Consolidation of Polymer Powders by Selective Laser Sintering

Transcription

1 Consolidtion of Polymer Powders y Selective Lser Sintering J.-P. Kruth 1, G. Levy 2, R. Schindel 2, T. Creghs 1, E. Ys 1 1 K.U. Leuven, Division PMA, Heverlee, Belgium 2 Inspire AG, IRPD Institute for Rpid Product Development, St. Gllen, Switzerlnd ABSTRACT: Selective Lser Sintering of polymers hs found widespred pplictions for rpid prototyping nd rpid mnufcturing. Well known mnufcturing pplictions re the production of hering ids, y thousnds dy, nd the series production of design lmp covers. However, the rnge of polymer mterils tht cn e processed y SLS tody is still very limited, restricting most SLS prts to e mde from nylon (polymide). This pper tries to understnd the sics of selective lser sintering of polymers. It gives n overview of the mterils tht were yet successfully processed y SLS or tht re eing investigted, including reinforced or filled polymers, degrdle polymers, iocomptile polymers, etc. It lso compres the properties of current SLS polymers to tht of injection molded ones. 1 INTRODUCTION Selective Lser Sintering (SLS) is one of the populr Rpid Prototyping nd Mnufcturing (RP&M) processes tht emerged in the lst two decdes [39, 45]. It uilds 3D ojects y pplying susequent lyers of powder mteril on top of ech other nd y consolidting ech lyer with lser em scnning the powder lyer in ccordnce with the CAD geometry of the prt, efore the next lyer is pplied nd consolidted (Figure 1). The min dvntges of SLS over other RP&M methods re: - SLS llows uilding prts in wide rnge of mterils, including polymers, metls nd vrious types of composites (e.g. metl-metl, metlpolymer, polymer-cermic, metl-cermic composites) [8, 41]. Reserch is lso going on to process pure cermic prts using SLS [41]. - Prts produced y SLS demonstrtes mteril properties tht re very close to the mteril properties otined y other mnufcturing processes, like injection molded plstic components, or cst nd mchined metllic prts. The SLS process is indeed not limited to the use of dedicted mterils, like photopolymers in stereolithogrphy or wx-like mterils in ink-jet printing processes. Theoreticlly, ny mteril tht cn e provided in powder form nd tht melts when rising the temperture (which mens lmost ny mteril) cn e processed y SLS, nd s the finl prt density pproches full density, their properties my rech those of similr ulk mteril. Figure 1 : Bsic lyout of SLS Even though SLS theoreticlly llows to process lmost ny mteril nd to rech norml ulk mteril properties, prctice is still fr from this idel sitution. Depending on the type of lser consolidtion pplied, the powder my not lwys e consolidted to full density or my even not e processle. Consolidtion mechnisms tht could e induced during lser heting re: solid stte

2 sintering, liquid phse sintering, prtil melting, full melting (often referred to s Selective Lser Melting or SLM) or chemiclly induced inding [41]. Solid stte sintering is tody hrdly used, ecuse it would require very slow scnning process to keep the powder prticles t high temperture for sufficient long time to llow diffusion of toms to occur, which minly pplies to metls nd cermics, ut not t speeds comptile with the demnd for economic RP&M. Chemicl induced inding is lso hrdly pplied tody, leving minly two consolidtion principles over for current prctice: liquid phse sintering or prtil melting (commonly referred to s SLS) nd full melting (commonly referred to s SLM). Lser consolidtion of polymers normlly involves melting of thermoplstics (prtil-sls or full-slm). A cler distinction should e mde etween (Figure 2): (semi-)crystlline thermoplstics morphous thermoplstics. When heted up from very low to very high tempertures, ll those thermoplstic mterils will chnge from hrd (solid nd glssy) structure to softer (tough lethery or ruery, solid or nonpourle) structure nd finlly turn into viscous flowing melt: see Figure 3. Figure 3 : Phses nd trnsition tempertures of some polymers Figure 2: Thermoplstic polymers (Red = mteril used in SLS) Tody, only few powder mterils re ville on the mrket for SLS or SLM processing. SLS/SLM of polymers mostly (±95%) involves polymide-12, i.e. typicl nylon grde. Only few other polymer powders re commercilly ville tody for SLS: limited success ws chieved with polycronte nd polystyrene (see elow). SLS nd SLM of metls is tody lso restricted to minly stinless steel nd few grdes of tool steel, while titnium (minly Ti6Al4V) nd luminum strt to rech industril pplictions. This pper focuses on the consolidtion of polymers y SLS nd SLM. It will lso give some exmples of other consolidtion mechnisms like chemicl induced inding or cross linking of polymers. It will descrie the fundmentls of polymer processing nd descries the vrious clsses of polymers tht cn e lser processed strting from deposited powder lyers. 2 POLYMER CLASSES AND BASIC CONSOLIDATION Even though polymer is the most processed type of mteril in SLS/SLM, the consolidtion phenomen invoked for polymers re proly still mongst the lest understood or t lest the lest descried in literture [6, 54, 64]. Mterils however differ in the wy nd the tempertures t which those trnsitions occur. Semicrystlline polymers hve glss trnsition temperture T g tht is elow or round room temperture (-1 to 5 C) nd distinct melting temperture T m tht is ove 1 C (etween 1 nd 4 C) t which significnt volume chnge hppens: see Figure 4. Amorphous polymers do not depict cler melting temperture rnge. They hve glss trnsition temperture T g tht lies round 1 C nd ove which the mteril will grdully evolve to lethery, ruery nd finlly liquid stte s temperture increses, without cler trnsitions. Figure 3 indictes the glss trnsition rnge ( T g ) nd melting rnge ( T m ) for the three most importnt semi-crystlline polymers (PE, PP, PA 6) nd indictes for three morphous polymers (PS, PC, PMMA) the glss trnsition temperture rnges ( T g ) nd the temperture rnge in which the trnsition to lethery mteril goes quickly ( T f ). This lst rnge ( T f ) cn physiclly not e defined exctly. Notice lso tht the T g nd T m vlues lrgely depend on the moleculr weight (MW) of the polymer. This explins why there might e significnt difference in SLS processility etween low nd high moleculr weight PA or PE.

