MOLD FOR METAL INJECTION MOLDING

TECHNICAL FIELD

This disclosure relates to the field of metal injection molding (MIM) molds for metallic powders and methods of using same.

BACKGROUND OF THE ART

Metal injection molding (MIM) is an advanced manufacturing process in which fine metallic particles are mixed with a polymeric binder to form a powder-binder mixture, also referred to as the feedstock, which is injected into a mold cavity in molten state to form a desired shape after solidification of the binder. This molded part, also referred to as the green part, is then debound and sintered to completely remove the binder and finally obtain a near-net shape dense metallic intricate part, in a relatively cost efficient manner. The MIM process can be divided into different branches according to the viscosity of the feedstock. High-pressure metal injection molding (HPIM) uses high-viscosity feedstocks, such as feedstocks falling within the 100-1000 Pa·s range requiring an injection temperature up to 220° C. and pressure ranging from 50-150 MPa, for example. Low-pressure metal injection molding (LPIM) uses low-viscosity feedstocks, i.e., feedstocks falling within the 0.1-20 Pa·s range requiring an injection temperature below 120° C. and pressure below 1 MPa, for example. Due to the low injection pressure used in the LPIM process, the size of the injection machines and the overall size of the tooling may be significantly reduced as compared to the HPIM process. The lower costs associated with this size reduction renders the fabrication of complex-shape metallic parts more cost-effective, either in low or in high production volumes. Despite all these advantages, the LPIM process is still in its infancy, especially with regard to metallic feedstocks. Efforts to optimize MIM and particularly LPIM of metallic materials typically involves changes in the injection, debinding, and sintering steps, while the optimum moldability of the feedstocks may be improved. Therefore, improvements in MIM processes are still desired.

SUMMARY

In a first aspect of the present disclosure, there is provided a metal injection molding method for producing a molded part, the method including the steps of: mixing a polymeric binder and a metallic powder to obtain a powder-binder mixture; heating the powder-binder mixture to a desired temperature; injecting the powder-binder mixture in a polymer mold at a desired pressure and/or a desired flow rate, the polymer mold having a portion of its internal surface coated with a metallic layer; and demolding the molded part.

Further in accordance with the first aspect, for example, following the demolding step, a sintering step can be performed on the molded part.

Still further in accordance with the first aspect, for example, the metal injection molding is low-pressure powder injection molding and the desired pressure is equal to or below 5 MPa.

Still further in accordance with the first aspect, for example, the desired temperature is from 60° C. to 180° C.

Still further in accordance with the first aspect, for example, before the step of mixing, the polymeric binder and the metallic powder are heated.

Still further in accordance with the first aspect, for example, before the step of injecting, the polymer mold is cooled or heated.

Still further in accordance with the first aspect, for example, the metallic layer has a thickness of from 100 nm to 200 nm.

Still further in accordance with the first aspect, for example, the metallic layer includes chromium, silver, cobalt, copper, iron, molybdenum, nickel, palladium, platinum, tungsten, or gold.

Still further in accordance with the first aspect, for example, the polymer mold is selected from polytetraluoroethylene, polycarbonate and polyethylene.

Still further in accordance with the first aspect, for example, the polymeric binder comprises low density polyethylene, paraffin wax, stearic acid, ethylene-vinyl acetate, beeswax, polypropylene, or polystyrene.

Still further in accordance with the first aspect, for example, the metallic powder is selected from a titanium alloy, a nickel alloy, an aluminum alloy, a stainless steel alloy, a nickel-based superalloy, and/or a steel alloy.

Still further in accordance with the first aspect, for example, the portion of the internal surface coated with the metallic layer has two opposite internal walls at a distance of less than 15 mm.

Still further in accordance with the first aspect, for example, the portion of the internal surface coated with the metallic layer has two opposite internal walls at a distance of less than 1 mm.

Still further in accordance with the first aspect, for example, the portion of the internal surface coated with the metallic layer has two adjacent walls at an angle equal to or less than 75°.

Still further in accordance with the first aspect, for example, the portion of the internal surface coated with the metallic layer has two adjacent walls at an angle equal to or less than 60°.

In a second aspect of the present disclosure, there is provided a metal injection mold comprising a polymer plate having a mold surface, and a metallic layer on a portion or the totality of the mold surface.

Further in accordance with the second aspect, for example, the metallic layer is on the totality of the mold surface.

Still further in accordance with the second aspect, for example, the portion of the internal surface coated with the metallic layer has two opposite internal walls at a distance of less than 15 mm.

Still further in accordance with the second aspect, for example, the portion of the internal surface coated with the metallic layer has two adjacent walls at an angle equal to or less than 75°.

Still further in accordance with the second aspect, for example, the metallic layer has a thickness of 100 nm to 200 nm.

