Patent Application
20180326492
The application relates generally to powder injection mold assemblies, and more particularly to metal injection molding mold assemblies and method for molding a metal element
In powder injection molding (PIM), the solidification of the feedstock can be achieved by relying on the heat transferred to the cooler mold, or by heating the mold during injection and then cooling it to solidify the part. In either case, there are many challenges in creating complex passages and thin walls inside of a molded part. As the feedstock cools, its viscosity increases until it finally solidifies. Frequent changes in velocity, direction, and/or cross-sectional area of the passages tend to increase the heat loss, thus reduce the time before the feedstock solidifies, which can create injections defects. Flow lines, trapped air, and/or incomplete filling of the mold can occur during cooling/solidification and can cause defects in the shaped element. For example, if the shaped element includes adjacent holes, very thin material portion between the holes can remains unfilled as, during injection, the feedstock rapidity solidifies in the thin cavity portion of the mold assembly due to the high thermal capacity and conductivity of the mold material.
Heating the mold to prevent defects in the shaped element greatly increases the energy and cost necessary to perform the PIM process, and affects the time required for injection because of the difficulties in attaining a steady state. In addition, careful control of heat transfer during filling and solidification of the feedstock is required in PIM.
In one aspect, there is provided a mold assembly for powder injection molding of a shaped element, the mold assembly defining a mold cavity for receiving a feedstock and comprising: a first mold portion having a first surface defining a first portion of the mold cavity and releasably engageable to the feedstock, the first mold portion having a first thermal capacity and a first thermal conductivity; and a second mold portion having a second surface defining a second portion of the mold cavity and releasably engageable to the feedstock, the second mold portion having a second thermal capacity and a second thermal conductivity; wherein at least one of the first thermal capacity and the first thermal conductivity is lower than a respective one of the second thermal capacity and the second thermal conductivity.
In another aspect, there is provided a mold assembly for powder injection molding of a shaped element, the mold assembly comprising: a first mold part and a second mold part cooperating to define a mold cavity, the first mold part and the second mold part being movable relative to one another to selectively open and close the mold cavity, the first and second mold parts being disengageable from the shaped element after molding; the first mold part comprising an integral mold portion having a surface defining the mold cavity, the mold portion made of a material different from solid metal and having at least one of a thermal capacity and a thermal conductivity lower than that of solid metal.
In a further aspect, there is provided a method of molding a green part, the method comprising: injecting a powder injection molding feedstock in a mold cavity defined in a mold assembly; extracting heat from the feedstock in the mold cavity through a first portion of the mold having a first surface in contact with the feedstock and through a second portion of the mold having a second surface in contact with the feedstock, a first heat flux through the first surface and first portion being lower than a second heat flux through the second surface and second portion to allow the feedstock to fill the mold cavity before solidification; solidifying the feedstock to create the green part; and disengaging the green part from the first and second portions of the mold by extracting the green part from the mold cavity.
Reference is now made to the accompanying figures in which:
Referring to
In a PIM process a feedstock comprising a material powder and a binder is injected in a mold assembly. Examples of possible powder materials include high temperature resistant powder metal alloys, such as a cobalt alloy or nickel-based superalloy, or ceramic, glass, carbide or composite powders or mixtures thereof. In MIM processes the powder material is a metal powder. Other high temperature resistant material powders which may include one material or a mix of materials could be used as well.
The binder can include one or more binding material(s). The binder can include various components such as surfactants which are known to assist the injection of the feedstock into the mold assembly for production of the shaped element. In a particular embodiment, the binder includes a mixture of binding materials, for example including a lower melting temperature polymer, such as a polymer having a melting temperature below 100° C. (e.g. paraffin wax, polyethylene glycol, microcrystalline wax) and a higher melting temperature polymer or polymers, such as a polymer or polymers having a melting temperature above 100° C. (e.g. polypropylene, polyethylene, polystyrene, polyvinyl chloride). Other suitable materials or mix of materials could be used as well.
In a particular embodiment, the solid loading of the feedstock (i.e. the proportion of binder/powder material) is of 60%, or of more than 60%, where the solid loading is determined on a volume basis as VP/(VP+VB) wherein VP is the volume of powder material and VB the volume of binder. In a particular embodiment, the solid loading is selected to facilitate heat transfer within the feedstock so as to facilitate cooling and solidifying of the portions of the part not in direct contact with the surfaces of the mold assembly.
Powder injection molding can be performed under low or high pressure conditions. In a particular embodiment, the PIM process is performed at a pressure range of less than 100 psi, preferably in a range of 50 to 100 psi. Lower pressures allow using mold portions of lower strength, such as hollow metal portions or plastic portions coated with metal.
