Anali PAZU - Letnik 1, leto 2011, številka 2 Time-Dependent Properties of Multimodal Polyoxymethylene based binder for Powder Injection Molding* asovno odvisne lastnosti multimodalnih, na polioksimetilenu temelje ih veziv praškov za brizganje Joamin GONZALEZ-GUTIERREZ**, Gustavo Beulke STRINGARI**, Barbara ZUPAN I **, Galina KUBYSHKINA***, Bernd VON BERNSTORFF**** and Igor EMRI** **Center for Experimental Mechanics, University of Ljubljana / Pot zaBrdom 104, Ljubljana, Slovenia ***Elektromaterial Lendava d.d / Kolodvorska 8, Lendava, Slovenia ****BASF Aktiengesellshaft / Carl-Bosch-Straße 38, Ludwigshafen, Germany E-mail: ie@fs.uni-lj.si Abstract: Powder injection molding (PIM) is one of the most versatile methods for the manufacturing of small complex shaped components from metal, ceramic or cemented carbide powders for the use in many applications. PIM consists of mixing the powder and a polymeric binder, injecting this mixture in a mold, debinding and then sintering. Catalytic debinding of polyoxymethylene (POM) is attractive since it shows high debinding rates and low risk of cracking. This work examines the possibility of using bimodal POM as the main component of the binding agent by studying its time-dependent and thermal properties and comparing them to monomodal POM. Furthermore, possible optimization of the binder formulation was investigated by the addition of short molecular weight polymeric chains (wax) to bimodal POM, as to create a multimodal material. It was observed that the magnitude of the complex viscosity for the commercial bimodal material was more than 2 times lower than for the chemically identical monomodal POM within the investigated frequency range. Viscosity values were observed to drop as the content of wax was increased, without compromising thebinders mechanical properties in solid state. A new formulation of bimodal POM plus 8 wt.% of added wax provided the most appropriate results from investigated combinations. This work has shown how the addition of short polymeric chains in POM influences its time-dependent properties in solid and molten state, which can be an important tool for the optimization of binders designed to be used in PIM technology. Key words: Polyoxymethylene; Powder; Technology; Binder; Time-dependence Povzetek: Praškasto brizganje (PIM) je ena najbolj vsestranskih metod za proizvodnjo majhnih kompleksno oblikovanih sestavnih delov iz kovine, keramike ali cementnega karbida v prahu za uporabo v številnih aplikacijah. PIM je sestavljen iz mešanja prahu in polimernega veziva, injiciranja te mešanice v kalup, razvezovanja in nato sintranja. Kataliti no razvezovanje od polioksimetilena (POM) je privla no, ker kaže visoko stopnjo razvezovanja in majhno tveganje za razpoke. To delo obravnava možnost uporabe bimodalne POM kot glavne sestavine veziva s študijem asovne odvisnosti in toplotnih lastnosti in jih primerjati z monomodalnim POM. Raziskana je bila možna optimizacija veziva z dodajanjem kratkih polimernih verig (vosek) bimodalnemu POM za ustvarjanje multimodalnega materiala. Ugotovljeno je bilo, da je obseg kompleksnih viskoznosti za komercialne bimodalne materiale ve kot 2-krat nižji kot za kemi no enake monomodalne POM znotraj opazovanega frekven nega obmo ja. Vrednosti viskoznosti so padale, ko se je vsebnost voska pove ala, ne da bi ogrozili mehanske lastnosti veziva v trdnem stanju. Nova oblika bimodalne POM plus 8 ut.% dodanega voska je najbolj ustrezala, glede na raziskane kombinacije. Klju ne besede: polioksimetilen; prašek; tehnologija; vezivo; asovna odvisnost 129 Anali PAZU - Letnik 1, leto 2011, številka 2 The process of Powder Injection Molding (PIM), invariably, consists of four steps: I) Feedstock preparation, II) injection molding, III) debinding and IV) sintering (7) (8). Figure 1 illustrates the main stages of PIM. During the feedstock preparation the metal or ceramic powder and an organic multicomponent binder are combined in a variety of compounding equipment, such as extruders and batch mixers. The mixture is then pelletized to an appropriate shape for feeding into the molding machine. The binder is simply a carrying medium for the powder and once a part is molded, the binder is removed in a subsequent step. The powder content usually ranges from 50 to 65% in volume, although there are claims of optimized commercial formulations in which even more than 80 % is used. If the powder content is found to be below 50 vol.