Sand cores and sand core manufacturing methods

Abstract

A sand core comprising a granular sand material, at least one inorganic binder, and an additive which is aluminum diethylphosphinate. A method for producing a sand core comprising a first contact or mixing step of a granular sand material and at least one binder, and a second curing step, wherein the granular sand material or a mixture of granular material and / or binder contains the additive.

Description

Technical Field

[0001] The present invention relates to a composition and method for producing a sand core.

Background Art

[0002] A sand core is generally manufactured by a conventional sand-making machine. In this case, the shape of the core (or cores) produced in each manufacturing cycle is determined by the mold. Next, the material used to make the core is poured into the mold, and the material is solidified or cured, resulting in a solid. The result is a sand core. The materials used are granular materials, binding compounds, and binders (for example, a type of resin). Examples of machines for producing sand cores in this way can be found, for example, in EP0494762A2 and EP2907601A1 of the same applicant.

[0003] Another method of manufacturing a sand core is by using an additive manufacturing method. This type of manufacturing method consists of several steps: a first step of alternately applying a plurality of layers of a granular sand material containing a binding compound and a plurality of layers of a binder or resin in an alternating arrangement on a working platform, and a second step of curing these layers by raising the temperature of the layers, preferably by a heat input method (heat or microwave), a dehydration method, or a combination of both methods. This type of manufacturing method is carried out in an additive manufacturing machine that includes a working table and a printing unit including a print head arranged on the working table and configured to deposit layers of granular material or binder.

[0004] Sand cores are typically used to manufacture metal castings with very special dimensional tolerances and very narrow tolerance ranges, often between 0.15 mm and 0.5 mm. Therefore, core manufacturers need to ensure the dimensional stability of the cores in use so that the metal parts obtained by casting meet the required dimensional tolerances in the manufacture of, for example, cooling jackets, cylinder head parts, engine blocks, disc brakes, or drive shafts.

[0005] US2013225718A1 discloses granular sand materials and sand cores containing organic binders, in which the core contains an additive selected from a phosphorus-oxyacid ester compound. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] EP0494762A2 [Patent Document 2] EP2907601A1 [Patent Document 3] US2013225718A1 [Overview of the Initiative]

[0007] The object of the present invention is to provide sand cores and methods for producing sand cores, as defined in the claims.

[0008] A first aspect of the present invention relates to a sand core comprising a granular sand material, at least one binder, and an additive selected from a group of phosphinate compounds, wherein the compound is aluminum diethylphosphinate and the binder is an inorganic binder.

[0009] A second aspect of the present invention relates to a method for producing a sand core, comprising a first contact or mixing step of a granular sand material and at least one binder on a support, and a second curing step, wherein the granular sand material or a mixture of the granular sand material and the binder comprises an additive selected from compounds of the phosphinate group, the compound being aluminum diethylphosphinate, and the binder being an inorganic binder.

[0010] One of the problems associated with sand cores is their dimensional instability due to the fact that, during their use in the process of obtaining metal parts by casting, the cores expand and / or deform under the influence of heat. This results in the production of defective metal parts that do not meet the required dimensional tolerances.

[0011] The addition of aluminum diethylphosphinate compounds achieves better dimensional stability of the sand core during use and during manufacturing. The sand cores of the present invention can withstand operating temperature conditions better and can delay the expansion and / or deformation of the sand core, so that when exposed to molten metal temperatures typically exceeding 600°C, the dimensions of the sand core remain within the specified range for a longer period of time. This makes it easier to adhere to dimensional tolerances of metal parts obtained by casting with sand cores, resulting in improved manufacturing performance and easier adherence to quality requirements for metal parts manufactured using these cores. The presence of additives in the granular sand material increases the sand's ability to absorb the binder more uniformly, improving the penetration of the binder into the sand, and therefore allowing for stronger control of the binder's effect and thus in the core solidification process.

