2023 Volume 64 Issue 5 Pages 1029-1035
In recent years, casting molds made by sand-type additive manufacturing (AM) technology are increasingly being used to build prototypes and small-lot production casting products.
The development of this technology aiming at Mass-production applications in the future is progressing through the advancement of AM technologies and casting technologies.
The application to Mass production is expected to increase the potentials of the whole casting, by realizing more complicated internal structure, reducing product thickness and weight by improving the cavity precision, etc. One method which can meet such needs is the current AM technique using furan sand molds. It is performed on large molding machines and is based on the organic self-hardening process utilizing mainly furan binder.
This 3D AM sand mold using furan binder (hereafter referred to as “furan AM sand mold”) contains sulfurous acid gas generated by the thermal decomposition of the catalyst used or organic gases generated by the thermal decomposition of the cured binder during pouring. In some cases, it is necessary to take measures such as removal of the gases from the molds. Other issues also need to be improved in terms of the working environment and casting quality. Examples of practically applied 3D AM methods using inorganic binders are; ink jetting water to plaster or cement, or ink jetting water to coated sand (laminated sand coated with water glass).
However, various challenges are met with these methods, such as lack of high-speed large 3D printers and mold collapsibility is difficult after pouring.
Under these circumstances, we attempted to fill a furan AM sand mold with an inorganic binder consisting of phosphate and sulfate, and then sinter the sand mold in an atmosphere of 850°C so that the furan AM sand mold changes into inorganic mold. With this mechanism, strong bonded layers are formed by the formation of polyphosphoric acid by the high molecularization of phosphate and the fusion of sulfate, while the thermal decomposition of the cured furan binder is progressing.
As a result, this transforms the sand mold into a mold composed of only the inorganic binder. The resultant mold has been found to be sufficiently practical with almost no gas generation even under the condition of 1000°C. As for the problem of mold breakage, when water is added to the mold, the sulfate that forms the adhesion layer dissolves in the water, and this action allows the mold to be collapsed easily.
These results confirm that this mold has a balanced combination of good heat resistance, reduced harmful gases, and good mold collapsibility.
This Paper was Originally Published in Japanese in JFS 94 (2022) 181–186.
In recent years, casting molds made by sand-type additive manufacturing (AM) technology are increasingly being used to build prototypes and small-lot production casting products.
The development of this technology aiming at mass-production applications is progressing through the advancement of AM technologies and casting technologies.
The application to mass production is expected to increase the potentials of the whole casting, by realizing more complicated internal structure, reducing product thickness and weight by improving the cavity precision, etc. One method which can meet such needs is the current AM technique using furan sand molds. It is performed on large molding machines and is based on the organic self-hardening process1,2) utilizing mainly furan binder.
This 3D AM sand mold using furan binder (hereafter referred to as “furan AM sand mold”) contains sulfurous acid gas generated by the thermal decomposition of the catalyst used or organic gases generated by the thermal decomposition of the cured binder during pouring. In some cases, it may be necessary to remove gases from the molds. Other issues also need to be resolved such as the working environment and casting quality. Examples of practically applied 3D AM methods using inorganic binders are; ink jetting water to plaster or cement, or ink jetting water to coated sand3,4) (laminated sand coated with water glass).
However, various challenges are met with these methods, such as lack of high-speed large 3D printers and difficulty of mold collapsibility after pouring.
Under these circumstances, we attempted to fill a furan AM sand mold with an inorganic binder consisting of phosphate and sulfate, and then sinter the sand mold in an atmosphere of 850°C so that the furan AM sand mold transforms into an inorganic mold. With this mechanism, strong bonded layers are formed by the formation of polyphosphoric acid by the high molecularization of phosphate and the fusion of the phosphate with sulfate, while the thermal decomposition of the cured furan binder is progressing.
As a result, this transforms the sand mold into a mold composed of only the inorganic binder. The resultant mold has been found to be sufficiently practical with almost no gas generation even under the condition of 1000°C. As for the problem of mold breakage, when water is added to the mold, the sulfate that forms the adhesion layer dissolves in the water, and this reaction allows the mold to be collapsed easily.
These results confirm that this mold has a balanced combination of good heat resistance, reduced harmful gases, and good mold collapsibility.