%, the sintering ability of the feedstock and the final density of the part are significantly lowered. From another standpoint, it is also important to keep the viscosity of the feedstock as low as possible in order to facilitate the injection molding process.This is the reason why a powder content higher than 65 vol.% should be carefully handled(9). The injection molding process is mainly identical to conventional plastic injection molding. Nevertheless, some machine hardware changes are usually required to process a specific feedstock based on its compressibility and viscosity. In addition, injection molding machines for processing of powdery materials are optimized with wearresistant components and screw geometry adopted to lower the compression rate and extend the compression zone as compared to screws used for thermoplastics(1). Control of the molding process is vital for maintaining tight tolerances in subsequent steps. Most design advantages of PIM technology are captured during molding by relying on the flexibility of incorporating complexities in the tool. A molded part is called a “green part” and is oversized to allow shrinkage during sintering (2). 1. Introduction Powder injection molding (PIM) is a technology for manufacturing complex, precision, net-shape components from either metal or ceramic powder. The potential of PIM lies in its ability to combine the design flexibility of plastic injection molding and the nearly unlimited choice of material offered by powder metallurgy, making it possible to combine multiple parts into a single one (1). Furthermore, PIM overcomes the dimensional andproductivity limits of isostatic pressing and slip casting, the defects and tolerance limitations of investment casting, the mechanical strength of die-cast parts, and the shape limitation of traditional powder compacts (2). Finally, the use of finer powder particles in PIM feedstock allows for the densification of parts during sintering, which yields high performance parts made of material systems that are difficult to sinter using conventional methods(3). PIM is generally best suited to produce parts less than 6 mm in thickness and weighting less than 100 grams(4). Therefore, industries that demand miniaturization of complex components can benefit from using PIM in their manufacturing process. Due to the demand of high performance materials and the miniaturization of complex components in various fields, PIM market has exceeded the USD $1 billion mark in 2007, becoming approximately six times larger than 15 years before(5). This impressive growth rate is not expected to slow down in the next few years. As the recent report from Global Industry Analysts announced, a combined world metal and ceramic powder injection molding market is forecasted to reach USD $3.7 billion by the year 2017(6). The metal powder injection molding (MIM) is still considered the largest segment of this market, accounting for more than 70% of global output. Although PIM is globally widespread, Europe and Asia-Pacific account for the major share of MIM segment, while USA is still the largest market forCeramic Injection Molding (CIM) (6). Binder Powder Feedstock I) Mixing of multi-component binder and metal (or ceramic) powder II) Injection molding of the mixture (feedstock) Binder Solvent Catalyst Heat TEMP Binder IV) Sintering of brown part III) Debinding of green part Fig. 1. Flow chart illustrating the main stages of PIM process. 130 Anali PAZU - Letnik 1, leto 2011, številka 2 Low viscosity at the molding temperature and a high drop in viscosity at high shear rate are very important properties of the binder for mold filling with less energy. This is particularly important when dealing with complicated geometries (14). When cooled, the binder should be strong and rigid to prevent distortion of the molded part. Finally, highthermal conductivity, low thermal expansion coefficient, good adhesion with powder, no orientation, and be chemical inertness with respect to the powder are also desirable properties that should be taken into account when selecting a binder (15). As previously mentioned, catalytic debinding is the fastest debinding method available. However, POM has a higher viscosity compared to other binder systems and in some instances this can complicate the injection molding process. On the other hand, POM is a polymer with a high degree of crystallinity in solid state and as such the molded part with this binder has very