Still further in accordance with the second aspect, for example, the portion of the mold surface is a protrusion or a cavity.

Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.

DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross sectional view of a mold according to an embodiment of the present disclosure.

FIG. 1B is a schematic exploded view of a mold according to an embodiment of the present disclosure, with a comparatively simpler geometry.

FIG. 1C is a schematic exploded view of a mold according to an embodiment of the present invention, with a comparatively more complex geometry.

FIG. 2 is a schematic exploded view of a mold according to an embodiment of the present disclosure.

FIG. 3A is schematic overview of a methodology used for the calculation of surface energy of flat molds using the contact angle of water and diiodomethane at room temperature.

FIG. 3B is a schematic overview of a methodology used for the calculation of surface tension of a binder and single constituent using pendant drop measured in the molten state.

FIG. 3C is a schematic overview of a methodology used for the calculation of surface tension of a binder using sessile drop measured in the molten state.

FIG. 4 is a bar graph showing a surface energy and water contact angle (points) for polycarbonate, Inconel, aluminum, steel, copper, and polytetrafluoroethylene.

FIG. 5 is a graph showing a surface tension of the binder, paraffin, stearic acid, and EVA (Ethylene-Vinyl Acetate).

FIG. 6A is a photograph showing a contact angle for a binder on a flat steel surface immediately after deposition at 20° C.

FIG. 6B is a photograph showing a contact angle for a binder on a flat polytetrafluoroethylene surface immediately after deposition at 20° C.

FIG. 6C is a photograph showing a contact angle for a binder on a flat steel surface after 135 s at 70° C.

FIG. 6D is a photograph showing a contact angle for a binder on a flat polytetrafluoroethylene surface after 480 s at 70° C.

FIG. 6E is a photograph showing a contact angle for a binder on a flat steel surface after 165 s at 70° C.

FIG. 6F is a photograph showing a contact angle for a binder on a flat polytetrafluoroethylene (PTFE) surface after 510 s at 70° C.

FIG. 6G is a graph showing a binder contact angle for a PTFE surface and a steel surface, in which gray bars are for a binder at 70° C. and mold at 20° C., and black bars are for a binder at 70° C. and mold at 70° C.

FIG. 7 is a graph showing a moldability of steel (rough) mold at 40° C. ( custom-character ); steel (polished) mold at 40° C. (); steel (rough) mold at 45° C. (); steel (polished) mold at 45° C. (); steel (rough) mold at 50° C. (); steel (polished) mold at 50° C. (); steel (rough & polished) mold at 70° C. (); and PTFE all conditions ().

FIG. 8 is a bar graph of a work of adhesion (black bars) and an interfacial energy (gray bars) for steel, uncoated PTFE or gold coated PTFE.

FIG. 9 is a bar graph showing the injected length for spiral molds having a mold cavity made of steel, PTFE, or PTFE coated with Cr.

FIG. 10A is a photograph of a demolded steel part produced by MIM with a spiral mold having a steel internal surface.

FIG. 10B is a photograph of a demolded steel part produced by MIM with a spiral mold having a PTFE internal surface.

FIG. 10C is a photograph of a demolded steel part produced by MIM with a spiral mold having a Cr coated PTFE internal surface.

DETAILED DESCRIPTION

Metal injection molding (MIM) is an advanced manufacturing method that allows the fabrication of metallic parts. MIM can be used to mold high value metals that are traditionally difficult to mold with conventional metal shaping methods. MIM may advantageously produce metallic parts that are dense and have desirable mechanical properties. Indeed, a metallic part with a substantially homogenous microstructure can be obtained from MIM. This desirable microstructure confers monotonic mechanical properties to the MIM part that may be similar to those that would have been obtained if the same metallic part was instead forged.

One of the limitations of MIM methods is the difficulty in maximizing the moldability while minimizing the adhesion of the molded part to the mold. The term “moldability” as used herein is to be understood as the capability of a powder-binder mixture to fill completely a particular mold without defects. The thermal conductivity of the mold is an important parameter that promotes moldability. The higher the thermal conductivity the faster the powder-binder mixture may cool down, which increases the viscosity of the powder-binder mixture thereby reducing moldability. The composition of the polymeric binder, the solid loading, the feedstock temperature, the metallic powder composition, and the shear rate applied on the powder-binder (i.e. feedstock) mixture are also parameters that affect the viscosity and in turn the moldability. Accordingly, mold filling is driven by the feedstock moldability and by phenomena occurring at the mold/feedstock interface, leading to feedstock spreading, sticking, solidifying, etc. on the mold cavity surfaces.