Artificial sintered sand (average particle size 106 µm) was heated to approximately 120°C as AM sand. A 60% aqueous solution of meta-xylene sulfonic acid was added to the heated sand at 0.3 mass%, and the mixture was stirred for 10 minutes.
After stirring for 10 minutes, when the sand temperature reached approximately 80°C, 0.3% anhydrous magnesium sulfate was added, and stirring was continued until the sand temperature reached approximately 25°C. The solid catalyst coated sand was then taken out.
2.1.2 Fabrication of furan binderBased on the specifications of the inkjet head, the physical properties required for ink-jetting the binder are a surface tension of 25.0 to 40.0 mN/m at 20°C, solution viscosity of 15 mPa·s, and density of 0.5 to 1.5 g/cm3.
Furfuryl alcohol [C5H6O2] has a high boiling point of 171°C and a low surface tension of 38.0 mN/m5) and can be easily polymerized using solid catalysts. Furthermore, the inkjet binder was added with resorcinol [C6H6O2], to improves the curing speed, and with N-(2aminoether)3-aminopropylmethyldimethoxysilane [H2NC2H4NHC3H6SiCH3(OCH3)2],6) to improves the adhesive strength with sand.
2.1.3 Mechanism of curing furan binder using catalyst coated sandMeta-xylene sulfonic acid [(CH3)2(C6H3)SO3H]7) has a melting point of 64°C an is solid at room temperature of 20°C. It is a catalyst that is capable of curing furfuryl alcohol-based binders at room temperature.
These binders mainly of furfuryl alcohol.
As shown in Fig. 1, the oxygen atom of the methylol group (–CH2OH) at the 2-position of the furan ring of furfuryl alcohol attracts a proton (H+) present in the acidic region and takes on a positive charge. C–O bond is then broken and carbocation6) is formed.
Generation of the carbocation in acidic region.
The carbocation is in an electron-deficient state, and a series of chain reactions occur in search of electrons.
As shown in Fig. 2, a methylene bond is formed by the dehydration of the hydrogen at the 5-position of the furan ring, and a methylene bond is formed by the condensation of two methylol groups, resulting in polymerization.8)
Generation of methylene bond.
Next, as shown in Fig. 3, reaction such as the formation of the levulinic acid structure by the cleavage of the furan ring proceed rapidly, resulting in the growth of a three-dimensional crosslinking structure.6)
Three-dimensional crosslinking structure.
Sodium dihydrogen phosphate (NaH2PO4) actively polymerizes at temperatures around 850°C to form sodium polyphosphate9) as shown in Fig. 4. Furthermore, a hydrogen bonding network is formed between the hydroxyl group and the oxygen atom of phosphine oxide, forming a solid adhesive layer as shown in Fig. 5.
Generation of sodium polyphosphate.
Generation of hydrogen-bond network.
Sodium sulfate (Na2SO4) has a melting point of 884°C, a boiling point of 1,429°C, and a solubility in water of 48.8 g/100 g (40°C).10) When mixed with sodium dihydrogen phosphate it becomes a molten salt.11)
Since the melting point of the molten salt is lowered, an adhesive layer is formed by melt adhesion. Furthermore, because sodium sulfate is soluble in water, the mold can be broken down by adding water when the mold is dismantled.
2.1.5 Mold strength testTest mold pieces measuring 10 mm × 10 mm × 60 mm in size were made by adding 2.0 mass% of furan binder to solid catalyst coated sand at the room temperature of 25°C and humidity of 50%, and compared. They were then filled with sand mixture and collapsed 24 hours later.
The test mold pieces were used for comparison immediately after removal. Their transvers strength was measured to JACT test method SM-1 (transvers strength test method).12)
Test mold pieces made of inorganic binders were also prepared. The inorganic binders used were sodium dihydrogen phosphate and sodium sulfate with the following mass ratios: 50:50 for high-strength molds with poor water collapsibility, 30:70 for molds with balanced functions of mold strength and water collapsibility, and 15:85 for molds with excellent water collapsibility (hereafter, only mass rations were used). Each mixture was dissolved in water to prepare a 20 mass% inorganic solution.