good mechanical properties and dimensional stability, which are very desirable to ensure a final part with the correct geometry.Therefore, it is important to find a way to decrease the viscosity of the molten binder without deterioratingits mechanical properties in solid state. A common method of lowering the viscosity of polymers is to decrease their molecular weight, but this in turn causes a drop in their solid mechanical properties. Thus other methods need to be investigated. One particular method is to modify the polymer molecular mass distribution by changing its modality, in other words to have more than a single peak in the molecular mass distribution of the polymeric material. Figure 2 shows a schematic representation of a bimodal and monomodal molecular mass distribution. The effects of bimodality have been previously studied in some polymers. For example, Emri & von Bernstorff(16) reported that bimodal Polyamide 6 (PA6) showed a significant improvement on the relaxation modulus as compared to conventional monomodal PA6; a difference of almost two decades in time dependency between the monomodal and bimodal PA6 materials. Similar results were observed for the same materials in terms ofcreep compliance by Kubyshkinaet al (17). Before sintering, the organic binder must be removed without disrupting the molded powder particles; this process is commonly referred as debinding. Organic polymers have to be removed completely from the “green part”, since carbon residues can influence the sintering process and affect the quality of the final product negatively. Moreover, binder removal is one of the most critical steps in the PIM process since defects can appear due to inadequate debinding, causing problems like bloating, blistering, surface cracking and large internal voids. It has been shown that the rate of binder removal plays a main role in the defect appearance due to structural changes in capillaries inside the green part (10). For these reasons, the debinding is the most expensive and timeconsuming stage in the PIM technology. Three main methods can be applied depending on the composition of the binder: thermal, solvent, and catalytic. Catalytic debinding is by far the fastest method of removing the binder from the molded part; it is based on the solid-tovapor catalytic degradation of polyoxymethylene (POM), which occurs when such polymer is exposed to high enough temperatures (110 to 150 °C) in the presence of nitric or oxalic acid vapor(11). It is observed that the debinding rate is proportional to the temperature at which the process is executed. However, this operating parameter is limited by the increasing dimensional distortion as the melting temperature of POM is approached (2). Sintering is the last stage of the PIM process; it is a thermal treatment that transforms metallic or ceramic powder into bulk material with improved mechanical strength that in the majority of cases has residual porosity. Sintering is performed at temperatures below the melting temperature of the major constituent in the metal or ceramic powder, generally within 70 to 90% of the melting point (12). The temperature inside the sintering furnace is high enough to start the recrystallization process of the metal or ceramic particles, but low enough so that the particles remain unmelted. At such temperatures, the particles recrystallize into each other causing them to fuse together(4).Depending on the material, debound parts are sintered at temperatures ranging from 1200 to 1600°C under a controlled atmosphere(13). 131 Anali li PAZU - Letnik 1, leto 2011, številka 2 stribution in polymeric Fig. 2. Schematic representationn for monomodal and bimodal molecular mass distr materials. Fig. 3. Complex viscosity of mono no- and bimodal PA6 as function of frequency. Viscosity measurements have been recently re performed by our group on monomodal and bimoodal PA6 and are shown in Fig. 3. As can be appreciated ed, the viscosity of bimodal PA6 is significantly lower lo than its monomodalcounterpart in all the rang nge of frequencies studied. Therefore, one can conclude tha that bimodality has brought the required properties toPA6, i.e., i.e lower viscosity in the molten state and high mechanical al properties in the solid state. Unfortunately, PA6 does not ot exhibit catalytic decomposition and could not be used inn ccatalytic PIM. For this reason, it was decided to investig tigate if a similar behavior could be