The adhesion is the interaction between the powder-binder mixture (i.e. feedstock) and the mold surface (i.e. the internal surface which is the surface defining the mold cavity), either in the molten state during injection or in the solid state after injection. Among the mold/feedstock interface phenomena influencing the quality of parts in production, the mold filling during injection, as well as the adhesion of feedstock with the mold surface after solidification, are two of the mechanisms to be controlled. The adhesion of feedstock with the mold surface after solidification, which is related to the demolding ability of the feedstock, is another practical parameter that can affect the manufacture method of the part. The adhesion magnitude can be assessed by measuring the amount of feedstock stuck on the mold surface after the feedstock solidification following an injection.

Polymeric materials such as polytetrafluoroethylene, also referred to as Teflon®, polycarbonate, or polyethylene, such as ultra high molecular weight polyethylene, as the material coating the mold enable high moldability because of their low thermal conductivity compared to traditional metallic molds (e.g. steel). However, polymeric molds have high adhesion which effectively negates the desirable high moldability for industrial purposes. The present disclosure provides a MIM method that improves the moldability while also reducing the adhesion. A polymer mold that has a portion of its internal surface coated with a metallic layer is provided to overcome the disadvantage of polymeric molds. The metallic layer can improve the adhesion without significantly increasing the thermal conductivity. In some embodiments, to limit the local effect on the thermal conductivity while sufficiently reducing the adhesion, the thickness of the metallic layer is about 150 nanometers. In further embodiments, at least 10% to 100%, at least 20% to 100%, at least 30% to 100%, at least 40% to 100%, at least 50% to 100%, at least 60% to 100%, at least 70% to 100%, at least 80% to 100% or at least 90% to 100% of the internal surface of the polymer mold can be coated with the metallic layer. The present disclosure further contemplates embodiments where the internal surface of the polymer mold is substantially entirely coated with the metallic layer.

In some embodiments, the portion that is coated is a complex shape whereas the uncoated portion of the mold is the simple geometry of the mold. The terms “complex” and “simple” in the context of mold and/or metallic part geometries are as detailed further herein below. A simple geometry may be defined by one or more of the following: a large volume such as more than 1 cm³; constant dimensions along an axis such as a cylindrical pin, a plate and the like; and the absence of complex geometry features described herein. A complex geometry may be defined by one or more of the following: a round surface or volume, a square, a semi-circle or semi-sphere, a tapered hole, an ellipsoidal hole, a tread, a marking, a knurled surface, a protrusion, an external undercut, an internal undercut, a thin wall, a tapered wall and a sharp edge. In one example, a complex geometry is a protrusion or cavity with a diameter smaller than 2 cm, smaller than 1 cm, smaller than 100 mm, smaller than 10 mm, smaller than 5 mm or smaller than 1 mm. The complex geometry may be characterized by a volume of less than 1 cm³, more particularly less than 0.5 cm³, even more particularly less than 0.1 cm³. A part can be made of any combination of simple and/or complex geometry features described herein.

Making reference to FIG. 1A, there is provided an exemplary polymer mold 1 adapted to receive a powder-binder mixture. The mold 1 can have sections of its internal surface, i.e. the surface of the mold cavity, with a simple geometry 10, and sections with complex geometries 11, 12. As illustrated in FIG. 1A by element 11, a complex geometry may be defined by the distance between two opposite internal walls of the polymer mold 1. For example, the distance between the two opposite internal walls can be less than 100 mm, less than 15 mm, less than 10 mm, less than 5 mm, less than 1 mm, less than 500 μm, or less than 100 μm. Further, as illustrated in FIG. 1A by element 12, a complex geometry may also be defined by an acute angle between two adjacent walls. For example, the angle can be less than 75°, less than 70°, less than 65°, less than 60°, less than 55°, less than 50°, less than 45°, less than 40°, less than 35°, less than 30°, less than 25°.

An example of a mold having a simple geometry is shown in FIG. 1B. As illustrated in FIG. 1B the mold plates 13 are of a simple geometry to shape the simple geometry part 14. The simple part 14 has an elongated constant geometry shaping a “T”. The simple part 14 can also have a complex feature such as the cylindrical protrusion 14a shown in FIG. 1B. Accordingly, predominantly simple parts may still contain one or more complex features. In some embodiments, a mold can be formed with plates according to the prior art for simple geometry section of the part in combination with plates according to the present disclosure for the complex geometry sections. In the example illustrated in FIG. 1B, the plate 13a can be according to present disclosure and the other plates 13 may be any suitable plates.

An example of a mold having complex geometries is shown in FIG. 1C. The mold 1 has a plurality of plates 15 which combine to define a cavity in the shape of the complex part 16. The complex part 16 has sharp edges and external rounded surfaces, along with internal square holes, internal cylindrical holes, and internal treads. Accordingly, in some embodiments, the entire mold including all the mold plates 15 are according to the present disclosure. Indeed, the reduced adhesion and increased moldability exhibited by the molds of the present disclosure, advantageously allows for the manufacture of complex parts, such as the complex part 16, by MIM.

