Powder Injection Moulding Feedstocks Research Articles

Powder Injection Molding an Alternative Method in the Manufacturing of Parts for Vehicles

Powder injection molding (PIM) is one of the most versatile methods for manufacturing small complex shaped components from metal, ceramic powders for the use in many applications. Interesting applications of PIM include the automotive and aerospace industries. PIM can be used to combine multiple parts into a single part with complex geometry, and parts are lighter since porosity remains in the final part while the fatigue resistance increases. Therefore, it can be said that PIM can help reduce the weight of moving vehicles, which is a major concern from the environmental and economic point of view. One of the main limitations of current PIM technology is the long debinding time, which is one of the steps within the process. This has been partially solved by introducing binder systems that undergo catalytic sublimation. However, current catalytic binders have the main problem of high viscosity. In this study, two ways to decrease the viscosity of PIM feedstock materials with polyoxymethylene were investigated. The first way was to reduce the average molecular weight of the binder and the second one to select a polydisperse particle size distribution with high maximum packing fraction.

Effects of jet milling on W–10 wt.%Cu composite powder for injection molding

Ultrafine W–10wt.%Cu powders synthesized by thermo-chemical method have a good sinterability compared with the mechanically mixed powers, which are suitable for powder injection molding (PIM). The deagglomeration, however, is necessary to improve the flowability of the ultrafine powder to obtain a PIM feedstock with high powder loading. Jet milling (JM) was employed in this study to reduce the strong agglomerations among the particles, which were subsequently used for PIM. Results showed that JM was a proper process to modify the ultrafine powders for mass production with negligible contamination. JM modified the powder characteristics, including agglomeration, size distribution, surface morphology, and tap density, which considerably improved the rheological properties of feedstock. With the increasing of powder fraction in the feedstock from 46vol.% to 52vol.%, the shrinkage of the parts during sintering reduced accordingly, which is favorable for dimensional stability. The resulted W–10wt.%Cu sintered composite has excellent physical properties, such as high thermal and electric conductivity, as well as low coefficient of thermal expansion, which can be attributed to its near full density and homogeneous microstructure.

Effect of Powder Shape and Size on Rheological, Thermal, and Segregation Properties of Low-Pressure Powder Injection Molding Feedstocks

Optimization of molding parameters at different filling stages requires that the characteristics of low-viscosity feedstocks must be properly known. In this study, the influence of the particle size (3, 7, and 12 μm) and powder morphology (gas- and water-atomized) was quantified using different feedstock formulations based on a 17-4 PH stainless steel powder, all combined with the same binder system. The specific heat capacity, viscosity, and segregation levels were assessed using a differential scanning calorimeter, a rotational rheometer, and a thermogravimetric analyzer, respectively. It was then shown that the feedstock prepared with the gas-atomized powder exhibits a higher specific heat capacity value when compared to a water-atomized powder (i.e., Cp varying from 0.6 to 0.3 J/g K, respectively). It was also shown that the feedstock viscosity profiles and the intensity of segregation both depend significantly on the powder shape and size used in the feedstock formulation. For short processing times (e.g., < 1 min spent in molten state), a feedstock formulated with coarse gas-atomized powder can be considered to be the best candidate for an injection process because its viscosity and segregation potential are relatively low (with a viscosity varying from 0.5 to 2 Pa s, and a volume fraction of powder close to 60 vol.%). Following this, the moldability index was used to predict that coarser water-atomized powder (i.e., 12 versus 7 μm) will not have a significant effect on the molding properties, and can be seen as a potential means of decreasing shrinkage during sintering without impacting the injection properties. For very long processing times, the magnitude of segregation remains insignificant for feedstock formulated with fine powder (i.e., a volume fraction of powder remaining close to 60 vol.%). However, this important gain (i.e., no segregation) comes with a significant increase in viscosity varying from 2 to 10 Pa s, resulting in a decrease in the molding properties.

Online Rheometry Investigation of Flow/Slip Behavior of Powder Injection Molding Feedstocks.

