Low-pressure Powder Injection 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.

Influence of powder size on the moldability and sintered properties of irregular iron-based feedstock used in low-pressure powder injection molding

Initially used for shaping irregular ceramic powders, manufacturing metallic parts via low-pressure powder injection molding (LPIM) from irregular powders has received very little attention. This study investigates the processability of irregular powders for fabricating high-density and low-cost metallic parts using the LPIM process. Three irregular iron-based powder lots were mixed with a low-viscosity binder, injected, debound, and sintered to evaluate their molding and sintering performances via viscosity measurements, real-scale injections, metallographic analysis, and tensile tests. The results revealed that feedstocks formulated from fine and coarse powders exhibit good moldability, predicted by their moldability indexes and validated by spiral flow distances. Furthermore, the feedstock formulated with the finer powder (i.e., 1250 mesh providing a powder particle size <10 μm) produced a homogeneous microstructure along with a low content of oxygen impurities, leading to the highest sintered relative density of ∼90%, an ultimate tensile strength of ∼225 MPa, and an elongation at break of ∼24%. These results confirm that the cost-effective irregular iron powders present a promising alternative to fabricating metallic parts via low-pressure powder injection molding compared to their relatively high-cost spherical powder counterparts (i.e., carbonyl iron powder).

Surface properties of molds for powder injection molding and their effect on feedstock moldability and mold adhesion

The surface energy of various mold materials for low-pressure powder injection molding was evaluated using values of contact angles (Owens–Wendt method), and correlated with the feedstock moldability and mold adhesion. The surface tension of the binder used to formulate a metallic-based feedstock was also measured in the molten state at a typical injection temperature using the pendant drop technique. Real-scale injection tests were performed into metallic and polymeric mold cavities to assess the feedstock moldability and its adhesion with the mold surfaces that were compared with theoretical predictions obtained from the surface energies values. The results confirmed that the adhesion was significantly affected by the interfacial energy between the mold and the binder—in this case, the metallic mold exhibited low adhesion as compared to the polymeric mold. It was finally demonstrated that the adhesion phenomenon is only related to the surface properties of the mold (i.e., they are not related to the solidification rate)—in this case, a gold-coated polymeric mold produced the moldability of a polymeric mold and the adhesion properties of a metallic mold, which translated into a high moldability potential, with no resulting adhesion of the feedstock with the mold.

Research Progress on Low-Pressure Powder Injection Molding

Powder injection molding (PIM) is a well-known technique to manufacture net-shaped, complicated, macro or micro parts employing a wide range of materials and alloys. Depending on the pressure applied to inject the feedstock, this process can be separated into low-pressure (LPIM) and high-pressure (HPIM) injection molding. Although the LPIM and HPIM processes are theoretically similar, all steps have substantial differences, particularly feedstock preparation, injection, and debinding. After decades of focusing on HPIM, low-viscosity feedstocks with improved flowability have recently been produced utilizing low-molecular-weight polymers for LPIM. It has been proven that LPIM can be used for making parts in low quantities or mass production. Compared to HPIM, which could only be used for the mass production of metallic and ceramic components, LPIM can give an outstanding opportunity to cover applications in low or large batch production rates. Due to the use of low-cost equipment, LPIM also provides several economic benefits. However, establishing an optimal binder system for all powders that should be injected at extremely low pressures (below 1 MPa) is challenging. Therefore, various defects may occur throughout the mixing, injection, debinding, and sintering stages. Since all steps in the process are interrelated, it is important to have a general picture of the whole process which needs a scientific overview. This paper reviews the potential of LPIM and the characteristics of all steps. A complete academic and research background survey on the applications, challenges, and prospects has been indicated. It can be concluded that although many challenges of LPIM have been solved, it could be a proper solution to use this process and materials in developing new applications for technologies such as additive manufacturing and processing of sensitive alloys.

Effect of powder particle shape and size distributions on the properties of low-viscosity iron-based feedstocks used in low-pressure powder injection moulding

ABSTRACT Low-pressure powder injection moulding (LPIM) is a cost-effective manufacturing technology used to fabricate complex-shaped parts with high mechanical properties at low- or high-volume production. This research work presents an experimental approach to investigate the debound/sintered properties of iron components produced by the LPIM process using iron-based powders exhibiting different particle shapes and size distributions. Four low-viscosity feedstocks were mixed and injected into a rectangular mould cavity before being thermally wick-debound and finally sintered using identical debinding and sintering cycles. This study confirms that both irregular and spherical iron powders can be shaped via the LPIM process. The solid loadings obtained with these two powder morphologies, varying from 58 to 62 vol.-%, represent expected values. The trend in the density values obtained with irregular (∼6.6 g/cm3) and spherical (∼7.5 g/cm3) powders was validated by metallographic observations.

