INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 18, No. 9, pp. 1275-1280

REGULAR PAPER

SEPTEMBER 2017 / 1275 DOI: 10.1007/s12541-017-0150-0 ISSN 2234-7593 (Print) / 2005-4602 (Online)

Effect of Humidity Changes on Dimensional Stability of 3D Printed Parts by Selective Laser Sintering Daeil Kwon1, Eunju Park1, Sangho Ha1, and Namhun Kim1,# 1 Department of System Design and Control Engineering, Ulsan National Institute of Science and Technology, 50, UNIST-gil, Eonyang-eup, Ulju-gun, Ulsan, 44919, South Korea # Corresponding Author / E-mail: [email protected], TEL: +82-52-217-2715, FAX: +82-52-217-2439 KEYWORDS: Selective laser sintering, Additive manufacturing, Dimensional stability, PA12

Across many industries, additive manufacturing, also known as 3D printing, is being recognized more and more as an innovative manufacturing technology for the next generation. The automotive industry, especially, has paid huge attention to additive manufacturing, and actively applies it to their design, research, development and manufacturing processes. Selective laser sintering (SLS) in additive manufacturing is emerging and popular, and is often used to produce jigs and fixtures for cost benefits and efficiency in the manufacturing industry. However, the application of additive manufacturing is still limited due to the lack of dimensional stability of 3D printed parts associated with changes in temperature and humidity. This paper examines the effect of humidity changes on the dimensional stability of 3D printed parts made by the SLS process. The test results demonstrated changes in relative humidity may result in design specification violation in terms of dimensional requirement while in use. Manuscript received: August 8, 2016 / Revised: November 6, 2016 / Accepted: May 12, 2017

1. Introduction

controlled laser scanning selectively fuses the powdered material according to the part design, which is referred to as partial melting. After finishing laser scans at the given layer, the workspace is lowered by a predetermined increment, and another layer of powdered material is spread over the previously sintered layer. This process repeats until all the layers are sintered. The powders that have not been sintered remain in place, supporting the neighboring layers. Once the printed part and the surrounding materials cool down below the glass transition and the oxidation temperatures, the 3D printed part can be obtained by brushing off the non-sintered powders. For the production of parts, the SLS process can take a variety of material types, including polyamide, thermoplastics, waxes, metal powder, ceramic powder and composite powder, which is considered to Among many be a competitive advantage over other processes. material types, polyamide is often used due to its good mechanical properties when produced by the SLS process. Also, SLS does not require additional supporting structures during the printing process unlike existing conventional manufacturing processes. Non-sintered particles can be used towards the next build for economical parts productions. On the other hand, some disadvantages of SLS include the limited part dimensions restricted by the workspace size, powder cleaning, and possible deformation. Residual stress, due to the temperature difference between the production and the use, is also of 3,6

Additive manufacturing, has recently emerged as a promising technology in biomedical science, automotive, manufacturing, defense, architecture, and many other industry fields. Recent development in biomedical industry has demonstrated stem-cells and human ears can be made using 3D printing technology. The automotive industry has been using 3D printed parts to produce prototypes for quick design evaluation, as well as to manufacture replaceable components, such as cooling ducts, intake manifolds, cylinder heads and cover flaps. In the manufacturing industry, Local Motors introduced the world’s first 3D printed car on the floor of the International Manufacturing Technology Show in 2014. Depending on application areas and operating conditions, there are a variety of 3D printing processes, including fused filament fabrication (FFF), stereo lithography apparatus (SLA), digital light processing (DLP), and selective laser sintering (SLS). Among these, SLS is a common and popular process. SLS utilizes powder bed fusion technology, which fuses powdered materials selectively over each successive layer. A schematic representation of the SLS process is illustrated in Fig. 1. The chamber of the SLS machine is pre-heated near the melting temperature of the material. The powdered material is then loaded into the chamber, in which a thin layer of the material is spread over a workspace. Computer1,2

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Fig. 1 The schematic of SLS process

concern associated with the dimensional stability of 3D printed parts. Shen et al. reported the inhomogeneity of the build rate and the workspace temperature distribution resulted in the shape deformation of 3D printed parts. Clare et al. studied the effect of heat conditions on the shape deformation of 3D printed parts by analyzing residual stress using finite element analysis. Regarding the mechanical property of material, Meredith, et al. reported the stress relaxation of nylon-based materials exposed to extra moisture. They claimed the addition of water molecules to non-crystalline regions of nylon broke the hydrogen bond between the chain molecules, and thus the material stretched. Quistwater et al. found the dynamic modulus of nylon fiber was reduced with breaking inter-chain hydrogen bond by water absorption. While many research papers investigated the characteristics of printing materials, process parameters, or as-produced 3D printed parts, there is a limited number of studies associated with examining the dimensional stability of 3D printed parts in use conditions. In field application, it is often the case that changes in environmental conditions, such as temperature and humidity, adversely affects the dimensional stability, and might result in unexpected shape deformation while in use. This paper examines the dimensional stability of 3D printed parts by SLS, particularly focusing on the effect of humidity on shape deformation. This paper is organized as follows. Section 2 introduces the material characteristics of 3D printed parts through the SLS process. Section 3 presents the experiments to examine the effect of humidity changes on 3D printed parts. Section 4 discusses the implication of experimental results. Section 5 summarizes the finding and presents the conclusions of this research along with a recommendation for future works. 11

