12.5 Improving powder injection moulding by modifying binder viscosity through different molecular weight variations J. Gonzalez-Gutierrez 1, P. Oblak 1, B.S. von Bernstorff 2, I. Emri 1 1

Center for Experimental Mechanics, University of Ljubljana, Slovenia 2

BASF AG, Germany

Abstract Powder injection moulding (PIM) is a versatile technology for manufacturing small metal or ceramic parts with complex geometry. Invariably, PIM consists of 4 stages: feedstock preparation, injection moulding, debinding and sintering. Debinding is the most time consuming step and in an effort to reduce debinding times, catalytic debinding was introduced, which rely on the sublimation ability of Polyoxymethylene (POM). Besides fast debinding, POM provides excellent mechanical strength to the moulded part. One major problem of POM-based binders is their high viscosity that can complicate the injection moulding process. This paper examines the possibility of lowering the viscosity of POM without affecting its mechanical strength by changing its average molecular weight (Mw). It was observed that POM’s viscosity increases with MW at a faster rate than impact toughness and it is suggested that a Mw of around 24000 g/mol provides the most appropriate combination of strength and fluidity.

Keywords: Impact Toughness, Molecular Weight, Polyoxymethylene, Powder Injection Moulding, Viscosity

1 INTRODUCTION Powder injection moulding (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 moulding 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 and productivity 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]. Due to the demand of high performance materials and the miniaturization of complex components in various fields, PIM market is expected to reach a value $ 3.7 billion by the year 2017 [3]. Metal powder injection moulding (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 a major share of MIM segment, while USA is still the largest market for Ceramic Injection Moulding (CIM) [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; some examples of these markets are the consumer electronics, medical devices and automotive industries. In Europe, the PIM production is dominated by automotive applications and the so called consumer market (which includes watches and eyeglasses), while the North American production is mainly applied to the medical/healthcare field. On the other hand, the Asian production, considered the largest one, is dominated by

consumer electronics and information technology applications [3]. When comparing manufacturing of metal parts by traditional methods, such as casting, warm extrusion and machining, with PIM, it has been found that raw materials used and energy consumption are decreased substantially. The waste is reduced due to the nature of the feedstock leftovers from the injection moulding which can be reused by re-melting the polymer matrix and feeding once again into the processing equipment. Energy consumption is reduced since parts are formed at the melting temperature of polymers rather at the melting temperatures of metals which is at least one order of magnitude higher. The latest variant, nPIM, which utilizes nanoparticles in its feedstock, could further increase the benefits of powder injection moulding by lowering sintering temperatures, producing finished parts with surfaces similar to polished products and reducing the porosity of the final part. Thus this technology could be used in high precision components or even jewellery. All of these benefits will improve the sustainability of manufacturing metal parts with complex geometry. The process of Powder Injection Moulding (PIM), invariably, consists of four steps: I) Feedstock preparation, II) injection moulding, III) binder removal and IV) sintering [5][6]. During the feedstock preparation the metal or ceramic powder and an organic multicomponent binder are combined in a variety of compounding equipment, the mixture is then pelletized to an appropriate shape for feeding into the moulding machine. The injection moulding process is mainly identical to conventional plastic injection moulding. Nevertheless, some machine hardware changes are usually required to process a specific feedstock based on its compressibility and viscosity. A moulded part is called a “green part” and is oversized to allow shrinkage during debinding and sintering [2].

G. Seliger (Ed.), Proceedings of the 11th Global Conference on Sustainable Manufacturing - Innovative Solutions ISBN 978-3-7983-2609-5 © Universitätsverlag der TU Berlin 2013 393

J. Gonzalez-Gutierrez, P. Oblak, B.S. von Bernstorff, I. Emri

Binder removal is one of the most critical steps in the PIM process since defects can appear due to inadequate debinding. 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 moulded part; it is based on the solid-tovapour 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 vapour. 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 [7].

The goal of this paper is to determine the maximum molecular weight of POM that will provide adequate viscosity (160 °C) before debinding. Currently available POM-based feedstock materials fulfil the second requirement very well; however, the first condition, which is related to processability, is partially not meet since neat POM has much higher viscosity than other binders based on polyolefins [7]. It has been suggested that the binder should have a viscosity lower than 10 Pa s at a shear rate of 100 s-1 [8], which is 20 times lower than currently available POM-based binders. In order to reduce the viscosity of POM-based binders, blending of POM with other polymers has been investigated but the viscosity is still orders of magnitude higher than other binders [7]; therefore other methods are needed to reduce the viscosity of POM-based binders. Another way to lower the viscosity of polymers is to lower their molecular weight [9][10][11], thus in an effort to decrease the viscosity of binders used in PIM, POM materials with distinct molecular weights have been synthesized and their viscosity and impact toughness have been investigated.

