Patent Application
20250214133
The present disclosure relates to a method of casting metal parts such as valve seat inserts which can be made of corrosion and wear-resistant alloys with high hardenability and sound elevated temperature applicability.
In conventional casting systems, liquid metal is directed through a vertical sprue, horizontal distribution sprue, runner, and gate into a casting cavity. In manufacturing valve seat inserts (VSIs), such a system can be used with sand molds. In some VSI casting processes, shrinkage and hot tear susceptibility can be a problem even with riser type gating systems.
There is a need for improved VSI casting systems which minimize shrinkage and hot tear susceptibility of the cast VSIs.
In an embodiment, a method of casting metal parts, comprises pouring molten metal into a gating system of a mold plate stack wherein mold plates including mold cavities are located between a cover mold and a bottom mold, the gating system including a casting header, down-sprue, at least one distribution runner, at least one up-sprue, runners and gates in fluid communication with mold cavities configured to form metal parts, and the gating system including at least one internal passage in the cover mold in fluid communication with the up-sprue and the down-sprue; filling the mold cavities with the molten metal while expelling air trapped in the mold plate stack to surrounding atmospheric air via the up-sprue, the internal passage and an upper end of the down-sprue; and solidifying the molten metal to form cast metal parts interconnected by solidified metal in the down-sprue, runners, gates, up-sprue, internal passage and down-sprue.
In various embodiments, (a) the cover mold is a 3D printed sand composition, (b) the mold plates have a plurality of up-sprues, the bottom mold has a plurality of distribution runners in fluid communication with the down-sprue and the up-sprues, and the cover mold has a plurality of internal passages in fluid communication with the up-sprues and the down-sprue, whereby during filling of the mold cavities with the molten metal air trapped in the mold plate stack is expelled to the surrounding atmospheric air via the up-sprues, the internal passages and an upper end of the down-sprue; (c) the internal passages extend radially outward from the down-sprue in the cover mold, the cover mold including a plurality of vertically extending recesses in a lower surface thereof, each of the vertically extending recesses in fluid communication with one of the internal passages and one of the up-sprues, whereby air escaping the mold cavities passes into the up-sprues and then through the recesses, the internal passages and the down-sprue before being expelled to the surrounding atmospheric air; and/or (d) the cover mold prevents surrounding atmospheric air from contacting the molten metal during filling of the mold cavities.
In an embodiment, each of the mold plates is a circular sand mold plate having a central opening corresponding to the down-sprue extending vertically between upper and lower surfaces of the mold plate, at least two circumferentially spaced openings corresponding to up-sprues extending vertically between the upper and lower surfaces of the mold plate, at least two ring-shaped mold cavities extending into the upper surface of the mold plate, at least two circular recesses extending into the upper surface of the mold plate at locations such that each ring-shaped mold cavity surrounds one of the circular recesses, at least two runners arranged such that at least one of the runners extends from each of the circumferentially spaced openings and each of the runners/gates is in fluid communication with one of the ring-shaped mold cavities, the method including solidification of the molten metal and forming a mold stack of parts comprising valve seat inserts.
In another embodiment, each of the mold plates is a circular sand mold plate having a central opening corresponding to the down-sprue extending vertically between upper and lower surfaces of the mold plate, at least four circumferentially spaced openings corresponding to the up-sprues extending vertically between the upper and lower surfaces of the mold plate, at least eight ring-shaped mold cavities extending into the upper surface of the mold plate, at least eight circular recesses extending into the upper surface of the mold plate at locations such that each ring-shaped mold cavity surrounds one of the circular recesses, at least eight runners arranged such that at least two of the runners extend from each of the circumferentially spaced openings and each of the runners/gates is in fluid communication with one of the ring-shaped mold cavities, the method including solidification of the molten metal and forming a mold stack of parts comprising valve seat inserts.
In a further embodiment, each of the mold plates is a circular sand mold plate having a central opening corresponding to the down-sprue extending vertically between upper and lower surfaces of the mold plate, at least six circumferentially spaced openings corresponding to the up-sprues extending vertically between the upper and lower surfaces of the mold plate, at least eighteen ring-shaped mold cavities extending into the upper surface of the mold plate, at least eighteen circular recesses extending into the upper surface of the mold plate at locations such that each ring-shaped mold cavity surrounds one of the circular recesses, at least eighteen runners arranged such that at least three of the runners extend from each of the circumferentially spaced openings and each of the runners/gates is in fluid communication with one of the ring-shaped mold cavities, the method including solidification of the molten metal and forming a mold stack of parts comprising valve seat inserts.