Wall slip in the flow of powder injection molding (PIM) compounds can be the cause of unrealistically low viscosity values, and can lead to a failure of flow simulation approaches. Regardless of its importance, it has been considered only scarcely in the rheological models applied to PIM materials. In this paper, an online extrusion rheometer equipped with rectangular slit dies was used to evaluate the slip velocity of commercial as well as in-house-prepared PIM feedstocks based on metallic and ceramic powders at close-to-processing conditions. The tested slit dies varied in their dimensions and surface roughness. The wall-slip effect was quantified using the Mooney analysis of slip velocities. The smaller gap height (1 mm) supported the wall-slip effect. It was shown that both the binder composition and the powder characteristic affect slip velocity. Slip velocity can be reduced by tailoring a powder particle size distribution towards smaller particle fractions. The thickness of the polymer layer formed at the channel wall is higher for water-soluble feedstocks, while in the case of the catalytic polyacetal feedstocks the effect of surface roughness was manifested through lower viscosity at smooth surfaces.

Rheological properties of titanium-hydroxyapatite with powder space holder composite feedstock for powder injection moulding

Metal-ceramic composites such as titanium-hydroxyapatite have been extensively studied as a new generation of implant material in medical application. The powder injection moulding (PIM) process is one the viable processing method in producing the titanium-hydroxyapatite composite which offers extreme geometry part at a lower cost of production. The final powder injection moulding (PIM) product was influenced by the feedstock rheological and thermal properties that related to the binder selection and the critical solids loading. The emphasis of this study was towards the rheological and thermal properties of Ti6Al4V-hydroxyapatite composite feedstock with the powder space holder using a low-density polyethene and palm stearin binder system. The critical solids loading for 90:10 of Ti6Al4V-hydroxyapatite with 20% of the space holder was selected for capillary rheometer tests. The solids loading was obtained at 73 vol.% with optimal solids loading in the range of 2–5% below the critical solids loading which were 68, 69 and 70 vol.%. All feedstocks exhibited shear thinning behaviour, with the shear rate and viscosity values in the range of the PIM feedstock. The work in this study found that the feedstock of 68 vol.% solids loading possessed the best rheological properties with a higher flow behaviour index.

Development of Hypereutectic AlSi Alloy Powder Injection Molding Feedstocks by Rheological Analysis

The comprehensive properties of a feedstock have a critical influence on the powder injection molding process. Proper feedstock with homogeneous structure, favorable flow characteristic, and moldability is the prerequisite for obtaining a final part with excellent comprehensive properties. The objective of the present work was to develop an optimal feedstock for fabrication of hypereutectic AlSi (20 wt.%) alloy parts by the powder injection molding technique. For this purpose, micron-sized hypereutectic AlSi (20 wt.%) alloy powder was mixed with different amounts of a binder which consisted of 35 wt.% high-density polyethylene, 62 wt.% carnauba wax, and 3 wt.% stearic acid. The binder contents of the feedstocks were in the range from 13 wt.% to 21 wt.%. The influences of binder content, shear rate, and temperature on the rheological behaviors of feedstocks have been investigated via a capillary rheometer. The feedstock with 21 wt.% binder exhibited a variable flow behavior and was culled. The rest of the feedstocks showed a pseudoplastic behavior. Comprehensive analysis of rheological parameters such as the flow behavior index, yield stress, flow activation energy, and the general moldability index, the feedstock with 17 wt.% binder exhibited the best rheological performance and favorable moldability. The molded part with 17 wt.% binder had constant density, good shape retention, and stiffness as well as homogeneous distribution of the powder and binder. After solvent debinding, the debound item showed a homogeneous porous structure which is suitable for the subsequent thermal debinding and sintering processes.

An Analysis of Rheological Properties of Inconel 625 Superalloy Feedstocks Formulated with Backbone Binder Polypropylene System for Powder Injection Molding

Abstract Binder formula is one of the most significant factors which has a considerable influence on powder injection molding (PIM) processes. In the study, rheological behaviors and properties of different binder systems containing PIM feedstocks, Inconel 625 powder commonly used in space industry, were investigated. The feedstocks were prepared 59%-69% (volume) powder loading ratios with three diversified binder systems by use of Polypropylene as backbone binder. The average particle size of the Inconel 625 powder used was 12.86 microns. Components used in the binder were mixed for 30 minutes as dry in three dimensional mixing to prepare binder systems. Rheological features of the feedstock were characterized by using a capillary rheometer. Viscosities of the feedstocks were calculated within the range of 37.996-1900 Pa.s based on the shear rate, shear stress, binder formula and temperature. “n” parameters for PIM feedstocks were determined to be less than 1. Influences of temperature on the viscosities of the feedstocks were also studied and “Ea” under various shear stresses were determined within the range of 24.41-70.89 kJ/mol.

