Wax-based Binder System Research Articles

Injection Flow Rate Threshold Preventing Atypical In-Cavity Pressure during Low-Pressure Powder Injection Molding

Controlling injection parameters is paramount when it comes to producing high-quality green parts using powder injection molding. This work combines experimental and numerical approaches to study the impact of injection parameters on mold in-cavity pressure and on the overall quality of green parts produced by low-pressure powder injection molding. The properties of two low-viscosity feedstocks (formulated from a water-atomized stainless-steel powder and wax-based binder system) were measured and implemented in an Autodesk Moldflow numerical model to quantify the molding pressures, which were finally validated using experimental real-scale injections. The results confirmed that an increase in mold temperature, an increase in feedstock temperature, and a decrease in solid loading decrease the mold in-cavity pressure, which was correlated with the feedstock viscosity. As a key result, real-scale injections confirmed that a minimum flow rate was required to avoid atypical high in-cavity pressure leading to several visual defects such as weld lines, flow marks, cracks, sinks, and incomplete filling. Due to differences in its thermal transfer properties, this flow rate threshold value decreases as the feedstock solid loading increases. For injection speeds higher than this value, the injection pressure measured experimentally was linearly correlated with the injection flow rate.

Smoothed Particle Hydrodynamics Approach for Simulation of Non-Newtonian Flow of Feedstocks Used in Powder Injection Molding

The present work aims to explore the ability to simulate flow patterns and the velocity field in the powder injection molding (PIM) process using the smoothed particle hydrodynamics (SPH) method. Numerical simulations were performed using the DualSPHysics platform. A feedstock formulated from 17-4 PH stainless steel powder (60 vol. % of powder) and a wax-based binder system was prepared to experimentally obtain its rheological properties that were implemented in DualSPHysics using two different viscosity models. The numerical simulations were calibrated, and then validated with real-scale injections using a laboratory injection press. During the calibration step, the feedstock flow momentum equation in the DualSPHysics code was modified and boundary friction coefficients at different injection rates were adjusted to create a frictional effect. During the validation step, these calibrated conditions were used to simulate the flow behavior into a more complex shape, which was compared with experimental measurements. Using an appropriate boundary friction factor, both the frictional effect of the boundaries and the stability of the numerical solution were taken into account to successfully demonstrate the ability of this meshless SPH method. The flow front length and feedstock velocity obtained in a complex cavity were satisfactorily predicted with relative differences of less than 15%.

The effect of debinding and sintering parameters on the mechanical and microstructural properties of Fe-2Ni metal injection molded compacts

This paper investigates the effect of debinding and sintering parameters on the density, ultimate tensile strength (UTS), hardness and grain size of the Fe-2Ni MIM compact. For this purpose, a wax-based binder system containing PE, PP, PW and SA was adopted for developing the feedstock. After optimizing the feedstock powder loading based on the critical solids loading and studying the homogeneity of feedstock, green parts were injected. The samples were then immersed in the n-hexane solution for different times to investigate the effect of debinding time on the weight loss of binder system. After observing the samples shrinkage behavior, sintering temperatures were selected and the samples were sintered at temperatures of 1050, 1125, 1200, 1275 and 1350 °C. Finally, the microstructure, density and hardness of the sintered samples were evaluated and the UTS of the samples was measured using the DIC technique. The results showed that immersion of samples for 12 h in n-hexane with 50 °C temperature applied as optimum parameters for solvent debinding. In addition, appropriate thermal debinding cycle was designed through the TGA analysis to produce flawless samples. It was observed that the density increases with the sintering temperature increasing, until where in the temperature of 1350 °C the relative density was 97.2%. UTS and hardness values of the sintered samples increased to (∼548 MPa) and (∼144 HV) for the sintered specimen at 1275 °C and decreased at higher temperature. This phenomenon relates to the substantial grain growth in the sintered sample at 1350 °C, as proved by the results of EBSD analysis. Finally, proper sintering condition for Fe-2Ni samples derived through the density, UTS, hardness and microstructural evaluations, so that sintering of the samples at 1275 °C under hydrogen atmosphere showed the optimum condition due to an excellent balance between the mechanical properties and grain size in the samples.

