3dp process for fine mesh structure printing

Available online at www.sciencedirect.com

7 (2008) 11–18www.elsevier.com/locate/powtec

Powder Technology 18

3DP process for fine mesh structure printing

Kathy Lu ⁎, William T. Reynolds 1

Virginia Polytechnic Institute and State University, Materials Science and Engineering Department, 211B Holden Hall-M/C 0237, Blacksburg, VA 24061, USA

Received 12 September 2007; received in revised form 18 December 2007; accepted 20 December 2007Available online 4 January 2008

Abstract

Three dimensional printing (3DP) is a unique technique for creating complex shapes. However, printing feature sizes at less than 500 μm with highintegrity and intricate structures have not been possible. In this study, TiNiHf shape memory alloy (SMA) powder was printed into 3D mesh structures of300 μm wire width. Effects of printing layer thickness and binder saturation level on the integrity and dimensional accuracy of the 3D mesh structureswere evaluated. 35 μm printing layer thickness and 170% binder saturation level offer the highest mesh structure integrity. Also, 35 μm printing layerthickness results in the smallest dimensional deviation from the designed 200 μm mesh width with the smallest standard deviation. Overall, 35 μmprinting layer thickness and 170% binder saturation level are the most preferred printing condition for the designed 3D mesh structure.© 2008 Elsevier B.V. All rights reserved.

Keywords: 3D printing; Mesh structure; Printing layer thickness; Binder saturation level; Integrity; Dimensional accuracy

1. Introduction

Since the mid-1990s, various digital forming techniqueshave emerged at the realization of digital control, moreadvanced materials, and the demand for very specificperformances and structure designs [1–6]. Three dimensionalprinting (3DP) is a unique technique that prints complex 3Dstructures that cannot be produced by other means, especiallyfor rapid prototype purpose [7–9]. During 3DP, the solid model,formatted into a [.stl] file (standard triangle language), isconverted by a slicing routine into a compilation of two-dimensional slices representing the 3D part. The slice file isfurther formulated into instructions that control the movementof the 3D printing components. The powder is spread by acounter-rotating roller onto a build platform inside a build box.By means of ink-jet printing technology, a printhead, containingan array of binder fluid jets, rasters across the layer of thepowder and deposits binder droplets in those locations definedby the current 2D slice of the solid model. Subsequently, thebuild platform advances downward by one layer thickness and anew layer of powder is spread, which is then printed by the

⁎ Corresponding author. Tel.: +1 540 231 3225; fax: +1 540 231 8919.E-mail address: [email protected] (K. Lu).

1 Tel.: +1 540 231 3225; fax: +1 540 231 8919.

0032-5910/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.powtec.2007.12.017

printhead. This procedure is repeated layer after layer until the3D part is completed. After the designed 3D geometries areprinted, the particles are held together by the binder used. Theprinted part can be removed from the surrounding unboundpowders. However, the printed structures are not strong enoughto be used directly and need to be sintered to densify the matrix.

3DP has demonstrated the capability of fabricating parts of avariety of materials, including ceramics, metals, and polymerswith an array of unique geometries [9,10]. However, substantialwork is still needed to explore the minimum feature sizes thatcan be printed and the ability of forming intricate structures.This is mainly because the minimal feature sizes and theintegrity of the structures are affected by numerous factors suchas binder droplet size and printing layer thickness. Also,suitable techniques need to be developed to evaluate the qualityof the fine features.

Shape memory alloy (SMA) is a group of novel materialsthat demonstrate the ability to return to some previously definedshape when subjected to appropriate thermal or stress-inducedprocedures [11]. The phase transformation strains have thepotential to relieve the thermal and mechanical stresses duringthe repeated thermal cycles and restore the distorted matrixshape to its original by shape memory effect. Currently, TiNiHfis considered to be the most attractive candidate as elevatedtemperature SMA due to its excellent workability, thermal

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http://dx.doi.org/10.1016/j.powtec.2007.12.017

Fig. 1. Particle size distribution of gas-atomized TiNiHf powder.

Fig. 2. Designed 3D mesh structure to be printed.