The metallic layer can be made of a metal that reduces the adhesion to the mold and that is capable of being deposited in the complex geometries of the mold cavity. One suitable method for the deposition of the metallic layer is physical vapor deposition (PVD). Exemplary metals for the metallic layer include but are not limited to chromium, silver, cobalt, copper, iron, molybdenum, nickel, palladium, platinum, tungsten, and gold. On the other hand, the polymer mold can be made of polymer that sustains the heat of the MIM process, for example polytetrafluoroethylene or polycarbonate, without substantially deforming or melting.

Accordingly, there is provided a MIM method of receiving a powder-binder mixture in a polymer mold to obtain a molded part, the polymer mold having a portion of its internal surface coated with a metallic layer, or made of a metal. Then, the molded part is demolded to obtain a demolded part. The powder-binder mixture can be obtained by mixing a metallic powder and a polymeric binder. The metallic powder may be any suitable metallic powder for MIM such as a titanium alloy, a nickel alloy, a nickel-based superalloy, a stainless steel alloy, a copper-based alloy, an aluminum alloy, and/or a steel alloy. More specific examples include but are not limited to austenitic, ferritic, and martensitic alloys, precipitation hardened alloys, duplex alloys, ASTM A801 Types 1 & 2, ASTM A753 Types 2 & 4, MIM 430L, ASTM F15Kovar, ASTM F 1684, ASTM F30, MIM 2200, MIM 2700, MIM 4605, carbon steel, ASTM F75, copper and bronze alloys. The polymeric binder may be selected based on its compatibility with the metallic powder. Examples of polymeric binders include a mixture of different polymeric constituents, and may include organic binders. A non-exhaustive non-limitative list of five exemplary binder compositions is detailed herein: a first binder example can be a low density polyethylene (LDPE) with paraffin wax and stearic acid; a second binder example can be polypropylene with microcrystalline wax and stearic acid; a third binder example can be polystyrene with polyethylene and stearic acid; a fourth binder example can be paraffin wax with EVA and stearic acid; and a fifth binder example can be polypropylene with paraffin wax and carnauba wax. In some embodiments, the polymer mold and/or the powder-binder mixture are heated to improve the fluidity of the powder-binder mixture and thus the moldability. The molten feedstock is heated to about 10-30° C. higher than the binder melting point (e.g., 60 to 180° C. according to the binder formulation) and then injected into a warm mold cavity (e.g. 60 to 180° C.) or a cold mold cavity (e.g. 5 to 60° C.) before solidification inside the mold. The green part is ejected from the mold to be debound (solvent, thermal, or catalytic) and sintered at high temperature (generally between 75-95% of the powder melting point) under vacuum, protective or reactive atmosphere to densify the metallic part.

Based on the above, the moldability of the polymer-binder mixture may not be affected by the surface energy of the polymer mold, but may mainly be driven by the solidification rate of the polymer-binder mixture. Since the polymer mold is a poor heat conductor, a high moldability may be obtained regardless of the surface finish, the mold temperature, and the powder-binder mixture temperature. In contrast, in a steel mold, an increase in the injection temperature and/or in the mold temperature enhanced moldability due to a decrease in powder-binder mixture viscosity and a delay in powder-binder mixture solidification associated with any increase in the binder temperature. An increase in the mold surface roughness may thus result in higher moldability with the metallic mold, regardless of the mold temperature or the powder-binder mixture temperature. This result was related to a change in the solidification rate of the powder-binder mixture, that may be explained by air trapped in the grooves of the mold surface, producing an insulating layer at the mold/mixture interface. The powder-binder mixture is injected in the polymer mold at a desired pressure or a desired flow rate. In other words, the injection can be controlled by the pressure or the flow rate in order to provide adequate moldability and an adequate amount of powder-binder mixture so as to fill the mold. The pressure can for example be of less than 5 MPa, less than 3 MPa or around 1 MPa. In some embodiments, the pressure is from 0.5 to 5 MPa, from 0.5 to 3 MPa, or from 0.5 to 1.5 MPa.

The adhesion of the powder-binder mixture may be directly influenced by the interfacial energy between the mold and the binder. A low interfacial energy indicates a high level of compatibility between the binder and the mold, which could be used to predict difficulties in removing the part from the mold cavity after its injection and solidification. In this respect, the high and low interfacial energies were correlated to the non-adherence or full adherence behavior of the polymer-binder mixture. For the steel mold, no adherence of the powder-binder mixture with the mold was visible for all typical injection conditions because of the high interfacial energy resulting in a less stable bond. Since the adhesion affects the demoldability of the injected parts, the high moldability potential obtained with the polymer mold of the present disclosure may be significantly counterbalanced by its high feedstock adhesion. The adhesion phenomenon may not be affected by the solidification rate, but rather, may be related to the surface properties of the mold. The surface modification of the polymer mold according to the present disclosure produced high moldability and no or minimal adhesion of the powder-binder mixture with the mold.

