ORIGINAL ARTICLE

Laser polishing of thermoplastics fabricated using fuseddeposition modelling

Yuan Chai1 & Rachel W. Li1,2 & Diana M. Perriman3& Song Chen4

& Qing-Hua Qin4& Paul N. Smith3

Received: 12 November 2017 /Accepted: 14 March 2018 /Published online: 23 March 2018# Springer-Verlag London Ltd., part of Springer Nature 2018

AbstractFused deposition modelling (FDM) is a technique of additive manufacturing (AM) which is capable of fast constructionof plastic prototypes. AM technology has been utilised in orthopaedics and traumatology to fabricate patient-specificmodels, surgical guides, and implants. However, the layering build-up by FDM usually generates a deleterious roughsurface that limits its application to items such as surgical guide and surgical operating guide, because these itemsrequire a highly resolved surface quality. The advancement of laser polishing has been offering a cost-effective and fastmanufacturing solution for FDM-constructed patient-specific guides and implants. This investigation explores applica-tion potential using a contactless laser scanning to improve the surface quality of FDM-fabricated thermoplastics. Theresults show that a maximum [68%] reduction in surface roughness was achieved at 3 W CO2 laser power, 150 mm/sscan speed, 30 ms scan delay and 0.025 mm line gap. Laser polishing is suitable to treat the surface of polylactic acid(PLA). This study provides data which supports a new approach to the manufacture of AM-fabricated thermoplasticsutilising a laser scanning technique to improve the surface quality.

Keywords Laser polishing . Fused depositionmodelling . Additive manufacturing . Thermoplastics

1 Introduction

Additive manufacturing (AM), or 3D printing, is an ad-vanced manufacturing technique that builds items in alayering manner by reshaping the source material [1, 2].The most popular AM technology is fused depositionmodelling (FDM) which has undergone significant devel-opment over the past decade. Over 70% of AM-related

companies employ the FDM technique [3]. However, thelayering build-up employed in FDM usually generates adeleterious rough surface that limits its application to itemssuch as toys, figures, and prototypes because these onlyrequire shape realisation [4, 5]. A number of publicationshave addressed the issue of poor surface quality [6, 7].Wohlers highlighted the fact that the lack of surfacefinishing quality of FDM items is a major limitation ofthe technology [3].

Research aimed at improving the surface smoothnessof FDM has been conducted. Techniques which have beenemployed involve optimising the manufacturing parame-ters, such as the melting temperature [8], build-up direc-tion [9], and layer thickness [10]. Post-processing strate-gies have also been investigated. Epoxy or polyurethanesealing [11], chemical polishing [12], electroplating, metalcoating [13, 14], and bead blasting [have] been used tosmooth the surface of AM polymers [15, 16]. [However,none of these methods has resulted in sufficient surfaceresolution to satisfy the needs of manufacturing.]

Secondary melting of the surface relief by laser scanning isa technique of improving surface quality which has been usedfor metal AM prototypes. However, only minor research has

* Rachel W. Lirachel.li@anu.edu.au

1 Trauma and Orthopaedic Research Laboratory, Department ofSurgery, The Medical School, The Australian National University,Building 131, Garran Road, Canberra, ACT 2601, Australia

2 Department of Immunology and Infectious Disease, John CurtinSchool of Medical Research, The Australian National University,Acton, ACT 0200, Australia

3 Trauma and Orthopaedic Research Unit, The Canberra Hospital,Yamba Drive, Garran, ACT 2605, Australia

4 Research School of Engineering, College of Engineering andComputer Science, Australian National University,Acton, ACT 2601, Australia

The International Journal of Advanced Manufacturing Technology (2018) 96:4295–4302https://doi.org/10.1007/s00170-018-1901-5

Fig. 1 Schematic of surface melting regimes on FDM object surface. a Melted area of surface over-melt is notably deeper than surface protrusion.b Melted layer of surface shallow-melt is thinner than the surface protrusion

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been reported on applying a laser scanning method to changethe surface morphology of a polymer AM object [17].Importantly, laser treatment allows contactless and wearlessmachining with a high degree of flexibility. These features arefavoured by many industries including medical devicemanufacturing [4, 5, 18–24]. Therefore, laser polishing hasthe potential to become the state of the art for optimisingsurface quality while being cost effective, simple, and reliable.

