In situ, Non-contact monitoring of Powder Compacts during Polymer Removal

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1 Euro PM2004 Powder Injection Moulding In situ, Non-contact monitoring of Powder Compacts during Polymer Removal R. P. Koseski, C. Binet, Randall M. German Center for Innovative Sintered Products, Pennsylvania State University, 147 Research West, University Park, PA Abstract: A non-contact system has been developed to monitor the dimensional change of powder compacts during thermal debinding. The dimensional change can be monitored along an axis by positioning the sample in the apparatus as desired. Any axis can be monitored by orienting the sample appropriately. The system has the added advantage of operating under a nitrogen atmosphere. A brief description of the apparatus with accompanying photographs is provided. A measurement of two compact systems with the same powder and different admixed binders is presented. The results show the dimensional changes that effect the dimensional instability found during thermal debinding. The apparatus operates under typical debinding temperatures; from room-temperature to roughly 500 C. 1. Introduction: Powder compaction and injection molding are commercially viable processes for manufacturing complex components. Dimensional tolerances in both methods can be held to ±0.3% when optimized, and can be lowered to ±0.05% in some cases, without post-sintering machining [1]. The major limitation to production occurs largely with injection molded components, as the ability to create large, asymmetric components within tolerances is difficult. Several iterations of pre-production prototypes are ineffective in most cases due to the high cost of mold machining, making the understanding of dimensional change during the process of key importance. Processing of powder compacts is typically a four step process. In the first step, powders are mixed with a lubricant or binder system that is composed of polymer. These mixes are then either compressed in a die or injected into molds in much the same manner as polymer injection molding. The significant difference in the two methods is the weight percentage of binder required. Compaction polymer percentages are often between 0.5% and 1.5% by weight, while injection molding is higher, around 10% by weight binder for some injection molded stainless steel compacts [1]. In the second and third steps, prior to sintering, the lubricant or binder that helps mechanically maintain the shape of the compacted loose powder must be removed. The second step might involve removing some of the binder by dissolution in a solvent. Due to polymer swelling and surface wetting, dimensional change can occur during this process [2]. In the third step, polymers are removed by melting and subsequently evaporated out through the pore system, escaping from the compact largely as a gaseous substance [3]. This requires the addition of energy as heat. The process is called thermal debinding (sometimes called thermal de-polymerization, or thermal de-lubrication). The

2 thermal expansion of the powder and binder, coupled with the internal stresses caused by melting and vaporizing of the binder can cause severe dimensional change of the component. These internal stresses can be the root cause of many defects that are not detected until sintering [2]. Previous work has contended that dimensional change occurring during this process is responsible for the bulk of the dimensional instability of the entire process [3]. The fourth step is sintering in a number of different ways based on the material in use. Compacts are heated and shrink to full density, giving a net-shaped product. Ideally, no post-sintering machining or working is necessary, making this process cost effective and capable of almost 100% material use. Typical materials that are utilized for this process are alloys and ceramics [1]. Current methods of describing dimensional change during thermal debinding in powder compacts depend on the weight percentage of binder present in the compact, as well as the pressure profile of compaction [1]. Conventional rod dilatometry is not easily adapted to measure dimensional change during thermal debinding. Many rod dilatometry units cannot be used under the conditions of polymer melts and high molecular weight gases that can condense. Also, rod dilatometry unavoidably introduces forces that impede true investigation of dimensional change during thermal debinding. This is magnified by the delicate state of the compact prior to pre-sintering onset. To measure, and subsequently model accurately the dimensional change of the compact during thermal debinding, a non-contact method is desirable. In this paper, a method for non-contact, in situ measurement during thermal debinding is presented. A laser dilatometer, operating through an in-house designed furnace with optically clear windows is used to accomplish this goal. The thermal debinding of stainless steel powder compacts will be observed with the new equipment under a nitrogen atmosphere. Previous work has been performed on ceramic compacts, under air using similar ideas [6]. Powder metal compacts oxidize readily and production methods rarely utilize an oxygen-rich atmosphere. The goal of this paper is to describe the non-contact measurement of thermal debinding in powder compacts, and identify binder characteristics that may affect the dimensional stability of the product. 2. Experiment 2.1 Apparatus: The apparatus is built around a laser micrometer. The laser micrometer utilizes a horizontal array of lasers and a receiver. The receiver translates the missing lasers in the array as the shadow cast by the part, and returns the dimension of that shadow. A simple schematic of the laser array in use in the micrometer is shown in Fig. 1. The resolution in air is +/-1μm. The resolution at 500 C in nitrogen has been estimated at +/-4μm. The laser array is passed through the hot zone of a tube furnace that has optically clear quartz windows on either side. The part being analyzed is placed on a pedestal in the furnace and situated to impede the laser array.

3 Figure 1: Simple Schematic of Laser Dilatometer as Visualized from Above Figure 2: Fully assembled apparatus, prepared to be inserted into laser array. Laser micrometer receiver is shown in the right foreground, the transmitter is obstructed by the furnace. A thermocouple being used to both automatically control the temperature profile of the furnace cycle, and return the part temperature, is placed directly below the part. The thermocouple almost makes physical contact with the sample and is embedded in the pedestal under the part to give a reliable sample temperature. A photograph of the apparatus is shown in Fig. 2. The horizontal laser array is emitted through the horizontal window. Another window is on the rear of the apparatus and cannot be seen in Fig Sample Preparation and Monitoring: Die Compacted Parts: The samples used for the preliminary study of this apparatus are die compacted, right cylinders. The cylinders were pressed to 9 metric tons. This geometry affords the ease of measuring the diameter of the cylinder without difficult alignment of a flat face in the laser array. Two different lots were compacted. Both lots utilized a 316L stainless steel powder. The particle size has a D50 of 15.3μm, with an irregular shape due to water atomization. Two binders, denoted as Binder A and Binder B were used. Binder A is a standard wax. Binder B has a polypropylene backbone, and has a molecular weight of approximately 40,000, with a tight distribution. Five cylinders were pressed of each mix at 1 wt% binder. Each cylinder was pressed to 11.78TSI. Dimensions and characteristics of the cylinders are given in Table 1 below. Table 1: Sample Dimensions Binder A (Part #) Height (mm) Diameter (mm) Mass (g) Binder B (Part #)