achieved inn POM through multimodality in its molecular mass distributi tion. The present work focusess oon the characterization of the time-dependent properties of o monomodal POM and different formulations of bim imodal POM-based materials. The aim is to understand how h the addition of short polymeric chains influences the th macroscopic properties of POM and thus design a binder bin more suitable to the requirements of PIM technolog ogy. 132 Anali PAZU - Letnik 1, leto 2011, številka 2 min: minimum p: polar ω: angular frequency Nomenclature D: diameter l: length I: moment of inertia J: creep compliance M: shearing torque T: Temperature t: time η*: complex viscosity : rotational angle 2. Materials and Methods 2.1 Materials Monomodal (MO-0) and bimodal (BI-0) POM were supplied by BASF (Germany). Their notation used in this study and corresponding thermal properties are specified in Table 1. As it can be seen in Table 1, thermal properties of the two types of POM have no significant difference. Subscripts max: maximum Table 1. Notation and thermal properties of monomodal and bimodal POM(18). Notation Supplier Molecular Mass Distribution Melting Temperature (°C) Crystallization Temperature (°C) MO-0 BASF Monomodal 165.8± 0.7 134.0± 1.1 BI-0 BASF Bimodal 164.3± 3.2 134.9± 2.8 components in a twin-screw extruder at the maximum concentration (16 wt.%). This master batch was subsequently diluted with previously extruded bimodal POM until the remaining concentrations were achieved. All the materials were extruded seven times in total to ensure a homogeneous distribution of the short chains among the long ones. Neat bimodal POM was also extruded (BI-00E) and characterized in order to investigate the effects of extrusion (Fig. 4). In the second step of our investigation the bimodal POM was further modified by adding different concentrations of a polymer with low molecular weight to obtain material with multimodal molecular weight distribution. Bimodal POM was modified by adding different concentrations (1, 2, 4, 8 and 16 wt.%) of a compatible polymer with low molecular weight (referred as ‘wax’ due to confidential terms). The blends of bimodal POM and wax were prepared by mixing the two Fig. 4. Flow chart for the blending of bimodal POM with wax at different concentrations. 133 Anali li PAZU - Letnik 1, leto 2011, številka 2 The list of multimodal materials (bimo modal + wax), their notation and thermal properties are show own in Table 2. As with the unmodified monomodal and bi bimodal POM, the thermal properties of the POM M-wax blends did not differ significantly among each other. er. Table 2. Notation and thermal the properties of extruded bimodal POM and its waax blends(18). Notation Wax Conte ntent (wt.%) Molecular Mass Distribution Melting Temperature (°C) Cry rystallization Temp mperature (°C) BI-00E 0 Bimodal 164.6 ± 2.4 13 135.5 ± 1.2 BI-01E 1 Multimodal 166.6 ± 3.2 13 135.4 ± 0.5 BI-02E 2 Multimodal 166.9 ± 1.4 13 136.1 ± 0.7 BI-04E 4 Multimodal 164.3 ± 1.3 13 135.7 ± 1.0 BI-08E 8 Multimodal 165.3 ± 1.9 13 135.6 ± 0.9 BI-16E 16 Multimodal 163.6 ± 0.9 13 135.6 ± 1.2 °C. The heater was moved at a relatively slow rate of 2 mm/min. After melting and pr pressing on the glass cylinder, the polymeric materials were le left to naturally cool down to room temperature. Schemati atics of the procedure are (21 provided elsewhere (16) (17) (18) (21) . After each specimen was prepared, p they were annealed at a temperature of 120 °C ° to erase their thermomechanical history. After aannealing, torsional creep measurements were performed ed at the temperature of 110 °C in the time scale of 3 h, under a constant shearing torque (M). The rotational angl ngle caused by this torque was recorded as a function of tim ime (ϕ(t)) and then the creep compliance (J(t)) of each spe pecimen was calculated using the following equation: 2.2 Viscosity measurements Viscosity measurements of all materi erials in the molten state were performed by means off small amplitude oscillatory shear tests according to ASTM M D-4440(19) using a Haake MARS II rotational rheometer (Thermo (T Scientific, Karlsruhe, Germany). Measurements w were performed at 190 °C, which is a commonly usedd temperature for injection molding of POM. The geometr try used was coneplate with a 20 mm in diameter. Two frequencies fr sweep were performed in each measuremen ent, the first one increasing from 0.1 s-1 (0.628 rad/s) to 100 s-1 (628.32 rad/s) and the