12 K. Lu, W.T. Reynolds / Powder Technology 187 (2008) 11–18

cycling stability, and the ability to absorb large amount of strainenergy [12–16]. One of the promising applications for theTiNiHf SMA is a metal/glass composite that integrates a definedTiNiHf SMA mesh structure into a glass/ceramic matrix forhigh temperature thermal cycling applications such as solidoxide fuel/electrolyzer cells. The phase transformation strainsfrom the TiNiHf can be used to offset the thermal stresses in theglass matrix imposed by other solid oxide cell componentsduring cooling. The shape memory effect during heating to theoperating temperature has the potential to close cracks that mayhave formed in the glass matrix during a previous cooling cycle.

To realize these predicted functions, the first step is to createdesired 3D mesh structures from the TiNiHf alloy. Since TiNiHfalloy powder can be obtained by gas atomization, 3DP presentsitself as a promising technique for creating the mesh structuresneeded. This study is focused on 3D printing of TiNiHf particlesinto mesh structures for such purpose. The printing variables areexamined based on the originating sources: 3D printer related,powder-related, and binder-related. Printing layer thickness andbinder saturation level (binder to pore volume ratio for a givenprinting volume) are identified as the primary variablesaffecting the 3DP capability and systematically evaluated inorder to achieve the highest 3D mesh structure integrity andminimum dimensional variation.

2. Experimental Design

2.1. TiNiHf powder

In this study, gas-atomized Ti35Ni50Hf15 (mol%, abbreviatedas TiNiHf) powder (Crucible Research Co., Pittsburgh, PA) ofsmaller than -635 mesh size was used for 3D mesh structureprinting. The TiNiHf particle size distribution was characterizedby laser light scattering (Horiba, LA-950, Irvine, CA). Themeasured particle size distribution is shown in Fig. 1. It can beobserved that 100% particle sizes are below 20 μm and themean particle size is about 5.50 μm. The particle sizedistribution is close to normal distribution.

2.2. Mesh structure design

A gradient 3D mesh structure was designed based on theapplication requirement of TiNiHf SMA mesh structure in

glass/ceramic matrix. The AutoCAD design of the 3D meshstructure is shown in Fig. 2. The diameters of the three ringsshown are 5.0, 10, and 15 mm, respectively. The numbers of theradial wires in the horizontal layers are 16, 8, and 4 from the topto the bottom. Square wires of 200 μm width are used in eachhorizontal layer. Round wires of 200 μm diameter are used in-between the layers. The reason for using the round wires in-between the horizontal layers is to ensure that the horizontal andthe vertical wires is connected with a known interfacial area (theround wire cross-section area). Otherwise, the curvature fromthe horizontal wire can create more complicated connectingareas.

2.3. Mesh structure evaluation

The printed 3D mesh structures are loosely bound by thebinder used and do not have high integrity. A small load cell hasto be used in the integrity evaluation. Since the printed samplesare brittle and experience compressive stress during the meshstructure-glass composite application, a Texture Analyzer testconsole equipped with a 5 kN load cell (Stable Micro Systems,Surrey, UK) was used for compression failure test. The consolewas set to record compressive force and the crosshead waslowered monotonically at a speed of 0.1 mm/min. The shape ofthe force vs. displacement curve was recorded and the peakforce was determined from the curve as the 3D mesh structurebreaking force. Breaking strength is calculated as the ratio ofbreaking force to the average cross section area of the threelayers in the mesh structure horizontal direction. To evaluate thedimensional accuracy of individual wires, an optical micro-scope (ME300TZ-3P, Amscope, Chino, CA, USA) was used tomeasure the wire width from the top-down view. Each reportedwire width was measured at ten different radial wire locations.

3. Results and Discussion

3.1. 3D printing variable evaluation

As mentioned, 3DP process is affected by numerousvariables. In order to obtain 3D printed structures with highintegrity, small feature sizes, and accurate dimensions, eachfactor needs to be considered. By the correlation with the majoraspects of 3DP, the variables can be classified into threecategories: 3D printer-related variables, powder-related vari-ables, and binder-related variables. These three groups ofprinting variables are listed in Tables 1, 2, and 3, respectively.