3 Figure 4 : Comprison of reltive volume etween morphous nd semi-crystlline polymers [6]. The temperture trnsitions nd melting rnge of polymers cn e oserved y recording DSC plot y Differentil Scnning Clorimetry nlysis. It mesures the difference in the mount of het required to increse the temperture of smple of the given polymer nd of reference piece or pn s function of temperture (Figure 5). Both the smple nd reference re mintined t very nerly the sme temperture throughout the experiment. Figure 6 shows typicl DSC plot of PA 12. It clerly shows melting pek during the heting cycle nd re-crystlliztion pek during the cooling cycle of the DSC test. Figure 7 gives the DSC plots of Durform PA 12 nd PA 6 during the heting cycle [58]. Notice tht the sign of the differentil het flow my e inverted in the DSC plots. Figure 7 illustrtes the much smller melting rnge (smll width of pek round 187 C) nd the smller melting temperture (T m =187 C) of PA 12 s compred to PA 6 (T m =223 C). This explins, esides viscosity differences (see 3), why PA 12 is more prone to SLS thn PA 6 (or PA 66 hving T m = 262 C nd wider pek thn PA 12). rpidly chnge into viscous liquid. The molten polymer flows in etween the powder prticles, forming sintering necks. With enough het, the complete lyer is fully molten nd overlps to the previous lyer. As the molten polymer cools down elow T m, polymer crystls nuclete nd grow, recreting regions of ordered moleculr chins (crystllites) mixed up with disordered morphous regions. Aove T m, the polymer depicts reltively low viscosity, s compred to morphous polymers, which fvors the rte nd mount of consolidtion. The densities otined will e close to full density nd the mechnicl properties will e close to those of molded polymers. However, the freezing of the polymer t T m coincides with n importnt shrinkge (phenomenon not occurring with morphous polymers), tht my induce geometricl inccurcies nd distortion of the prt (Figure 4). A good wy to prevent this is to prehet the polymer powder to temperture slightly elow its melting temperture nd keep it there for certin time fter consolidtion [66]. Idelly the powder should e preheted nd post-heted to temperture etween the crystlliztion nd melting temperture. In such wy, no mjor shrinkge will occur during lser melting, nd loclized shrinkge (nd hence prt distortion) cn e voided: the shrinkge will occur in uniform wy when the prt will e slowly cooled down in uniform wy fter the whole uild is finished. To fcilitte the control of pre- nd postheting nd of shrinkge, it is recommended to hve certin temperture rnge etween the crystlliztion nd melting peks in the DSC plot, s depicted in Figure 6 for PA 12 (see further 3). Figure 5 : Differentil Scnning Clorimetry nlysis [72] The lser consolidtion of (semi-)crystlline polymer powders will hppen y heting ove their melting temperture T m. Semi-crystlline mterils hve highly ordered moleculr structure with shrp melt points. They do not grdully soften with temperture increse ut rther remin hrd until given quntity of het is sored nd then Figure 6 : DSC-curve for melting nd crystlliztion of PA12 [4,74] The consolidtion of morphous polymer powder occurs y lser heting ove the glss trnsition temperture, t which the polymer is in much more viscous stte thn semi-crystlline polymers t similr temperture. Amorphous polymers hve rndomly ordered moleculr structure. They do not hve shrp melt point ut insted soften grdully s the temperture rises. The viscosity of these mterils chnges when heted, ut they seldom re s esy flowing s semi-

4 crystlline mterils. The flow nd sintering rte will e less, resulting in lower degree of consolidtion, higher porosity, less strength, ut lso lower shrinkge tht is fvorle in cses of SLS of ptterns for producing mould for molding or csting (i.e. Indirect Rpid Tooling, investment csting ptterns, etc.). The higher porosity is lso fvorle to void reking the cermic investment csting shell when heting the SLS pttern during the deinding cycle intended to relese the mold cvity. In severl pplictions post-infiltrtion of the pores of the SLS prt is pplied to consolidte it. polymer powder my provide essentil dditionl informtion to the DSC plots in order to chrcterize it SLS processility (see elow). Figure 7 : DSC plot of PA 12 (Durform) nd PA 6 In conclusion for semi-crystlline polymers we rther pproch full melting consolidtion (even though the prts my still depict some porosity), wheres for morphous polymers we rther see prtil melting inding mechnism. The former cn e clled Selective Lser Melting, while the ltter coincides with liquid phse Selective Lser Sintering. However, unlike metl powder processing [4], this terminology is rrely used for lser processing of polymer powders: one generlly refers ll polymer vrints s Selective Lser Sintering. Severl reserchers tried to nlyze nd model the complex ehvior of polymers during lser sintering, including spects like viscosity, sintering (neck formtion, viscous flow), lser sorption, temperture distriution, therml conductivity, etc. [8, 13, 34, 42]. The complex therml ehvior of polymers is mplified in SLS processing due to severl fctors. Among others, the costly pulveriztion either y cryogenic milling or precipittion-process (Figure 8) limits the vilility of polymer powders. The pulveriztion cn influence lso the consolidtion process: the powder lyer s therml chrcteristic differs. Moreover, the moleculr weight of the polymer will influence the melt viscosity. Chrcterizing the melt flow index (MFI) of Figure 8 : Powder prticle shpes REM, ) precipittion, ) Cryogenic milling The mteril is in powder form with certin prticle size distriution (Figure 9) nd the induced energy, heting the fine powder prticles, my led to evportion or disintegrtion which in turn disturs the process. It shields off the lser window nd the IR heting elements. In short, this hs negtive influence on the process consistency nd the economy of the process. Further the fine prticles my crete either dusty tmosphere or cogultions clustering preventing the homogeneous deposition of lyers, thus preventing good lyer consolidtion. Figure 9 : Typicl prticle size distriution of Durform PA 12 (Source FHSG St. Gllen) A vriety of commercilly ville technicl polymers with excellent properties for SLS hve een tuned y lending or y specil design of the