second one decreasing from om 100 s-1 to 0.1 s1 , in order to monitor any possible flow low instability. The applied stress was set at 200 Pa; such valu alue was previously determined to be within the linear viscoela elastic region of the materials at the specified temperature. The Th reported results are the average of six repetitions for each ach of the materials studied. (1) whereIp is the polar moment ntum for the circular crosssection geometry of the cylind indrical specimen and l is the length of the specimen. The ap applied torque was selected to be within the linear viscoel elastic region for all POM materials at the specified tempperature. Finally, the reported results are the average over two wo repetitions carried for each of the materials studied. 2.3 Creep compliance measurements Creep measurements in the solid state ate were performed following the methodology developed aat the Center for Experimental Mechanics at the Universit sity of Ljubljana(20). Cylindrical specimens with diameter D = 5.80 ± 0.05 mm and length l = 40.0 ± 1.0 mm were we prepared by gravimetrical casting from all of the m materials listed in Table 1 and 2. The casting procedure sta tarts by filling up a glass tube with polymer pellets. The mat aterial is melted by a movable heater positioned around thee glass tube and a pressure of about 1 MPa is applied to the molten polymer using a piston. The temperature of the hea eater was set at 220 3. Results and Discussion 3.1 Viscosity The viscosity of all mat aterials was investigated in frequency domain. Based on these measurements, it is clear that all the materials ls show a nearly constant magnitude of the complex viscosity vi at low frequencies, 134 Anali li PAZU - Letnik 1, leto 2011, številka 2 followed by a drop at higher freque uencies, which is commonly referred as a shear-thinning bbehavior. Figure 5 shows the shear thinning behavior for or monomodal and bimodal POM. The viscosity results can be summa marized using two parameters: and . Th The first parameter represents the magnitude of th the complex viscosity at the lowest angular frequency at which experiments were performed (0.628 rad/s), while ile the second parameter is the magnitude of the complexx viscosity at the highest frequency reachable by the eexperimental set up (628.3 rad/s). The viscosity results are re shown in Fig. 6. Fig. 5. Shear thinning behavior of monomodal and bimodal POM at 190 °C. Fig. 6. Complex viscosity at the minimum m and maximum frequency studied for all mat aterials investigated at 190 °C. 135 Anali li PAZU - Letnik 1, leto 2011, številka 2 It is clear that the addition of short rt polymeric chains resulted in materials with better flowability. The commercial bimodal POM (BI-0), whi hich has a higher content of short polymeric chains, sho hows a significant decrease in viscosity in both low and high hi frequencies in comparison to monomodal POM (MO-00). At the lowest frequency bimodal POM has 4 times lowe wer viscosity and at the highest frequency the viscosity is tw twice smaller. The reduction of viscosity is a consequencee of the introduced short polymeric chains, which can act ass internal i lubricants reducing the friction between the longg chains and thus, increasing the flowability of the m molten polymer, particularly when subjected to hig high shear rates. Theextrusion protocol used to preparee P POM/wax blends led also to a reduction on viscosity; itt was w observed that BI-00E had about 17% lower viscosity tha than BI-0 due to the breaking of longer chains during extrusion e cycles. Regarding the new formulations propose sed in this work, it is possible to notice that the flowabilityy of o POM is clearly improved by increasing the concentratio tion of wax added. For instance, by adding 16 wt.% of wax (BI-16E), is reduced by 34% when com ompared to BI-00E and by 59% when compared to BI-0. Ass already discussed in this work, improving flow propertie rties is of extreme importance to mold filling off complex c geometries as those usually found in PIM. 3.2 Creep compliance In Fig. 7 the creep compli pliance determined at 110° C after 10000 s (∼3h) for eachh oof the POM-based materials investigated is shown. Creep co compliance values at selected time and temperature enable a direct correlation to process conditions, since both paramet eters are very similar to those commonly applied during thee catalytic debinding process of POM. Therefore, a low valu lue of creep