New thermoplastic powder for selective laser sintering
Dominik Rietzel Ernst Schmachtenberg et at |
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Contrary to classical plastic processing methods, the choice of materials for selective laser sintering (SLS) - and thereby also the range of possible application areas - is limited. Recent work has shown that plastics, which previously were unsuitable for SLS, can now be processed into powders for the fabrication of complex components. First trials with POM resulted in parts with appealing surfaces and good mechanical properties.
Additive processes such as selective laser sintering (SLS) permit layered buildup of three-dimensional components directly from CAD data. Contrary to conventional forming methods like injection molding, this toolless approach offers practically unlimited geometric freedom. Thanks to new developments, this technique has outgrown the established application area of prototyping and is now suitable for manufacturing small series of functional parts.
Although many different processes are available, their breakthrough into production applications has been prevented by material-related restrictions. As soon as specific demands are placed on components and their properties processors are frequently forced to compromises. However, for the manufacture of mechanically stressable prototypes in particular, selective laser sintering has become accepted.
 | Fig. 1. SEM image of the powders used.
Left: Commercially available PA 2200. Right:
Cryogenically ground. |
A prerequisite for SLS processing is that the material is available in the form of a fine powder. So far, partially crystalline PA12 was mainly used for mechanically highly stressed plastic components. Although other thermoplastic materials such as polystyrene (PS) or polycarbonate (PC) are available as powders, the results are highly porous components with poor mechanical properties. Such materials are typically used for manufacturing lost cores in precision casting. In addition, thermoplastic elastomers (TPE) are used in a limited number of applications.
Research work at the Institute of Polymer Technology (Lehrstuhl fŸr Kunststofftechnik, LKT) at the University of Erlangen-Nuremberg is focused on eliminating these material-related restrictions. As a number of requirements are placed on the material by the manufacturing process, material characterizations derived from knowledge of the interactive mechanisms are decisive for successful processing of new polymers.
 | Fig. 2. Assessment of the temperature
difference between melting and crystallization
-shown here as a DSC curve for PA12-permits
the process window to be defined. The two
peaks represent the limits of the permissible
construction volume temperature. |
Demands placed on plastic powders plus thermal constraints
The properties of laser-sintered components, e. g. density, surface structure are determined by the interaction of the process parameters with the material. Investigations have shown that particle geometry affects the componentÕ s surface roughness. Similarly, melt behavior and pourability of powders as well as their application determine the componentÕ s density. Size distribution and the geometry of PA12 particles have a great influence on the porosity of the sintered components, as these depend greatly on the packing density in the powder bed. As a result, higher bed packing density leads to improved strength, density, and dimensional accuracy of the sintered components. Commercially available, free-flowing laser sintering powders consist of spherical particles with a close size distribution of about 60mm (Fig. 1).
The utilized plastics must be specially stabilized against thermal decomposition, as they must be maintained close to the crystallization temperature for several hours. At the end of the manufacturing phase, the unmelted but thermally aged powder is separated from the component. Ideally, the powder should not be agglomerated after the buildup process, so that during removal it separates from the component simply through gravity.
Apart from the application of material, the properties of the finished components depend on precise temperature control during the buildup process. The temperatures are different from partially crystalline and amorphous thermoplastics. Amorphous plastics soften over a wide temperature range, whilst partially crystalline plastics have a far narrower melting range and their viscosity curve drops more sharply after the crystallite melting temperature is exceeded.
 | Fig. 3. Based on these measurements
(for POM, PA 2200, and PE), the energy
density required for complete fusion of a
layer can be calculated. Only the wavelength
range relevant for CO2 lasers is shown. |
An ideal buildup process reaches the exemplary state of quasi-isothermal laser sintering. The laser then only supplies the extra energy required to exceed the phase transition. Hereby, the temperature increase in the surrounding bed should be as low as possible.