Effect of Thermal Debinding Conditions on the Sintered Density of Low-Pressure Powder Injection Molded Iron Parts

Low-pressure powder injection molding (LPIM) is a cost-effective technology for producing intricate small metal parts at high, medium, and low production volumes in applications which, to date, have involved ceramics or spherical metal powders. Since the use of irregular metal powders represents a promising way to reduce overall production costs, this study aims to investigate the potential of manufacturing powder injection molded parts from irregular commercial iron powders using the LPIM approach. To this end, a low viscosity feedstock was injected into a rectangular mold cavity, thermally wick-debound using three different pre-sintering temperatures, and finally sintered using an identical sintering cycle. During debinding, an increase in pre-sintering temperature from 600 to 850 °C decreased the number of fine particles. This decreased the sintered density from 6.2 to 5.1 g/cm3, increased the average pore size from 9 to 14 μm, and decreased pore circularity from 67 to 59%.

Mold filling simulation and experimental investigation of metallic feedstock used in low-pressure powder injection molding

Mold filling simulation and experimental investigation of metallic feedstock used in low-pressure powder injection molding

Impact of binder constituents on the moldability of titanium-based feedstocks used in low-pressure powder injection molding

This work presents an experimental and numerical approach to study the impact of binder constituents and solid loading on spherical titanium powder-based feedstocks used in low-pressure powder injection molding. The viscosity profiles of 35 feedstocks were used to quantify the threshold proportions of each constituent. The binder which maximizes the moldability and demolding while minimizing the segregation is constituted of 1, 1 and 3 vol% of stearic acid, ethylene-vinyl acetate and carnauba wax, respectively, and paraffin wax making up the balance. The maximum solid loading capable of producing intricate parts with minimal defects was found to be 64 vol%. Finally, numerical simulations were carried out along with experimental validation to study the injection flow in an intricate mold cavity. Results show that numerical simulations are in good agreement with experimental values in predicting the melt front position during injection of high solid loading feedstock in complex shape cavity.

Impact of rheological model on numerical simulation of low-pressure powder injection moulding

ABSTRACT The aim of this study is to quantify the impact of different rheological models on the accuracy of LPIM numerical simulation. Two different feedstocks were formulated, and rheological profiles were characterised using a rotational rheometer from which the parameters of different rheological models were extracted and then implemented in an Autodesk Moldflow package. Mould filling patterns into a rectangular shape mould cavity were simulated, and then validated using real-scale injections. Depending on the feedstock rheological behaviour, models are not always well adapted to predict the mixture viscosity over the entire shear rate range. Because injections were performed at constant volumetric flow, all the rheological models were successfully used to predict the injected length. However, the simulated pressure through the mould cavity was only accurately simulated using the second-order or matrix models since these viscosity models were able to adequately capture the feedstock behaviour in the shear rate range representing the injections.

Molding properties of titanium-based feedstock used in low-pressure powder injection molding

The moldability of titanium-based feedstocks used in low-pressure powder injection molding process was investigated using numerical simulation and real-scale injections. Feedstock characterization and injection were conducted using a feedstock formulated from a Ti-6Al-4 V powder and a low-viscosity binder system. The thermal characteristics of the feedstock, including the specific heat capacity and the thermal conductivity, were measured over a 20–120 °C temperature range, while rheological profiles were experimentally measured at 80, 90, and 100 °C to establish the constitutive equations to be implemented in the model. The mold filling stage of a complex shape part was numerically simulated using Autodesk Moldflow Synergy 2019. The predicted filling time and magnitude of powder segregation after an injection were validated using a laboratory injection press and thermogravimetric analysis. The melt front location and filling completion predicted by the numerical model at different short shots were in good agreement with the experimental observations, with relative differences below 9%. Moldflow was also used to identify the high shear rate zones experienced by the feedstock, in which a predicted segregation of 2 vol% was validated by experimental tests. A halo shape segregation pattern was determined as resulting from a solidified skin and/or a segregation band passing near the interface of the shear rate gradient.