Fig. 2 DSC curve for PA12 powder

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2. 3D Printed Parts using SLS Process SLS process parameters are dependent upon the thermal properties of the powdered material, such as melting and recrystallization temperatures. A Differential Scanning Calorimeter (DSC) was used to identify the thermal properties of polyamide 12 (PA12) powder, which is a common material for the SLS process. Fig. 2 shows heating and cooling cycles of the material. A melt peak was observed to be 186.5°C indicating PA12 underwent an endothermic phase transition from solid to liquid, and a recrystallization peak was observed to be 152.4°C indicating PA12 underwent an exothermic phase transition from liquid to solid. The range between the melt and the recrystallization peaks was

Fig. 3 PA12 powder morphology with multiple magnifications

determined to be the processing window of PA12 powder for the SLS process. Fig. 3 shows the morphology of PA12 powder with 500 times and about 3000 times magnifications using a scanning electron microscope (SEM). The powder particles showed various shapes, but were mostly in potato-like shapes. The longest diameter was observed to be less than 100 µm, which affects the layer thickness of a 3D printed part, as well as the workspace increment during the printing process. For part characterization, 3D printed samples were produced using PA12 powder by an sPro60 SD printer from 3D Systems. A laser power of 9 W and a layer thickness of 100 µm were used after considering the material characteristics. Based on the DSC characterization results, the

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Fig. 6 RH cycle profile Fig. 4 Dimensions for each direction of the SLS part spherical particles were found on the sintered surface, indicating that the powder particles had not been completely melted. Porous microstructure could have affected by powder characteristics, as well as SLS process parameters, such as powder particle size, chamber temperature, and laser power. Less porous microstructure can be obtained with powder materials with smaller particle size due to higher interfacial diffusion per unit volume. Also, the chamber temperature and the processing time during melting and recrystallizing needs to be high or long enough for gases generated during the sintering process to Laser power, scan speed, distance, escape out of the molten polymer. as well as layer thickness are the parameters that affect the porosity variation of the sample part. Due to the porous structure of the sample part, moisture absorption can result in part deformation through direct contact with water or exposure to moisture. Many researchers have also reported the deformation of 3D printed parts, and the loss of mechanical strength In the following section, this paper due to moisture absorption. discusses an experimental study to examine the dimensional stability of 3D printed parts under changes in the humidity of the environmental conditions. 15,16

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3. Experiment: Effect of Humidity on 3D Printed parts

Fig. 5 Microstructure of a sintered sample (a) at the surface and (b) in the middle

powder bed temperature was set to be 160°C in order to prevent localized solidification shrinkage and part distortion. There were three samples designed in a simple hexahedron shape with 60 mm × 30 mm × 30 mm for each x, y and z direction respectively, as shown in Fig. 4. Multiple rectangular bumps were engraved on each surface as reference points for dimension measurement. Fig. 5 shows the morphology of sintered PA12 powder taken from a surface of the part and in the middle of the sample part. The morphology in the middle was obtained by breaking the sample in half and observing the interface. It was observed that more powder particles at the surface were fused compared to the ones in the middle. The morphology in the middle was more porous than that at the surface. Moreover, some

Humidity cycling was conducted in order to examine the effect of humidity on the dimensional stability of 3D printed parts. According to ASTM-D6207, the highest and the lowest humidity levels were 95 ± 5% RH and 15 ± 5% RH, respectively. The test conditions in this experiment were determined to cycle between 20% and 90% RH. The ramp up and down time was determined to be 1 hour considering the control limit of the environmental chamber, ESPEC ARS-0390, used in this experiment. The dwell time at each extreme RH level was 12 hours, which was long enough to saturate and to dry the samples. A complete humidity cycling profile is shown in Fig. 6. Strain gages were attached to both the top and bottom surface of the samples to track the part deformation during humidity cycling as shown in Fig. 7. The strain values were recorded every minute using instrumental control software during humidity cycling. Five humidity cycles were performed with in-situ monitoring of strain values. Fig. 8 shows the strain history of the samples on the primary axis, and the humidity levels on the secondary axis during a humidity cycle. The strain values were observed to follow the humidity level changes with positive correlation during every humidity cycle. Although there was

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Fig. 7 Samples with strain gauges

Fig. 9 RH vs. strain hysteresis loop during humidity cycling

Fig. 8 Strain history during a humidity cycle (a) at the top surface and (b) at the bottom side

sample to sample variation, the strain history of each cycle was almost identical to each other except that of the first cycle, which was considered as a stabilization period for both the environmental chamber and the sample. Thus, the strain history from the first cycle was excluded for the following analysis. Fig. 9 shows hysteresis loops of the top and bottom surface strain values from sample 1. Changes in the RH and the strain value were consistent over multiple humidity cycles, indicating the deformation was in the elastic regime. It was also observed that the top surface expanded and shrank more than the bottom surface due to the proximity of the top surface to the moisture outlet of the environmental chamber. The highest and the lowest strain values were recorded to be 0.08% and -0.13%, respectively. Considering that RH levels of the test conditions are commonly observed levels in a typical use condition, these experimental results imply the shape of a 3D printed part may not be consistent in field applications.