In the field of surface secondary melting, there are tworegimes of surface modification, surface over-melt(SOM), and surface shallow-melt (SSM) [20]. Laser scan-ning on the surface of an AM-fabricated object can em-ploy both regimes [25–28]. The SOM mechanism is de-termined by the depth of melt pool and surface roughness.The depth of melt pool must be higher than the surfaceroughness as shown in Fig. 1a [27, 28]. The principle ofthe SSM is shown in Fig. 1b, in which the melt zonedepth is much smaller than that of the surface relief.

This project aimed to apply CO2 laser scanning tech-niques in the improvement of the surface quality ofFDM-fabricated polymer objects. Specifically, to assessthe effects of CO2 laser scanning on the most widely usedFDM polymer-acrylonitrile-butadiene-styrene (ABS) andpolylactic acid (PLA).

2 Materials and methods

PLA 3D Filament 1.75 mmWhite (Sunhokey-tec, GuangdongProvince, China) and ABS 3D Filament 1.75 mm White(Makerbot, New York, USA) were purchased to produce 3Dprinting samples for the laser scanning. An aluminium framereinforced self-assembly deltabot FDM 3D printer was used tomanufacture polymer samples. Square samples 50 × 50 mm2

were printed vertically with 3-mm thickness. A 40-W CO2

industrial laser scanning machine (King Rabbit HX-40A,Shandong Province, China) was set up at a 38-mm focal lengthf of the lens with a spot size of 73 μm and the source laser spotsize D of 2.5 mm. We selected this machine because of itsexcellent energy absorption rate with polymers [29, 30].

2.1 CO2 laser scanning process

The process of laser scanning on polymerAM is summarisedin Fig. 2a. The CO2 laser beam scans the layered surfaceproduced using FDM 3D printing technique, where thewarmer colour in the laser beam represents higher energy.The anisotropy polymer surface is melted and thenre-solidified to form a new surface profile. The melt is usu-ally flattenedbygravity and surface tension [30]. Theoreticalenergydepositionof apulse laser scan is demonstrated inFig.2bwhere scan linegap, scandelay, and scan speed (the periodthe laser spot remains at an area) greatly influence the laserenergy deposition at a spot. The Fig. 2c is a schematic of theheat distribution of CO2 laser beam scanning onto anFDM-fabricated plastic substrate, which is simulated fromthe theoretical model in Comsol Multiphysics (COMSOL,Inc.MA,USA). This figure shows that the heat-affected areais greater than the laser spot size, which indicates that severalquick scans at a low-energy level may better melt the surfaceevenly than a single high-energy scan.

In this study, the laser performance was subject to theworking condition of a single machine and the great potentialof improving the surface quality of FDM items might be dem-onstrated. Differences in laser scanning machines may need toapply slightly different parameters to achieve the same result.

2.2 Basic laser scanning conditions

There is no reliable literature which addresses the selec-tion of parameters required for safe application of lasersurface modification of FDM-fabricated ABS and PLAsamples. Therefore, we used a theoretical calculation ofmetal laser scanning for a safe initial laser output pow-er. The transverse intensity distribution (TID) of 40 WCO2 laser beam is known to have a Gaussian distribu-tion [31, 32]. When the laser irradiates the materialsubstrate, the total energy of the incident laser can becalculated as follows:

E0 ¼ Er þ Ea þ Et ð1Þ

where Er, Ea, and Et are laser energy reflected, absorbed, andpassed through the material, respectively [33].