Effect of Binder Constituents and Solids Loading on the Rheological Behavior of Irregular Iron-Based Feedstocks

Abstract This work presents an experimental approach used to evaluate the influence of binder constituents and solids loading on the rheological behavior and molding properties of irregular shape iron-based feedstocks used in low-pressure powder injection molding (LPIM). Thirty-six (36) different feedstocks formulated from one new in-development iron-based powder and different wax-based binder systems (paraffin wax (PW) with surfactant and thickening agents) were obtained for solids loading varying from 50 to 68 vol%. The viscosity profiles were experimentally measured at different temperatures using a rotational rheometer in order to quantify the threshold proportions of each ingredient in the binder systems, identify the best feedstock candidates, and calculate their moldability indices, which were finally validated using real-scale injections. Results confirmed that the best feedstock formulation was the one containing paraffin wax with 1 vol% stearic acid (SA) used as a surfactant, 2 vol% ethylene-vinyl acetate (EVA) used as a thickening agent, and 2 vol% carnauba wax (CW) used as a shrinking agent. An irregular shape iron-based feedstock with maximum solids loading of 58 vol% was successfully injected.

Fabrication of titanium components by low-pressure powder injection moulding using hydride-dehydride titanium powder

The injection moulding of metal powder is one of the newest processes for producing titanium components. In this research, challenges of fabrication titanium component by low- pressure powder injection moulding (LPIM) were studied. A hydride-dehydride titanium powder was kneaded with paraffin wax based binder system in 53 vol% powder loading. Powder-binder was injection moulded using low pressure injection moulding machine to prepare the green part. This process was followed by debinding and sintering. The injection results showed that feedstock with high powder loading was not appropriate for LPIM. The suitable parameters of solvent debinding were obtained from soaking the samples in n-hexane at 60 °C temperature. The open channels were observed in solvent debound components which can help remove the binder in subsequent stages. The hardness and density of the sintered part were 490 HV and 4.11 g/cm3, respectively. Coarse α plates and α/β colonies were observed in the microstructure.

Numerical simulation and experimental investigation of mold filling and segregation in low-pressure powder injection molding of metallic feedstock

The mold filling stage of the low-pressure powder injection molding process and the segregation of the injected part were numerically simulated and validated by an experimental approach. The numerical simulations were performed using the Autodesk Moldflow Synergy 2019 package to specifically investigate the injected length, the front velocity, the filling time, the pressure, and the shear rate, as well as to assess the magnitude of powder segregation after an injection. Furthermore, thermal and rheological characteristics of feedstock were experimentally measured to establish the constitutive equations to be provided to Moldflow. Numerical simulation results were validated with real-scale injections (with feedstock temperatures varying between 80 and 100 °C), using a laboratory injection press. Experimental characterization and injection were conducted using a feedstock formulation based on 17–4 PH stainless steel powder (60 vol% of powder) combined with a low-viscosity binder system. The injected length, the melt front velocity, and the filling times predicted by the numerical model were in good agreement with the experimental observations, with relative differences varying from 3.1 to 4.4%. Since the injections were performed and simulated at constant volumetric flow in a constant cross-section mold cavity, the mold filling results confirm that the feedstock temperature has no influence on the injected lengths, but rather, on the injection pressure. Moldflow also captured the absence of segregation occurring during injection, with experimental measurements of the local solid loading confirming the ability of the numerical model to successfully predict the homogeneous distribution of the powder (i.e., no segregation) within a simple shape mold cavity. Although theses interesting simulation capabilities of the injection and segregation behaviors were obtained for simple-shape components, the present work represents, to the best of the authors’ knowledge, the first simulation of low-pressure injection molding of metallic feedstock.

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.

Molding Properties of Cobalt-Chrome-Based Feedstocks Used in Low-Pressure Powder Injection Molding

Molding Properties of Cobalt-Chrome-Based Feedstocks Used in Low-Pressure Powder Injection Molding

Experimental study on moldability and segregation of Inconel 718 feedstocks used in low-pressure powder injection molding

Moldability and segregation of feedstock are linked to the rheological behavior of the powder-binder mixture. In this study, the impact of binders on viscosity and segregation of feedstocks was investigated. The experiments were conducted on several feedstocks obtained by mixing Inconel 718 powder with paraffin wax-based binder systems. The viscosity of feedstocks was measured by a rotational rheometer while the segregation within green parts was evaluated using a thermogravimetric analyzer. It was demonstrated that the variation in solid loading within a molded part can be measured with a sensitivity of at least ±0.25 vol% of powder. The results indicated that the predominant powder-binder separation appears clearly at the top and the bottom of the molded part. It was also shown that the viscosity profiles of feedstocks and the intensity of segregation depends significantly on the binder constituents used in feedstock formulation. The mixture containing only paraffin wax produced the best trade-off between high moldability and low segregation for an injection process requiring an extended time range between injection and solidification of the part (e.g. up to 10 min). For a short processing time (e.g. <1 min spent in molten state), the feedstocks containing paraffin wax with stearic acid or small amount of ethylene vinyl acetate can be also considered as good candidates for LPIM process because their viscosity and segregation potential are relatively low.

