Impact of sintering time and temperature on mechanical properties in projection sintering of Polyamide-12

Highlights

• Polyamide 12 (PA12) powder was sintered for varying times and temperatures using projection sintering.

• Strength increases up to ~50 MPa, but additional sintering does not increase strength.

• Elongation at break (>150%) increases with sintering time and is highest when sintering at 195 °C.

Abstract

In powder bed fusion additive manufacturing (AM), the fusing process is temperature and time dependent. However, little work has been done to understand how different processing temperatures and times might impact the mechanical properties at longer sintering times than are typical in laser sintering (LS) systems. Prior results with projection sintering have shown that heating for longer times (>1 s) improves part toughness compared to laser sintering. In this work, Large Area Projection Sintering (LAPS) is used to sinter entire layers of material simultaneously over the course of a few seconds with spatial control of layer temperature. This work evaluates the effect of time and temperature on the mechanical properties of parts sintered from PA 2202 (Polyamide 12) powder. Toughness is shown to increase significantly with longer sintering times (5–8 s) and higher temperatures (195–205 °C) with elongations at break (EaB) over 100% and strengths of up to 51.7 MPa. This represents a small increase in strength and an order of magnitude increase in elongation at break relative to LS datasheet values for the material. Peak mechanical properties are achieved sintering at lower temperature (195 °C) and longer times (5–8 s). The density of the samples that maximize toughness and strength is greater than 1.02 g/cm³ compared to datasheet values of 0.98 g/cm³. Porosity elimination could be a mechanism for improved performance though it does not fully explain the improvements in strength and ductility.

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:

$${\left(\frac{a}{R}\right)}^2=\frac{\gamma t}{R{\eta }_0}$$

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.