5 chin length (nlogy to moleculr weight rnges). Often they result in multi-pek DSC digrm. SLS cn hndle only one fixed controlled consolidtion temperture or severl very close peks (typiclly 5-1 o C difference), which mens tht polymers with more thn one tight melting rnge re not processle with SLS. Figure 1 demonstrtes the DSC-curve of commercil pulverized lend with no rel full melting SLS processility, due to two different melting points t 178ºC nd 185ºC. We cn ssocite seven min SLS polymer powder mterils with different ppliction fields. The four min thermoplstic SLS pplictions re:. SLS of polymer prts: semi-crystlline polymers ( 3). SLS of investment csting ptterns: morphous polymers ( 4) c. SLS of metl or cermic prts using scrificil polymer inder: thermlly degrdle morphous polymers ( 5) d. SLS of reinforced or filled semi-crystlline polymers for highly loded prts ( 6) SLS tody lso llows production of prts using: e. Flexile elstomeric mteril prts ( 7) f. Polymer-polymer lends (used in the rcing industry) ( 8) g. Thermosetting mterils ( 9) Those different mteril ctegories re discussed in more detil in following sections. Cooling curve polymers is not totlly cler yet. Some semicrystlline polymers re le to crystllize much fster thn others, s result of their chin structure. The rte of crystlliztion is minimum ner T g nd T m, nd reches mximum etween those two tempertures. Crystlliztion rte should however e kept reltively slow (reltive to the verticl uild rte) in order to void prt distortion due to grdient freezing shrinkge which would e loclized in upper lyers. Good crystlliztion nd uniform shrinkge is then otined y keeping the powder ed t high temperture for sufficient long time fter sintering. Figure 11 : Exmple of PA 12 prt produced y SLS As lredy introduced in 2, the DSC plot of well processle thermoplstic mteril for SLS should depict: - nrrow melting rnge (nrrow melting pek in heting curve): Figure 7 - non-overlpping melting nd re-crystlliztion peks: Figure 12, Figure 13, Figure 14 - nd should void doule melting peks: Figure 1. Figure 7 lredy illustrted the difference of PA 12 nd PA 6 in terms of melting rnge. Heting curve Figure 1 : Commercil pulverized lend with no rel SLS processility: melt curve (left) nd re-crystlliztion curve (right) 3 SEMI-CRYSTALLINE POLYMERS Tody, the production of prototypes (RP) nd functionl prts (RM) y SLS is siclly limited to nylon, i.e. polymide (PA) [2, 11, 12, 13, 36, 78]: Figure 11. Quite some reserch is going on using other (semi-)crystlline thermoplstics: polyethylene (PE) [55, 57], POM [54, 74], PEEK [52, 53], PCL [7, 71, 75], HDPE [29]. PEEK nd PCL re of specil interest s iocomptile polymers [52, 65, 75]. The fundmentl reson why PA sinters well, while more difficulties re experienced with other Figure 12 : DSC plots (heting nd cooling) of new reinforced PA 12 powder contining elongted rther thn sphericl eds for etter strength A powder with slow re-crystlliztion rte, s depicted y non-overlpping or slightly overlpping endothermic nd exothermic peks during the heting nd respective cooling phses of the DSC nlysis (Figure 12), might result in nerly fully