Influence of capillary die geometry on wall slip of highly filled powder injection molding compounds

Uneven distribution of solid particles contained in the feedstocks used in the process of powder injection molding (PIM) is observed in the close vicinity of the walls. A particle-free thin layer adjacent to the walls is formed by the binder only and is characterized by so-called wall slip. Wall slip is a key to successful modeling of injection molding step of PIM. For its determination we used capillary rheometers equipped with the dies of different entrance angles applied to four PIM feedstocks. The entrance angle has been found to be a crucial parameter to intercept wall slip. Conical dies are more suitable to obtain reliable slip velocity values of highly filled compounds than capillaries having plane entrance, which are used in the majority of studies.

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.

Rheological and thermal debinding properties of blended elemental Ti-6Al-4V powder injection molding feedstock

The rheological and thermal debinding properties of a feedstock play an important role in the molding and debinding stage of powder injection molding (PIM), and can directly determine the quality of final PIM product. This work focuses on the rheological and thermal debinding properties of blended elemental Ti-6Al-4V (BE Ti64) PIM feedstock with a comparison study of pure titanium (Ti) feedstock. The effects of 60Al-40V alloy addition, on the attributes of BE Ti64 feedstock; including critical solids loading, rheological behavior, and binder decomposition behavior, were studied using torque rheometry, capillary rheometry, and thermogravimetric analysis. The critical solids loading of BE Ti64 (66vol.%) was somewhat lower value than the Ti feedstock (69vol.%), due to the different in particle characteristics. Both BE Ti64 and Ti feedstocks exhibited non-Newtonian shear-thinning behavior. The difference in flow activation energies for the two feedstocks indicates that the BE Ti64 feedstock is more sensitive to temperature compared to the Ti feedstock. During thermal debinding, both the feedstocks exhibited a dual-sigmoid, binder-decomposition behavior with only minor differences in activation energy.

Development of feedstocks based on steel matrix composites for metal injection moulding

Feedstock behaviour during processing via powder injection moulding (PIM) has a critical influence on the properties of the final parts. The objective of the present study is to analyze the effect of vanadium carbide (VC) addition (1, 3, 6, 10 and 14wt.% VC) on the rheological response and stability of AISI M2 high speed steel PIM feedstocks. A binder system consisting of high density polyethylene (HDPE) and paraffin wax (PW) is proposed. The effects of VC concentration, powder loading, apparent viscosity and temperature have been investigated via capillary rheometer. The feedstocks with a 14wt.% of VC, which exhibited a variable flow behaviour was discarded. The rest of the feedstocks showed a pseudoplastic behaviour which is suitable for injection moulding and in particular, those reinforced with 6 and 10wt.% of VC exhibited the best rheological responses. In order to maximize the powder loading and ensure safety conditions, the feedstocks reinforced with 6wt.% (5.9vol.%) and 10wt.% (9.8vol.%) of VC with 68vol.% of powder loading and 3wt.% (2.8vol.%) of VC with 62vol.% of powder loading were selected to continue the rheological analysis. Rheological parameters such as flow behaviour index (n), yield stress (ơy), flow activation energy (Ea) and the general mouldability index (αstv) have been analyzed for the above mentioned feedstocks and showed suitable values for the injection moulding. The green parts produced using the three feedstocks have constant weights and dimensions, good mouldability, density, shape retention and stiffness. Therefore, the developed feedstocks are suitable for PIM.

Influence of binder system and temperature on rheological properties of water atomized 316L powder injection moulding feedstocks

Hatasiz toz enjeksiyon kaliplama (TEK) islemi icin besleme stoklarinin reolojik ozellikleri iyi bilinmelidir. Bu calismada 316L paslanmaz celik tozunun reolojik ozelliklerine baglayici sisteminin etkisini belirlemek amaciyla 2 farkli baglayici sistemi kullanilarak besleme stoklari hazirlanmistir. Deneysel calismalarda 20 mikron alti su atomize 316L paslanmaz celik tozu kullanilmistir. Heptan icerisinde cozunebilen parafin wax (PW) esasli ve su icerisinde cozunebilen polietilenglikol (PEG) esasli iki farkli baglayici sistemi hazirlanmistir. Her iki baglayici sistemi icerisinde iskelet baglayici olarak polipropilen (PP) ve yaglayici olarak stearik asit (SA) kullanilmistir. Baglayici sistemleri kuru olarak turbula marka 3 boyutlu karistiricida 30 dakika karistirilarak hazirlanmistir. Toz ve baglayici sistemleri kullanilarak hazirlanan besleme stoklarinin reolojik ozellikleri kilcal reometre kullanilarak karakterize edilmistir. Kilcal reometrenin sicakligi 150-200 °C araliginda basinc degerleri ise 0.165-2.069 MPa arasinda degistirilmistir. PEG esasli baglayici sistemi kullanilarak hazirlanan besleme stoklarinda toz tasima kapasitesi hacimce %55’e, PW esasli baglayici sistemi kullanilarak hazirlanan besleme stoklarinda ise %61’e cikartilmistir. PEG esasli besleme stoklarinda en dusuk viskozite degeri 304.707 Pa.s olarak, PW esasli besleme stoklarinda en dusuk viskozite degeri 48.857 Pa.s olarak elde edilmistir.