This finding leads to a further material-related precondition: The recrystallization temperature of the plastic should be significantly lower than the crystallite melting temperature. Dynamic difference calorimetry (DSC) permits the difference between the two temperatures to be represented (Fig. 2). This is a characteristic of the SLS process window: If the upper temperature limit is exceeded, the powder melts uncontrollably; if the temperature falls below the lower limit, the previously generated polymer melt begins to crystallize, and the component shrinks (warpage due to internal stresses, 'curling' ).
Laser-induced phase conversion
CO2 lasers are mainly used for forming purposes, because many plastics exhibit a high absorbance in their wavelength range. According to the Lambert-Beer Law, the absorption coefficient in the IR range can be determined directly by measuring the extinction. The results are relative values, which depend greatly on temperature, particle geometry, and measurement method.
 | Fig. 4. Laser-sintered POM samples
exhibit higher strength with lower breaking
elongation than PA 2200. |
The examination of three exemplary materials reveals very different behavior regarding their absorption properties (Fig. 3). POM absorbs most of the laser energy. From this, widely varying material-specific processing parameters can be derived. Positioned between these two systems is the commercially available laser sintering powder PA 2200 with a medium absorption. Consequently, the interactions between geometric, rheological, and thermodynamic influences are relevant for the buildup process.
Laser Sintering of Powdered POM
The possibility of obtaining spherical particles directly from polymerization is not given for all plastics. Regarding process control and reproducibility, cryogenic grinding exhibits significant advantages. First investigations were carried out with various partially crystalline thermoplastics supplied in granular form by BASF AG, Ludwigshafen and so on Aim of the investigations was to obtain a high yield of particles with less than 100mm diameter with the most economic processing. A disadvantage of the grinding method is the demonstrable polymer degradation due to mechanical chain disintegration under impingement, which also occurs at low temperatures.
For numerous application areas, POM is an interesting material, as it exhibits very good mechanical properties, is physiologically harmless, and resistant to chemicals. Based on DSC and IR measurements, it was possible to estimate the process temperatures and energy densities required for laser sintering. Campus tensile rods were sintered as samples for the mechanical characterization using tensile tests (DIN EN ISO 527), and for the microscopic assessment of layer binding and morphology. Compared with injection-molded parts, components manufactured by means of an additive process generally exhibit less strength and higher brittleness.
 | Fig. 5. Morphology of tensile rods made
of PA 2200 (left) and POM (right), shown by
microtome sections in polarized light. |
As the melt temperature is maintained above the crystallization temperature for a long time during the sintering process, followed by slow cooling to room temperature, high crystallinity levels (CPOMmax ~ 85%) and therefore higher strengths are achieved, whereby breaking elongation is reduced (Fig. 4). Mobility of the molecular chain is ensured during a comparatively long time, so that highly crystalline structures can be grown. Transmission light microscopy images of microtome sections of tensile rods in polarized light show that POM exhibits less lamellar and more pronounced orderly spherulitic superlattices than PA 2200 (Fig. 5). The consistent crystalline structure through several layers confirms that quasi-isothermal conditions exist at least temporarily.
Transcrystalline areas are detectable in the upper boundary area of the POM tensile rods. These are fronts of oriented crystal growth, which occur typically at phase boundaries. The component's top surface is such a phase boundary, as powder layers are still added during the cooling phase following the buildup process. In case of process instabilities, transcrystalline fronts with different origins can also occur within the component. One out standing feature of laser-sintered POM components is their even and homogeneous surface.
Outlook
The first-time use of POM in a selective laser sintering process has shown that economical manufacturing of plastic powders from granulates with reproducible properties is possible. Because of the materialÕ s good chemical resistance, high dimensional accuracy ect it should be possible to significantly extend the product range of the process. Potential application areas for this new laser sintering powder are vehicle construction, household goods, and medical technology.
Present research work is focused on the optimization of particle geometry as well as modifications of the powder composition The modifications improve the powders' pourability as well as the density and functionality of the components.
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