Table 13D printer-related printing variables

Variables Definition Effect Range ValueAdopted

Cap CleaningFrequency

Number of printing cyclesbetween cleaning of printhead cap

Cleanliness of printhead cap 1-5 cycles/cleaning 2 cycles

Wipe CleaningFrequency

Number of printing cycles between cleaning of thesponge wipe for printhead

Cleanliness of printhead sponge wipe 1-5 cycles/cleaning 2 cycles

Full PrimeFrequency

Number of printing cycles between fully refillingprinthead with binder

Consistency of binder drop volume 1-5 cycles/cleaning 1 cycle

SpreaderReverse Speed

Travel speed of printbed after dispensing of binder Dwell time of binder before drying;affect the dimensional tolerance of the meshes

0.5-5.0 mm/sec 2.0 mm/sec

SpreaderTraverse Speed

Travel speed of printbed after drying Smoothness of powder layer for next printingcycle; related to powder packing density and particle size

0.5-5.0 mm/sec 2.0 mm/sec

13K. Lu, W.T. Reynolds / Powder Technology 187 (2008) 11–18

Tables 1–3 also list the range of the 3D printing variables andhow they affect printed structure quality.

Even though all the above variables affect the printedstructure integrity and resolution, some of the printing variablescan be pre-determined. First of all, 3DP process requires carefulselection of particle size and size distribution. Suitable particlesize range is dependent on the feature sizes to be printed, theprinted mesh structure integrity desired, and the availability ofspecific powders. If the particle size is too large, it will notcreate strong enough mesh structures and the fine featuresdesired. For example, if a 50 μm thick feature is desired, theneach printing layer would only be one particle size thick for a50 μm size powder and is not practically possible. However,other difficulties can arise if the particle size is too small. Smallparticles, due to their poor flowability, cannot be spread intothin and even layers. The spreader roller can press the particlesinto the previous layers in the printbed. Also, the particlesshould have a reasonable range of size distribution. The largestsize should meet the integrity and feature size requirement andthe smallest particle size should not affect powder flow. Thenormal size distribution used in this study is preferred for higherparticle packing density in comparison to monosize distribution.

Binder is another important variable to consider. It shouldhave low viscosity and high stability during the 3DP process. Ifthe binder viscosity is too high, it can easily clog the printheadnozzles, which have about 74 μm size. Also, the binder shouldbe stable during the 3DP process with no significant chemicalreaction. Otherwise, it can affect the wetting capability on theparticle surfaces and subsequently the integrity of the meshstructures. In this study, an acrylic-based aqueous binder with

Table 2Powder-related printing variables

Variables Definition Effect

Particle Size and SizeDistribution

– Feature size, powder p

Powder Packing Density Ratio of powder volume to totalvolume

Binder saturation

Printing Layer Thickness Thickness of each layer to beprinted

Dimensional tolerance

Feed Powder to LayerThickness Ratio

Ratio of feed powder to layerthickness

Should be high enoughshifting of layers

17 wt% concentration was used for printing the SMA powderand is pre-determined based on the binder availability.

Cap cleaning frequency, wipe cleaning frequency, full primefrequency, spreader reverse speed, and spreader traverse speedwere determined by trial and error before the designed 3D meshstructures were printed and the values are given in Table 1: 2cycles/cleaning for the cap, 2 cycles/cleaning for the wipe, 1cycle for each full prime. The spreader reverse speed and thespreader traverse speed are 2.0 mm/s. Although the theoreticalpacking density should be ~64% for the smaller than –635 meshpowder based on loose random packing principle, actualpacking density of the SMA powder in the printbed is farbelow that. The powder packing density is measured to be 35%.This is because the SMA powder in the printbed is not asdensely packed as the bulk powder, likely due to the rollerspreading effect and the loose packing of particles in the spreadlayer when the printing layer is thin. Also, feed powder toprinting layer thickness ratio needs to be properly pre-set so thatan even layer is spread for each printing cycle. For the SMApowder, this value is again identified by trial and error to be 1.5(Table 2). For the variables listed in Table 3, drying time isdetermined to be sufficient at 60 s and drying power to besufficient at 90% based on printing of simple wires with theSMA powder used. Binder drop volume is determined to be at211.5 pico-liter by the 3D printer based on the powder packingdensity, printing layer thickness, and binder saturation level. As-sprayed binder droplet size is directly determined by theprinthead nozzle size and reflected by the binder drop volume.This value is the same for all the conditions studied,approximately 74 μm.