Influence of binder system and temperature on rheological properties of water atomized 316L powder injection moulding feedstocks

In order to obtain a proper powder injection molding the rheological behavior of feedstocks should be known. To determine the binder effect on the rheological behavior of 316L stainless steel powders feedstock two different feedstock were prepared. In the current experiments water atomized 316L stainless steel powders (-20 µm) were used. Two types of binders, one of which is mainly paraffin wax can be dissolved in heptane and the other Polietilenglikol (PEG) based and can be dissolved in water, were used. Polypropylene was used as binder and steric acid was used as lubricant for both binder systems as skeleton binder. Dry binder system were mixed for 30 min in a three dimensional Turbola. Capillary rheometer was used to characterize the rheological properties of feed stocks at 150-200 °C and a pressures of 0.165-2.069 MPa. Powder loading capacity of PEG and PW based feed stocks were found to be %55 and %61 respectively. The lowest viscosity of PEG and PW based feed stocks were found to be 304.707 Pa.s and 48.857 Pa.s respectively. Keywords: PIM, Binder, Rheological properties

The development of powder injection moulding binders: A quantification of individual components' interactions

The study of interactions between binder system components is critical for improving the processing properties of powder injection moulding (PIM) feedstocks. In this paper the interactions between acrawax (AW) and polyethylene glycol (PEG) were analysed and compared with those obtained for carnauba wax (CW). Due to the complexity of interaction mechanisms, the polymers were substituted with their basic low molecular weight analogues and analysed by FTIR and calorimetry. Self-interaction energies and association energies were determined using calorimetric analysis. Shifts of FTIR absorption peaks (C–O stretch and N–H stretch) served as evidence of the presence of interactions between the components. The calorimetric study of AW/PEG analogues showed a temperature increase during mixing, indicating the presence of strong interactions. The combined data from FTIR and calorimetry allowed a quantitative evaluation, which indicated about two times stronger interactions between AW (with CO and N–H groups) and PEG (with C–O and –OH groups), as compared to CW (with CO and C–O groups) and PEG analogues.

Thermo-Gravimetric Analysis and Debinding Study of Powder Injection Moulded Titanium Alloy

An investigation into the solvent and thermal debinding behaviour of a typical paraffin wax-polymer binder system for titanium alloy Ti-6Al-4V powder injection moulding (PIM) feedstock was conducted. Details of the mixing parameters, powder loading, temperature and mixing speed were evaluated for optimal compounding of the feedstock. The feedstock was injection moulded into 22 mm diameter by 3 mm thick disks. Debinding is conducted in two stages, an initial solvent debinding step to remove the waxes, followed by a thermal debinding step to remove the polymers. Solvent debinding was conducted by immersing the injection moulded specimens in n-heptane heated to 50 °C for a period of 5 hours. The mass loss rate was logged during solvent debinding in order to determine the efficiency of the solvent debinding step. From these measurements, it was determined that 94 wt% of the wax (paraffin wax and stearic acid) components of the binder were removed during solvent debinding. A thermo-gravimetric analysis (TGA) was conducted in air on the Ti-6Al-4V powder, as well as the green and solvent debound feedstock. These results show the effect of debinding the feedstock in air and were used to design a thermal debinding cycle for the system.

Optimization of 67% Powder Loading Co-30Cr-6Mo µMIM Part by Taguchi Method

This research studies the effect of injection moulding parameters on the density of green body of Cobalt-30Chromium-6Molybdenum (Co-30Cr-6Mo) for powder injection moulding (PIM) feedstock. In this paper 20 micron Co-Cr-Mo powder was mixed with a palm stearin and polyethylene binder system. L18 orthogonal array by Taguchi Method was used to optimize and predict the future performance. Several injection parameters were optimized such as injection temperature, holding pressure, injection temperature, and mould temperature and injection time. The result shows that the optimum combination of these parameters will produce higher density micro parts. The optimum parameters for 67% powder loading of 20µm Co-30Cr-6Mo powder is 180oC injection temperature, while injection pressure, mold temperature, packing time and injection time are 10 bar, 100oC, 5 s and 7 s respectively.