The effect of PSD on properties of unimodal and bimodal powders using 17-4 PH stainless steel powder

17-4 PH is a martensitic precipitation-hardening stainless steel that possesses an outstanding combination of superior physical and mechanical properties, good toughness, resistance to corrosion, and is easily machinable and weldable. It is widely used in the chemical, petrochemical, and aerospace industries. In this paper, we study the effect of the particle size distribution of unimodal and bimodal powders on properties of 17-4 PH stainless steel parts manufactured through metal injection molding (MIM). Unimodal and bimodal injectable MIM feedstocks of 60 vol% solids loading were prepared using powders of three different particle sizes with a wax-based binder system. The results show that coarser particle sizes (unimodal or bimodal) exhibit relatively lower shrinkage levels which is desirable for dimensional control but inferior “as-sintered” mechanical properties. The finer particle sizes (unimodal or bimodal) exhibited inferior rheological feedstock properties but comparatively superior “as-sintered” mechanical properties. The study suggests that bimodal feedstocks of finer particle sizes exhibit a favorable combination of rheological properties and “as-sintered” mechanical properties.

Rheological and Thermal Characterization of AISI 4605 Low-Alloy Steel Feedstock for Metal Injection Molding Process

In this study, rheological and thermal decomposition behavior of AISI 4605 MIM feedstock investigated using capillary and thermogravimetric analysis. For this purpose and in order to find the critical solids loading, the feedstock was prepared by compounding AISI 4605 micro powder (~ 4 µm) with an adopted multi-component wax-based binder system at various powder loadings (53–61 vol%). After preparing the feedstock with optimal solids loading, fundamental rheological characteristics of homogenized feedstock, including flow behavior index (n), flow activation energy (E), and general moldability index (αstv) were studied. Furthermore, thermal analysis of feedstock, including apparent debinding activation energy and master decomposition curve was investigated. The results showed that critical solids loading of feedstock is around 58 vol%. The flow behavior index of developed feedstock was acceptable, since it came out to be less than one. Also, flow activation energy which shows the sensitivity of material to temperature changes, came out to be 70.406 kJ/mol. Using these two parameters, general moldability index was calculated to be 2.3147E−06. Finally, the master decomposition curve of feedstock was constructed based on calculated apparent debinding activation energy and verified using decomposition curves at different heating rates of 2, 5 and 10 °C/min.

Investigation on Feedstock Preparation for Micro-Cemented Carbide Injection Molding

Abstract This research was focused on mixing of submicron cemented carbide (WC-Co-VC) powder and binder. WC-Co-VC powder particle size and morphology were analyzed by laser diffraction and field emission scanning electron microscopy. The WC-Co-VC powder was kneaded with a paraffin wax based binder system. Based on critical solid loading, the feedstock with different solid loadings between 49 to 51 vol.% was prepared. Finally, the flow behavior of different feedstocks was investigated. Morphology of powder revealed that the particles of powder are slightly agglomerated and irregular in shape. The result of mixing indicted that the torque value increases as the solid loading increase from 49 vol.% to 51 vol.%. The feedstock exhibited homogeneity and the powder particles are homogenously coated with binder. The feedstock with solid loading of 51 vol.% is sensitive to temperature and showed high viscosity values. The feedstock with solid loadings of 49 and 50 vol.% had good compatibility and flow characteristics.

Effect of nanoparticles in bimodal powder on physical and mechanical properties of powder injection molded parts

This paper presents investigation of how nanoparticles in bimodal powder that contains both nanoparticles and microparticles affect the physical and mechanical properties of injection-molded parts. As the starting materials, a pure-micropowder, and three different nano/micro-bimodal powders were prepared and mixed with a wax-based binder system. Addition of nanoparticles slightly decreased the critical solids loading and the feedstock homogeneity, and significantly increased the flexural strength and strain of green bodies. Addition of nanoparticles increased the density, decreased the grain size and provided superior surface finish of the sintered samples, and thereby improved their mechanical properties. The Vickers hardness of the samples increased as the nanoparticles content increased. Tensile strength was also improved by addition of small amounts of nanoparticles. However, the large passivation area of the nanoparticles raised the oxide content in the bimodal powder samples and degraded the strength and elongation. As a result, tensile strength was the highest in the sample fabricated from the bimodal powder that contained 12% nanoparticles.