In this study, laser scattering can be neglected because bothABS and PLA are amorphous materials [30]. The changes ofthe laser spot size due to surface roughness are also ignored.According to the equation above, the effect of laser radiationonmaterial surface integrity andmelt quality strongly dependson the optical properties of the material, such as the reflectionand absorption coefficients [30]. However, it is difficult totrace the actual properties of the continuously changing ther-moplastic designed for the FDM process. Each of the 3Dprinting materials demonstrates slight variations compared toprevious plastic formulas [34]. In our research, the propertiesof pure ABS and PLA were assumed in order to design theinitial experiment conditions. These assumptions were that theemissivity of both polymer is 0.9 [35], the thermal conductiv-ity of ABS at 0.17 W/(m K) and PLA at 0.14 W/(m K) [36,37], the specific heat capacity of ABS Laser polishing of AMthermoplastics 3 and PLA are both at 2.1 J/(g K) [38], and thedensity of ABS at 1.05 g/cm2 and PLA at 1.25 g/cm2. Laserheat flux follows Gaussian distribution, we assume the laserspot range D contains 68.27% of total energy:

Heat Flux W=mm2� � ¼ 2P

D2πexp −

2r2

D2

� �� �ð2Þ

The energy density (ED) of laser polishing can beexpressed as follows:

Energy Density J=cm2� � ¼ 6000� P

Vf � Dð3Þ

where P is the laser power (W), Vf is the feed rate (mm/min), ris the distance to the centre (mm), and D is the laser spotdiameter (mm).

Based on the pilot theoretical calculation above, initial laseroutput power 3 and 4 W were selected to ensure safety andmaterial melting. The power was confirmed as effective onour trial experiment. We used laser power rather than ED tocharacterise the process because ED largely describes the en-ergy of a single scan instead of several quick scans as wereused in this study. This is illustrated in Fig. 2c.

Depending on exploratory trial, the basic experimental con-ditions were set by machine availability at 150 mm/s scan

speed, 30 ms scan delay, and 0.025 mm line gap. In this re-search, 15 × 10 mm2 areas were scanned by laser on layeringsurface of FDM-made sample, with each scan was 15 mm inlength.

2.3 Surface observation

The surface roughness of all samples before and after lasertreatment was observed using an optical profiling system(Veeco Wyko NT1100, AZ, USA). Magnification at ×5 wasset for the observation of overall surface (1.3 × 0.9 mm2) con-taining five protrusions. The magnification at ×50 was set forlocal surface (126 × 94 μm2), which is a single protrusion.Both conditions are without any stitching.

2.4 Statistical analyses

All statistical analyses were performed using the computerprogram Graphpad Prism 7.0 (GraphPad Software, Inc.CA, USA). The difference between two means wasanalysed through a student t test. All data were assessedfor Gaussian distribution using the D’Agostino-Pearsonnormality test before analysis with these parametric tests.The level of statistical significance was fixed at p ≤ 0.05.The n numbers of each individual in the test (n = 3) wereprovided in the caption legends.

3 Results and discussion

The initial and final surface topographies in this study arecharacterised by surface roughness (Sa) (μm) as shown inEq. (4) [39] and calculated according to the standard ISO4288. The topography optimization is characterised by theroughness reduction of surface (%) and shown in Eq. (5),

Sa μm½ � ¼ 1

A∬A Z x;yð Þ

�� ��dxdy; ð4Þ

Sareduction %½ � ¼ Sainitial−SapolishedSainitial

� 100 ð5Þ

Fig. 2 Schematic of surface laserscanning. a Laser treatmentprocess. b Schematic of laserenergy depositionwhile scanning.c Heat distribution while laserscanning

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This study represents the surface roughness ofFDM-fabricated heterogeneous surface waviness of ABS

processing conditions were tested and maximum surfaceroughness was reduced from 14.42 to 4.64 μm (67.83%).However, laser-irradiated ABS samples did not notablyimprove surface quality as none of the results showed thereduction of roughness exceeded 5%. The results show thatPLA was superior to ABS under the same fabrication con-ditions using FDM, in terms of its suitability for laser sur-face morphology modification. Next, we investigated the

influence of changing parameters of laser treatment pro-cess on surface roughness. The change of delay time be-tween scans did not affect the surface roughness, but scanspeed and line gap between scans can be optimised.