Range Value Adopted

acking density – 5.5 μm, normaldistribution

0-100% 30%

of printed mesh structures N2 times ofparticle size

20 μm35 μm50 μm

to spread new layers but not so high to cause 1.0-2.0 1.5

Table 3Binder-related printing variables

Variables Definition Effect Range ValueAdopted

BinderSaturation Level

Volume percent of binder printed vs.pore volume among particles

Higher saturation creates stronger bonds between particles; but too high asaturation creates lateral spreading and poor dimensional tolerance

40-200% 55%110%170%

Drying Time andPower

Drying time and power betweensuccessive layers

Integrity and shape retention of 3D mesh structures Time: 0-90 sPower: 0-100%

60 s,90%

Binder dropvolume

Average binder drop volume per jetspray

Integrity and dimensional resolution of mesh structures Binder specific 211.5pico-liter


The major variables that need to be further studied in the 3Dprinting of the TiNiHf powder are printing layer thickness andbinder saturation level. These two parameters also playimportant roles in determining the integrity, dimensionalaccuracy, and minimal feature sizes that can be printed for theTiNiHf powder. In order to print the 3D mesh structures withgood integrity and accurate shape, thin printing layer thicknessshould be used. However, it has to be greater than the SMAparticle size. Since the average particle size is 5.5 μm and themaximum particle size is 17.0 μm, the printing layer thicknessis chosen at 20, 35, and 50 μm in this study for furtherevaluation. Binder saturation level is closely related to powderpacking density, binder-particle interaction, and thus binderdistribution in the 3D printed meshes. Under the same printinglayer thickness, higher saturation level results in higher volumeof sprayed binder. However, there is no specific knowledgerelated to the TiNiHf powder. Since the binder saturationvarying range is 40-200%, three levels were selected in thiswork: 55%, 110%, and 170%. It should be pointed out that the3D printer allows greater than 100% binder saturation level(excessive binder amount than what is needed to fill the poresamong the particles).

After the 3D printing, the mesh structures are pre-cured forone hour at 100% heater power of the 3D printer before they areremoved from the printbed. A subsequent cure of one hour at170 °C is done in an oven to completely remove the water in thebinder solution and fully bind the particles.

3.2. Mesh structure integrity evaluation

For a given powder with fixed particle size and sizedistribution, higher printing layer thickness generally ensures

Fig. 3. 3D printed TiNiHf mesh structure with 50 μm printing layer thicknessand 55% binder saturation level.

better powder spreading. However, our aim is to print meshstructures with 200 μm wire width. Higher printing layerthickness will result in fewer printing layers. To achieve high3D mesh integrity, the number of printing layers should be ashigh as possible. With this consideration, printing layerthickness of 50 μm is selected to evaluate three bindersaturation levels: 55%, 110%, and 170%. Six samples wereprinted for each condition. However, 55% binder saturationlevel produced 3D mesh structure that is too fragile to beremoved from the printbed. All the samples broke into piecesduring transfer from the printbed to the sample container, asshown in Fig. 3. This means that 55% binder saturation level istoo low to form the designed 3D mesh structure.

When the binder saturation level was increased to 110%, the3D mesh structures printed broke partially. The framework ofthe 3D structure was intact but some vertical connecting wiresbroke. The image of the printed 3D mesh structures at 110%binder saturation level is shown in Fig. 4(a). This means 110%binder saturation level produces improved mesh structures thanthe 55% binder saturation level. However, the vertical, round-shaped wires connecting the horizontal layers are still weak.When the binder saturation level was increased to 170%, all thesamples were strong enough to be removed from the printbed asshown in Fig. 4(b).

