Research PaperImpact of sintering time and temperature on mechanical properties in projection sintering of Polyamide-12
Graphical Abstract
Introduction
Additive manufacturing (AM) is becoming a staple in the manufacturing industry because it offers benefits in geometry, lead time, and customization. Additionally, the material options are expanding and the properties of the parts are approaching the quality desired by design engineers. These improved properties have shifted the focus of AM to direct fabrication of parts for the end user. However, parts created by AM typically suffer from anisotropy [1], [2] and reduced mechanical properties when compared to well-established manufacturing processes, such as injection molding [3].
Laser sintering (LS) is an AM technology commonly used to create components for prototyping and end use. LS is a powder bed fusion technology, as defined by ASTM [4], that uses a high power laser which scans over the surface of a powder bed. LS scans the cross sections at high speeds (~1–10 m/s [5]), fusing powder particles as it scans. The material most commonly used in LS is Polyamide 12 (Nylon 12, PA12). PA 12 is an engineering thermoplastic with high strength, chemical resistance, abrasion resistance, and a moderately high melting temperature [6], [7]. Although LS provides good quality and resolution in PA12, the rapid scanning required for economical build times are known for producing parts with porosity [8] and unsintered particle cores which decreases the bulk strength and ductility [9], [10].
In LS, the laser scans over a point on the powder bed in less than a millisecond. The short exposure time requires rapid heating with significant local gradients across and through the layer. After the laser passes, cooling occurs. As such, a steady, uniform sintering temperature is never reached [11]. Small differences in parameters such as the delay between successive laser passes can impact the resulting mechanical properties [12]. Traditionally, these processes are monitored based on the energy input which has been shown to be a good predictor of density, fracture toughness and strength [13], [14], but ductility may be more dependent on the total temperature history [15]. While sintering is understood to be a time and temperature dependent process, these time-temperature effects have not been studied due to the difficulty of controlling the temperature when the heating process occurs as rapidly as observed in laser scanning. Variations of standard LS processing parameters have not significantly expanded the mechanical performance beyond datasheet values [16].
High speed sintering [17], [18] and multi jet fusion [19], [20] are alternative powder bed fusion processes that print a radiation absorber into the bed to selectively melt targeted regions when a heat lamp passes over. Heating is much slower than in LS (~1 s vs ~ 1 ms), but time/temperature tradeoffs are still difficult to study. Some limited control in local energy input is possible by varying printing saturation [21].
Large Area Projection Sintering (LAPS) is an alternative approach which sinters powder particles with significantly longer exposure times (~2–8 s in this work) compared to microsecond time scales with LS) [22], [23]. This is achieved by utilizing a high intensity projector which projects a thermal pattern on the surface, heating and fusing the entire cross section simultaneously. A new layer of powder is then deposited and spread by a blade or roller mechanism. In this work, a thermal camera has been integrated to provide closed loop feedback control, updating the image being projected to obtain a specified final temperature. The use of closed loop feedback control allows temperature targets to be set and maintained for a specified time and to correct for spatially varying thermal boundary conditions within or between layers that cause spatial temperature variations in traditional LS processes. A schematic of the system can be seen in Fig. 1. Additional process heat is provided from below via a heated build plate and above via a halogen heat lamp. Preheat temperature in the part region is controlled via feedback from the IR camera to improve the uniformity of the preheat temperature in the image region.
Prior results have shown that this approach produces comparable strength, but higher density and elongation at break than achieved with other powder bed fusion methods (laser sintering, Multi Jet fusion/high speed sintering) [22]. However, in the prior work, only one processing condition was tested and the heating temperature was not controlled. In this paper, the exposure time and temperature are varied and will be correlated with the resulting mechanical properties and part density through tensile testing and Archimedes density measurements respectively.
In commercial polymer powder bed fusion systems, the powdered feedstock is generally semi-crystalline and has both a glass transition temperature, and a melting temperature range over which the polymer melts. In these systems, the melting transition is relatively sharp with a decrease in viscosity of several orders of magnitude as the crystalline regions transition from a solid to a liquid over just a few degrees Celsius. Above this transition temperature, viscous sintering of the polymer proceeds rapidly. Viscous sintering is driven by minimization of surface energy through the reduction in surface area when there is sufficient liquid [24], [25].