4 The parts were thermally debound at a rate of 5 C/min up to 500 C. The atmosphere used was 99.5% pure nitrogen. After measuring the samples during thermal debinding, the data was analyzed as part diameter with temperature. To identify change in diameter and rate of change, both binders were analyzed in powder, at 1wt% binder, by thermogravimetric analysis (TGA). These tests monitor mass loss of a sample as a function of temperature. Mass loss is a consequence of binder pyrolisis in these systems. The TGA tests were conducted under the same atmosphere and rate of temperature increase as the thermal debinding experiments conducted. Finally, a debound sample was heated through a thermal debinding cycle to identify any effects of the heating cycle on the samples with no binder present. 3. Results Fig. 3 Depicts the TGA readout for binder A. The mass loss (pyrolisis) peak is seen in the temperature range of 300 to 320 C. Figure 3: TGA of binder A (wax) and powder. Figure 4: Diameter of compact containing binder A, measured in situ during thermal debinding. Fig. 4 depicts the in situ dimensional change as binder A is removed from the stainless steel sample. Of special interest is the slight change in slope after temperature ranges coinciding with the phase changes and subsequent mass loss seen in Fig. 3 (~300 C). The signal noise seen in the raw data above is likely attributed to power irregularities. The trends are still visible, though the signal to noise ratio is low. After heating beyond the temperatures coinciding with the pyrolisis point of binder A, the slope of the line decreases slightly. The region of heating above the pyrolisis range (above ~300 C) shows less of a deviation in data around the mean sample size, as well as a slightly lower average rate than through the melted binder temperature range (~ C). Fig 5 depicts the mass loss characteristics found in binder B mixed with powder to 1% by weight. Note the difference from binder A.

5 Figure 5: TGA data for binder B (polypropylene) and powder. Figure 6: Diameter of compact containing Binder B measured in situ during thermal debinding. The mass loss range in the range of approximately 440 to 470 C. Fig.6 shows a similar dimensional change as seen in Fig. 4. The slopes (analogous to a thermal expansion coefficient) of the different regions follow the same trend, decreasing as binder is removed. Binder B shows much more deviation in the data. This is mostly attributed to poor distribution of the polypropylene particles during admixing, which can be seen in the compact with the naked eye. Figure 6: Dimensional change measured in situ of a previously debound compact. This part contained binder B when it was initially debound. Fig. 7 shows the steady dimensional change of the compact that has been previously debound. The slope of the data trend line throughout the cycle is similar to the slope of both binder samples after the binder has been burned out, and may provide a thermal expansion coefficient for stainless steel as a powder compact. This indicates that the dimensional change and rate of dimensional changes seen in the samples containing binder are effected by polymer burnout. 4. Summary: An in situ system for monitoring dimensional change of powder during thermal debinding using a non-contact laser dilatometer has been described. Measurement of

6 dimensional change of die compacted stainless steel with two different lubricants has been demonstrated. Comparing dimensional change with the phase changes shown in the TGA/DTA data draws some conclusions. i.) ii.) iii.) The dimensional change of a powder compact appears to be effected by the pyrolization of the binder during debinding. The absence of the dimensional disturbance and rate change in the preciously debound samples demonstrates that the instability is likely a function of the binder characteristics. The thermal expansion coefficient of the compact is affected by the presence of binder. It has a larger value when the polymer is still present in the compact. A broader range of binders inspected in the same manner may show a single binder characteristic or a set of characteristics that can be used to model the dimensional change, and also the severity over time of the dimensional change of powder compacts during thermal debinding. Acknowledgements: The Authors acknowledge the Ben Franklin Technology Development Authority and the Center for Innovative Sintered Products. Also acknowledged are Tim Mueller and Tracy Potter for their guidance in design of the apparatus. References: 1. Randall M. German and A.Bose, Injection Molding of Metals and Ceramics, Metal Powder Industry Federation, Princeton, NJ E.Westcott, In Situ Observation and Evaluation of Solvent Debinding. Pennsylvania State University, Engineering Science and Mechanics, Masters of Science, Dec Y. Shengjie, Y.C.Lam, Simon C.M. Yu, K.C. Tam, Thermomechanical Simulation of PIM Thermal Debinding, Intern. J. Powd. Metall., vol. 38, no.8, Y.Mizuno, A.Kawasaki, R. Watanabe, In situ measurement of sintering shrinkage in powder compacts by digital image correlation method, Powder Mettalurgy, v.38, No.3, Y.C.Lam, Y.Shengjie, S.C.M.Yu, K.C.Tam, Simulation of Polymer Removal from a Powder Injection Molding Compact by Thermal Debinding. Metall. Mater. Trans. vol.31a, P. K. Lu, J.J. Lannutti, Density Gradients and Sintered Dimensional Tolerance in Compacts Formed from Spray-Dried Alumina, J. Am. Ceram. Society, 8 [6], 2000, pages P.K. Lu, J.J. Lannutti, Effect of Density Gradients on Dimensional Tolerance During Binder Removal, J. Am. Ceram. Society, 8 [10], 2000, pages