[0012] These and other advantages and features of the present invention will become apparent with reference to the drawings and detailed description of the invention. [Brief explanation of the drawing]

[0013] [Figure 1] This figure shows the steps of a method for producing a sand core according to one embodiment of the present invention. [Figure 2] This figure shows a work box including a work platform having multiple cores, which is generated according to an embodiment of the method of the present invention. [Figure 3] This graph shows the deviation of measurement points at the core of a compound that does not contain additives. [Figure 4] This graph shows the deviation of the core measurement point for formulations containing aluminum diethylphosphinate. [Figure 5A] Figure 5 shows two graphs illustrating the deviation of core measurement points for a formulation containing zeolite and a formulation containing both zeolite and aluminum diethylphosphinate. [Figure 5B]Figure 5 shows two graphs illustrating the deviation of core measurement points for a formulation containing zeolite and a formulation containing both zeolite and aluminum diethylphosphinate. [Figure 6A] Figure 6 shows two graphs illustrating the deviation of core measurement points for a formulation containing wollastonite and a formulation containing wollastonite and aluminum diethylphosphinate. [Figure 6B] Figure 6 shows two graphs illustrating the deviation of core measurement points for a formulation containing wollastonite and a formulation containing wollastonite and aluminum diethylphosphinate. [Figure 7A] Figure 7 shows two graphs illustrating the deviation of the core measurement point for formulations containing lithium carbonate and formulations containing lithium carbonate and aluminum diethylphosphinate. [Figure 7B] Figure 7 shows two graphs illustrating the deviation of the core measurement point for formulations containing lithium carbonate and formulations containing lithium carbonate and aluminum diethylphosphinate. [Figure 8A] Figure 8 shows two graphs illustrating the deviation of the core measurement point for a formulation containing sodium borate and a formulation containing sodium borate and aluminum diethylphosphinate. [Figure 8B] Figure 8 shows two graphs illustrating the deviation of the core measurement point for a formulation containing sodium borate and a formulation containing sodium borate and aluminum diethylphosphinate. [Figure 9A] Figure 9 shows two graphs illustrating the deformation of core test specimens containing aluminum diethylphosphinate and those without diethylphosphinate. The first graph represents data from test specimens obtained by additive manufacturing, and the second graph represents data from test specimens obtained by blow molding. [Figure 9B] Figure 9 shows two graphs illustrating the deformation of core test specimens containing aluminum diethylphosphinate and those without diethylphosphinate. The first graph represents data from test specimens obtained by additive manufacturing, and the second graph represents data from test specimens obtained by blow molding.

BRIEF DESCRIPTION OF THE INVENTION

[0014] A first aspect of the present invention relates to a core sand comprising a granular sand material, a binder, and an additive selected from the group of phosphinate compounds, preferably a metal salt of phosphinic acid. In a preferred embodiment, the compound is aluminum diethylphosphinate, which is sold, for example, under the trade name Exolit OP 1230 (trademark) (hereinafter Exolit). The inventors have discovered, as shown in Example 3, that the use of phosphinates exhibits surprising data regarding dimensional stability in the use of cores. Cores containing phosphinates have greater resistance to embrittlement, expansion, and deformation under casting temperature conditions than cores without phosphinates.

[0015] "Dimensional stability during use" means the ability of a core to maintain its dimensions within the established dimensional specifications during the casting process. In the case of the present invention, it is determined by a hot deformation test.

[0016] In one embodiment, the core sand comprises a second additive selected from zeolite and / or wollastonite and / or lithium carbonate and / or sodium borate or a mixture thereof. As shown in Example 2, this second additive is advantageous in cores, particularly cores obtained by an additive manufacturing method.

[0017] With respect to the sand-like material that constitutes the majority of the core composition, in one embodiment, the type of sand can be selected from silica sand, chromite sand, zircon sand, olivine sand, bauxite, mullite and / or sand with any artificial properties, or a mixture of one or more of these. With respect to the particle size of the sand, the diameter is preferably between 100 μm and 425 μm. The particle size will depend on the type of manufacturing method by which it is obtained. Therefore, in a preferred embodiment, if the sand core is obtained by an addition process, the diameter of the sand grains may be between 100 μm and 300 μm, preferably between 100 μm and 160 μm. In another embodiment, if the sand core is obtained by a conventional sand-making machine, the diameter of the sand grains may be between 160 μm and 425 μm.

[0018] In this invention, the diameter, particle size, or particle diameter is measured by a sieving method.

[0019] Regarding the binder, in a preferred embodiment, the binder is inorganic.