EXAMPLE 1: GOLD PLATED POLYMERIC MOLD

Water-atomized stainless steel 17-4PH powder (Epson Atmix Corporation, Japan), with a typical near-spherical or ligament shape and nominal particle size of 12 μm were used for the formulation of feedstock. This precipitation-hardening stainless steel is widely used in the aerospace, chemical, petrochemical, and many other sectors for its high strength and good corrosion resistance. The dry powder was combined with molten binder (90° C.) in a laboratory mixer and blended for 1 hour under vacuum. The solid loading was set at 60 vol. % of powder to prepare a feedstock from a low melting point binder system formulated from 34 vol. % of paraffin wax, 1 vol. % of stearic acid, and 5 vol. % of ethylene vinyl acetate. These binder constituents were selected due to their extensive use in LPIM, to help with the mold filling, to promote the surfactant effect enhancing chemical links between the powder and binder, and to produce the thickening effect needed to control the segregation of powder.

6.35 mm thick aluminum, copper, Inconel 625, polycarbonate (PC), polytetrafluoroethylene (PTFE), and steel plates were first used for the surface energy measurements. The PTFE and steel plates were then selected for the moldability and adhesion tests. These interchangeable plates were prepared using two different surface finish conditions to investigate the effect of roughness on moldability and adhesion during injections. The first condition (polished surface) was prepared by manual high-speed polishing using a 0.05 μm silica solution, while the second condition (rough surface) was obtained by shot peening. The arithmetical mean roughness values (Ra) resulting from polishing or shot peening operations were obtained from three measurements taken on each specimen at random positions using a profilometer (Mitutoyo Surftest SJ-400) with a 0.000125 μm minimum resolution over a range of 8 μm. According to ASME B46.1, the measuring speed, pin diameter, and pin top angle of the tool were 0.5 mm/sec, 2 μm, and 90°, respectively. The moldability and adhesion tests were performed using a rectangular mold cavity formed by one support plate (steel), two thick base plates (steel), and two thin interchangeable plates (PTFE or steel). FIG. 2 illustrates the experimental mold 2 which has thermocouples 20, a support plate 21 made of steel, two base plates 22 also made of steel, a rectangular cavity 23 defined between two interchangeable plates 24 made of steel or PTFE. The rectangular cavity 23 was used to assess the injected length values for different feedstock temperatures and mold conditions. The feedstocks were injected at 70, 75, and 80° C. using a controlled constant volumetric flow of 1.15 cm³/s, leading to an injection pressure varying from 0.1 to 0.5 MPa until a sudden increase in pressure, indicating excessive friction with the mold walls or complete solidification. The mold temperatures were set at 40, 45, 50, and, 70° C., and monitored using three thermocouples 20 inserted into the mold.

The surface energy of the six flat molds was calculated by the Owens and Wendt method. Contact angles values measured using a goniometer (VCA Optima, AST Products, Inc.) formed by liquid droplets of water or diiodomethane deposited on each surface (FIG. 3A) were used for that purpose. The measurement of the contact angle was repeated five times on each plate to calculate the surface energy of these six different materials, and was then used to identify two materials (i.e., steel and PTFE) exhibiting different surface properties to be used as interchangeable plates for the injection and adhesion tests. The surface tensions of the binder and its single constituents, namely, paraffin wax, stearic acid, and ethylene vinyl acetate alone, or combined as a binder, were calculated using the same goniometer and the molten pendant drop technique (FIG. 3B). Since the binder constituents were solid at room temperature, a nozzle band heater combined with a plastic syringe was set up to produce a liquid pendant droplet at 70-75° C., as illustrated in FIGS. 3A-3B. The mold and binder surface energies were then used to calculate the work of adhesion and interfacial energy. Finally, the measurement of the contact angle formed by direct binder deposition on hot mold (FIG. 3C) was also investigated. The surface of the mold was heated up to 75° C. using the same nozzle band heater to create an open environmental chamber to melt a solid binder droplet over the mold surface (FIG. 3C). After injection into steel or PTFE mold cavities, the adhesion or demoldability of feedstock was estimated using a sticking ratio calculated as the ratio of the feedstock remaining on the mold surface over the total injected length. This metric was categorized into four groups with the following distributions: 0% (no adhesion), 0-10% (light adhesion), 10-30% (moderate adhesion), and >30% (high adhesion).