In order to quantitatively compare the integrity of the 3Dmesh structures, three printing layer thicknesses, 20, 35, and50 μm, are evaluated at 110% and 170% binder saturationlevels. All the other printing parameters are kept the same asdiscussed in 3.1. From Fig. 5, it can be observed that at all the

Fig. 4. 3D printed TiNiHf mesh structure with (a) 110% binder saturation, and(b) 170% binder saturation. The samples are printed with 50 μm printing layerthickness.

Fig. 5. Breaking strength of printed TiNiHf 3D mesh structures.

Fig. 6. Printed wire width measured by optical microscopy.


printing layer thicknesses, the 3D mesh structures printed using170% binder saturation level have higher breaking strength thanthose of the mesh structures printed using 110% bindersaturation level. The error bar represents the standard deviationof the breaking strength. Mesh structures with 170% bindersaturation level have smaller breaking strength variation thanthose of the mesh structures printed using 110% binder saturationlevel. This means higher binder saturation level is beneficial forthe studied 3D wire meshes and offers more binding functionamong the particles. Also, 170% binder saturation level offersmore consistent 3D mesh structure integrity.

The effect of the printing layer thickness on the integrity ofthe 3D mesh structures can also be analyzed from Fig. 5. Forboth 110% and 170% binder saturation levels, the 3D meshstructures printed with 35 μm printing layer thickness arestronger than those with 20 μm and 50 μm printing layerthicknesses. Also, the 3D mesh structure printed with 35 μmprinting layer thickness and 170% binder saturation level showsthe smallest breaking strength variation. Based on ourobservation, this can be explained from the lateral and verticalbinder spreading differences. In general, the binder spreadingdistance in the horizontal direction is much longer than that inthe vertical direction. This is because the binder drop has certainsize when it reaches the powder bed and the spreading is in allhorizontal directions. For the vertical spreading, the binder onlyprogresses from the top to the bottom of the printing layer. Thedetailed lateral and vertical binder spreading rates will bedetailed in future studies using 3D microscope. For the 20 μmprinting layer thickness, the wire width is 10 times of thevertical spreading distance. The vertical direction will besaturated with the binder before the lateral binder spreading iscomplete. The poor lateral spreading causes low integrity 3Dmesh structures. For the 35 μm printing layer thickness, thebinder is able to complete the binder spreading in both thelateral and vertical directions within similar time span. Thiscontributes to the highest breaking strength observed. For the50 μm printing layer thickness, the wire width is only 4 times ofthe printing layer thickness. Binder spreading in the lateraldirection will be complete before that in the vertical direction.As a result, the binder will likely have incomplete penetration inthe vertical direction. Based on the breaking strength evalua-tion, it can be concluded that 35 μm printing layer thickness and

170% binder saturation level are the desired condition forprinting the highest integrity 3D mesh structures.

3.3. 3D printing dimensional accuracy

In addition to the 3Dmesh structure integrity, the printed wirethickness and its deviation from the designed 200 μm width areother important factors to study. Fig. 6 shows the average wirewidth and the corresponding standard deviation under differentprinting conditions. All the printed wire width is larger than thedesigned 200 μm value. Actually, the dimensional deviation is~50% or higher. While the dimensional deviation should besubstantially less when the printed feature size increases, it is stillimportant to be able to predict the printed wire width. Futureeffort will be devoted to narrow the dimensional gap between thedesigned and the printed structures.

Under the same binder saturation level, 35 μm printing layerthickness yields the smallest dimensional deviation from thedesigned 200 μm width in comparison to the other two printinglayer thicknesses. The wires printed with 20 μm printing layerthickness show the largest deviation from the designed 200 μmwire width. Also, the wire width printed with 35 μm printinglayer thickness has the smallest standard deviation overall. Thisdifference can be understood as follows. For the 20 μm printinglayer thickness, the designed wire width is 10 times of theprinting layer thickness. The binder penetrates quickly to thebottom of the layer, but the previous layer printed prevents thebinder from further spreading. In the lateral direction, the binderspreads without such limitation. This results in larger wire widththan desired. For the 50 μm printing layer thickness, the binderis able to spread in the vertical direction before beingcompletely consumed. This results in lower wire widthdeviation. 35 μm printing layer thickness allows the binder tofinish spreading in both directions at approximately the sametime and offers the closest dimension to the designed wirewidth. From Fig. 6, it can be concluded that 35 μm printinglayer thickness is preferred for the realization of good wiredimensional tolerance. Combined with Fig. 5, it confirms that35 μm printing layer thickness is the desired 3D printingcondition for the TiNiHf powder.