Investigation and modeling of binder removal process in nano/micro bimodal powder injection molding

Nano powder injection molding is an appropriate technique for the mass production of micro components. However, the large surface area of nano powder increases the processing challenges, and as a result, nano/micro bimodal powder has been introduced into the process to reduce the problems. Although the understanding of the debinding behavior directly contributes to a fabrication of defect-free parts with a high production rate, most studies for the bimodal powder injection molding focused on the feedstock rheology and properties of sintered samples. In this study, nano powder effects on both of solvent and thermal debinding processes have been investigated with nano/micro bimodal feedstocks, and the master decomposition curve was developed. To prepare the bimodal feedstocks, premixed Fe nano/micro bimodal powders were blended with a wax-based binder system as the optimal volume ratios. Solvent and thermal debinding behaviors were analyzed by observing the weight change of the samples depending on the process time and temperature, respectively. The result indicated nano powder increased immersing time required in the solvent and the amount of the residual binder. It also narrowed the decomposition temperature range of the binder system by affecting the decomposition activation energy. From the result, the master decomposition curves of each bimodal feedstock were constructed and verified.

Evaluation of Wear Behaviour of Metal Injection Moulded Nickel Based Metal Matrix Composite

Metal injection moulding (MIM) is a near-net shape manufacturing technology for producing intricate parts, cost-effectively. MIM comprises combined techniques of plastic injection moulding and powder metallurgy. The present study focused on the development of the binder and fabrication of defect free MIM component. A wax-based binder system consisting of paraffin wax (PW), low density polyethylene (LDPE), polyethylene glycol (PEG-600) and stearic acid was established for MIM of NiCrSiB (70% Wt.) + Cr3C2-NiCr (30% Wt.) nickel based metal matrix composite (NMMC) powder. The feedstock was characterised through rheological properties at different temperatures. Injection temperature was determined from the rheological investigation of the feedstock having the 56% powder loading and 44% binder by volume. Sintering process was carried out with the temperature cycle in the range of 1250–1300 ∘C under hydrogen purged atmosphere. The MIM components showed good and acceptable shrinkage in linear dimensions. The mechanical properties and wear behaviour of NMMC was studied using a pin-on-disc apparatus with alumina disc.Tests were performed under dry sliding conditions at room temperature and elevated temperatures of 200 and 400 ∘C. Results shows that wear rate is maximum at 400 ∘C for 40 N and lowest at room temperature for 10 N. Further wear mechanism was analysed using scanning electron microscope (SEM).

New Injection Moulding Techniques for Automotive Aluminium-Based Foams - Part I

The raw material to be compacted by moulding is represented by aluminium alloy (ALUMIX 321) powder particles as metallic matrix and carbamide as foaming agent. The raw material to be injected is represented by the mixture (feedstock) between the wax-based binder system (40-60% mass) and the aluminium alloy (ALUMIX 321) powder particles (balance). The binder system is made of paraffin wax and stearic acid. The foaming effect is generated by addition of carbamide as foaming agent. Both categories of raw samples were washed in the ultrasonic machine and the aim of research was to study the physical properties and the macroscopic analysis of this materials.

Capillary rheological studies of 17-4 PH MIM feedstocks prepared using a custom CSIR binder system

This paper reports on an attempt to establish the rheological properties of 17-4 PH stainless steel MIM feedstocks prepared using a proprietary CSIR wax-based binder system. The influence of powder and feedstock characteristics on the rheological properties of 17-4 PH stainless steel MIM feedstocks were investigated using a capillary rheometer. The effects of bimodal powder distributions on the rheological properties were also studied. Three starting 17-4 PH stainless steel powder materials (5μm, 15μm, and −45μm) were used to formulate with five feedstocks investigated in this work. Results are presented herein are discussed in terms of know feedstock rheological characteristic parameters (flow behaviour index, activation energy, and high mouldability index). Effects of blending size distributions on feedstock stability and mouldability are reported. Fine powder size distributions and their bimodal blends exhibit superior mouldability.

Development of powder injection molding process for sintered soft magnet with the addition of Fe-17 at.% P powder

Soft magnetic powder injection molding is a net-shape production technology to fabricate sintered soft magnets. Powder injection molding technology for soft magnetic materials can be expected to develop a new type of electromagnetic components with the miniaturization and complexity design. In this work, 8 wt.% of iron-phosphorus powders (Fe-17 at.% P) were added to carbonyl iron powders to improve the magnetic performances with PIM process. Soft magnetic feedstocks were fabricated by optimized PIM process using a wax-based binder system. The critical solids loading of feedstocks were measured to determine the optimal solids loading, and injection molding process was conducted for the PIMed soft magnetic parts. The effect of Fe-17 at.% P addition to carbonyl iron powder was investigated on the sintering behaviors and magnetic properties. It was observed that sample with Fe-17 at.% P addition had optimum relative density (99.8%) and magnetic polarization (28,299 Gauss) at sintering temperature 1400°C in a hydrogen atmosphere.