The surface energies and contact angle formed by drops of water on the different materials used for the molds are presented in FIG. 4. All metallic materials and one polymeric material (the PC) exhibited a hydrophilic behavior, with water contact angles less than 90°, leading to higher surface energy values. With a contact angle of around 105°, the PTFE plate could be considered as a hydrophobic material, resulting in the lowest surface energy obtained in this study. Due to their high and low surface energies, steel and PTFE were identified as good candidates for further real-scale injection tests. Since high-strength steel or tool steel are often used for the fabrication of LPIM molds, tool steel was preferred over aluminum or Inconel even though it presented higher surface energy values.

The surface tensions of the binder and its single constituents calculated from the pendant drop technique are presented in FIG. 5. The surface tension of the binder was similar to that of pure paraffin wax, indicating that this constituent was more concentrated at the surface of the binder molten droplet. Also, paraffin wax, stearic acid, and ethylene vinyl acetate represented 85, 3, and 12 vol. % of the binder, respectively. Therefore, paraffin wax (i.e., the low surface tension component) was expected to migrate to the surface and reduce the overall surface tension of the system. A comparison of the values of the surface energies obtained for the mold (FIG. 4) and the surface tension of the binder (FIG. 5) showed that the surface tension of the binder was about two-fold lower than the surface energy of the steel plate, but only slightly lower than the surface energy of the PTFE plate. This indicated that the binder will tend to wet both surfaces, but the wetting will be more favorable on steel since the difference between the surface tension of the binder and the surface energy of steel is greater than that between the surface tension of the binder and the surface energy of PTFE.

The analysis above considered the values of the surface tension of the binder and the surface energy of the mold separately, and did not take into account other mold surface characteristics such as chemical heterogeneity. In order to observe the binder spreading directly on the surface of the molds, contact angles formed by drops of molten binder on steel and PTFE interchangeable plates were evaluated, as illustrated in FIGS. 6A-6F. When the plates were at room temperature, the molten binder dropped on the metallic surface quickly formed a solidified droplet, resulting in a high contact angle value (FIG. 6A), while a liquid binder dropped over the polymeric surface took a longer time to solidify, producing a lower contact angle (FIG. 6B). The solidification of the binder stopped the liquid spreading regardless of the thermodynamic equilibrium state estimated by surface energy analysis. The difference in solidification time between the metallic and plastic molds can be explained by their differences in thermal conductivity, where k_steel≈10-50 W·m⁻¹·K⁻¹>>k_PTFE≈0.3 W·m⁻¹·K⁻¹. To discretize the effect of solidification on the contact angle, the molds were heated at 70° C. (i.e., 10° C. higher than the melting point of the binder) to maintain the molten binder droplet in liquid state and evaluate its spreading over the mold surfaces after several time periods (FIGS. 6C-6F). Using this near-adiabatic condition (i.e., mold artificially maintained at 70° C.), the heat transfer between the binder and mold was hindered and the difference in thermal conductivity of the plates did not play a significant role on the final contact angle. Therefore, the difference in surface energies reported above may explain why the binder formed a low contact angle on the steel surface as compared to the PTFE surface. On the one hand, the thermal conductivity played a predominant role when the difference in temperature between the binder and the mold was high (where the influence of the surface energy was low or simply absent, as represented by grey bars in FIG. 6G). On the other hand, the surface energy of the mold was the predominant mechanism for spreading when the difference in temperature between the binder and the mold was low (where the influence of thermal conductivity was low or simply absent, as represented by black bars in FIG. 6G). From a practical perspective, a slight heating of steel molds, usually at temperatures varying from 35 to 50° C., is often used to promote feedstock spreading and mold filling, while preventing feedstock adhesion. In summary, the results obtained using the sessile drop approach (illustrated by the black bars in FIG. 6G, where an average binder contact angle of 11 and 32° was obtained with steel and PTFE plates, respectively) corroborated the theoretical pendant drop approach (FIGS. 4 and 5) with a significant difference in surface energy between the mold and the binder), finally predicting a higher spreading with the steel surface.