[0020] In embodiments containing an inorganic binder, it is common to include at least one binder compound. Therefore, in this embodiment, as one relationship, the core comprises a binder compound consisting of a set of metal oxide particles selected from the group of silicon dioxide, aluminum oxide, titanium oxide, and zinc oxide, with particle sizes between 0.10 μm and 1 μm, preferably the binder compound being silicon dioxide. These particles bond to the surface of the sand grains and the binder, establishing bonds with each other. The particles of the binder compound surround the sand grains, thereby causing the binder to react with the sand grains and form bridges between different particles located in different sand grains.

[0021] A second aspect of the present invention relates to a sand core, more specifically, a method for producing a sand core of the present invention, which may be a conventional machine-based or additive manufacturing method. The production method comprises a first contact or mixing step of a binder on the surface of a support, a granular sand material, and optionally a binding compound if the binder is an inorganic binder, and a second curing step. The mixture of the granular sand material or binder, the granular material and the binding compound, if present, contains an additive selected from a compound of the phosphinate group, preferably a metal phosphinate salt, most preferably aluminum diethylphosphinate. With respect to the amount of this compound from the phosphinate group, in a preferred embodiment, the weight percentage of the compound relative to the granular sand material is between 0.01 and 0.045%, preferably between 0.01 and 0.03%, and very preferably between 0.01 and 0.02%.

[0022] In one embodiment, in addition to the first additive, the granular material, or a mixture of a binder, granular material and, if applicable, a binder compound, preferably a mixture of granular material and, if applicable, a binder compound, comprises a second additive selected from zeolite and / or wollastonite and / or lithium carbonate and / or sodium borate or a mixture thereof. In one embodiment, the weight percentage of the second additive relative to the granular sand material is between 0.1 and 0.6%.

[0023] In the case where the manufacturing method is an additive manufacturing method for producing sand cores, as shown in Figure 1, the first contact step involves applying multiple layers of a mixture 2 (granular mixture 2) of granular sand material, if any, a binder compound, and a phosphinate group compound, and multiple layers of a binder 3 to the surface 1 of the work platform 10 in an alternating arrangement, and then subjecting the layers to a hardening step by increasing the temperature of the layers, preferably by a heat input method or a dehydration method. More specifically, the granular mixture 2 is first obtained by mixing granular sand material, if any, a binder compound, and a phosphinate group compound, and the first layer of this granular mixture 2 is deposited on the work platform 1. Next, the binder 3 is applied to the required areas of the previously deposited granular mixture 2 layer to produce a layer of solidified granular material, and the previous two steps are repeated as many times as necessary to create as many layers as necessary by stacking the layers of solidified granular material on top of each other, and these layers of solidified granular material are stacked on top of each other to form a sand core, which then undergoes a second hardening step. The deposition of the layers of granular mixture 2 and binder 3 is usually carried out using individual printing press heads 20 known to those skilled in the art, which are placed on a work platform 1 and configured to deposit the layers of granular mixture 2 and binder 3 on the corresponding layers of granular mixture. Figure 2 shows a work box 10 including a work platform having a plurality of cores 9 surrounded by unhardened granular mixture 2, manufactured according to this embodiment of the present invention. After the hardening step, the unhardened granular mixture 2 is removed. The method of the present invention can be applied by any apparatus for additive manufacturing of sand parts using a binder.

[0024] In a preferred embodiment, the first and / or second additives are incorporated into the granular material. The fact that the additives are incorporated into the granules rather than the binder facilitates the additive manufacturing process. Mixing the additives into the granular material is much easier than mixing them into the binder, and any solidification of the sand in the outlet port of the header, which is typically used to apply the binder coating, is avoided or minimized.

[0025] As shown in Example 2 and Figures 3 to 8, in additive manufacturing, the addition of the first and / or second additives has the additional advantage of providing better dimensional stability of the core manufacturing.

[0026] Manufacturing dimensional stability will be understood as the ability of the core to meet dimensional tolerances after the solidification and / or curing process. In the present invention, the dimensions of the manufactured core are determined by comparing them with an ideal model of the core represented by CAD software design.