Mold assemblies 10-310′ can be used for molding green parts for obtaining metal elements, such as gas turbine engine components for aircraft. In a particular embodiment, the metal element to be produced, and accordingly the corresponding molded green part, has at least one portion that has low thickness or complex geometry. For example the metal element can be a gas turbine engine component having a plurality of adjacent holes therein, the holes forming a complex network of channels in the mold cavity, where the flow of the feedstock may be difficult. The metal element can also comprise thin portions, molded with corresponding thin portions of the mold cavity, where the flow of the feedstock may also be difficult. In a particular embodiment, the metal element is a heat shield panel including a plate having a small thickness, for example about 0.036 inch. In another embodiment, the metal element is a swirler for a fuel nozzle, including a plurality of differently angled holes, with a thickness of material between adjacent holes being small, for example 0.010 inch or less. Other types of elements are also possible.
The mold assemblies 10-310′ are used to mold the shaped element as a green part. The green part, once separated from the mold assembly 10-310′, is then debound to produce a brown part, then sintered to produce the final element.
Referring to
In a particular embodiment shown in
The first mold part 14 and the second mold part 16 are disengageable from the shaped element after molding. Once the feedstock is molded (i.e. the feedstock is cooled until the binder has reached a solid state), the first mold part 14 and the second mold part 16 are disengaged from one another and from the shaped element. Therefore, the shaped element is removed from the mold assembly 10 when it is in an open configuration and none of the first mold part 14 or second mold part the 16 remains engaged with the shaped element.
The mold assembly 10 includes a first mold portion 18 and a second mold portion 20 which have different thermal capacities and/or different thermal conductivities from each other. Each portion can be defined by a respective one of the mold parts 14, 16, or both portions can be defined in a same one of the mold parts 14, 16. In
The first mold portion 18 has a first surface 22 defining a first portion 24 of the mold cavity 12. The second mold portion 20 has a second surface 26 defining a second portion 28 of the mold cavity 12. The first and second surfaces 22, 26, are in contact with the feedstock when the feedstock is injected and flows in the mold cavity 12 so that heat can be transferred from the feedstock to the first and second mold portions 18, 20 through the first and second surface 22, 26, respectively.
The first mold portion 18 and second mold portion 20 are integral elements of the mold assembly 10 and releasably engageable to the feedstock. Therefore, the first surface 22 and the second surface 26 enter in contact with and engage the feedstock when the feedstock flows in the mold cavity 12. Heat is then removed through the first surface 22 and through the second surface 26 and the feedstock solidifies. However, when the shaped element is removed from the mold assembly 10, the first portion 18 and the second portion 20 are disengaged from the shaped element.
The first mold portion 18 has a first thermal capacity and a first thermal conductivity. The second mold portion 20 has a second thermal capacity and a second thermal conductivity. In a particular embodiment, the first thermal capacity is lower than the second thermal capacity and/or the first thermal conductivity is lower than the second thermal conductivity. As used herein, “thermal capacity” refers to the ratio of heat added/removed from an object to the resulting temperature change, i.e. the ability of a material to absorb heat, while “thermal conductivity” refers to the rate at which heat flows through a material, i.e. the ability of a material to transfer heat therethrough; accordingly, a lower thermal capacity results in a material having a lower ability to absorb heat, and a lower thermal conductivity results in a material having a lower ability to transfer heat therethrough, both of which resulting in a lower quantity of heat being removed from a component (e.g. feedstock) adjacent to that material in a given period of time. Therefore, the flux of heat through the first surface 22 and first portion 18 is lower than the flux of heat through the second surface 26 and second portion 20 so that a smaller amount of heat is removed from the feedstock in the first portion 24 of the mold cavity 12 than from the feedstock in the second portion 28 of the mold cavity 12.
In a particular embodiment, the first mold portion 18 having the lower thermal capacity and/or thermal conductivity is made of a material different from solid metal. Therefore, the first mold portion 18 has a thermal capacity that is lower than the thermal capacity of solid metal and/or a thermal conductivity that is lower than the thermal conductivity of solid metal. The thermal capacity and thermal conductivity of the first mold portion 18 can both be lower than that of solid metal, or only one of the thermal capacity and conductivity can be lower than that of solid metal. As a consequence, the heat flux through the first surface 22 and first portion 18 is lower than through a similar portion made of solid metal material.
In the embodiment shown in
Referring to
In a particular embodiment, the core 30 is made of metal, and the layer 32 is made of plastic, ceramic or glass. In that case, the layer 32 has a lower conductivity than solid metal and insulates the metal core 30 so that the heat flux through the first surface 22 and first portion 18′ is reduced with respect to that through the second surface 26 and second portion 20.
In another embodiment, the core 30 is made of plastic, ceramic or glass, and the layer 32 is made of metal, so as to allow a polished finish of the molded surface, have increased wear, and/or reduce chemical interactions between the core 30 and the feedstock; other advantages could also motivate the use of a layer 32 of different material. The presence of the core 30 allows for the first portion 18′ to have a lower total thermal capacity than a first portion 18′ completely made of solid metal, so that the heat flux through the first surface 22 and first portion 18′ is lower than through the second surface 26 and second portion 20.