4. Discussion In the automotive industry, additive manufacturing technology is popularly used to produce parts, components, and assemblies, such as head lamp assemblies, dash board parts, and door trim parts, using SLS. Also, the use of additive manufacturing is drawing more attention in the automotive aftermarket due to its unlimited design flexibility. For

Fig. 10 RH distribution in 2015

the practical and feasible use of additive manufacturing in the automotive industry, the dimensional requirements and use conditions of 3D printed parts should be critically considered. Fig. 10 shows a daily average RH distribution without having rain collected from a data station of KMA (Korea Meteorological Administration) in 2015. The average humidity level was recorded to be as low as 20.9% RH in March and as high as 93.6% RH in July. During summer, i.e., from June to August, daily deviation was observed to be as high as 55% RH. These statistics indicate the RH cycle profile used in the experiment can be observed in a typical use condition, and can cause dimensional requirement violation in field applications. Many automotive part suppliers, producing structural components or modules for vehicle assembly, generally set the maximum dimensional tolerance of each part to be at most 0.5 mm in terms of RMS error with respect to the original design. For illustration, suppose there is a door trim assembly, shown in Fig. 11, produced by the SLS process. From the local RH history data, the maximum RH difference is approximately 73%, resulting in the %-strain change from 0.05% to -0.15%. Given that the distance between two holes in Fig. 11(b) is required to be 300 mm, and that the same geometric tolerance of the dimensional accuracy of 0.5 mm and the locational accuracy of 0.5 mm, changes in RH can yield dimensional change of 0.15 mm and -0.45 mm, respectively. The total displacement would be 0.6 mm, which is out of specification compared to the 0.5 mm RMS displacement error of the general specification. As shown in Fig. 11(b), with consideration of the inspection criteria between holes in MMC (maximum material condition), the probability

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reported between 50 and 100 ppm, a 20°C rise or drop in temperature results in a strain change of as high as 0.2. In this sense, using 3D printed parts made by SLS in engineering applications may require careful consideration of field environment, including temperature and humidity conditions.

5. Conclusions

Fig. 11 An example of dimensional specifications of (a) a sample door trim design, and (b) its geometric dimensions and tolerances

of having a good part under the dynamic environment of the specified RH changes can be calculated using the following equation, P {Good Part} = ∏ [P {Good dimension }*P {Good Location } ] all

Suppose 0.05% strain change decreases the probability of having a good dimensional or locational feature of a part by 1%. With four features (two locational and two dimensional requirements) the probability of having a good part is calculated to be between 89% and 96% under the %-strain change from -0.15% to 0.05%. As a result, displacement changes in SLS parts can cause residual stresses between joining parts, which may adversely affect the performance, lifetime, quality and reliability of assembly parts under humidity variations. Temperature should be another factor to be considered for SLS parts in field use conditions. While the temperature changes during the laboratory experiment presented in section 3 were almost negligible, field application may induce significant temperature changes. Considering that the coefficient of thermal expansion (CTE) of PA12 is

Additive manufacturing technology is becoming more and more recognized as a leading technology of the new industrial revolution, recently. It’s been also widely used in various industries including biomedical, automotive, manufacturing, defense, and architecture. However, the reliability and quality issues surrounding 3D printed products have not been cleared yet. As part of the dimensional quality and reliability study, this paper examined the effect of humidity changes on 3D printed parts by SLS. Multiple 3D printed sample parts have been produced using an SLS 3D printer with PA12 powdered material. Humidity cycling tests have been conducted while monitoring the stain history of the 3D printed sample parts with respect to changes in humidity levels. The test results demonstrated a change in humidity from 20% RH to 90% RH yielded a total %-strain change of 0.2%. A survey on RH history showed that the minimum and the maximum RH in a year was recorded as low as 20.9% and as high as 93.6%, respectively, and the humidity change could, as an example, result in the violation of dimensional requirement for part design specification. Thus, careful consideration in terms of dimensional stability due to humidity level is required for 3D printed parts from the design stage to field application. To promote the new industrial applications of additive manufacturing, many quality and reliability issues considering safety and use conditions should be investigated. In the future, thus, dynamic material properties, lifetime of the printed parts under severe and realistic use conditions, as well as examination of porosity-controlled deformation due to moisture absorption need to be studied.

ACKNOWLEDGEMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B03028604).

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