Effect of Mixing Process Parameters and Suitability of Backbone Polymer for Aluminum Powder Injection Molding Feedstock

A suitable and cost-effective microfabrication technique for processing aluminum micropart is required, as the choice of aluminum microparts for aerospace, electronics and automobile components is preferred over other metals due to its excellent properties. Meanwhile, powder injection molding (PIM) is identified as an economical manufacturing technique for processing ceramic and micro-metal powders into microparts and or components. Therefore, this study investigates formulation and processing of aluminum PIM feedstock using a custom-made machine. The investigation is focused on the effect of mixing process parameters (powder loading, rotor speed and mixing temperature) and the suitability of the backbone polymer. The formulated PIM feedstock constituents are paraffin wax (PW), stearic acid (SA), high-density polyethylene (HDPE)/ medium-density polyethylene (MDPE) alternatively and aluminum micro-metal powder. Taguchi method is used for the design of experiments (DOEs) and analysis. In addition, response surface methodology (RSM) is employed to develop empirical viscosity models. The optimum powder-binder mixing ratio of 58:42 vol. % with rotor speed of 43 rpm were determined for preparing aluminum PIM feedstock using mini-lab mixer developed. The empirical model developed for aluminum PIM feedstock viscosity shows a good fit with R2 values of 0.84 using HDPE and 0.96 for MDPE binder system. This investigation demonstrates preparation and suitability of aluminum PIM feedstock using wax-based binder system.

Influence of nano powder on rheological behavior of bimodal feedstock in powder injection molding

Rheological behavior of feedstock, which highly affects the condition of green body, is substantial issue in powder injection molding process. In this paper, the influence of nano powder contents in bimodal feedstock on solids loading and rheological behavior has been investigated. The bimodal powders were fabricated with 100nm and 4μm sized 316L stainless steel powders, and mixed with wax-based binder system. The critical solids loading for each powder was measured by torque rheometer. In order to analyze net effect of nano powder ratio, all feedstocks were formulated as solids loading of 42vol.%. Feedstock homogeneity was evaluated by rotational rheometer. Capillary rheometer test was conducted to measure the viscosity of feedstock with different temperatures, shear rates and nano powder ratios. Rheological parameters, like flow index, flow activation energy and moldability index, were calculated based on the capillary rheometer test. The results indicated the critical solids loading and feedstock homogeneity decreased as nano power ratio increased in bimodal feedstock. Rheological parameters were also affected by nano powder ratio, and the optimal mixing ratio of nano powder for moldability index was investigated as 12vol.%.

Effects of different backbone binders on the characteristics of zirconia parts using wax-based binder system via ceramic injection molding

In this work, various backbone binders were used in wax-based binder system to formulate zirconia parts by ceramic injection molding (CIM). The effect of different backbone binders on the molding, debinding, and sintering behaviors was investigated. After blending process, the feedstock using multi-polymer components exhibited more homogeneous structure compared with that using the mono-polymer ones due to the synergistic effect of multi-polymers. During solvent debinding, some defects such as “slumping” and “peeling” appeared in the parts containing ethylene-vinyl acetate copolymer (EVA), but they were not found in the parts with other thermal polymers. Also, as for the parts after sintering, the one using low density polyethylene (LDPE) and high density polyethylene (HDPE) as backbone binders presented a more uniform microstructure with finer zirconia grains among all the investigated compositions, and thus obtained the highest flexural strength (~949 MPa) and relative density (~98.9%).

Influence of Specimen Dimensions and Temperature on the Debinding Behavior of Alumina Feedstock

This work investigated the influence of specimen dimensions and temperature on the debinding behavior of alumina feedstock for powder injection process. Polyethylene glycol (PEG) was used as a based binder. It is soluble in water therefore the use of wax-based binder system can be avoided. PEGs with a molecular weight of 1500 and 4000 were mixed with polyvinyl butyral (PVB) a minor component. Stearic acid was added during feedstock preparation to act as a lubricant. Debinding process was carried out using a water leaching test at 30 and 70 °C to study for PEG removal rate. Specimens retained their shapes after leaching of the binders. In general, the rate of binder removal increased at initial stage then slowed down at longer leaching times. A faster debinding rate was achieved at 70 °C when compared to 30 °C. In addition, samples with a higher surface area to volume ratio (As/V) led to an increase in the %PEG removal rate. This study also showed the relationship between %PEG removal as a function of different surface area to volume ratio (As/V) which can be helpful in predicting %PEG removal for any other samples having cylindrical shape.

