Livarski vestnik, letnik 67, št. 1/2020 43 Uporaba 3D tiskanih polimernih modelov v livarstvu Applications of 3D printed polymer patterns in casting technologies Povzetek Delo opisuje raziskave različnih materialov za 3D tisk in natisnjenih modelov za uporabo pri različnih livarskih tehnologijah. Za izdelavo modelov in vzorcev sta bili uporabljeni dve tehnologiji 3D tiska, in sicer tehnologija ciljnega nalaganja materiala (FDM) ter stereolitografska tehnika (SLA). Natisnili so se trije različni materiali, kot so material na osnovi polilaktične kisline (PLA), polivinil alkohola (PVA) ter akrilatne fotosenzitivne smole (Castable). Natisnjeni vzorci so bili analizirani z metodami, kot so diferenčna vrstična kalorimetrija (DSC), dilatometrijska analiza ter določevanje vsebnosti pepela. S temi analizami smo okarakterizirali materiale za uporabnost pri tehnologiji precizijskega litja, kjer je temperatura tališča modela, temperaturna razteznost ter ostanek pepela po izžiganju modela ključnega pomena za uspešno izdelavo ulitka. Rezultati so pokazali, da so temperaturi tališča PLA in PVA materialov nižji kot pri smoli Castable in znašajo med 140 in 180 °C, kar je primerno za proces izžiganja modela. Podobno je temperaturna razteznost omenjenih materialov ugodna, saj pri segrevanju do 120 °C ne izkazujeta znatnega širjenja, nasprotno pa se smola Castable med segrevanjem do 120 °C širi, kar lahko povzroči pokanje školjk med segrevanjem. Delež pepela je bil največji pri PLA materialu, pri PVA in Castable pa je bil najmanjši. Iz rezultatov je moč zaključiti, da je za proizvodnjo modelov za precizijsko litje najugodnejši material PVA. Ključne besede: 3D tisk, polimerni materiali, precizijsko litje Abstract This paper describes a research of different 3D printing materials and printed models used for different casting technologies. In the research the Fused Deposition Modelling (FDM) and Stereolithography (SLA) 3D printing techniques were used with different printing materials such as Polylactic Acid based material (PLA), Poly Vinyl Alcohol based material (PVA) and Acrylate Photopolymer resin (Castable). Printed models were characterised by dilatometric analysis, Differential Scanning Calorimetry (DSC) and by ash content determination in order to evaluate the possibilities of such produced models being used at investment casting technology. The analysed properties such as thermal expansion, melting point of polymer material and ash content after burnout process are crucial at achieving good quality of cast parts. Results showed that the melting points of PVA and PLA materials are at 140 to 180 °C which is suitable for the burnout process. Thermal expansion is the most suitable for PVA and PLA materials since they are not expanding during the heating of the samples. The Castable resin has the highest melting point and is expanding until 120 °C which makes it M. PETRIČ 1 , R. CERC KOROŠEC 2 , M. VONČINA 1 , P. MRVAR 1 , S. KASTELIC 1 1 Univerza v Ljubljani, Naravoslovnotehniška fakulteta (SI) 2 Univerza v Ljubljani, Fakulteta za kemijo in kemijsko tehnologijo (SI) 44 Livarski vestnik, letnik 67, št. 1/2020 less interesting for investment casting models production. The ash content was the highest at PLA material and the lowest at PVA an Castable resin. According to presented results the PVA material is the most suitable for use in investment casting technology. Keywords: 3D-printing, polymeric materials, investment casting 1 Uvod 3D tisk je tehnologija, ki je vse bolj v uporabi za proizvodnjo različnih delov iz različnih materialov. Ta tehnologija je veliko hitrejša pri izdelavi omejenih, majhnih serij proizvedenih kosov v primerjavi z konvencionalnimi tehnologijami, ni pa še primerna za proizvodnjo velikoserijskih izdelkov. Največkrat je uporabljena za proizvodnjo prototipnih izdelkov. Poznanih je veliko aditivnih 3D tehnologij, najbolj uporabljeni materiali pa so polimerni. V zadnjem času se pojavljajo tudi tehnologije tiskanja kovinskih izdelkov, a v omejenem obsegu zaradi lastnosti kovin, kot je krčenje, ki vodijo v krivljenje, notranje napetosti, fazne transformacije itn. Možnost uporabe 3D tiska je pri tehnologiji precizijskega litja. Konvencionalni postopki precizijskega litja uporabljajo voščene modele, ki