As the binder saturation level increases at the same printinglayer thickness, the wire width generally becomes thicker even


though the difference is small. This is because the bindersaturation level affects binder spreading distance. Higher bindersaturation level leads to longer binder spreading distance. Ingeneral, 35 μm printing layer thickness shows minimal wirewidth at different binder saturation levels.

Optical micrographs of the printed wire meshes underdifferent conditions are shown in Figs. 7–9. It can be seen thatunder the same binder saturation level, the wire meshes printedwith 20 μm printing layer thickness show more dimensionalvariation in comparison to those printed with 35 and 50 μmprinting layer thicknesses. There are some large protruded areasfrom the 20 μm printing layer thickness wire. Even though the

Fig. 7. Optical micrographs of 3D printed TiNiHf square wire with bindersaturation level at (a) 55%, (b) 110%, and (c) 170%. The printing layer thicknessis 20 μm.


micrographs only show local areas, the general trend is the samefor all the wires examined.

3.4. Optimal 3D mesh structure

The binder saturation level and the printing layer thicknesscan be understood schematically as shown in Fig. 10. During3D printing, binder is jetted from the printhead with a certaindrop size. However, the total binder amount jetted per layer isproportional to the total volume of the powder to be printed.After it reaches the printbed, the binder spreads into theinterstitial sites of the particles in vertical and lateral directions.


Fig. 10. Different printing layer thick

Fig. 11. 3D mesh structure printed with 35 μm printing layer thickness and170% binder saturation level.


When the powder layer is too thin, such as the case for the20 μm printing layer thickness, there is less than desired powderthickness to accommodate the binder at the studied saturationlevels since the powder layer beneath the current printing layeris saturated with the binder already from the prior printing cycle.This causes excessive lateral flow of the binder and larger anduneven wire width. When the printing layer thickness is toolarge, such as 50 μm, uneven lateral spreading becomes less.However, there is more binder jetted into the printbed becauseof the larger powder volume per layer. The binder might haveless than sufficient time to diffuse vertically to the previouslyprinted layer while the binder drop is large enough for lateralspreading. As a result, larger wire width still results, with muchless wire width variation. At the optimal printing layerthickness, such as 35 μm, binder vertical spreading and lateralspreading proceed and finish at approximately the same time.This very desirably leads to smallest wire width and the smallestwire width variation. Since the binder is distributed all withinthe wire mesh structure, it also leads to stronger 3D wire meshstructures. The actual binder spreading process requires in-situobservation, which will be discussed in future work.

Based on the above observations and understanding, the 3Dmesh structures are printed as shown in Fig. 11 under theoptimal conditions: 35 μm printing layer thickness, 170%binder saturation. It reproduces the designed 3D mesh structuresin Fig. 2 with high accuracy and integrity. Clearly, 3D printingis capable of producing near net-shape intricate mesh structureswhen the printing condition is optimized. There are no othertechniques available to produce mesh structures in suchcomplexity and resolution.

ness and binder spreading cases.


4. Conclusions

In this work, 3DP variables are systematically analyzed inorder to print TiNiHf 3D mesh structures. Two major 3Dprinting variables, printing layer thickness and binder saturationlevel, are evaluated. At the same printing layer thickness,breaking strength increases with the binder saturation level up to170%. At the same binder saturation level, 35 μm printing layerthickness yields 3D mesh structures with the highest integrityand the lowest dimensional deviation. 35 μm printing layerthickness and 170% binder saturation level are the optimal 3Dprinting parameters for the TiNiHf 3D mesh structures. Thisstudy demonstrates the unique capabilities of the 3DP techniquein printing intricate and 300 μm wire width structures.

Acknowledgment

Thismaterial is based uponwork supported by the Departmentof Energy under Award Number DE-FC07-06ID14739.

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3dp process for fine mesh structure printing

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