In this pilot study, processing parameters were treatedas independent factors. A detailed analysis of each pa-rameter has not yet been thoroughly investigated. Amultivariate analysis of the relationships between theseparameters is required in the future.

Fig. 3 Original FDM samplesurface. a MIT invert microscopeimages of PLA (top) and ABS(bottom) at 0.2 mm slicing height.b Local surface profile of PLA(top) and ABS (bottom). c SEMimages of PLA at 0.1 mm slicingheight show the manufacturingerror (top) and the unevendistribution of layering substrate(bottom)

Fig. 4 Laser-treated ABS surface made by FDM. a Surface roughness oflaser-treated ABS samples. Values represent the means ± SD. Y axisstands for different processing conditions: (1) 4 W output power, 30 msdelay between scans, treatment repeat twice; (2) 4 W, 30 ms delay, repeatfour times; (3) 5W, 50ms delay, repeat twice; (4) 5W, 50ms delay, repeatfour times; (5) 5.5 W 200 ms delay without repeat; (6) 5.5 W, 350 msdelaywithout repeat. Vertical dash line stands for the surface roughness of

original ABS samples (11.75 μm, n = 3). The highest roughness valuewas found at condition (6), while lowest roughness was found atcondition (4). b Surface morphology of ABS samples treated by CO2

laser scanning and observed using MIT inverted microscope. From topto bottom: original surface, shallow-melt, over-melt, and laser scan edge.c Local surface morphology of original and shallow-melt ABS

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and PLA before and after laser surface treatment. Different

3.1 Initial topography

We first observed the overall surface of samples made by twothermal plastics. Areas of 1.3 × 0.9 mm2 containing over fiveprotrusions were analysed. The comparison of surface profilesbetween ABS and PLA samples at 0.2 mm slicing height isshown in Fig. 3. We can see from Fig. 3a, b that the PLAsurface was smoother than that of ABS. Interestingly, overallmeasurement of the surface roughness (measured from fiveprotrusions) of ABS was on average 13% lower than that ofPLA. These contrasting results may have been influenced bymanufacturing errors because some minor voids and unevenlayer distribution were detected using a scanning electron mi-croscope (SEM), see Fig. 3c.

3.2 Effect of laser treatment on FDM-fabricated ABSsamples

In laser material treatment, the heat-affected zone is theresult of a combination of laser optical parameters such asspot size, depth of field, and laser incident power. It isalso influenced by operating conditions, such as scanspeed, delay of each scan, and gap between scans [40].We observed both shallow-melt regime and over-melt re-gime using laser treatment on the surface of ABS samplesas reported in the literature [28].

Laser-treated surface of FDM-fabricated ABS underdifferent process conditions are presented in Fig. 4a. Asshown in Fig. 4a conditions (1) and (2), alteration of scanspeeds and line gaps for achieving a shallow-melt regimedid not result in identically improved surface smoothness.The repeated laser treatment on the same area did notincite multiple melts to further smooth the surface. Theshallow-melted surfaces had slightly reduced overall sur-face roughness (maximum reduction 3%), whichcorresponded to surface roughness (see Fig. 4a at condi-tion (3) and (4)). Under our experimental conditions, therewas no strategy to reduce the overall surface roughness bymore than 5%. When laser power was increased to 5.5 Win conditions (5) and (6), over-melt regime was observed.The highest roughness value was found at condition (6),while lowest roughness was found at condition (4). Thesurface started to collapse and the colour turned fromwhite to yellow in condition (5) and (6). Notably, theoverall surface roughness had deteriorated dramaticallybecause of plastic spheroidization from liquid to solid.However, the local surface morphology of shallow-meltsurface (observed at the area of 126 μm × 94 μm) shownin Fig. 4c demonstrated the laser-melted trace and im-proved surface morphology with a reduction of surfaceroughness at 15%. Laser power over 6 W was not selectedin this study because it caused oxidation on the surface, asevidenced by surface colour change from white to black.