so sestavljeni v drevo, s pomočjo katerega se izdela keramična školjka. Danes so na voljo različni materiali, ki bi se lahko uporabili namesto voska in se lahko natisnejo. Taka tehnologija je lahko primerna za izdelavo prototipnih kovinskih izdelkov preko precizijskega litja. Tehnologija precizijskega litja zahteva materiale, ki so dobro iztaljivi in izgorljivi, kjer je delež pepela zelo nizek ter širjenje in krčenje pri izžiganju čim manjše. Take lastnosti lahko zagotavljajo samo polimerni natisnjeni materiali. Polimerni materiali so najpogosteje natisnjeni s tehnologijo ciljnega nalaganja materiala (FDM), kjer je filament potisnjen skozi segreto šobo in se odlaga na prej natisnjenih slojih, s stereolitografsko tehniko (SLA), kjer se 1 Introduction 3D printing is a more and more used technology for production of various parts from various materials. This technology can be much faster at small batch series than conventional technologies, but it is not yet appropriate for large scale productions. It is often used for production of prototype parts. There are many 3D technologies known and the most used materials are different polymers. Recently also metal powders can be used for 3D printing but with limited capabilities due to material properties such as contraction factors leading to distortions and stresses, phase transformations, etc. One option to use 3D printing technology is in the field of investment casting. Conventional investment casting technology uses wax patterns assembled together in a tree to produce ceramic shells used for casting. Nowadays different materials are available which can be used instead of wax and could be printed instead of poured or injection moulded. Such technologies would be very suitable for rapid prototyping of real metal cast parts. Investment casting technology has demands for pattern materials such as good burnout with low ash content and low contraction or expansion at burnout process. In the case of such use of 3D printed models only polymeric materials are suitable. Polymeric materials are usually printed by Fused Deposition Method technology (FDM) where filament is pushed through a heated nozzle and deposited layer by layer in order to build a model, by Stereolithography (SLA) where photo Livarski vestnik, letnik 67, št. 1/2020 45 fotosenzitivna smola utrjuje z laserskim žarkom, z brizganimi polimeri itd [1]. Delo predstavlja karakterizacijo različnih 3D natisnjenih materialov z namenom izbire najustreznejšega materiala za uporabo v tehnologiji precizijskega litja. 2 Eksperimentalno delo Analizirani so bili trije različni materiali, natisnjeni z dvema metodama. Dva sta bila natisnjena s FDM metodo, eden pa s SLA metodo. Prva dva sta bila polilaktična kislina (PLA) in polivinil alkohol (PVA), tretji pa je bila akrilatna fotosenzitivna smola (Castable)[2]. Vzorci so bili natisnjeni v različne oblike, primerne za določevanje pepela, za dilatometrijsko analizo ter diferenčno vrstično kalorimetrijo (DSC). Za dilatometrijsko analizo so bile natisnjene kvadratne palice z robom 5 mm in dolžino 50 mm. V primeru PLA in PVA materialov so bile notranjosti palic različno zapolnjene, ena je bila polna, dve pa sta bili zapolnjeni 20-odstotno, enkrat z rombično in enkrat s heksagonalno strukturo. Za določanje deleža pepela so bile natisnjene kocke z robom 10 mm in z maso med 2 in 3 g. Za DSC analizo pa so bili natisnjeni valjčki premera 4 mm in višino 3 mm. Dilatometrijska analiza je bila izvedena na napravi Bähr DIL 801, do temperature 120 °C. Delež pepela je bil določen z žganjem materiala v keramičnih lončkih pri temperaturi 600 °C 2 uri. Delež pepela je bil nato izračunan po enačbi: % % . ash      NN N cupa ch cup SAMPLE 100 DSC analiza je bila izvedena z namenom določitve likvidus in solidus temperature na napravi Mettler Toledo DSC1 do temperature 250°C s segrevalno hitrostjo 10 K/min v zračni atmosferi. polymeric resins are used and cured by laser, Jetted photopolymer etc. [1]. The presented study deals with the characterization of different 3D printing materials in order to determine most suitable material and technology of production of patterns used for investment casting technology. 