Moldability and adhesion tests were performed to validate whether the interfacial energy between the mold and binder can be used to predict the behavior of feedstock after real-scale injections. The moldability was quantified using the injected length over a rectangular mold cavity, and is reported in FIG. 7 as a function of mold temperature (40, 45, 50, and 70° C.), mold material (steel or PTFE), surface finish (polished or rough), and injection temperature (70, 75, and 80° C.). The results obtained with the PTFE plates show high moldability regardless of the surface finish, mold temperature, and feedstock temperature since the injected length reached the experimental limit (i.e., complete mold filling) for all injection conditions. Note that for better clarity in FIG. 7, the black marks for PTFE were slightly shifted from their horizontal positions, meaning that the feedstock was, in fact, really injected at 70, 75, and 80° C. For the steel plates, the results reported in FIG. 7 confirmed that an increase in injection temperature and/or mold temperature produces an increase in moldability generally explained by a decrease in viscosity associated with any increase in binder temperature and by a delay in feedstock solidification during injection. It is interesting to note that the moldability obtained with a high feedstock temperature and steel mold temperature (i.e., T_mold=70° C. and T_feedstock>75° C.) was similar to that obtained with the PTFE plates. This result confirmed that the solidification of the binder determined the final moldability of the feedstock, where a faster cooling rate lead to a decrease in the injected length. For similar mold cavity geometries, the rate of heat transfer for thermal conduction (Q/t) depends on the material thermal conductivity (k) and the difference in temperature between the feedstock and the mold (ΔT). Under the same injection condition (i.e., same cavity, injection temperature, mold temperature, volumetric flow, etc.), the high thermal conductivity of steel plates will produce a lower moldability as compared to the PTFE plates simply due to the fact that the steel plates have a faster binder solidification rate. Since PTFE is a poor conductor of heat, an injection using this material maintains the binder in the molten state for a longer time period as compared to steel plates. However, it is demonstrated below that this high moldability potential is counterbalanced by a high adhesion of the feedstock (Table 1). From a practical perspective, these two parameters must be evaluated simultaneously to determine an adequate combination of mold/feedstock/injection parameters producing the highest moldability and the lowest adhesion of the feedstock.

FIG. 7 also shows an increase in mold surface roughness results in higher moldability (dashed vs. continuous lines), regardless of the mold temperature or feedstock temperature. According to the Wenzel wettability model, an increase in surface roughness may lead to higher binder spreading (lower binder contact angle) and a possible increase in moldability. However, the increase in surface area associated with a rougher surface would also result in a higher contact surface between the mold and feedstock, leading to a higher solidification rate and lower moldability. Given the important role played by heat transfer, the dynamic occurring during an injection may contribute to preventing the penetration of the feedstock into the grooves of the surface, which are rather filled with trapped air, producing an insulating layer which delays the feedstock solidification, and ultimately leads to an increase in moldability. Although the surface energy of the mold can be used to predict the spreading of feedstock at the mold interface, it may not be a critical parameter for describing the moldability during the LPIM injections. In fact, the surface energy analysis and binder contact angle reported above predicted a higher moldability for steel (i.e., opposite to real-scale injections) without considering the solidification rate of the feedstocks related to the mold material.

The adhesion of the feedstock with the mold is also an important parameter describing the behavior of the injected part after its solidification. This phenomenon affects the demoldability (i.e., the capability to demold the part from the mold cavity), and consequently, the production of parts. The adhesion was quantified by the amount of feedstock stuck on the mold surface following the injection of molten feedstock, the solidification, and the demolding sequence. The injections were performed in the same rectangular mold cavity used previously for the moldability tests and the adhesion results are reported for different conditions in Table 1 below. In general, the feedstocks did not adhere to the steel plates in most mold and feedstock temperature conditions, while only a few combinations of high feedstock temperature (75 or 80° C.) with high mold temperature (45° C.) produced adhesion between the mold and the feedstock. Conversely, the feedstock fully adhered to the PTFE mold under all conditions. From a practical perspective, no adhesion is generally tolerated for the LPIM process, meaning that although they presented very high moldability, the PTFE plates could not really be used as is for the fabrication of a mold cavity.

TABLE 1

Adhesion results (−: no adhesion (0%), +: light adhesion

(<10%), ++: moderate adhesion (10-30%), +++: high adhesion (>30%))

T_mold= 30° C.
T_mold= 35° C.
T_mold= 40° C.
T_mold= 45° C.

T_feedstock(° C.)
T_feedstock(° C.)
T_feedstock(° C.)
T_feedstock(° C.)

Material
Surface
70
75
80
70
75
80
70
75
80
70
75
80

Steel
Polished
−
−
−
−
−
−
−
−
+
−
++
+++

Rough
−
−
−
−
−
−
−
−
−
−
−
++

PTFE
Polished
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++

Rough
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++
+++

Polish +
−
−
−
−
−
−
−
−
−
−
−
−

gold

coating

Given the importance of adhesion during the LPIM injection stage, the work of adhesion and interfacial energy at the mold/binder interface were used to better understand the mechanism of adhesion. The work of adhesion W, or, in other words, the work required to separate two adhered surfaces, is a measurement of the contact strength between these two phases—the binder and plate—and can be estimated by Eq. (1):

$\begin{matrix} W = γ_{1} + γ_{2} - γ_{1 2} & (1) \end{matrix}$

- where γ₁₂represents the interfacial energy between phases 1 and 2 calculated with the harmonic mean equation (Eq. 2) based on the surface energy values γ₁and γ₂calculated above in FIG. 4 and FIG. 5, which are the sum of the dispersion (d coefficient, γ_steel^d≈24 mJ/m²and γ_PTFE^d≈19 mJ/m²) and polar components (p exponent, γ_steel^p≈4 mJ/m²and γ_PTFE^p≈1 mJ/m²).