[0027] The inventors found that compounds from the phosphinate group, preferably aluminum diethylphosphinate, exhibit remarkable data regarding manufacturing dimensional stability, as shown in Example 2. During the experimental period, the inventors found that aluminum diethylphosphinate increased the viscosity of the granular mixture, accelerating the solidification process of the sandy granules and causing it to occur in a short time. As a result, the consistency of the granular mixture layers after printing is improved, affecting the dimensional stability of the final core. On the other hand, it was also found to affect the wettability of the granular material. The presence of this additive in the granular sand material increases the sand's ability to absorb the binder more uniformly, improving the penetration of the binder into the sand, and therefore allowing for stronger control of the binder's effect, and thus in the solidification process.

[0028] Regarding the type of sand in the granular material, in a preferred embodiment of the addition method, the sand is selected from silica sand, chromite sand, zircon sand, olivine sand, bauxite sand, mullite sand, and / or any synthetic sand. For best optimization, the grain size or particle diameter of the sand is between 100 μm and 300 μm, preferably between 100 μm and 160 μm.

[0029] Regarding the binder, in one embodiment, the binder is an inorganic binder. In a preferred embodiment, the binder is a resin containing sodium silicate and water, with a volume ratio of sodium silicate / water ranging from 20:80 to 45:55, encompassing all possible ratios, but including the minimum or maximum values ​​of the sodium silicate portion as described above. Regarding the ratio of the binder to the granular sand material, in a preferred embodiment, the weight percentage of the binder to the granular sand material is between 1.5% and 6%. In the case of additive manufacturing methods, the preferred range is between 2.5% and 6%. In the case of conventional mechanical methods, this range is preferably between 1.5% and 2.5%.

[0030] Regarding the binder compound, it is selected from the group consisting of silicon dioxide, aluminum oxide, titanium oxide, and zinc oxide, and in preferred embodiments, silicon dioxide. In terms of size, the binder compound may have a particle size between 0.10 μm and 1 μm. The binder compound is preferably used in embodiments where the binder is inorganic.

[0031] Regarding the ratio of the binder compound to the granular sand material, in preferred embodiments, the weight percentage of the binder compound to the granular sand material is between 0.4% and 1.8%. In the case of additive manufacturing methods, the preferred range is between 0.4% and 1.2%. In the case of conventional mechanical methods, this range is preferably between 0.6% and 1.8%.

[0032] Another aspect of the present invention relates to sand cores obtained according to the manufacturing method of the present invention for use in metal castings, preferably iron, aluminum, copper, or aluminum castings. These cores can be used in the manufacture of castings of cooling jacket components, cylinder head components, engine blocks, disc brakes, or drive shafts.

[0033] Another aspect of the present invention relates to a composition for producing sand cores, preferably by an addition process, comprising granular sand material, optionally a binding compound, and a first additive selected from the group of phosphinates, and / or zeolite and / or wollastonite and / or lithium carbonate and / or sodium borate or a mixture thereof. This composition of the present invention can be used in the method of the present invention for obtaining the cores of the present invention.

[0034] In a preferred embodiment, the composition contains a first additive comprising a compound from the group of phosphinates, preferably aluminum diethylphosphinate. In another embodiment, the composition comprises the first additive and a second additive selected from zeolite and / or wollastonite and / or lithium carbonate and / or sodium borate or a mixture thereof. The first and second additives are dimensional stabilizers. The advantages associated with these additives are described in the embodiments of the method of the present invention.

[0035] In the context of the present invention, a dimensional stabilizer is a substance that, when added to a composition, prevents the deterioration of the product obtained from that composition. In the context of the present invention, the product is a sand core, and deterioration means a reduction in the strength, density, or size of something, and in the context of the present invention, a reduction in the size or dimensions of a sand core.

[0036] In one embodiment, the composition ratio of the dimensional stabilizer is as follows: - A first additive consisting of phosphinate group compounds: 0.010% and 0.045% by weight relative to the granular sand material. In preferred embodiments, the compound is aluminum diethylphosphinate; - A second additive selected from zeolite and / or wollastonite and / or lithium carbonate and / or mixtures thereof: in amounts of 0.1 and 0.6% by weight relative to the granular sand material. In one embodiment, this second additive is optional; and - Binder compound: Between 0.0 and 1.8% by weight of the granular sand material, preferably between 0.4 and 1.8% by weight. In preferred embodiments, the binder is an inorganic binder.