compliance under these conditions is impor ortant to ensure minimal deformation of the molded par art resulting in a better quality of the sintered product. Bimoodal (BI-0) and monomodal POM (MO-0) showed no sig significant difference in their creep compliance at the m measuring conditions. This indicates that the addition of short molecules does not significantly affect the solid mechanical me properties of POM. It can be said that the extrusio sion protocol also showed no significant change in the creep ep performance of POM, since BI-00E shows only slightly lo lower creep compliance than neat bimodal POM in the condi ditions here defined. Fig. 7. Creep compliance measured red at 110 °C, after 104 seconds, for all materials investig stigated. From another standpoint, the addition on of up to 2 wt.% of wax yielded no significant change inn creep c compliance, it appears as if BI-01E and BI-02E 2E have a slight improvement in the resistance to deforma mation as compared to BI-00E, but the change cannot be consi nsidered significant. On the other hand, materials BI-04E and a BI-08E have shown creep compliances slightly higher er than BI-01E and BI-02E, but at the samelevel as the extrud uded bimodal POM. 136 Finally, the highest creep com ompliance, and consequently, the lowest resistance to defor ormation among all materials investigated was observed forr BI-16E. B The possible explanationn for the observed behavior could be that the smaller mole lecules arrange themselves in the spaces between the largerr oones when in the molten state. As the material solidifies a mo more closely packed structure is obtained, such tighter stru ructure prevents the material Anali PAZU - Letnik 1, leto 2011, številka 2 from creeping. This hypothesis is supported by the polarized micrographs presented by Emri and von Bernstorff(16)and Kubyshkina et al(17)for PA6 and by Stringari et al(18) for POM-wax mixtures, which shows a finer structure for the materials with the higher content of shorter chains. However, when the content of shorter molecules exceeds a certain concentration, they may actually facilitate the movement of larger chains and as a result the materials start to creep more. Acknowledgements This research work is co-funded by the European Commission Erasmus Mundus Executive Agency, Slovenian Research Agency- ARRS, and the Slovene Human Resources and Scholarship Fund – Ad futura. References (1) Hausnerová, B. (2011). Powder injection moulding- An alternative processing method for automotive items.In New Trends and Developments in Automotive System Engineering, Chiaberge M. (Ed.), InTech, Rijeka, Croatia, pp.129-145. (2) Tandon, R. (2008). Metal injection moulding. In Encyclopedia of Materials: Science and Technology, Buschow, K.H.J., Cahn, R.W., Flemings, M.C., Ilscher, B., Kramer, E.J., Mahajan, S. &Veyssiere, P. (Ed.), Elsevier Science Ltd, Amsterdam, The Netherlands, pp.5439-5442 (3) Ye, H., Liu, X.Y., and Hong, H. Fabrication of metal matrix composites by metal injection molding – A review. Journal of Material Processing Technology, Vol. 200 No. 1 (2011), pp. 12-24. (4) Boljanovic, V. 2010. Powder metallurgy. In Metal Shaping Processes: Casting and Molding, Particulate Processing, Deformation Processes, Metal Removal (2010), p.75-106, Industrial Press Inc. (5) German, R.M. (2008). PIM breaks the $1 bn barrier. Metal Powder Report, Vol. 63, No.3, (2008), pp.8-10. (6) GIA. (2011). Metal and Ceramic Injection Molding.Global Industry Analysts, Inc., San Jose, USA. (7) Gonçalves, A.C. (2001). Metallic powder injection molding using low pressure.Journal of Materials Processing Technology, Vol. 118, No.1-3, (2001), pp.193-198. (8) Tay, B.Y., Loh, N.H., Tor, S.B., Ng, F.L., Fu, G. & Lu, X.H. Characterisation of micro gears produced by micro powder injection moulding. Powder Technology, Vol. 188, No. 3, (2009), pp.179-182. (9) Merz, L., Rath, S., Piotter, V., Ruprecht, R., RitzhauptKleissl, J. and Haussel, J. Feedstock development for micro powder injection molding. Microsystem Technologies, Vol. 8, No. 2-3, (2002), pp.129-132. (10) Oliveira, R.V.B., Soldi, V., Fredel, M.C. &Pires, A.T.N. Ceramic injection molding: influence of specimen dimensions and temperature on solvent debinding kinetics. Journal of Materials Processing Technology, Vol.160, No.2, (2005), pp.213-220. (11) Mathew, B.A. and Mastromatteo, R. MIM Focus- Metal injection moulding for automotiveapplications. Metal Powder