$\begin{matrix} γ_{12} = γ_{1} + γ_{2} - \frac{4 γ_{1}^{d} γ_{2}^{d}}{γ_{1}^{d} + γ_{2}^{d}} - \frac{4 γ_{1}^{p} γ_{2}^{p}}{γ_{1}^{p} + γ_{2}^{p}} & (2) \end{matrix}$

The work of adhesion and interfacial energy for both the binder/steel and binder/PTFE contacts are reported in FIG. 8. Similar work of adhesion calculated for both the binder/steel and binder/PTFE contacts confirmed that this metric cannot be used directly to correlate the experimental adhesion results obtained in Table 1. However, the interfacial energies calculated for the binder/PTFE and binder/steel interfaces were significantly different at 1 and 6 mJ/m², respectively. The interfacial energy indicated the compatibility between the two materials composing the new interface, and the adhesion between the two phases increases when this parameter decreases. In many industrial sectors involving adhesives and bonding, a low value of the interfacial energy is often sought to achieve higher adhesion strength and durability of the assemblies. In this respect, the high and low values of the interfacial energy measured respectively for the binder/steel and binder/PTFE interfaces can be used to predict the level of incompatibility or compatibility, which will contribute to an easier or difficult separation of the feedstock from the mold. In other words, a high interfacial energy between the feedstock and the mold is expected for the LPIM process in order to promote the demolding/ejection of the injected parts. From a practical perspective, this experimental tool could be used to predict the adhesion behavior of new feedstocks (i.e., new binder recipes) and new mold materials.

To confirm that the adhesion of feedstock is mainly related to the interfacial energy, and not to the solidification rate, PTFE plates were gold-coated using sputtering coating. Since the gold layer is very thin as compared to the PTFE plates (t_PTFE=6.35 mm vs. t_gold≈5×10⁻⁵mm), these coated plates should have similar thermal behaviors as the uncoated PTFE plates. The results presented in Table 1 for different conditions, show that the injected parts were more easily removed from the gold-coated section of the mold, than from the uncoated section of the PTFE mold. In other words, the significant increase in interfacial energy achieved by the gold coating on PTFE plates (FIG. 8) notably changes the adhesion behavior of the feedstock, even for a bulk material producing a low solidification rate.

EXAMPLE 2: CHROMIUM PLATED POLYMERIC MOLD

A PTFE spiral shaped mold was coated with a chromium layer by PVD (˜150 nm). A PTFE mold was left uncoated (negative control) and a steel mold was used as the positive control. A steel powder 17-4 PH (60 vol %) with a binder containing paraffin wax (34 vol. %) and stearic acid (1 vol. %) was used to test the chromium plated mold. The powder-mixture was heated to 80° C. and the mold was heated to 40° C. The results are shown in FIG. 9, FIG. 10A (steel), FIG. 10B (PTFE), and FIG. 10C (PTFE coated with Cr). FIG. 9 presents the average injected length (i.e., the feedstock moldability) obtained for the three mold coating conditions, i.e., steel mold, uncoated polymeric mold and coated polymeric mold). The steel mold produced the lowest moldability, while the uncoated polymeric mold and coated polymeric mold exhibited the best conditions in terms of moldability. In industry, MIM injection stage is generally performed into steel molds to avoid the sticking phenomenon. However, as demonstrated in FIG. 9, this choice is made to the detriment of the moldability performances. During the injection stage, the maximum pressure (i.e., 2 MPa in this example) was reached much faster for injection into the steel mold as compared to the uncoated or coated polymeric molds. This was explained by the faster feedstock solidification in the steel mold which ultimately prevents the feedstock to flow. This was measured by a premature increase in injection pressure. FIG. 10 shows that the spiral injected in the steel mold (FIG. 10A) was short (about 1.5 turn) but easily demoldable (i.e., no sticking phenomenon), while the spiral injected in the uncoated polymeric mold (FIG. 10B) was long (about 2.5 turns) but not demoldable (i.e., significant sticking phenomenon, where it was not possible to take off the part from the mold without breaking it). In other words, the high moldability observed with the uncoated polymeric mold (2 times higher than steel mold) cannot be, in fact, used due to the excessive sticking making this mold ultimately unusable. However, the spiral injected in the coated polymeric mold (FIG. 10C) simultaneously produces high moldability (about 3.5 turns, producing an injected length 3 times higher than steel) and no real evidence of the sticking phenomenon.

MOLD FOR METAL INJECTION MOLDING

Information

Publication Number

Date Filed

Date Published

Inventors

Original Assignees

CPC

International Classifications

Abstract

Description

Claims

CROSS REFERENCE TO A RELATED APPLICATION

PCT Information

Provisional Applications (1)