6 dense prts with miniml distortion, while highly overlpping peks will yield d results (Figure 13). Figure 14 [74] illustrtes tht PA22 (i.e. PA 12 powder specilly developed for SLS) hs more seprted melting nd re-crystlliztion peks, thn norml milled PA 12 powder. A lrger dt rnge etween those peks llows for lrger process window in term of fluctutions of temperture in time nd spce within the processing chmer nd uild prt [54]. Figure 13 : DSC plots (heting nd cooling) of IP6 powder showing overlp etween melting nd re-crystlliztion peks [US ptent 5,648,45] Figure 15 : Trnsmission light microscopy imges of microtome sections of PA (left) nd POM (right) [54] To otin fully dense prts, the melt viscosity should e low enough to llow complete consolidtion within the time scle of the process. Pure polymers with melt viscosity in the rnge of few tens to few thousnds poise (e.g. nylon nd wxes) cn e processed successfully nd rech ner full density [6]. However, even for PA 12, the SLS prt still contins smll mounts of porosity nd the process is something etween prtil nd full melting. The densities otined in SLS of PA 12 re just slightly elow those of compression molded prts ( versus 1.4 g/cm 3 ), yielding very similr mechnicl properties under compression, ut somewht lower tensile strength nd notched Izod vlues tht re sensitive to smll voids. ( ) Figure 14 : DSC plots of PA nd POM The Chir of Polymer Technology (LKT) of the University of Erlngen successfully produced polyoxymethylene powders (POM) y cryogeniclly milling for ppliction in SLS. Although the intervl etween the melting nd re-crystlliztion peks is smller thn for PA 12/PA22 (Figure 14), they were le to produce prts in POM nd luminum filled POM (3% Al y volume) tht demonstrted good mechnicl properties nd even etter surfce qulity thn commercil PA 12 SLS powders (i.e. PA22). Figure 15 illustrtes the improved surfce roughness of POM prts s compred to PA22 [54]. This figure lso illustrtes the higher crystlline structure of POM s compred to PA22, tht results in slightly higher strength (47 vs. 45 N/mm²), higher E-modulus (371 vs. 198 N/mm²), ut lower elongtion t rek (out 3% for POM, compred to 9% for PA22 nd 2% for injection molded POM). Figure 16 : ) The rise in viscosity of re-used PA 12 mteril, ) resulting ornge peel texture on SLS prt The melt viscosity is linerly relted to the moleculr weight (MW). For good interfusion of the polymer chins, sufficient to provide prticle necking nd lyer to lyer dhesion, low melt viscosity is desirle. However, low viscosity results in high shrinkge nd poor prt ccurcy. Thus n optimum MW rnge exists ut is not esily controllle, neither t the polymer production nor during ging in process.

7 In contrst to SLS or SLM of metls, one hs to e wre of the therml degrdtion of polymer powders over time when re-using the sme powder repetedly. The long exposure to het leds to chin growth, rise in MW nd so rise in viscosity. This cuses non-constnt consolidtion conditions nd shift in the melting temperture. The empiricl melting temperture rmps up s function of uild height nd ge of the powder. This results in the decy of the MFI (Mold Flow Index) hence drop in powder flowility nd rise in melt viscosity preventing proper sintering with resonle qulity. In the limit this cn led to cretion of unsmooth ptterns on the surfce known s ornge peel texture (Figure 16) fter certin numer of runs (Figure 16). A drop in mechnicl properties cn e oserved s well. The consolidtion temperture my e rised to compenste for those phenomen, ut only in nrrow rnge nd with limited effectiveness. A etter wy is to limit those prolems y lwys using mixture of virgin nd re-used powder (typiclly 7% virgin 3% reused): see elow. difficulty in plcing nd forming the mteril ecuse of the high viscosities, ut it cn reduce shrinkge nd thus improve the dimensionl ccurcy. When polymer powder is reused in SLS, the MFI is lredy out three times lower thn the virgin powder (Figure 17). This leds to higher viscosity, nd my deteriorte the qulity of the product s descried ove. With PA 12, process stility nd repetility in the consolidtion process cn e otined y keeping the MFI within constnt rnge y working with n pproprite mix of virgin nd recycled powder [43]. All those phenomen re mplified for glss filled PA 12 mteril, leding to rpid decline in mechnicl properties, cler evidence of different consolidtion conditions (Figure 18). The ging nd the resulting chnge in consolidtion phenomen nd finl component properties re very minor with morphous mterils like PS nd elstomeric polyester sed mterils tht re discussed in sections 4 nd 7. Virgin Recycled Interior qulity MFI Build (run) QA MFI Liner (MFI) Figure 17 : Melt flow index of powder s function of numer of uilds in which powder is used (Source FHSG St. Gllen) Melt Flow Index (MFI) is criticl chrcteristic. It is the flow in grms tht occurs in 1 minutes through stndrd die of 2.95 ±.5 mm dimeter nd 8. ±.25mm in length when fixed pressure is pplied to the melt vi piston nd lod of totl mss of 2.16 kg t temperture of 19 C (some polymers re mesured t higher temperture, some use different weights nd some even different orifice sizes). MFI is n ssessment of verge moleculr mss nd is n inverse mesure of the melt viscosity; in other words, the higher MFI, the more polymer flows under given test conditions. Hence the MFI of polymer is vitl to nticipting nd controlling its processing. Generlly, higher MFI polymers re used in injection molding, nd lower MFI polymers re used with low molding or extrusion processes. To chieve functionlly strong SLS prts, polymers tht hve high moleculr weights should e fvored. High moleculr weight cn led to Figure 18 : Tensile strength nd elongtion of PA-GF polymers s function of numer of uild in which nonrefreshed powder is used PA polymer powders for SLS re normlly supplied without dditives. Some dditives my e dded to fvor the powder fluidity nd powder spreding, ut those hve no influence on viscosity nd sinterility. Some SLS tests hve een performed with POM, PE, PBT nd PPS hving een modified y ddition of stilizers nd fillers [54]. The University of Erlngen is lso testing PE- UHMW powders tht re modified with sorers nd fillers [74]. The use of fillers will e further discussed in 6. Alterntively, SLS prts in PA 12 nd other polymers my e strengthened pplying crosslinking gents. Those cross-linking gents re often pplied on the finished "green" SLS prt y impregntion nd y pplying high level rdition [74]. The University of Erlngen demonstrted n