2 Experimental Three different materials were test printed by two different methods. Two materials were printed by the FDM method and one by the SLA method. First two were Polylactic Acid (PLA) and Poly Vinyl Alcohol (PVA) and the third one was Acrylate Photopolymer resin Castable [2]. Samples were printed in different shapes suitable for different testing such as dilatometry, ash determination and Differential Scanning Calorimetry (DSC). For dilatometric analysis the square rods with dimensions of 5 mm × 5 mm × 50 mm were printed. In case of PLA and PVA the inner structure of rods was in one case full in one it was rhombic and in one it was faceted with 20 % of fill. For ash content determination a cube with mas from 2 to 3 g were printed and for DSC analysis the discs with diameter of 4 mm and height of 3 mm were printed. Dilatometric analysis was made using a Bähr DIL 801 dilatometer up to 120 °C. Ash content was determined by burning the samples in ceramic cups at temperature of 600 °C for 2 hours. The ash content was then calculated by equation: % % . ash      NN N cupa ch cup SAMPLE 100 A DSC analysis was made in order to determine the melting point of sample using a Mettler Toledo DSC1 device up to 46 Livarski vestnik, letnik 67, št. 1/2020 3 Rezultati in diskusija Rezultati dilatometrijske analize polnih vzorcev so prikazani na sliki 1. Opazi se, da se PLA in PVA materiala ne širita veliko med segrevanjem. PVA se sploh ne širi, kar pomeni, da je pri 30 °C že plastičen in se krči zaradi vzmeti dilatometrijskega senzorja, ki stiska vzorec. V primeru PLA pa se vzorec razširi za približno 200 µm do temperature 55 °C. Širjenje je linearno s temperaturnim razteznostnim koeficientom α 0,000067 /K, po doseženi temperature pa se zmehča in se začne krčiti zelo hitro. Vzorec iz smole Castable pa kaže linearen trend širjenja cel čas segrevanja do temperature 120 °C in kaže, da bi se ob dodatnem segrevanju še širil. Temperaturni razteznostni koeficient α je v tem primeru 0,000156 /K. Iz predstavljenih ugotovitev se lahko zaključi, da material Castable ni primeren za uporabo v tehnologiji precizijskega litja, ker se preveč širi, kar bi predstavljalo nevarnost za nastanek razpok na školjki med žganjem le-teh. temperatures of 250 °C with heating rate of 10 K/s in air atmosphere. 3 Results and Discussion The results of dilatometric analyses of fully filled samples are presented in Figure 1. It can be noticed that PLA and PVA material do not expand much during heating. PVA is not expanding at all which means that at the 30 °C material is already soft and is shrinking due to a spring of dilatometric sensor which compresses the sample. In the case of PLA, it expands for about 200 µm until temperature of 55 °C with a linear thermal expansion coefficient α 0,000067 /K and then it gets soft and starts to compress very fast. The Castable resin sample is showing constant expansion during the whole range of heating up to 120 °C and it seems it would expand some more. The α is 0,000156 /K. From this we could conclude that Castable resin is expanding too much and would not be suitable for investment Slika 1. Dilatometrjska analiza vzorcev Figure 1. Dilatometric analysis of samples Livarski vestnik, letnik 67, št. 1/2020 47 Določitev deleža pepela je predstavljena v tabeli 1. Izvedene so bile tri ponovitve eksperimenta in izračunane povprečne vrednosti. Najvišji delež pepela ima vzorec PLA, ki ima skoraj 1 mas. %. V vzorcih Castable in PVA sta deleža pepela desetkrat manjša in znašata okoli 0,1 mas. %. Visok delež pepela v PLA vzorci pomeni, da tudi PLA ni najbolj primeren material za uporabo pri tehnologiji precizijskega litja. Smola Castable je razvita za uporabo v zlatarski industriji za precizijsko litje nakita, kjer se za formarski material uporablja mavčne forme. To je razlog, da je delež pepela v tem materialu najnižji. Tabela 1. Delež pepela v vzorcih Material Delež pepela (mas. %) Castable 0,095 PVA 0,090 PLA 0,980 Na sliki 2 so prikazane segrevalne DSC krivulje vseh treh testiranih materialov. Krivulja PLA vzorca kaže prvi negativni vrh pri 56 °C, ki najverjetneje predstavlja temperaturo steklastega prehoda TG [3, 4]. Ta temperatura je opazna tudi pri dilatometrijski krivulji (slika 1), kjer material postane plastičen in se začne krčiti zaradi delovanja vzmeti dilatometrskega senzorja. Pri