Orientation of Carbon Fibers in Copper matrix Produced by Powder Injection Molding

Fiber orientation is a big challenge in short fiber reinforced composites. Powder injection molding (PIM) process has some intrinsic fiber alignment associated with it. During PIM process fibers in skin region of moldings are aligned as these regions experience higher shear flow caused by the mold walls. Fibers in the core region remain randomly aligned as these regions are far from mold walls and experience lesser shear flow. In this study short carbon fiber (CF) reinforced copper matrix composite was developed by PIM process. Two copper composite feedstock formulations were prepared having 5 vol% and 10 vol% CFs and a wax based binder system. Fiber orientation was controlled during injection molding by using a modified mold that has a diverging sprue. The sprue creates converging flow when feedstock enters into the mold cavity. Fiber orientation was analysed after molding using FESEM. The orientation of fibers can be controlled by controlling flow of feedstock into the mold.

Powder Injection Moulding of Alumina Using PEG/PVB Binder Systems

Powder injection moulding (PIM) is a process that is suitable for a fabrication of small and complex shape components. It consists of 4 main steps: feedstock preparation by mixing powder and binder, injection moulding of the prepared feedstock into the desired mould, removal of the binder and finally sintering to obtain materials with specific properties. In this study, powder injection moulding of alumina (Al2O3), using polyethylene glycol (PEG) based binder systems, was investigated. PEG is soluble in water; therefore, the use of organic solvents required for debinding of wax-based binder system can be avoided. PEG with a molecular weight of either 1500 or 4000 was used as a major constituent together with polyvinyl butyral (PVB) as a minor component. Stearic acid was also added during feedstock preparation to act as a lubricant. After mixing the powder with the binder, a variety of Al2O3 feedstocks were injected into the moulds. The mouldings were prepared by a laboratory-scaled plunger-typed machine. Debinding was carried out using a combination of solvent extraction and thermal debinding. Water leaching tests were performed at 30 and 50 °C to study PEGs removal rate. The pyrolysis of PVB was completed during ramping up of the mouldings to the sintering temperature. The mouldings were subjected to sintering at 1500 °C in air. It was found from the study that PEG/PVB binder systems can be used for the preparation of alumina powder injection moulding feedstocks. Specimens retained their shapes during and after leaching of the PEGs.

Critical Solid Loading and Rheological Study of WC-10%Co

Micro powder injection molding (µPIM) is a preferred technology for the production of micro parts or micro structured parts which derived from the well known thermoplastic injection molding technique. It is suitable for a large-scale production of ceramic and metallic parts without final machining. In the hardmetal industry, submicron and ultrafine hardmetals are the most demanding and also the fastest growing grades in production and application. Four stages involve in µPIM are mixing, injection, debinding and sintering. The volumetric ratio of solid powder to the total volume of powder and binder, which is usually called powder loading, largely determines the success or failure of subsequent processes. Critical solid loading of the powder can be estimated by torque variation, density, melt flow, density and viscosity versus composition. In this paper, critical solid loading of WC-10%Co is determined using torque variation method and its rheological behavior is studied. During the process, the wet surface of the powder particle WC-10%Co will cohesive together and resulted to the torque. Progressive powder is added-in after torque decrease and critical solid loading is identified when torque becomes unstable. Hence, critical solid loading WC-10%Co with WC (APS < 1 µm) is 46% and 42, 43 and 44 vol% of powder loading are selected to mix with wax-based binder system. The viscosity of feedstock show the pseudoplastic behavior and flow index (n) are 0.444, 0.491 and 0.492 for powder loading 42%, 43% and 44% respectively.

Characterisation of micro gears produced by micro powder injection moulding

The paper describes the fabrication of micro gears of diameter and height 1 mm by micro powder injection moulding. 316L stainless steel powder of 2.4 µm ( d 50) is mixed with a multi-component wax-based binder system. The feedstock is then injection-moulded into micro gears followed by debinding and sintering. The microstructure of the micro gear at its tooth and hub is studied by electron backscattered diffraction. The average grain size at the hub is ~ 5 µm. However, a high content of annealing twin and a significant grain growth is found at the tooth.