According to an embodiment, the molten metal is a wear and corrosion resistant alloy, nickel-base alloy, cobalt-base alloy, or intermetallic-base alloy, the method further comprising maintaining a substantially uniform temperature distribution of the molten metal in a vertical direction during solidification of the molten metal.
In another embodiment, the method includes solidifying the molten metal after the molten metal fills the internal passages, the cover mold allowing escape of trapped air into the down-sprue and providing sufficient thermal insulation in a vertical direction to improve surface quality of the cast parts.
In an embodiment, an apparatus for casting metal parts, comprises a mold plate stack comprising mold plates located between a cover mold and a bottom mold, and a gating system including a casting header, down-sprue, at least one distribution runner, at least one up-sprue, runners, and gates in fluid communication with mold cavities configured to form the metal parts; the mold cavities located in upper surfaces of the mold plates; the cover mold including at least one internal passage in fluid communication with the up-sprue and the down-sprue, the internal passage in the cover mold allowing trapped air to be expelled through the down-sprue during a casting operation in which molten metal sequentially fills the down-sprue, the distribution runner, the up-sprue, runners, gates, and mold cavities in each mold plate, and after filling the up-sprue of each mold plate the liquid metal fills the internal passage and contacts the liquid metal in the down-sprue of the cover mold.
In embodiments, (a) the cover mold is a 3D printed sand composition; (b) the mold plates have a plurality of up-sprues, the bottom mold has a plurality of distribution runners in fluid communication with the down-sprue and the up-sprues, and the cover mold has a plurality of internal passages in fluid communication with the up-sprues and the down-sprue; (c) the internal passages extend radially outward from the down-sprue in the cover mold, the cover mold including a plurality of vertically extending recesses in a lower surface thereof, each of the vertically extending recesses in fluid communication with one of the internal passages and one of the up-sprues; and/or (e) each of the mold plates is a circular sand mold plate having a central opening corresponding to the down-sprue extending vertically between upper and lower surfaces of the mold plate, at least two circumferentially spaced openings corresponding to up-sprues extending vertically between the upper and lower surfaces of the mold plate, at least two ring-shaped mold cavities extending into the upper surface of the mold plate, at least two circular recesses extending into the upper surface of the mold plate at locations such that each ring-shaped mold cavity surrounds one of the circular recesses, at least two runners arranged such that at least one of the runners extends from each of the circumferentially spaced openings and each of the runners/gates is in fluid communication with one of the ring-shaped mold cavities, the ring-shaped mold cavities configured to form valve seat inserts.
In an embodiment, each of the mold plates is a circular sand mold plate having a central opening corresponding to the down-sprue extending vertically between upper and lower surfaces of the mold plate, at least four circumferentially spaced openings corresponding to the up-sprues extending vertically between the upper and lower surfaces of the mold plate, at least eight ring-shaped mold cavities extending into the upper surface of the mold plate, at least eight circular recesses extending into the upper surface of the mold plate at locations such that each ring-shaped mold cavity surrounds one of the circular recesses, at least eight runners arranged such that at least two of the runners extend from each of the circumferentially spaced openings and each of the runners/gates is in fluid communication with one of the ring-shaped mold cavities, the ring-shaped mold cavities configured to form valve seat inserts.
In another embodiment, each of the mold plates is a circular sand mold plate having a central opening corresponding to the down-sprue extending vertically between upper and lower surfaces of the mold plate, at least six circumferentially spaced openings corresponding to the up-sprues extending vertically between the upper and lower surfaces of the mold plate, at least eighteen ring-shaped mold cavities extending into the upper surface of the mold plate, at least eighteen circular recesses extending into the upper surface of the mold plate at locations such that each ring-shaped mold cavity surrounds one of the circular recesses, at least eighteen runners arranged such that at least three of the runners extend from each of the circumferentially spaced openings and each of the runners/gates is in fluid communication with one of the ring-shaped mold cavities, the ring-shaped mold cavities configured to form valve seat inserts.
In a further embodiment, the cover mold includes a central opening extending vertically between upper and lower surfaces of the cover mold, each of the internal passages including a horizontal section extending radially outward from the central opening and a vertical section extending vertically from the lower surface of the cover mold. The horizontal section can be formed by a cylindrical passage having an inner end extending from the central opening and an outer end spaced inwardly of an outer periphery of the cover mold, and the vertical section can be formed by a cylindrical recess having a lower end extending through the lower surface of the cover mold and an upper end extending from the outer end of the cylindrical passage.
In a further embodiment, the mold cavities extend vertically into an upper surface of each mold plate.
Disclosed herein is an improved casting system useful for mass production of valve seat inserts made of high alloy compositions.