Table 1. Average molecular weight of POM copolymers

T = 190 °C Mw = 81100 g/mol

100

Oscillatory Rheometry Constant Shear Rheometry 10 0,01

1 100 Angular frequency, .  [rad/s] Shear rate, γ [1/s]

10000

Figure 1. Applicability of Cox-Merz rule for POM

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Improving powder injection moulding by modifying binder viscosity through different molecular weight variations

2.3

Impact toughness measurements

Charpy tests were performed at room temperature in order to measure the impact toughness of all selected POM copolymers. Non-notched cylindrical specimens were prepared via twin screw extrusion in a PolyLab Haake OS (Thermo Scientific, Germany). A glass tube (external diameter = 9 mm, internal diameter = 6 mm and length = 200 mm) was placed at the end of the extrusion die and filled with the extrudate up to a minimum length of 80 mm. The melt temperature at the die was measured to be 190 °C. After extrusion, extrudates were left to cool to room temperature inside the glass tube for at least 4 h before performing the impact tests. Two hammers were used, 1 and 3.9 J, which provide an estimated impact velocity of 2.7 and 2.8 m/s, respectively. All measurements were repeated six times. 3 3.1

RESULTS AND DISCUSSION Viscosity

The magnitude of the complex viscosity as a function of angular frequency for all the different POM materials is shown in Figure 2. It is clear that as Mw increases so does the viscosity, also it can be seen, that almost all of the materials investigated display Newtonian behaviour in the frequency range investigated. Only the materials with the higher molecular weight (MW6 to MW8) show a clear deviation from Newtonian to shear thinning behaviour starting at approximately 30 rad/s. This is not unexpected, since as the molecular weight increases, it is expected that the level of entanglement increases and the amount of free volume decreases, which reduce the chain mobility and as a consequence increases the viscosity [13]. However, as the frequency of excitation or shear rate increases these entanglements break and the viscosity starts to decrease, i.e. shear thinning behaviour. Polymers with higher Mw have a higher number of entanglements and thus are more susceptible to shear leading to an onset of shear thinning at lower frequencies, as observed in Figure 2.

Polyoxymethylene can be classified as a linear entangled polymer and it is well known that for this type of polymers the shear Newtonian viscosity, 0 and the average molecular weight, Mw are related by a power law function (2) of the form proposed by Fox and Flory [11]: 0 = K Mwb ,

where the K parameter quantifies the temperature and pressure dependence of the Newtonian viscosity of molten polymers, and a is related to the level of entanglement of the polymers, for the conditions here tested K = 5 x 10-17. Figure 3 shows that the above equation applies also for the POM copolymers here investigated. The value of b has been reported for several polymers to be between 3.3 and 3.7 when Mw > Mc and b ≈ 1 when Mw < Mc, where Mc is a critical average molecular weight [13][14][15]. Below Mc the flow units are single macromolecules while above Mc the flow units are chain segments since the macromolecules are entangled [14]. As can be seen in Figure 3, all the POM copolymers investigated appear to be above the critical molecular weight, since the value of b is approximately 3.7; this was expected since it has been estimated in the literature that the molecular weight for entanglement Me of POM is 3100 g/mol [16] and it is generally believed that Mc is between 2 and 3 times larger than Me [13][14][15]. In this particular study the lowest molecular weight available is around 10000 g/mol, which is more than 3 times the estimated molecular weight for entanglement, Me. With respect to the viscosity required for PIM (< 10 Pa s), it appears that one could select a POM material with an average molecular below or equal to 36340 g/mol, i.e. MW1, MW 2 and MW3. However, the decision cannot be taken without considering the solid mechanical properties of the polymer, in particular the impact toughness of the material, since it is desirable that the moulded part exhibits good toughness in order to be easily handled after injection moulding without fracturing. 1000

Newtonian viscosity, η0 [Pa s]

Complex viscosity, |η*| [Pa s]

1000

MW 8 MW 7 MW 6 100 MW 5 MW 4 10

MW3 1

0,1

MW 2

MW 1

0,01 0,01

(2)

T = 190 °C  = 100 Pa

1

100

10000

Angular frequency,  [rad/s]

Figure 2. Viscosity of POM copolymers with different average molecular weights, Mw

T = 190 °C  =100 Pa

100

10

1

y = 5E-17x3,7277 R² = 0,9955

0,1

0,01 1000

10000

100000

1000000

Average molecular weight, Mw [g/mol]

Figure 3. Effect of average molecular weight, Mw, on viscosity of POM copolymers

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J. Gonzalez-Gutierrez, P. Oblak, B.S. von Bernstorff, I. Emri