8 increse of E-modulus (from 165 to 1935 N/mm²) nd tensile strength (from 38 to 46 N/mm²), with drop in elongtion t rek (from 9 to 4%) [74]. Atmospheric control in the SLS chmer is very criticl in terms of temperture (control of sintering nd shrinkge), humidity (powder fluidity) nd oxygen content (to void therml degrdtion of the polymer y oxidtion resulting in depolymeriztion). A nitrogen tmosphere is normlly used, together with creful control of the mount of rest oxygen. Typiclly polymers like PE nd PP re more prone to rpid oxidtion thn romtic ckone polymers like PET (semi-crystlline) or PC (morphous). Powder prticle morphology lso influences the lser consolidtion. Figure 19 shows powder (PA 12) with ner to sphericl shpe. This yields lower density during lyering ecuse ir voids remin (Figure 19). When no pressure is pplied during consolidtion (s the one occurring during injection molding), the voids nd ir trps remin in the solid prt. This reduces the effective cross section nd the tensile strength compred to molded prts from the sme polymer (1-2% reduction). Figure 19 (PA 12) nd Figure 2 (PS) depict the tensile rek cross section demonstrting prtil melting regime, which is more pronounced in cse of the morphous PS (Figure 2). Comprison etween SLS nd molded prts concerning strength is discussed in 1. Figure 19 : ) PA 12 Powder, ) tensile rek cross section showing some ir voids (Source FHSG-St. Gllen) Severl reserchers investigted SLS of iocomptile semi-crystlline thermoplstics like PEEK nd PCL (Poly-ε-cprolctone) [5, 51, 53, 6, 65, 71, 75]. PCL is iodegrdle liner polyester, while PEEK is not iodegrdle. Their respective melting tempertures lie round 62 C for PCL nd 34 C for PEEK. A polymer hs never one specific moleculr weight, ut consists of mixture of molecules with different chin lengths, nd so, different moleculr weights. The men moleculr weight nd the moleculr weight distriution re importnt properties of polymer. Typiclly SLS of PCL with MW of 5. g/mol demonstrted good lser sinterility [75], while PCL with MW of 4. g/mol could not e lser sintered well [7, 71] due to ig distortions cused y shrinkge. This shrinkge is due to the lower moleculr weight of the second PCL powder, nd due to the purity differences compred to the first powder. Figure 2 : ) PS Powder (CstForm), ) Tensile rek cross section demonstrting prtil melting regime 4 AMORPHOUS POLYMERS Amorphous polymers with strongly temperturedependent viscosities (or high ctivtion energy for viscous flow) hve een redily processed using SLS [6, 7, 24, 32, 49]. Typicl exmples re polycronte (PC) nd polystyrene (PS). Depending on the powder size nd MW, they re generlly preheted to temperture ner or even ove the glss trnsition temperture Tg nd will melt s the temperture further rises ove Tg. They minly hve the dvntge tht their volumetric shrinkge, when cooling down, does not show jump, s it does for semi-crystlline polymers (Figure 4). This fct provides high ccurcy with little distortion. However, morphous polymers show lower level of consolidtion (higher residul porosity) nd hence do not rech the strength of their molded equivlent (Figure 2). The higher degree of porosity, low therml shrinkge/expnsion nd high ccurcy

9 mkes them well suited for investment csting ptterns. 5 THERMALLY DEGRADABLE AMORPHOUS POLYMERS FOR SLS OF METAL OR CERAMIC PARTS Vrious SLS pplictions mke use of powder comining degrdle polymer inder nd structurl mteril (metl or cermic) tht produces green prt in which the structurl prticles re ound into degrdle polymer mtrix [9]. These green prts re further processed in furnce to urn out the polymer ( de-inding ) nd to consolidte the remining polymer-free metl or cermic prt y post-sintering, infiltrtion, or HIPing. The polymers used s the scrificil inder in those pplictions re different from the polymers descried in the previous nd susequent sections. They should llow de-inding, i.e. therml depolymeriztion. Polymers used for this purpose re PMMA nd copolymers MMA-BMA tht cn e deinded in furnce t tempertures round C. They should not e wter solule [6, p. 112] nd should hve sufficient strength, even when heted up in the de-inding furnce, in order to void tht the green prt would collpse efore some solid stte sintering cn consolidte the metllic or cermic prticles together. The polymer must lso e fully de-indle (i.e. de-polymerized nd sulimted) nd should not yield excessive contmintion of the remining prt with cron residuls (importnt e.g. for steel prts). If the strength of the polymer in the green stge is insufficient, the green prt cn e consolidted y cross-linking the inder. This cn e done y impregnting the green prt with wter solule thermosetting crylic emulsion nd drying it in oven t 5 C, efore it goes to the de-inding furnce [6, p. 139]. Alterntively, cross-linker cn e dded into the degrdle polymer coting of the powder [6 p. 139]. This will eliminte the step of impregnting the green prt. When using scrificil inders, cre should e tken ecuse de-polymeriztion could lso occur during SLS [6 p. 18, 35, 37]. Indeed, these polymers re deliertely designed to de-polymerize s n id to their therml removl in post-sls processing. Polymers sed on copolymers of n- utyl methcrylte (BMA) nd methyl methcrylte (MMA) hve een oserved to lso de-polymerize during SLS processing [68, 69]. The polymer inder my e dded to the structurl mteril in two wys: mixing [2, 28, 46] or coting [61, 62, 67, 77]. Typicl volume of inder in the powder is 5% for mixed powders [2] s well s coted ones [6, p136]. The ltter typiclly corresponds to 5 µm polymer coting on 55 µm 18 cron steel grin [68]. Figure 21 [78] demonstrtes three stges in the SLS process of Aptite-Wollstonite (AW) glss cermic with 5 wt% MMA-BMA inder: ) powder mixture efore processing (white prticles re AW glss cermic, grey prticles re MMA-BMA), ) highly porous prt fter SLS processing (The few polymer connections re just enough to give the 'green' prt sufficient strength to e trnsferred to the post-tretment furnce) c) 'rown' prt fter polymer de-inding nd 1 hour firing t 115 C. During firing, AW is prtly melted. The prt still is not totlly dense nd my need further densifiction to convert the rown prt into finl one. c Figure 21 : SLS of AW glss cermic with scrificil inder: () Powder mixture () Green prt (c) Brown prt Figure 22 : Injection mold mde t K.U.Leuven y SLS of polymer-coted steel powder (LserForm ) nd molded connector housings