temperaturi 140 °C se pojavi drugi negativni vrh, ki pa predstavlja začetek taljenje PLA materiala. Pri PVA vzorcu ni izrazitih vrhov, le pri temperaturi 170 °C se začne pojavljati negativen vrh, ki doseže najnižjo vrednost pri 205 °C in najverjetneje predstavlja taljenje. Castable material na segrevalni DSC krivulji kaže velik negativen vrh, ki pa je najverjetneje vpliv nehomogenosti casting technology since there would be a danger for cracking of the shells during the burnout process. The ash content determination is presented in Table 1. There were three repetitions made and average values calculated. The highest ash content is in PLA sample and is nearly 1 wt. %. In Castable and in PVA samples the ash content is ten times lower and is around 0,1 wt. %. This would make PLA material unappropriated for investment casting patterns production. Castable material is intended for investment casting patterns production for jewellery purposes, where investment is usually plaster based that is why the ash content is low. Table 1. Ash content in samples Material Ash content (wt. %) Castable 0,095 PVA 0,090 PLA 0,980 Figure 2 is presenting DSC heating curves of all three materials. The curve of PLA material is showing first negative peak at 56 °C, which should correspond to glass transition TG [3, 4]. This temperature is visible also on dilatometric curve where material becomes soft and starts to shrink due to a spring of a dilatometric sensor. At temperature of around 140 °C another negative peak appears which corresponds to melting of a PLA. PVA material is showing no significant peaks only at 170 °C a negative peak is visible which is presenting solidification. Castable material is showing a heating DSC curve with large negative peak but it seems to be an influence of non- homogenous material, first negative peak 48 Livarski vestnik, letnik 67, št. 1/2020 materiala. Prvi, bolj izraziti, negativni vrh se pojavi pri 150 °C in traja do 200 °C, kar predstavlja taljenje vzorca. 4 Zaključki Delo predstavlja rezultate analiziranih materialov. Dilatometrijska analiza je pokazala, da material Castable ni primeren za uporabo pri klasičnem postopku precizijskega litja s keramičnimi školjkami, ker ima prevelik temperaturni razteznostni koeficient. PLA in PVA materiala kažeta boljše rezultate glede raztezanja. Delež pepela je v materialu Castable in PVA nizek, je pa v PLA materialu kar 1 mas. %. Temperature začetka taljenja so v vseh materialih podobne in so v območju med 140 in 200 °C, kar je glede na temperaturo taljenja voska visoko, so pa temperature žganja keramičnih školjk tudi do 1000 °C, kar pomeni, da so omenjeni materiali še vedno primerni za uporabo v ta namen. Zaključek je, da je najprimernejši material za uporabo v tehnologiji precizijskega litja, glede na izvedene analize, PVA. V prihodnje pa je potrebno appears at 150 °C and lasts until 200 °C and it represents the melting of a sample. 4 Conclusions The research is presenting results of analysed materials. Dilatometric analysis showed that Castable material is not very suitable for use at classical investment casting technology with ceramic shells, since it expands too much during heating. PLA and PVA are showing promising results in this way. Ash content is good at Castable and PVA material, but it is nearly 1 wt. % at PLA material. Melting points for all materials are approximately the same and they are in the range between 140 to 200 °C, which is higher than for usual waxes but still acceptable for the investment casting technology where firing temperatures for shells can reach over 1000 °C. Conclusion is that the most suitable analysed material is PVA but, the research should be also expanded and dedicated to dimensional stability of printed patterns and 3D printability of investigated materials. Slika 2. DSC segrevalne krivulje vseh treh vzorcev Figure 2. DSC heating curves of all three samples Livarski vestnik, letnik 67, št. 1/2020 49 preveriti ustreznost materialov tudi z vidika dimenzijskih točnosti 3D natisnjenih modelov. 5 Literatura / Literature [1] [online]: [2] Formlabs appliction guide, Recommended Burnout Process and Casting Guide[online] Založba Formlabs, 2018. Dostopno na svetovnem spletu: . [3] ŠTURM, R. GRADIVA - Polimeri [online]: .