Freeman Tech
As 3D printing becomes more established for finished part production, the demand for new feed materials intensifies.
Meeting the performance profile for demanding aerospace, automotive, and medical applications calls for the ability to tailor properties such as weight, strength, flexibility, heat resistance, colour and biocompatibility.
Polymers are often the answer with significant untapped potential for further development, particularly when fillers are factored into the material design equation for added functionalities. This raises the critical question of how best to characterise new candidate materials. What can we measure to predict printing performance and by extension the quality of the finished component? To what extent can testing, as opposed to a trial, answer the question ‘Is this a good material for printing?’
Examination of the technology used for printing polymers provides context for answering this question. In 3D printers, polymers are used as filaments, deposited as droplets, bonded together in the form of laminated sheets or selectively cured from a bulk liquid phase. Here though the focus is on characterising materials for powder-based processes. Such processes include binder jetting (BJ), in which successive layers of powder are selectively joined by dropping binder, effectively a glue, into the powder bed, and powder bed fusion (PBF) which involves the selective melting of powder layers with a laser. A defining feature of these processes is a requirement to rapidly and efficiently spread powder in layers around 100 microns thick.
For commercial applications, PBF is the more common approach for polymers with a wider range of printers and materials to choose from. This is despite the relatively high build speeds associated with BJ and its potential for full colour printing, as commercially realised by printers such as the 3D Systems ProJet CJP x60 range. Polymers for BJ tend to be acrylate-based, with coloured printing achieved using coloured binders on white powders. For PBF, polyamides (including nylon) are a popular option; alternatives include polystyrene, polypropylene and glass-, carbon- and aluminium-filled materials. The introduction of the EOS P 800, the first commercial printer for high temperature laser sintering (HT-LS) has extended the range of polymers for which printing is feasible. New introductions include a polyetherketone (PEK) specifically for high temperature printing commercially known as EOS PEEK HP3, that competes with metals in terms of the properties of finished components.
When it comes to assessing the physical characteristics of new powders then it is the defining requirement for rapid, consistent flow and layer formation that is crucial. Powder must discharge freely and reliably from the feed hopper and then settle into a uniform, densely packed layer. Flowability and the speed at which powder layers can be spread has a direct impact on build rate, while packing behaviour influences the properties of the finished component. Powders with low bulk density that give rise to layers with substantial voidage are undesirable from the perspective of heat transfer during fusing (PBF) and the properties of the final product. And it is important to look beyond the flow properties of virgin material. Determining the impact of passage through the printer and, in the case of PBF, any localised heat-related degradation of loose powder close to the path of the laser is essential. An effective recycling and blending strategy for polymer powder not incorporated into a finished component is vital for economic production.
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Freeman Tech
Differentiating a powder that can perform efficiently in the build chamber from one that will not, calls for powder testing technology that can reliably characterise flow with a high level of sensitivity. The challenge of quantifying powder flowability is shared with a number of industries and it is helpful to examine a standard method for powder testing, used routinely by the AM industry to assess metal powders, to illustrate the issues that can compromise measurement. The Hall Flow test (ASTM B213) involves measuring the time taken for a known mass of powder to flow through an orifice of defined diameter. It is a simple, manual, intuitive test and relatively quick to carry out but has some important limitations.
Since powder is simply poured into the discharge funnel by the operator, prior to testing, there is no real control over the state in which the powder is tested. For example, powders entrain and release air to varying degrees, and the presence of air directly impacts flowability. An aerated sample extracted from a process can release air on standing and consequently exhibit different flow properties depending on the length of time between sampling and testing. With the Hall Flow test, and other simple methods, the absence of any sample preparation, along with the manual nature of measurement tend to result in relatively low repeatability and reproducibility, compromising the ability of the test to differentiate closely similar samples. Furthermore, some powders simply fail to flow at all, producing a somewhat uninformative ‘null’ result. All samples that fail to flow are classified as identical though they may, in fact exhibit quite different levels of cohesivity.
Dynamic powder testing with a powder rheometer is a more sophisticated approach with a track record of relevance for 3D printing. Dynamic properties are evaluated by measuring the axial and rotational forces acting on a blade as it rotates through a powder sample.
Figure 1: With dynamic testing, powders can be measured precisely, under highly relevant conditions to directly assess their compatibility for 3D printing processes.
Dynamic testing provides a far more complete assessment of powder behaviour than can be obtained from a single number test. For example, a downward traverse of the blade pushes the powder against the confining base of the test vessel, generating the metric Basic Flowability Energy (BFE). This quantifies how the powder will flow when in a low stress state and subject to forcing flow conditions. BFE is a particularly sensitive metric and can often identify differences that impact a process, that other powder testing methods fail to detect.