[0037] The technical features described in various embodiments of the method and core of the present invention are applied to this aspect of the present invention and give rise to various embodiments thereof. [Examples]

[0038] Several exemplary embodiments that clearly demonstrate the features and advantages of the present invention are described below, but they are defined in the claims and should not be construed as limiting the scope of the present invention.

[0039] Example 1: Preparation of sand cores according to an embodiment of the method of the present invention: Sand cores containing an inorganic binder were produced using an additive manufacturing method with a Voxeljet VX1000 printing press, both without the first and second additives (white) and with the first and second additives.

[0040] Materials used: Granular sand material: AFS 100 silica sand with an average particle size of 150 μm. Binder: Sodium silicate-based binder with a sodium silicate / water ratio of 35 / 65. 3.5% by weight of sand. Bonding agent: Silicon dioxide. 0.4% relative to the sand.

[0041] Manufacturing conditions: Printing time: 1 hour 42 minutes Dehydration curing time: 16 minutes Removal of unhardened sand Additional curing time: 6 minutes at 180°C.

[0042] Core's ultimate strength: 330 N / cm 2 .

[0043] Cores were prepared using the following additive ratios, and one core from a formulation without additives was used as a control (Z):

[0044] [Table 1]

[0045] Example 2: Study on the dimensional stability of sand cores obtained in Example 1: Manufacturing dimensional stability is measured by comparing a core manufactured based on Example 1 with a set of points defined in an ideal CAD core model. After comparing the two cores using a 3D core measuring device, the deviation of the manufactured core at each of these points relative to the ideal model is measured. The deviation values ​​at these points are collected to obtain a quantitative view of dimensional stability.

[0046] If all points have deviations within the set limits, the core is considered to meet the required dimensional stability. There are two types of points: reference points and measurement points. On the one hand, there are a total of 10 reference points, which serve as the reference for positioning the core in space on the 3D measuring machine. These points are the most important and therefore have the smallest deviation tolerance. On the other hand, there are 33 measurement points, which are located on different parts of the core to evaluate only the difference from the model part. The following table shows the allowable dimensional tolerances in millimeters:

[0047] [Table 2]

[0048] For measurement, a structured light-based 3D measuring device (Solutionix C500) is used, which captures images of the fabricated core and combines them to create a virtual model of the core for comparison with an ideal CAD model. For accurate reconstruction of the virtual image of the fabricated core, the reference points must be clear and strict dimensional stability must be observed, because if the reference points deviate significantly from the tolerance range, the reconstructed image of the core will not be entirely reliable.

[0049] Figures 3 to 8 show graphs obtained from these measurements of the core obtained in Example 1.

[0050] The x-axis indicates the measurement point, and the y-axis shows the deviation from the ideal tap, measured in millimeters. The graph also includes a line representing the tolerance for the required core.

[0051] As the data shows, incorporating the additive improves the dimensional stability of the resulting core. A synergistic effect of aluminum diethylphosphinate is also involved when combined with other additives.

[0052] Example 3: Dimensional stability analysis under casting plant conditions: To reproduce the temperature conditions and quantify the behavior of the core under these conditions, we followed these steps: - Core fabrication, including a series of test pieces made from the same material as the core to verify print quality. The core and test pieces were printed in white (no additives), and the core and test pieces were printed with aluminum diethylphosphinate (containing 0.02% aluminum diethylphosphinate relative to the weight of the sand). - Perform hot deformation tests on specimens of different dimensions (6.5 mm × 115 mm and 25 mm × 115 mm) suitable for hot deformation testing, always within the same given time.

[0053] Hot deformation testing is a commonly used test in the foundry industry to predict the reaction of a core to rising temperatures. In this way, it is possible to observe the resistance of a particular core formulation to brittleness, expansion, and deformation at the same casting temperature.