An increase in either the temperature of the material (through a decrease in viscosity) or with an extended period of time at a specified temperature increases sintering. As the material flows, densification occurs and pores are eliminated [26]. The basic sintering model developed by Frenkel [27] and later corrected by Eshelby [26] to satisfy the continuity equation defined as:where a is the radius of the growing neck between two spherical particles of radius R, γ is the particle’s surface energy, η0 is the zero shear viscosity, and t is time. The amount of time required for a material to densify is based on its zero-shear viscosity as there are negligible external forces [28]. As higher temperatures are reached, lower viscosities can be achieved.
At extended time periods (often measured in minutes and hours [29]), the PA12 powder is known to post-condensate. This is an aging effect that increases the molecular weight of the polymer and viscosity [30]. However, in the time spans evaluated in this study and minimal temperature increase above the melting temperature this is assumed to be negligible. While the general impact of time and temperature on polymers below the melting point are well-documented, little work exists which directly studies time and temperature effects on sintering. To minimize the potential impact of powder property changes on the results, all tests were conducted with virgin PA12 powder.
The goal of this work is to correlate LAPS extended exposure times and the sintering temperature to part properties. This is of interest as the extended exposure times could allow more time for the polymer to melt and flow, leading to decreased porosity (increased density) and fully melted polymer particles. In this study, tensile test specimens were created under various time and temperature regimes to compare density and mechanical properties.
Section snippets
Materials
Each of the specimens was created from PA2202, which is a dark colored polyamide 12 powder (PA12, Nylon 12) produced by EOS for the LS industry [31]. This powder was utilized as it absorbs the visible light from the projector much better than white powders such as PA 2200 sold by EOS. To determine the preheat temperature and sintering temperature, a differential scanning calorimetry (DSC) test was conducted on virgin powder with a TA Instruments Q20 DSC system. The results (Fig. 3) show there
Methods
The LAPS system in this study has the capability of sintering entire layers with a single exposure from a high intensity projector [22]. The projector was modified to produce an intensity of 2.4 W/cm2 with a maximum image size of 2.1 cm by 1.6 cm. A FLIR A325sc long wave infrared thermal camera is mounted to the system to observe the sintering area. The thermal camera is used in a feedback control loop with the projector to maintain the set point temperatures and target a uniform temperature
Impact of target sintering temperature
This experiment consists of multiple temperature targets with a constant 2.5 s of exposure at varied target temperature settings (185, 190, 200, 205 and 215 °C). The results of the tensile testing can be seen below in Fig. 4 and tabulated with density measurements in Table 1.
The ultimate tensile strength (UTS), and elongation at break (EaB) are graphed as a function of the target temperature for a constant exposure time of 2.5 s in Fig. 5. The sample UTS appears to increase almost linearly with
Conclusion
It is well understood that sintering rates depend strongly on both time and temperature but in LS systems, the sintering temperature is highly transient and the heating time is constrained by the need to scan the laser rapidly. These highly transient conditions make direct application of sintering models to LS difficult. Further, only the input laser power and scan speed can be controlled in LS systems making experimentation difficult.
This work evaluated the effect of both sintering time and
CRediT authorship contribution statement
Justin Nussbaum: Investigation, Writing - original draft. Taranjot Kaur: Investigation, Writing - review & editing. Julie Harmon: Methodology, Validation, Resources, Writing - review & editing, Supervision Nathan B. Crane: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Funding Acquisition.
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: One of the authors of this paper (N. Crane) is a member of the editorial board of this journal. J. Nussbaum is owner and director of Ascend Manufacturing who is trying to commercialize products based on some of the technology used in this work. J. Nussbaum and N. Crane have pending patent applications related to the work described in this paper.
Acknowledgements
This work is supported in part by the National Science Foundation of the United States through award CMMI1851728.
Conflict of interest
One of the authors of this paper is a member of the editorial board of this journal. To avoid potential conflict of interest, the responsibility for the editorial and peer-review process of this article lies with the journal’s other editors. Furthermore, the authors of this paper were removed from the peer review process and had no, and will have no access to confidential information
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