Unless otherwise indicated, all numbers expressing quantities, conditions, and the like in the instant disclosure and claims are to be understood as modified in all instances by the term “about.” The term “about” refers, for example, to numerical values covering a range of plus or minus 10% of the numerical value. The modifier “about” used in combination with a quantity is inclusive of the stated value.
In this specification and the claims that follow, singular forms such as “a”, “an”, and “the” include plural forms unless the content clearly dictates otherwise.
The terms “room temperature”, “ambient temperature”, and “ambient” refer, for example, to a temperature of from about 20° C. to about 25° C.
Valve seat inserts can be made from various alloy compositions which have been cast and machined. Large scale production of valve seat inserts is typically done by using stacked mold plates with multiple castings in each mold plate. With modern valve seat inserts, high alloy compositions are used to meet the high temperature, high stress, and harsh combustion environment conditions. Valve seat insert castings made of high performance alloys for heavy-duty engine applications preferably have uniform and desired solidification sub structures. However, solute distribution in a high alloy often involves solute element redistribution which affects the final solidification substructural formation and morphology. For example, with intermetallic strengthened cobalt-based alloys, it can be very difficult to achieve uniformly distributed solidification substructure such as between soft cobalt solid solution phases and intermetallic Laves phases. In some high alloys, eutectic reaction phases can form after formation of primary dendritic structures with the result being eutectic phases interdendritically distributed. Fine and uniform distribution of solidification structures including eutectic reaction phases is preferred from a product performance and component shaping related process (e.g., machining) consideration.
In order to improve yield of cast valve seat inserts, it is desirable to improve machining characteristics of the cast parts. For parts made by casting in conventional molds, an off-set adjustment of cutting tools needs to be performed after machining 30 cast parts. In contrast, using an improved thermal jacket mold design, it is possible to produce cast parts wherein the off-set adjustment is not needed until after machining 150 cast parts. Disclosed herein is a closed circuit liquid metal flow system for casting parts in a stacked mold plate apparatus which is designed to provide improved casting cavity-fill conditions resulting in a better cavity yield, finer casting surface appearance, and more consistent casting quality.
With the new flow system design, liquid metal flow starts from the casting header of the stacked mold plate apparatus down through a down-sprue to distribution mold, then through a distribution runner to an up-sprue followed by filling casting molds layer by layer in the stacked mold plate apparatus. When all the casting molds are filled, the liquid metal will be directed by a channel in a cover mold which links the up-sprue to the down-sprue. Hence, a circuit of liquid metal flow is achieved with the new flow system design. Due to a high velocity liquid metal stream flowing during a liquid metal/alloy pouring in the down-sprue, air present in the up-sprue prior to up-sprue being filled by liquid metal can be sucked into the down-sprue region and released to the atmosphere through air gaps between liquid metal and walls of the down-sprue. Hence, low air pressure is created in the casting stacked mold plate system due to the low pressure cover mold design. Low pressure casting forms a full loop of liquid metal flow contrasted to common static casting process for which liquid metal flow always has an open end. For metal/alloy casting, an advantage of full loop liquid metal flow includes enhancement of cavity/casting yield capability compared to open end liquid metal flow.
For small size and high-volume casting manufacturing, such as valve seat insert (VSI) manufacture, a mass production method has been commonly applied for cost-effectiveness and sustainable manufacturing considerations.
The new flow system design of the stack mold plate assembly 30 can be illustrated in
As shown in
The casting stack 50 shown in
The mold stack 30 can include various arrangements of sprues, runners/gates and mold cavities. Depending on the size of the valve inserts, one or more up-sprues may feed one, two, three, four or more mold cavities in each mold plate. In an example, a mold plate 14 may have six up-sprues 22 and three mold cavities 24 in communication with each up-sprue 22 via runners 23, as shown in
The cover mold and mold stack casting apparatus can optionally include a thermal barrier (jacket) as disclosed in commonly-owned U.S. Pat. No. 10,421,116. With or without the advantage of the thermal barrier which provides an even and low temperature gradient along the radial direction in the mold plates, the cover mold can contain heat in the stack during metals/alloys pouring hence a low temperature gradient distribution especially along the casting stack vertical orientation can be obtained and thereby achieve a desired solidification condition of the molten metals/alloys and better casting quality.
As noted above, one or more channels in the cover mold connect one or more up-sprues to the down-sprue. Due to the 3D printing technique, the one or more channels can be located entirely inside the cover mold rather than extend into the lower surface of the cover mold. With such arrangement, the air originally in the mold cavities and up-sprue will be released through the internal channel(s) of the cover plate to thee down-sprue. Because the high flow rate of liquid metal during pouring down through down-sprue, localized low pressure can be created which draws the air from the up-sprue(s) toward down-sprue area.