3.3 Effect of laser treatment on FDM-fabricated PLAsamples

The influence of laser parameters on surface modification ofPLA samples were investigated including laser output power,scan speed, gap between scans, and delay between laserpulses. Surface roughness of laser treated PLA samples madeusing the FDM method are presented in Fig. 5. We observedvery exciting results that the laser scan effectively reducedsurface roughness of FDM-fabricated PLA samples underthe basic experimental conditions at 150 mm/s scan speed,30 ms scan delay, 25 μm line gap, and low laser output power(3 W) or high laser output power (4 W).

In the optimization of parameters, the surface roughnessversus the scan speed is presented in Fig. 5a. It is worthwhileto note that under the condition of 4 W, an increase of the scanspeed to 150 mm/s caused a significant decrease of surface

Fig. 5 Surface roughness of PLA samples treated using laser scanningunder different variables. Basic experimental conditions were set at150 mm/s scan speed, 30 ms scan delay, 25 μm line gap, and low laseroutput power (3 W) or high laser output power (4 W). Vertical dash linestands for original surface roughness (14.42 μm) of PLA samples. Twomeans comparison was performed using Student’s t test, Prizm 7. Bars(n = 3) represent the means ± SD. a Surface roughness measured atdifferent scan speeds when laser output power and scan line gap wereset at 4 W and 0.025 mm, respectively. b Surface roughness measured atdifferent scan delay and laser output power while scan line gap remainedat 0.025 mm. * P < 0.05 versus 1000 ms scan delay group at 3 W laseroutput power. c Surface roughness measured at different scan line gap andlaser output power while scan delay and scan speed remained at 30 msand 150 mm/s, respectively. **P > 0.05 versus 0.2 mm line gap group at3 W laser output power

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roughness (p < 0.05). There is no statistically significant dif-ference on the surface roughness among the samples treatedwith low-speed scanning (P > 0.05, data not shown). Underour experimental conditions, a fast scan speed at 150 mm/s isrecommended for further research. Considering scanning onan area that multiple scans superimposed together at a timeinterval and a gap between scans, these two factors needed tobe taken into account simultaneously.

We next investigated the influence of different scandelays from 30 to 1000 ms in both 3- and 4-W laserpower groups on the surface roughness of laser-treatedPLA surfaces. As shown in Fig. 5b, there was no sig-nificant difference in surface roughness detected at thescan delays of 30, 90, and 500 ms in both powergroups (P > 0.05). The surface melting was critically in-sufficient when the scan delay was set up at 1000 mswith a low laser output power (3 W). An optimisedcondition was found at 30 ms scan delay and 3 Wenergy consumption. This condition was effective andfast in scanning with a 53% reduction of surface rough-ness. The results provide new information in the opti-mization of parameters using laser scanning techniquesin the surface improvement of FDM-fabricated PLA ob-jects. The same finding was also observed from ourABS experiment, in which a shallow-melt surface wasexcited by a single laser scan (Fig. 4c). The single laserscan did not reduce the surface roughness, but multiplequick scans deeply heated the material, generated a sur-face melt, and reduced surface roughness.

We further investigated the influence of different linegaps between scans on the surface roughness. An opti-mum scan line gap at 0.05–0.1 mm under our basicexperimental conditions (Fig. 5c), achieved a 68% re-duction of surface roughness when compared to theoriginal roughness (p < 0.001). In addition, a clear

concave trend on average value at both energy groups(3 versus 4 W) implies a minimum surface roughness isachievable, though it is not statistically significant(P > 0.05). Optimization by expanding the sample sizedeserves further investigation. Considering that abroader line gap can reduce scan repetition and timeconsumption in finishing a scanning in the same sizeof areas, we would recommend a scan line gap at0.1 mm for further investigation.

The surface profiles presented in Fig. 6 reveal that thelaser scanning process reduced the amplitude of the originalwaviness to a lower level in both the SSM and SOM groups.SSM was distinguished by the reduced waviness but it wasstill present as evidenced by the contrast of blue and blackamplitudes in Fig. 6a–c. The amplitudes of the surface ir-regularities were increased when SOMwas observed, whichis shown by the red line in Fig. 6a. The red line demonstratesthat surface roughness increased when the laser energy wastoo high. The height was reduced averagely 6.96 μm at alow energy level, i.e 150 mm/s scan speed, 30 ms scandelay, 0.2 mm scan line gap, and 3 W laser output power.Higher energy density further reduced the average height ofthe surface.