Influence of segregation on rheological properties of wax-based feedstocks

The rheological behavior of wax-based feedstocks has a direct impact on successful mold filling for parts produced by low-pressure powder injection molding. During a rheological test, segregation may occur within low-viscosity feedstocks, leading to errors in results, and to an improper evaluation of the flowability of powder-binder mixtures. The segregation occurring during rheological tests is not generally considered when measuring the viscosity profiles of such mixtures. In this study, the impact of testing time, testing protocol (increasing or decreasing shear rate), polymer degradation, powder deagglomeration, and temperature on the rheological properties of feedstocks was investigated. The segregation occurring during rheological testing produces a meaningful effect in low-viscosity feedstocks. Because the segregation is driven by the time spent in the molten state, the effects of segregation on viscosity were visible, particularly at the end of a long rheological test. During a short rheological test, the pseudoplastic effect occurring in wax-based feedstocks was only attributable to the binder molecules' orientation with respect to the flow. In this respect, and contrary to the information reported in literature, it was demonstrated that the dilatant effect observed was in fact more likely due to segregation occurring during the rheological test, rather than as a result of the elimination of the preferential layer formed during the pseudoplastic regime. Because the rheological properties of low-viscosity feedstocks were significantly influenced by the test duration, a rheological test duration of a few minutes must be used to precisely quantify the flowability of such feedstocks.

Low-pressure powder injection molding using an innovative injection press concept

In low-pressure powder injection molding, the injection presses are equipped with an interconnecting pipe in which the segregation of feedstocks may occur during the dead time of the process. This segregation phenomenon limits the use of low-viscosity feedstocks leading to a decrease in the capacity to produce more complex-shaped components. The full moldability potential of the process is still limited by the injection method proposed in the commercial machines. A new injection concept was developed to increase the moldability of powder-binder mixtures, while avoiding segregation within the injection press. This innovative injection system consists of eliminating the interconnecting pipe and injection valve by using a sliding platform concept which was adapted for the transportation of the feedstock from the container to the mold cavity. Validation of the injection system was performed using three wax-based feedstocks. Injection profiles obtained with two different molds were correlated with the rheological behavior of the feedstocks, and thermogravimetric analyses were used to quantify the segregation within the system. An experimental simulation of the segregation within the machine confirmed that the injection method avoids segregation for a feedstock exhibiting a viscosity as low as 0.1 Pa·s. The best moldability properties were obtained with the feedstock containing paraffin wax and stearic acid.

Investigation of Rheological Behavior of Low Pressure Injection Molded Stainless Steel Feedstocks

The purpose of this research is to investigate the influence of different powder loadings of 316L stainless steel (SS) powders on rheological behavior of feedstocks required for low pressure powder injection molding (L-PIM) process. The main idea consists in development of various formulations by varying 316L SS powder contents in feedstocks and evaluating the temperature sensitivity of feedstock via flow behavior index and activation energy. For this purpose, the irregular shape, spherical shape, and combination of both shapes and sizes (bimodal approach) of 316L SS powders are compounded with wax based composite binder. Moreover, the influence of elemental nanosized boron (nB) addition (up to 1.5 wt.%) on rheological properties of irregular shape 316L SS powders is also evaluated using capillary rheometer method. It is observed that rheological parameters for solid powder loading of powder gas atomized (PGA) and bimodal powder P25/75 316L SS underwent sudden change from PGA-69 vol.% to PGA-72 vol.% and P25/75-67 vol.% to P25/75 316L SS 70 vol.%, respectively. Thus it is concluded that PGA-69 vol.% and P25/75-67 vol.% are optimal powder solid loadings corresponding to the lowest values of activation energies.

Molding Properties of Inconel 718 Feedstocks Used in Low-Pressure Powder Injection Molding

The impact of binders and temperature on the rheological properties of feedstocks used in low-pressure powder injection molding was investigated. Experiments were conducted on different feedstock formulations obtained by mixing Inconel 718 powder with wax-based binder systems. The shear rate sensitivity index and the activation energy were used to study the degree of dependence of shear rate and temperature on the viscosity of the feedstocks. The injection performance of feedstocks was then evaluated using an analytical moldability model. The results indicated that the viscosity profiles of feedstocks depend significantly on the binder constituents, and the secondary binder constituents play an important role in the rheological behavior (pseudoplastic or near-Newtonian) exhibited by the feedstock formulations. Viscosity values as low as 0.06 to 2.9 Pa·s were measured at high shear rates and high temperatures. The results indicate that a feedstock containing a surfactant agent exhibits the best moldability characteristics.