10 Figure 22 shows typicl mould mde y SLS of polymer-coted steel powder (LserForm ). After uilding the prt, the green prt ws de-inded in furnce nd infiltrted with ronze. 6 SEMI-CRYSTALLINE REINFORCED POLYMER FOR REINFORCED PLASTIC OR COMPOSITE PARTS Tody there exist severl SLS powders imed t production of reinforced polymer prts. Applictions include prts mde from glss reinforced PA [14, 16], Cu filled PA [1], Al filled PA [1, 22, 47], SiC- PA [27, 33], HA-HDPE, i.e. hydroxyptite reinforced high density polyethylene [31, 57], HA- PA [58, 59] nd other filled or reinforced polymers [3, 15, 18, 19, 25, 26, 3, 38, 73, 8]. Unlike the scrificil polymer inders descried in 4, the polymers used here should withstnd therml degrdtion nd should e durle. Figure 23 illustrtes n injection mold mde from Cu reinforced polymide. reinforced PA powder hving elongted filler eds. This new powder turned out to e etter thn the trditionl GF-PA powder: tensile strength x 1.8; E- modulus x 1.3; elongtion x 3.3. Figure 24 : Glss-nylon powder mixture (Source: K.U.Leuven) Aluminum filled PA 12 often shows cogultion, since mixing the luminum nd PA powder prticles isn t lwys very successful. This cuses segregtion during lyering due to the differences in size nd specific weight (Figure 28). This fct is even more striking in PA 12-Cu. The inhomogeneous solid with voids cretes mechnicl defects nd deteriortes the tensile strength. The locl het concentrtion on the filler hs negtive influence on the inder polymer cusing very short recycle ility of 2-3 times only (for PA 6-8 times depending on judgment criteri). Figure 23 : Injection mold mde from Cu-filled Polymide nd Polypropylene molded prts (injected t 2.76 MP nd 23 C) The powder for producing such polymer mtrix composite cn e provided in 3 different wys: - The initil powder my e mixture of polymer prticles nd reinforcement eds. Figure 24 shows such mixture of sphericl PA nd glss prticles: on the right side of the picture, the glss prticles re colored lue with Prussic ink for etter visiility - Alterntively, the single powder prticles my lredy e composite contining of polymer mtrix (typiclly PA or PE) contining filler prticles [57, 58]: see Figure A 3 rd lterntive is to use polymer-coted metl or cermic powder [1, 48]. Figure 26 shows the frcture surfce of prt mde with populr commercil glss filled nylon powder. One cn see tht the connection etween the glss eds nd PA inder ws not perfect, tht mny glss eds hve een pulled out nd tht the PA ws stretched during rekge, resulting in rough spongy surfce. Figure 27 shows new Figure 25 : Cross section of glss filled nylon prt (Source: K.U.Leuven) Figure 26 : PA-12 GF One of the most populr glss eds reinforced HTD mterils (Source FHSG St. Gllen)

11 Figure 27 : New glss filled PA powder (Source FHSG St.Gllen) Figure 29 : Polyester sed elstomer ) Green prt fter sintering t low hrdness ) Infiltrted with polyurethne (Source FHSG St. Gllen) The influence of lser processing prmeters on Shore A hrdness nd ductility is shown y Figure 32. Clerly the prts processed with higher lser power ecome more ductile nd hve higher Shore A hrdness. The influence of lser processing prmeters on elstic modulus E nd the tensile strength R t rupture is shown y Figure 33. Figure 28 : ) PA 12 nd 3% Al lend; ) tensile rek cross section with defects due to inhomogeneity (Source FHSG St. Gllen) 7 ELASTOMERIC MATERIALS Elstomeric polymers hve structure with long chins nd only few cross-links etween them. Below the glss-trnsition temperture they re rittle, ove they re very elstic. When the density of cross-links enlrges, strength nd stiffness lso enlrge, nd ductility lowers. A new elstomer powder mteril for SLS ws recently relesed [44]. Figure 29 demonstrtes this polyester sed elstomer (ptent pending). A typicl prt is depicted in Figure 3. The most significnt properties in the thermoplstic mteril selection were considered nd compred with reference to elstomeric mterils s Neoprene, EPDM nd nturl ruer. Figure 31 gives comprison of the different elstomeric mterils for Shore A hrdness nd elongtion t rek: mterils 1 to 4 re SLS mterils. Figure 3 : Exmple of flexile shoe mde y SLS of polyester-sed powder 8 POLYMER BLENDS Polymeric lends offer n lterntive men to otin SLS prts with specific structure nd properties, permitting the development of new pplictions. Most polymeric lends re multiphse systems nd, therefore, their properties lrgely depend on their microstructure [17, 76]. Slmori, for instnce, used lends of PA nd HDPE (lend rtio s of 8/2, 5/5 nd 2/8 wt%) to chieve dedicted

