sintering system.
As a powder production technology for polyesters in general and PBT in particular, melt emulsification however presents critical limitations. It could be demonstrated that the polymer degraded considerably over the course of the powder production process. This initially translated to a narrower process window. For parts that could be built, inferior mechanical properties were found for both laser sintered and compression molded parts made from powder, compared to injection-molded parts made from the starting granulate.
The initial melt emulsification process step in particular leads to degradation, first of all because a PBT grade without heat and hydrolysis stabilization was used in this study. A similar PBT grade with additives showed comparable degradation effects, which may be traced back to the water content in the matrix phase and washing out of stabilizers. The selection of a different water-soluble matrix polymer such as polyvinyl alcohol (PVA) or polyvinylpyrrolidone (PVP) could help reducing hydrolysis during extrusion. Enhanced drying, for example in a circulating hot air dryer would also lead to an improvement. Finally, the addition of more or, other additives can reduce thermal degradation and hydrolysis during all process steps.
This study could validate the viability of the melt emulsification process as a means to produce laser sintering powders with outstanding particle shape, size and flowability. The process is scalable and can be extended to other material systems to allow the production of a wide spectrum of laser sintering powders. The economically viable production of powders on an industrial scale can be realized by combination of individual process steps. Extrusion, washing, and sedimentation can be performed in a single continuous step for example. By such optimizations, the method need not be much more expensive than other powder production processes, such as cryogenic milling. Possible additional costs for producing the powder with melt emulsification can be offset by the benefit of obtaining powders with spherical particles.