The cover mold and mold plates can be made of a sand/binder composition. For 3D printing of the cover mold, various sans compositions can be used which include silicon oxide sand, river sand or lake sand.
The mold plates can have any desired number of mold cavities and up-sprues. For example, the mold plates can have 4, 5, 6, 7, 8, 9 or 10 up-sprues depending on the OD of the mold plates and size of the valve seat insert castings.
In the process of casting parts, as the molten metal rises from the bottom to fill the mold cavities of each mold plate, the molten metal is preferably not under any pressure except gravitational force. For static casting, the driving force is gravitational force only. For conventional mold stack designs, the force needs minus atmospheric pressure from up-sprue opening(s). However, the atmospheric pressure has been substantially reduced with low pressure cover plate concept.
During a casting operation, air is forced out of the mold cavities by the rising molten metal as the mold fill occurs layer by layer. Any remaining air in the up-sprues is forced into the down-sprue as the liquid metal fills the channels in the cover plate as the down-sprue region becomes a low pressure region when using the low pressure cover plate. With the new low pressure cover plate concept, escaping air is not exposed to atmospheric air until it leaves the casting header. This is possible because the liquid metal stream during pouring will not occupy the entire space of up-sprue, thus allowing escaping air to pass upwardly through the down-sprue.
The new stack mold apparatus 30 can be used for mass production of cast metal parts such as vale seat insert castings wherein circular mold plates 14 made of sand are stacked vertically between the bottom mold 16 and a cover mold 32. The casting header 10 is located at the center of the cover mold 32 with the central opening 34 is aligned vertically with a central down-sprue 18 extending through the cover mold 32. The down-sprue 18 extends downwardly through each mold plate 14 and communicates with horizontal distribution runners 20 below the lowest mold plate 14. The distribution runners 20 communicate with up-sprues 22 extending upwardly through the mold plates 14. The up-sprues 22 communicate with runners 23 which communicate with one or more mold cavities 24 in each mold plate 14. Tops of the up-sprues 22 communicate with internal passages 36 in the cover mold 32. When molten metal is poured into the casting header 10, the liquid metal flows through the down-sprue 18, the horizontal runners 20, the up-sprues 22, the runners 23 into the mold cavities 24 and poring of molten metal is stopped when the liquid metal fills the internal passages 36.
The flow system can include various arrangements of sprues, runners/gates and mold cavities. Depending on the size of the cast metal parts such as valve seat inserts, one or more up-sprues may feed one, two, three, four or more mold cavities in each mold plate. In an example, a mold plate 14 may have six up-sprues 22 and three mold cavities 24 in communication with each up-sprue 22 via runners 23. As shown in
The cast metal parts such as valve seat inserts are made by pouring molten metal into a gating system of a mold plate stack wherein mold plates are located between top and bottom molds. If the stack of mold plates includes mold plates having 18 mold cavities in each mold plate, or if the stack includes 10 mold plates, 180 cast metal parts such as valve seat inserts can be cast in a single pouring. The mold plates are preferably made of conventional green shell sand for VSI casting applications and are designed such that during solidification of the molten metal in the mold cavities, the binder in the sand is volatilized and thin sand walls forming the inner surfaces of the valve seat inserts collapse as the valve seat inserts contract due to shrinkage upon solidification of the molten metal.
In another example, each of the mold plates can include four up-sprues with each up-sprue connected to two runners, each runner communicating with a single mold cavity. Thus, in a stack of 10 mold plates, 80 valve seat inserts could be cast with such an arrangement.
In another arrangement, the mold plate includes four up-sprues with each up-sprue connected to three mold cavities via runners. For large diameter valve seat inserts, a mold plate can have four up-sprues with each up-sprue connected to a single mold cavity.
In order to improve yield of cast valve seat inserts and/or lower costs of machining of the cast valve seat inserts, it is desirable to control the microstructure of the cast parts such that the microhardness distribution is more uniform. By improving uniformity of the microstructure, machinability of the cast valve seat inserts can be improved.
In a preferred casting system for mass production of valve seat inserts, mold plates made of sand and having a diameter of about 14 inches can have a central 1 inch diameter down-sprue, horizontal bottom distribution runners feeding an equal number of up-sprues having diameters of about ½ to ¾ inch, rectangular runners which taper in cross section, and mold cavity gates having heights of about ⅔ the valve seat insert height and widths of about 1.6 times the gate height.
It will be appreciated by those skilled in the art that the casting method and apparatus described herein can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