Absorbed energy by Charpy, Q [J]

10

T = Room Non-notched specimen Cylindrical specimen  ≈ 6 mm

1000

Absorbed Energy by Charpy

100

Newtonian Viscosity

1

10 1

0,1

0,1 0,01 1000

10000

100000

0,01 1000000

Average molecular weight, Mw [g/mol]

1

Figure 5. Effect of molecular weight, Mw, on viscosity and impact toughness of POM copolymers 10 0,1

0,01 1000

10000

100000

1000000

Average molecular weight, Mw [g/mol] Figure 4. Effect of average molecular weight, Mw, on impact toughness of POM copolymers 3.3

10

Newtonian viscosity, η0, at 190 °C [Pa s]

It is known that the impact toughness of polymeric materials is highly dependent on the molecular weight. When the molecular weight of polymers is increased, the mechanical response goes from brittle to ductile [17], i.e. the toughness increases with molecular weight [18]. For semi-crystalline polymers, like POM, this increase has been attributed to an increase in density of inter-lamellar tie chains and chain entanglements, which give higher craze fibril strength and, hence, a higher energy for fracture initiation is required [19]. Figure 4 shows that for POM, a similar behaviour has been observed, as the molecular weight increases the impact toughness increases: in the range between 10240 to 24410 g/mol the increase is very small and it appears that a plateau is present between 24410 and 60500 g/mol; and finally as the Mw increases beyond 60500 g/mol the increases in toughness is very pronounced. Similar behaviour has been reported in other polymers with respect to their mechanical strength [20]. It has also been reported that as the molecular weight increases beyond a very large molecular weight a decrease in the fracture toughness can be observed as in the case of ultrahigh molecular weight polyethylene, thus toughness is a nonmonotonical function of molecular weight with a maximum [20]. In this particular case, the maximum was not reached in the range of molecular weights investigated.

viscosity values between 1 and 30 Pa s, which correspond to an average molecular weight between 24410 and 60500 g/mol (MW2 and MW4); therefore by looking at these results it can be suggested that POM MW2 should be used as the main component for the binder since it has 3 times lower viscosity than POM MW3, but the same level of toughness. It is important to mention that the POM currently used as binder for PIM feedstock has a similar molecular weight to MW6, thus if we select MW2 as the new binder we can expect a decrease in viscosity of almost 200 times, while a decrease in toughness of approximately 10 times, which can be considered a significant improvement. Absorbed energy by Charpy, Q, at room temp & 6mm diameter samples [J]

Impact toughness

Binder selection

In order to select the POM material to be used as part of the PIM binder, it is important to take into account the viscosity of the material as well as its toughness. The viscosity should be as low as possible to allow easy moulding, while the toughness should be as high as possible to prevent damage to the moulded part before sintering. As it can be seen in Figure 5, the viscosity increases much more rapidly than the toughness; viscosity increases approximately 6 orders of magnitude, while at the same time the toughness increases only 3 orders of magnitude. Figure 6 also shows that the dependence of toughness (absorbed energy by Charpy) with viscosity follows a similar shape as its dependence with average molecular weight (Figure 5), showing a plateau at the

Absorbed energy, Q, by Charpy at room temp & 6mm diameter samples [J]

3.2

1

0,1

0,01 0,01

1

100

Newtonian viscosity, η0, at 190 °C [Pa s] Figure 6. Impact toughness measured by Charpy as a function of Newtonian viscosity for POM copolymers 4

CONCLUSIONS

POM used as a binder for powder injection moulding (PIM) has the major advantages that it can undergo catalytic debinding which is much faster than other debinding processes and that the moulded part has good mechanical

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Improving powder injection moulding by modifying binder viscosity through different molecular weight variations

strength (i.e. high toughness). However, currently used catalytic binder has high viscosity that can bring difficulties to the injection moulding process. In this investigation the viscosity and toughness of different POM copolymers has been studied [22]. It has been observed that both properties increase as the average molecular weight Mw increases. However, the viscosity increases much more rapidly than impact toughness. Viscosity increases with molecular weight as a power law function, with an exponent a ~ 3.7, as it has been reported for other polymers [13][14][15]. Therefore, for an increase in Mw of approximately 10 times there is a viscosity increase of almost 12000 times. The impact toughness measured by Charpy tests increases approximately 130 times as the Mw increased from 10240 to 129000 g/mol. The increase in toughness does not follow a simple relationship with molecular weight and it appears that there is a plateau at small molecular weights. With the information here gathered, it possible to suggest that a POM copolymer with an average molecular weight of around 24000 g/mol could be used as the main component of a binder used in PIM. As compared to the currently available binder, using POM with the suggested Mw can lead to a decrease in viscosity of 200 times, while reducing toughness only by 10 times; this can be considered a significant improvement on the performance of POM-based binders for PIM [22] and a step in the right direction for the sustainable manufacturing of metal parts with complex geometry, since there will be a reduction on energy consumption during the injection moulding process.