12 properties. Depending on the lend rtio, different phses nd micro-structures were oserved using SEM, EDX nd XRD nlysis [56]. Others re mixing PS nd PA [63]. Shore A Hrdness Shore A (low) Shore A (high) Elongtion % Mteril Elongtion % E Modulus [MP] R [Mp] Sintflex: E module vs. LS Lser Power not infiltrted infiltrted Lser Power [W] Sintflex: Rupture t rek R vs. E Module R R (inf.) Figure 31 : Developed elstomeric mteril for SLS in reltion to commonly used elstomers (Source FHSG St. Gllen) Shore A Hrdness Shore A Hrdness Sintflex not infiltrted 5W 7W 9W 11W 13W 15W Elongtion (%) Sintflex infiltrted 5W 7W 9W 11W 13W 15W Elongtion (%) Figure 32 : Hrdness Shore A vs. Elongtion [%] for infiltrted nd not infiltrted prts processed with different lser powers (Source FHSG St. Gllen) In US ptent [21] the following is climed: A prticle for use in selective lser sintering (SLS), including core (1) formed from t lest one first mteril, nd t lest prtil coting (2) of the core (1) with second mteril (further components re optionl), the second mteril hving lower softening point thn the first mteril, wherein the softening point of the second mteril is lower thn pproximtely 7 C E Modulus [MP] Figure 33 : () The elstic modulus [Mp] ginst lser power for infiltrted nd not infiltrted prts () The tensile strength R [MP] t ruptures ginst elstic modulus (Source FHSG St. Gllen) The coting (2) generlly contins polymer, preferly thermoplstic polymer, e.g. polyvinyl cetl, preferly polyvinyl utyrl. It my consist of lloys with low softening point which re used e.g. in fuses. Moreover sturted liner croxylic cids with chin length of 16 (e.g. heptdecnoic cid, melting point 6-63 C.) or polymers in the rodest sense my lso e suitle. The softening point of the 2 nd mteril of pproximtely 7 C or elow, llows lser sintering to e crried out t significntly lower tempertures compred to prticles which hve een used hitherto, nd therefore lso llows significntly lower temperture difference etween irrdited prticles nd stndrd room temperture. Tests hve shown tht the lower mximum temperture difference lso improves the temperture homogeneity of the uilding spce s whole. 9 THERMOSETTING MATERIALS Thermosetting polymers cn e used in different steps of the SLS process. These mterils cn e used s n infiltrnt [23, 5], ecuse it is reltively inexpensive pth to fully dense, stiff, net shpe, polymer mtrix composite prt. In ddition this is very useful intermedite step for fully functionl mterils, s criticl surfces nd tolernces my e chieved esily t this stge. An exmple of infiltrtion with thermosetting polymers is the production of metl-epoxy molds vi SLS indirect processing [6, p143]. In this process, SLS is used to form green mold cvity inserts from metl

13 powder tht is coted with fusile thermoplstic inder. In susequent steps, the inder is thermlly removed nd the metl powder is oxidized to form porous metl/cermic cvity tht shows little shrinkge nd generlly excellent retention of geometry, reltive to the green prt. The cvity is then strengthened nd seled y infiltrtion nd cure of n epoxy tooling resin. Figure 34 : Binding mechnism for epoxy resin to iron prticles It is lso possile to produce metl prts y lser sintering mixture of metls with thermosetting polymers [46]. When thermosetting mteril is exposed y lser source, the thermosetting mteril turns into viscous liquid instntly. When for instnce lser sintering n epoxy resin mixture with iron powder, the polr groups (e.g. epoxy group) in the molecule of the resin re ctivted simultneously. The liquid flows penetrte the pipes mde from pores nd wet metl prticles. As such, they mke ridges from one prticle to nother. The inding effect, depicted in Figure 34, is dominted y the interfcil chrcteristic etween the resin nd the iron. Iron surfces commonly ttch some ctive hydrogen toms ecuse of its ttrction of some molecules such s H 2 O nd HCl due to its high polrity. Hydrogen onds occur etween the electronegtive oxygen toms in polr groups in resin molecules nd the electropositive ctive hydrogen ones on iron surfces. Iron prticles re strongly onded ecuse hydrogen ond ttrction is more intensive thn tht of inter-moleculr ction existing on the interfces etween iron surfces nd other non-polr polymers. The resin viscosity is lowered t higher temperture (i.e., lser energy) nd its viscous liquid cn spred esily. Thus mny more iron prticle surfces cn e ttched y resin. However, degrdtion of resin, induced y excessive lser energy, might occur nd reduce the onding cpility. 1 PROPERTIES OF MOLDED AND SLS POLYMERS Although gret efforts hve een mde nd some results were chieved, the resons for the reltive few SLS mteril options re mny. In future we would like to hve nerly the sme choice s e.g. in injection molding. Tody we cn only compre the differences nd compenste for deficits. Figure 35 shows ll the commercilly ville SLS mterils (in yellow) in comprison to some molded polymer mterils (in lue): the comprison is shown for tensile strength, tensile modulus nd elongtion. We find some deficits. While some SLS mterils hve comprle properties to molded ones, none of them is le to rech the highest vlues of tensile modulus nd strength, nor elongtion. The low elongtion cpility cn e explined y the mny short inding necks creting high stiffness with little elongtion. Tensile modulus [MP] Tensile strength [MP] c Injection vs. SLS Mterils d e f 8 Injection SLS, 1, 2, 3, 4, Elongtion [%] c d e f 8, 5, 1, 15, 2, 25, 3, 35, 4, Elongtion [%] Injection 9 SLS Figure 35 : Commercil ville SLS mterils (yellow) in comprison to some injection polymer mterils (in lue) Figure 36 : Boundry etween loose powder nd solid prt A further interesting finding indicted y Zrringhlm [78] is the oundry etween the loose powder nd the solid prt (see Figure 36, which