In contrast to ABS, it was difficult to see any colourchange on the laser-scanned PLA surface regardless ofthe power used. Samples were weighed before and afterlaser treatment, and weight loss was found in bothlaser-treated groups indicating that PLA decompositionoccurred. Considering the height reduction, SSM wasindeed produced by the combination of evaporated layer(height reduction) and heat-affected layer (SSM layer).The edges of surfaces of PLA and ABS samples treatedusing laser scanning are shown in the bottom figure ofFigs. 6b and 4b, respectively. This demonstrates that theheat-affected zone on both the PLA and ABS surfaces

Fig. 6 Observation of surfaceamplitudes and profiles of PLAbefore and after laser treatment. aSurface amplitudes of PLA atoriginal, shallow-melt, and over-melt condition. b MIT Invertedmicroscope images of CO2 laserscanned PLA surfacemorphology. From top to bottom:original surface, over-meltsurface, shallow-melt surface,laser scan edge

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from the laser scanning trace to the remote meltedboard was less than 0.2 mm (layering height).

In summary, we found that the best reduction in surfaceroughness was achieved when the laser scan speed was150 mm/s and scan line gap was 0.05 mm at 3 W of laserpower. The surface roughness was reduced from 14.42 to4.64 μm, which was 68% smoother than that of originalroughness.

3.4 Comparison of laser-treated ABS and PLA

The influence of laser parameters on surface modification ofPLA samples were investigated including laser output pow-er, scan speed, gap between scans, and delay between laserpulses. There is a profile difference between SOMABS andPLA, which can be seen on the third figure of Figs. 4b and6b. The SOM ABS material tended to ball up rather thanflatten during the melt. This effect is possibly because ABShas a higher surface tension than PLA. The spheroidizationseen in ABS might be eased with material modification, arise in the environmental temperature and a wider melt pool.Future studies should include detailed thermodynamic anal-ysis of melt pool dynamics, surface tension, and cohensiveenergy density. Further studies may also include laser scan-ning effect on different FDM layer thicknesses, and laserenergy irradiation and reflection on the anisotropy surface.

4 Conclusion

This study proposed a CO2 laser scanning method for modi-fying surface morphology of thermoplastic products made byan additive manufacturing method. Based on our results, wecan conclude that the laser polishing method is a cheap andfast contactless method of improving the surface quality ofselected AM-fabricated thermoplastics. The study showedthat the application of laser treatment to improve a PLA sur-face was more effective than to an ABS surface, and SSM inPLAwas achieved with the most reduction of surface rough-ness (68% in this study, when applied 150 mm/s scan speed,30 ms scan delay, 0.05 mm scan line gap, and 3 W laserpower). However, laser treatment of ABS resulted in only a5% reduction in surface roughness.

Future studies should focus on reducing process timesby examining the theoretical relationship between opera-tion parameters and laser energy density. Parameters thatmay also influence the scan results but were not included inthis study include depth of field and focal distance. It isimportant to acknowledge that the change of surface mor-phology treated by laser may potentially alter mechanicalproperties, despite the laser treatment essentially being avariation on the application of heat energy which was theinitial fabrication process. We anticipate that further

material roughness reduction could be achieved by furtherparameter optimization.

Acknowledgements The authors are thankful to Mr. Khu Vu in theResearch School Physics, Dr. Hua Chen in the Centre of AdvancedMicroscope, and Chao Hu in the Research School of Engineering at theAustralian National University for the technical assistance.

Funding information This work has been financially supported by theAustralian Research Council PhD Scholarship (LP150100343).

Compliance with ethical standards

Conflict of interest The authors declare that they have no conflict ofinterest.

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4302 Int J Adv Manuf Technol (2018) 96:4295–4302