Alternately, the core 30 may be replaced by an insulating cavity filled with air or with any other suitable gas, so that the first mold portion 18′ is configured as a thermal insulating storage container. As air is a good thermal insulator, the flux of heat through the first surface 22 and first portion 18′ is reduced compared to a solid mold portion made of the same material as that of the layer 32.
Referring to
In this embodiment, the first mold portion 118 is integral to the first mold part 114 and the second mold portion 120 includes the remainder of the first mold part 114 and an entirety of the second mold part 116. The first mold portion 118 defines a plurality of pins, protruding within the mold cavity 112 to allow formation of holes in the shaped element. The first mold portion 118 defines a first portion 224 of the mold cavity 112 that has a cross-section that is smaller than the cross-section of a second portion 228 of the mold cavity 112 which is defined by the second mold portion 120. In a particular embodiment, a smaller cross-section causes change in the direction and/or velocity of the flow of feedstock injected and flowing in the mold cavity 112. As shown in
Referring back to
Alternately, the insulating cavity 142 may be replaced by a core made of a different material than that of the layer 138, where the core or the layer 138 may be made of the same material as the second mold portion 120. As previously mentioned, various combinations are possible, including, but not limited to, a metal core with a plastic, ceramic or glass layer, and a plastic, ceramic or glass core with a metal layer.
Referring to
Referring to
Referring to
In this embodiment, the first mold portion 318 correspond to the first mold part 314 and the second mold portion 320 corresponds to the second mold part 316; the two mold parts 314, 316 as a whole thus have different thermal capacities and/or different thermal conductivities from each other. Alternate configurations are also possible.
The first surface 322 of the first mold portion 318 and the second surface 326 of the second mold portion 320 are in proximity to each other when the mold assembly 310 is closed, so as to define a mold cavity 312 having a small thickness, defining for example a plate or sheet like shaped element, for example a platform of a metal heat-shield panel used in gas turbine engine. In this embodiment, the first mold portion 318 is made of solid material having a lower thermal capacity and thermal conductivity than the material of the second mold portion 320 and/or than that of solid metal. For example, the first mold portion 318 can be made of plastic, ceramic, glass or a combination thereof while the second mold portion 320 is made of metal. Other combinations of materials are also possible.
Referring to
It is understood that although the first mold portion 18, 18′, 118, 218, 318, 318′ is depicted in the Figures as being an element of an upper mold part of the assembly, it is understood that the mold assembly 10-310′ can have any other suitable orientation.
In a particular embodiment, mold assemblies 10-310′ using mold portions having a conductivity and/or a thermal capacity lower than that of solid metal and/or lower than that of another mold portion of the mold assembly 10-310′ allow complete filling of the mold cavity, elimination of weld lines and/or elimination of air entrapment, by effectively slowing the cooling and thus slowing the increase in viscosity and the subsequent solidification of the feedstock, thus facilitating flow of the feedstock in restricted regions of the mold cavity.
In use in a particular embodiment, a shaped element is molded using powder injection molding, for example metal injection molding, in accordance with the following. The feedstock having a composition and a solid loading as defined above is injected in the mold cavity defined in the mold assembly 10-310′. The feedstock is injected as a viscous suspension to be solidified by extracting heat through the mold assembly 10-310′. Heat is extracted from the feedstock through the first mold portion 18, 18′, 118, 218, 218′, 318, 318′ having a first surface contacting the feedstock, and through a second mold portion 20, 120, 320 having a second surface contacting the feedstock. The first surface defines a first portion of the mold cavity and the second surface defines a second portion of the mold cavity.
In a particular embodiment, the heat flux through the first surface and first mold portion 18, 18′, 118, 218, 218′, 318, 318′ is lower than the heat flux through the second surface and second mold portion 20, 120, 320, so as to slow the increase in viscosity and the subsequent solidification of the feedstock in the first portion of the mold cavity as compared to what it would be if the two portions were made of material having the same thermal properties. The feedstock can therefore fill the entire mold cavity before solidifying, thereby preventing formation of flow lines, weld lines, trapping of air or other defects in the shaped element. This can also decrease the pressure required to fill the mold cavity and further decrease the number of possible defects.
The first mold portion 18, 18′, 118, 218, 218′, 318, 318′ is made of a material having a first thermal capacity and a first thermal conductivity. The second mold portion 20, 120, 320 is made of a material having a second thermal capacity and a second thermal conductivity. In a particular embodiment, at least one of the first thermal capacity and the first thermal conductivity is lower than a respective one of the second thermal capacity and second thermal conductivity, and/or at least one of the first thermal capacity and the first thermal conductivity is lower than that of solid metal.
Once the feedstock is solidified to create the green part, the green part is disengaged from the mold portions 18, 18′, 118, 218, 218′, 318, 318′, 20, 120, 320 by extracting the green part from the mold cavity. The green part can then be debound and sintered to create the metal element.
The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.