14 shows the microstructure of prt cross section). Under certin conditions unconsolidted prticles my remin in the solid prt. This cn e overcome esily y proper energy intke settings, correct scn strtegies like cross scnning (in X, Y, in X&Y nd so on) or multi times scnning of the sme lyers. The oundry conditions my e prtilly improved y the so clled out-line or skip scnning (i.e. scnning the lyer contours seprtely) imposing shrper order line etween loose powder nd consolidted prt. To conclude this pper on polymers, Tle 1 gives n overview of some commercil ville SLS polymers with their most importnt properties concerning the SLS process. Tle 1 : Overview of commercil ville polymers with most importnt properties for SLS Mteril T g (Glss trnsition temperture) ( C) T m (Melt temperture) ( C) PA MW (moleculr weight) (g/mol) PA 12 (41) PCL PEEK CONCLUSIONS This pper hs given survey of wys of consolidting polymer powders into solid mteril y Selective Lser Sintering. Although lot hs een chieved so fr mking SLS of polymers to one of the most populr RP&M techniques lot remins to e done to widen the scope of polymers nd polymer lends tht cn e processed y SLS nd to improve the properties of resulting SLS prts. 12 REFERENCES 1 3D Systems, 25, Durform AF dt sheet. 2 Ajoku, U., Hopkinson, N., Cine, M., 26, Experimentl mesurement nd finite element modelling of the compressive properties of lser sintered Nylon-12, Mterils Science nd Engineering, A/428: Alexndre, T., Giovnol, J., Vucher, S., Beffort, O., Vogt, U., 24, Lyered mnufcturing of porous cermic prts from cermic powders nd precermic polymers, Proc. LANE, Erlngen, Alscher, G., 2, Ds Verhlten teilkristlliner Thermoplste eim Lsersintern, PhD Disserttion, Universität-GH Essen, Shker Verlg, Achen. 5 Antonov, E.N., Bgrtshvili, V. N., et l., 25, Threedimensionl ioctive nd iodegrdle scffolds fricted y surfce-selective lser sintering, Advnced Mterils, 17/3: Bemn, J. J., Brlow, J. W., Bourell, D. L., Crwford, R. H., Mrcus, H. L., McAle, K. P., 1997, SFF: A new direction in mnufcturing, Kluwer Acdemic Pulishers, Msschusetts 261 USA. 7 Berzins M., Childs T.H.C., Ryder, G.R., 1996, The selective lser sintering of polycronte, CIRP Annls, 45/1: Bourell, D.L., Bemn, J.J., 25, Mterils issues in rpid prototyping, Proc. VRAP, Leiri: Bourell, D.L., Mrcus, H.L., Brlow, J.W., Bemn, J.J., 1992, Selective lser sintering of metls nd cermics, Int. J. Powder Metllurgy, 28/4: Burning S., 1998, Copper Polymide, 2 nd Europen SLS users meeting, K.U. Leuven, Belgium. 11 Culfield, B., McHugh, P.E., Lohfeld, S., 27, Dependence of mechnicl properties of polymide components on uild prmeters in the SLS process, J. Mterils Processing Technology, 182/1-3: Cheng, J., Lo, S., SLS processing studies of nylon 11 nnocomposites, Proc. SFF Symp., Austin: Childs, T.H.C., Huser, C., Tylor, C.M, Tontovi, A.E., 2, Simultion nd experimentl verifiction of crystlline polymer nd direct metl selective lser sintering, Proc. SFF Symp., Austin: Childs, T.H.C., Tontowi, A. E., 21, Selective lser sintering of crystlline nd glss-filled crystlline polymer: experiments nd simultions, Proc. IMechE Prt B, J. Engineering Mnufcture, 215/11: Chu, C. K., Leong, K. F., Tn, K. H., Wiri, F. E., Cheh, C. M., 24, Development of tissue scffolds using selective lser sintering of polyvinyllcohol/hydroxyptite iocomposite for crniofcil nd joint defects, J. Mterils Science: Mterils in Medicine, 15/1: Chung, H. Ds, S., 27, Processing nd properties of glss ed prticulte-filled functionlly grded nylon-11 composites produced y selective lser sintering, Mterils Science nd Engineering, A437/2: Cirdelli, G., Chiono, V., et l., 25, Blends of poly- (epsilon-cprolctone) nd polyscchrides in tissue engineering pplictions, Biomcromolecules, 6/4: Cruz, F., Coole, T., Bocking, C., Simoes, J., 23, Selective lser sintering of customized medicl implnts using iocomposite mterils, Tech. Vjesn., 1/2: Cruz, F., Simoes, J., Coole, T., Bocking, C., 25, Direct mnufcture of hydroxyptite sed one implnts y selective lser sintering, Proc. VRAP, Leiri: Dlgrno, K.W., Wood, D.J., et l., 25, Mechnicl properties nd iologicl responses of ioctive glss cermic processed using indirect SLS, Proc. SFF Symp., Austin: Dickens, D.E., Lee, B.L., Tylor, G.A., Mgistro, A.J., Ng, H., McAle, K.P., Forderhse, P.F., 1997, US ptent y DTM Corportion: Sinterle semi-crystlline powder nd ner-fully dense rticle formed therein, United Sttes Ptent EOS, Alumide dt sheet 23 Evns, R.S., Bourell, D.L., Bemn, J.J., Cmpell, M.I., 25, Rpid mnufcturing of silicon cride composites, Rpid Prototyping J., 11/1: Fn, K.M., W.L. Cheung, Gison, I., 25, Movement of powder ed mteril during the selective lser sintering of

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