Imbalance filling of multi-cavity tooling during powder injection molding

A shortened cycle time with improved productivity is a key justification for multi-cavity molds. However, filling imbalance is an offsetting barrier for multi-cavity molds. The sources for an imbalance are often subtle, in both conventional plastic injection molding and powder injection molding (PIM) industries. The greater thermal conductivity of the PIM feedstock and the subsequent large sintering shrinkage amplify the issue for PIM. Due to its characteristic, the size of PIM parts is relatively smaller than the conventional plastic injection molding and so multi-cavity molds are widely used. Despite the geometrically systematic delivery system and cavities, asymmetric branching of flow paths, uneven viscous heating, and cooling variations exist for the molten feedstock flow to the cavities. These variations significantly limit the benefit of a multi-cavity mold. We investigated the imbalance filling between the near- and far-side cavities in a geometrically symmetric multi-cavity mold for PIM. Via molding simulations and experiments we found that imbalanced filling arises from subtle factors such as non-uniform mold cooling, uneven conduction of the molten feedstock into mold, and uneven viscous heating and their competitive roles between conduction/convection and viscous heating in the multi-cavity molds mainly contribute to imbalance filling the same way in the conventional plastic injection molding.

Analysis of the Rheological Behavior and Stability of Inconel 718 Powder Injection Molding (PIM) Feedstock

In PIM process, the molding stage is a critical step for the fabrication of sound parts without cracks and distortions. So, this step requires specific rheological behavior. Rheological analysis can be made to quantify the stability of the PIM feedstock during molding process. In this study, an experimental rheological study has been performed to evaluate the influence of palm stearin on the rheological behavior and stability of Inconel 718 powder injection molding (PIM) feedstock. The effects of shear rate and temperature were investigated via capillary rheometer method. The viscosity and shear rate of the feedstock were measured at various range of temperature and shear rate across the L/D = 1 capillary rheometer. The results show that the formulation binder system PSPE 7030 is the best feedstock to be injection molded because it has moderate value of flow sensitivity index, lower value for flow activation energy and higher value for mold ability index. Besides that, the increasing of PS content in the binder system has found reduce viscosity and value of flow activation energy and increase value of mold ability index of feedstock.

Effect of the particle size and solids volume fraction on the thermal degradation behaviour of Invar 36 feedstocks

Abstract Degradation kinetics and the thermal stability of Invar 36 powder injection moulding feedstocks (PIM) based on cellulose acetate butyrate (CAB) and polyethylene glycol (PEG) binders were investigated using simultaneous thermogravimetric analysis (STA) and differential scanning calorimetry (DSC). The initial decomposition temperature (IDT) and the integral procedure decomposition temperature (IPDT) were used to analyse the thermal stability of the binder system as a function of the solid loading content and powder particle size. The degradation kinetics was studied, and the process apparent activation energies were assessed using isoconversional methods. All the methodologies revealed changes in the thermal degradation behaviours of the feedstocks for solid loadings that were previously determined to correspond to optimal solid loadings using other experimental procedures. The studies also contrast previous similar findings with a ceramic powder. Therefore these results strengthen the proposal of thermodynamic degradation studies of feedstocks as an alternative or complementary technique to determine optimal solid loading contents in metal injection moulding (MIM).

Thermal stability and degradation kinetics of feedstocks for powder injection moulding – A new way to determine optimal solid loading?

Degradation kinetics and the thermal stability of zircon powder injection moulding feedstocks (PIM) based on cellulose acetate butyrate (CAB) and polyethylene glycol (PEG) binders were investigated using simultaneous thermogravimetric analysis (STA). The initial decomposition temperature (IDT) and the integral procedure decomposition temperature (IPDT) were used to analyse the thermal stability of the binder system as a function of the solid loading content. The degradation kinetics was studied, and the degradation activation energy was assessed for varying zircon powder contents using isoconversional methods. All the methodologies revealed changes in the thermal degradation behaviours of the feedstocks for solid loadings that were previously determined to correspond to optimal solid loadings using other experimental procedures. These results may promote the proposal of thermodynamic degradation studies of feedstocks as an alternative or complementary technique to determine optimal solid loading contents in powder injection moulding (PIM). The studies in this paper also examined PIM process operation temperatures for zircon feedstocks.