Report, Vol.58, No.3, (2002), pp.32-35. (12) Lame, O., Bellet, D., Di Michiel, M. and Bouvard, D. In situ microtomography investigation of metal powder compacts during sintering. Nuclear Instruments and Methods in Physics Research B, Vol.200, (2003), 4. Conclusions This work has shown that macroscopic properties of POM are sensitive to the addition of short polymeric chains. For instance, the commercial bimodal POM (BI-0) which has a larger content of short polymeric chains presents time-dependent properties in solid state similar to those shown by standard monomodal POM (MO-0) with larger molecular chains. However, BI-0 exhibits a much better flowability, which is extremely important for filling the cavities of complex geometries as those usually encountered during the powder injection molding process. The extrusion protocol has shown no negative effects in both, binder’s flowability and its resistance to deformation. These results suggest that not only the ratio between long and short chains affects the time-dependent properties of POM, but also the distribution and interactions between them. Also, for the blends of bimodal POM and wax, it was observed that the addition of short polymeric chains to bimodal POM showed a further decrease in molten viscosity. At low wax content (up to 4 wt.%), the solid creep properties of the blends remained almostunchanged . However, the addition of higher concentrations, for instance 16 wt.%, already shows a relatively larger increase in creep compliance. The investigated mixture of bimodal POM and 8 wt.% wax had viscosity values (at 190 °C) up to 19% lower than the commercial bimodal material submitted to the same extrusion protocol. At the same time the creep compliance of the same mixture is only 9% higher at the conditions here studied (at 110 °C, after 104 s), which is still lower than the 14% increase when the content of wax reached 16 wt.%. So it can be said that a compromise was reached at 8 wt% wax content, lower viscosity and not so high increase in creep compliance. In general, it can be concluded that the addition of short polymeric chains may be an interesting tool for the optimization of binders designed to be used in powder injection molding technology, and that measuring the timedependent properties of binder in solid and molten state can be an important tool for determination of the optimal formulation of a PIM binder. 137 Anali PAZU - Letnik 1, leto 2011, številka 2 pp.287-294. (13) Krug, S., Evans, J.R.G. &terMaat, J.H.H. Differential sintering in ceramic injection moulding: particle orientation effects. Journal of the European Ceramic Society, Vol.22, No. 2, (2002), pp.173-181. (14) Ahn, S., Park, SS.J., Lee, S., Atre, S.V., and German, R.M. Effect of powders and binders on material properties and molding parameters in iron and stainless steel powder injection molding process, Powder Technologies, Vol. 193 (2009), pp. 162-169. (15) German, R.M., Powder injection molding. (1990), p.1015, Metals Powder Industries Federation. Princeton, New Jersey, USA. (16) Emri, I., and von Bernstorff, B.S. The effect of molecular mass distribution on time-dependent behavior of polyamides.Journal of Applied Mechanics, Vol. 73, No. 5 (2006), pp.752-757. (17) Kubyshkina, G., Zupan i , B., Stukelj, M., Grošelj, D., Marion, L. & Emri, I. The influence of different sterilization techniques on the time-dependent behavior of polyamides.Journal of Biomaterials and Nanobitechnology (accepted for publication, 2011). (18) Stringari, G.B., Zupan i , B., Kubyshkina, G., von Bernstorff, B., and Emri, I., Time-dependent properties of bimodal POM – Application in powder injection molding, Powder Technology ,Vol. 208 No. 3 (201), pp.590-595. (19) ASTM International, ASTM D4440-08 Standard Test Method for Plastics: Dynamic Mechanical Properties Melt Rheology, Annual Book of ASTM Standards (2008). (20) Metlikovi , P., and Emri, I., Device for measuring torsional creep for cylindrical polymeric specimens (in Slovenian), Journal of Mechanical Engineering (Strojniški vestnik), Vol. 36 (1990), pp. 101-104. (21) Prodan, T., Emri, I., von Bernstorff, B.S., and Voloshin, A., The effect of temperature on morphology and mechanical behavior of PA, Proceedings of 2003 SEM Annual Conference and Exposition on Experimental and Applied Mechanic, (2003). Available from: <http://semproceedings.com/03s/sem.org-2003-SEM-Ann-Confs83p05-The-Effect-Temperature-MorphologyMechanical-Behavior-PA.pdf>(accessed 2011-10-19). 138