[7]

[8] [9] [10]

[11]

[12] [13]

[14]

[15]

[16] 5

ACKNOWLEDGEMENTS

This research work is co-funded by Slovenian Research Agency - ARRS, and the Slovene Human Resources and Scholarship Fund - Ad futura. 6 [1]

[2]

[3] [4]

[5]

[6]

REFERENCES

[17]

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, Croatia, 129145. Tandon, R., 2001, 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, Netherlands, 5439-5442.

[18]

GIA, 2011, Metal and Ceramic Injection Molding. Global Industry Analysts, Inc., San Jose, CA. Boljanovic, V., 2010, Powder metallurgy. In Metal Shaping Processes: Casting and Molding, Particulate Processing, Deformation Processes, Metal Removal, Industrial Press Inc., New York, NY, 75-106. Gonçalves, A.C., 2001, Metallic powder injection molding using low pressure. J. Mater. Process. Technol., 118, 193-198. Tay, B.Y., Loh, N.H., Tor, S.B., Ng, F.L., Fu, G. & Lu, X.H., 2009, Characterisation of micro gears produced

[21]

[19]

[20]

[22]

by micro powder injection moulding. Powder Technol., 188, 179-182. Gonzalez-Gutierrez, J., Stringari, G.B., Zupančič, B., Kubyshkina, G., von Bernstorff, B., Emri, I., 2012, Timedependent properties of multimodal polyoxymethylene based binder for powder injection molding. Journal of Solid Mechanics and Materials Engineering, 6, 419-430. Mutsuddy, B.C., Ford, R.G., 1995, Ceramic Injection Molding, Chapman & Hall, London, U.K., 1-26. Nichetti, D., Manas-Zloczower, I., 1998, Viscosity model for polydisperse polymer melts, J. Rheol, 42, 951-969. Kanai, H., Sullivan, V., Auerbach, A., 1994, Impact modification of engineering thermoplastics. J. Appl. Polym. Sci., 53, 527-541. Fox, T.G., Flory, P.J., 1951, Further studies on the melt viscosity of polyisobutylene. J. Phys. Colloid Chem., 55, 221-228. Cox, W.P., Merz, E.H. 1958, Correlation of dynamic and steady flow viscosities. J. Polym. Sci., 28, 619-622. Grosvenor, M.P., Staniforth, J.N., 1996, The effect of molecular weight on the rheological and tensile properties of poly(ε-caprolactone), Int. J. Pharm., 135, 103-109. Colby, R.H., Fetters, L.J., Graessley, W.W., 1987, Melt viscosity-molecular weight relationship for linear polymers. Macromolecules, 20, 2226-2237. Ajroldi, G., Marchionni, G., Pezzin, G., 1999, The viscosity-molecular weight relationships for diolic perfluropolyethers. Polymer, 40, 4163-4164. Plummer, C.J.G., Cudré-Mauroux, N., Kausch, H.H., 1994, Deformation and entanglement in semicrystalline polymers, Polym. Eng. Sci., 34, 318-329. Han, Y., Gang, X.Z., Zuo, X.X., 2010, The influence of molecular weight on properties of melt-processable copolyimides derived from thioetherdiphthalic anhydride isomers, J. Mater. Sci., 45, 1921-1929. Garlotta, D., 2001, A literature review of poly(lactic acid). J. Polym. Environ, 9, 63-84. Andreassen, E., Nord-Varhaug, K., Hinrischen, E.L., Persson, A.M., 2007, Impact fracture toughness of polyethylene materials for injection moulding. Extended abstracts for PPS07EA, Gothenburg, Sweden. Baltá Calleja, F.J., Flores, A., Michler, G.H., 2004, Microindentation studies at the near surface of glassy polymers: influence of molecular weight. J. Appl. Polym. Sci., 93, 1951-1956. Nakayama, K., Furumiya, A., Okamoto, T., Yagi, K., Kaito, A., Choe, C.R., Wu, L., Zhang, G., Xiu, L., Liu, D., Masuda, T., Nakajima, A., 1991, Structure and mechanical properties of ultra-high molecular weight polyethylene deformed near melting temperature. Pure Appl. Chem., 63, 1793-1804. Gonzalez-Gutierrez, J., Oblak, P., Bernstorff, B.S., Emri, I., 2013, Processability and mechanical properties of polyoxymethylene in powder injection molding, 2013 SEM Annual Conference and Exposition on Experimental and Applied Mechanics, Lombard, IL.

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