Repeat BFE measurements quantify the physical stability of a powder via a Stability Index (SI), indicating susceptibility to change as a result of the repeat application of shear. BFE can also be measured at different blade speeds to determine flow rate sensitivity (Flow Rate Index (FRI)), which indicates whether the powder flows more easily when subject to higher shear rates. The action of the blade can also be reversed to exert a gentle, lifting action on the powder via an upward traverse, generating Specific Energy (SE) values that quantify unconfined flow properties in the low stress state, how a powder will behave when subject to gravity.
A key benefit of dynamic testing relative to other techniques it that it has been successfully used to differentiate powders that print well from those that don’t. For example, ExOne a global leader in BJ technology, uses dynamic testing to assess the printability of any new metal powder used, including alternative supplies or customer samples. The company’s specification for powders that perform well over multiple print cycles includes values for dynamic (SI and FRI), shear and bulk powder properties, all of which are measured with the FT4 Powder Rheometer®. Measuring these properties along with particle size and shape data provides the company with a robust protocol for differentiating poorly performing materials, significantly enhancing operational efficiency.
Researchers at the University of Exeter and the Centre for Additive Layer Manufacturing (CALM) recently used dynamic testing (FT4 Powder Rheometer, Freeman Technology) to evaluate the properties of novel filled polymers made specifically for 3D printing, polyetheretherketone (PEEK) nanocomposite powders filled with either C-coated Inorganic Fullerene-like WS2 nanoparticles (IF-WS2) or graphene nanoplatelets (GNP). Figure 2 shows a selection of the results obtained including comparative data for a commercial PEK product, PEK HP3 (commercially known as EOS PEEK HP3).
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Figure 2: BFE (a), Stability (b) and flow rate (c) test data highlight the WS2 filled PEEK composites as being closely similar to the commercial PEK HP3 product.
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Figure 2: BFE (a), Stability (b) and flow rate (c) test data highlight the WS2 filled PEEK composites as being closely similar to the commercial PEK HP3 product.
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Figure 2: BFE (a), Stability (b) and flow rate (c) test data highlight the WS2 filled PEEK composites as being closely similar to the commercial PEK HP3 product.
Pushing for performance in polymer powders
The BFE values of the WS2-based composites are closest to that of the PEK HP3 with the 1wt% WS2 exhibiting the highest value. Particle packing has a marked impact on BFE with efficiently packed particles presenting considerable resistance to flow. High BFE values, perhaps counterintuitively, therefore tend to be associated with good printing performance. These data identify the WS2-based composites, especially the 1% material, as the preferred choice in terms of printing performance.
SE values for all the materials (data not shown) are comparable indicating similar levels of performance in terms of unconfined flow behaviour, which is governed by mechanical interlocking and friction between the particles.
SI values lying predominantly in the range 0.9 to 1.1 indicate that all these powders are physically stable. There is no evidence of changes in flow behaviour as a result of, for example, agglomeration or attrition, electrostatic build-up or the uptake of moisture. FRI values for the majority of the composites are lower than for the commercial products, a beneficial effect since lower FRI values tend to be associated with lower interparticle cohesion. Low cohesion would be expected to improve performance in an HT-LS process and may be attributed to the lubricating effect of the partially exposed nanofillers, particularly at lower concentrations.
Figure 3: SEM images highlight a direct correlation between BFE data and the homogeneity/quality of printed composite samples (triple laser exposure; a) 1wt%GNP b) 5wt% GNP c) 1wt% WS2 d) 5wt% WS2).
These results demonstrate the comprehensive insight provided by dynamic testing and its ability to identify new materials as closely similar, or otherwise, to commercial materials with proven print performance. To confirm correlation between these data and printing performance, samples were printed with each of the composites using HT-LS (single powder layer). The WS2 composites (figure 3c and d), with higher BFE values, produce better quality samples than the GNP analogues (figure 3a and b). The 5wt%GNP composite, which has the lowest BFE, produced a sample with a rough, flaky surface while the1% WS2 composite, the novel material identified as having superior powder rheology produced a sample with a notably smoother surface than all of the others.
Those working at the forefront of powder printing are increasingly recognising the importance of flow properties and their critical role in defining material specifications for additive manufacturing. Technology that can precisely and relevantly quantify flowability has an important role to play in the development of new materials for the effective exploitation of 3D printing in an increasingly broad array of applications.
Jamie Clayton is the Operations Director at Freeman Technology and Professor Oana Ghita is part of the University of Exeter's College of Engineering, Maths and Physical Sciences.