[0054] Specimen composition and manufacturing parameters: a) Using conventional (blow molding) machine manufacturing methods: - Prepare a homogeneous mixture of granular sand material, binding compound, aluminum diethylphosphinate, and binder (resin). First, ensure that the solid material is properly homogenized, and then mix it with the resin. - Blow-molding the test specimens using the Morek Laboratory blow molding machine. The mixture prepared in the previous step is introduced into the machine head. Blow-molding and curing of the test specimens are performed based on the parameters specified below. - To enable firing of test specimens of standard dimensions (6.5 mm × 115 mm and 25 mm × 115 mm) for the "hot deformation" test, use a specific tool with traces adapted to the specified dimensions.

[0055] Composition of the test specimen:

[0056] [Table 3]

[0057] Blow molding parameters for laboratory blow molding equipment:

[0058] [Table 4]

[0059] b) Using additive manufacturing methods: Manufacturing parameters: Same as those described in Example 1.

[0060] Composition of the test specimen:

[0061] [Table 5]

[0062] c) Hot deformation test The hot deformation apparatus has a source of flammable gas for ignition, which provides the heat source, the support for the test specimen, and a distance sensor. For a set time, the sand specimen is exposed to the flame and adjusted to achieve the appropriate casting temperature, while the distance sensor records how far the specimen has moved from its original position. It should be noted that, as a natural tendency of the specimen, due to the effect of heat being concentrated at the central point, the specimen will gradually bend downward.

[0063] The parameters for the hot deformation test are:

[0064] [Table 6]

[0065] Results of hot deformation due to additive manufacturing: The average deformation of specimens containing aluminum diethylphosphinate was 9.42 mm, compared to 26.5 mm for the blank specimen at the end of the study. As shown in Figure 9A, the deformation was considerably lower due to the effect of aluminum diethylphosphinate, with the solid line corresponding to specimens containing aluminum diethylphosphinate and the dotted line corresponding to specimens without aluminum diethylphosphinate.

[0066] Results of hot deformation due to blow molding: The average deformation of specimens containing aluminum diethylphosphinate was 0.72 mm, compared to 19.2 mm for the blank specimen at the end of the study. As shown in Figure 9B, the deformation was considerably lower due to the effect of aluminum diethylphosphinate, with the solid line corresponding to specimens containing aluminum diethylphosphinate and the dotted line corresponding to specimens without aluminum diethylphosphinate.

Claims

1. A sand core comprising granular sand material and at least one binder, characterized in that the core comprises an additive in which aluminum diethylphosphinate is present, and the binder is an inorganic binder.

2. The sand core according to claim 1, wherein the granular material preferably comprises sand selected from silica sand, chromite sand, zircon sand, olivine sand, bauxite, mullite and / or any composite type of sand, with a particle size between 100 μm and 425 μm as measured by a sieving method.

3. The sand core according to claim 1 or 2, wherein the core comprises a second additive selected from zeolite and / or wollastonite and / or lithium carbonate and / or sodium borate or a mixture thereof.

4. A method for producing a sand core, comprising a first step of contacting or mixing a granular sand material with at least one binder, and a second step of hardening, wherein the granular sand material or a mixture of the granular material and the binder contains an additive which is aluminum diethylphosphinate, and the binder is an inorganic binder.

5. The method according to claim 4, wherein the weight percentage of aluminum diethylphosphinate relative to the granular material is between 0.01 and 0.045%.

6. The method according to claim 4 or 5, wherein the manufacturing method is by additive manufacturing, and comprises the first contact step of distributing and coating a work platform (1) with a plurality of layers of a mixture (2) of granular sand material and aluminum diethylphosphinate and a plurality of layers of the binder (3), and the second curing step of raising the temperature of the layers, preferably by a heat input method and / or a dehydration method.

7. The method according to claim 6, wherein the mixture (2) comprises a second additive selected from zeolite and / or wollastonite and / or lithium carbonate and / or sodium borate or a mixture thereof, and the weight percentage of the second additive relative to the granular sand material is between 0.1 and 0.6%.

8. The method according to claim 6 or 7, wherein the granular sand material comprises sand selected from silica sand, chromite sand, zircon sand, olivine sand, bauxite, mullite and / or any artificially occurring sand, the particle size measured by sieving is between 100 μm and 300 μm.

9. The method according to any one of claims 6 to 8, further comprising a third step of removing unhardened granular sand material.

10. The method according to any one of claims 6 to 9, wherein the weight percentage of the binder relative to the granular material is between 1.5% and 6%.

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