In addition to the above, test mold pieces made of furan binder, which were not used for strength tests, were impregnated with the aqueous solution of the above inorganic binder. After volatilizing the water solvent at 200°C for 30 minutes, the test mold pieces were fired for 15 minutes at 850°C. Only the test mold pieces with 5.0 mass% of inorganic binder were used for measuring the transvers strength (Table 1).
The transvers strength was measured according to JACT test method SM-1 (Transvers Strength Method)12) at the room temperature of 25°C and humidity of 50%.
2.1.6 Measurement of gas generation from moldTest molds 1,2,3 and comparison mold prepared in 2.1.5 were measured for gas generation at 1,000°C by the JACT test method M-5 (gas generation measurement method).13)
2.1.7 Mold collapsibility testTest mold pieces prepared using the materials as described in 2.1.5 were immersed in water and visually judged whether the molds collapsed or not.
The comparison mold was found to collapsed due to decomposition and combustion of the hardened furan binder in about 2 minutes when fired at 850°C.
2.2 Evaluation of properties of molds 2.2.1 Mold expansion rate testTo prepare 30ϕ × 50 mm cylindrical test pieces, 3D AM sand molding was performed on solid catalyst coated sand (comparison material) at a pitch of 280 µm by ink-jetting 2.0 mass% of furan binder to the sand in the X-axis, Y-axis, Z-axis direction. Specifically, these cylindrical test pieces were impregnated with sodium dihydrogen phosphate and sodium sulfate at a mass ratio of 50:50, 30:70, and 15:85 respectively.
Then, after volatilizing the water solvent at 200°C for 30 minutes, the test mold pieces were fired for 15 minutes at 850°C. Only those with 5.0 mass% of inorganic binder were used for measuring.
The expansion rate of the test mold piece was measured in an atmosphere of 1,000°C for 5 minutes according to the JACT test method M-2 (to measure the rate of rapid thermal expansion).14)
2.3 Mold casting evaluationTo prepare test pieces with a core diameter of 15 mm × 200 mm and a core print of 30 mm × 20 mm × 23 mm, 3D AM sand molding was performed at a pitch of 280 µm by ink-jetting 2.0 mass% of furan binder to the sand (Fig. 6). This furan core was used for Comparison test material.
Casting dimensions and core dimensions.
As test materials, these cores were impregnated with sodium dihydrogen phosphate and sodium sulfate at a mass ratio of 50:50, 30:70, and 15:85 respectively.
Then, after volatilizing the water solvent at 200°C for 30 minutes, the cores were fired for 15 minutes at 850°C. Only those with 5.0 mass% of inorganic binder were used for measuring.
The base molds were made by molding kneaded sand with solidum silicate solution (molar ratio of 2.5, baume 48° (20°C)) and adding 0.4 mass% of triacetin organic ester added to the sintered artificial sand (average particle size 106 µm) for AM sand molding.
Aluminum castings (material: AC4C) were cast in the molds at a pouring temperature of 700°C. Odor during pouring, mold collapsibility after pouring, and cast surface of castings were checked.
Figure 7 shows the results of the mold strength development test.
Transvers strength of furan mold and inorganic binder mold.
The result of the transvers strength of the test mold pieces after sintering at 850°C differed depending on the mass ratio of sodium dihydrogen phosphate and sodium sulfate.
Specifically, the results were 8.2 MPa at a mass ratio of 50:50, 4.1 MPa at a mass ratio of 30:70, and 2.2 MPa at a mass ratio of 15:85, when the total binder content was 5.0 mass% in relation to sand.
The transvers strength of the comparison material was 5.5 MPa, and the molds substituted with inorganic binders also had sufficient transvers strength. The transvers strength increased with increasing ratio of sodium dihydrogen phosphate. The transvers strength was confirmed to be 2.0 MPa or higher, which is a standard handling strength of casting cores, indicating that the molds are easy to handle.
3.2 Results of gas generation measurement from moldFigure 8 shows the results of the gas generation test.
Quantity of gas generated at 1,000°C.
There is almost no gas generation from all the molds substituted with the proposed binder material (inorganic binder). On the other hand, gas generation is observed from the organic molds made of the comparison material after 15 seconds. This suggests that molds with almost no gas generation allow for a better working environment and can be expected to reduce casting gas defects.
3.3 Mold collapsibility testTable 2 shows the results of the mold collapsibility test.
The test material were placed in water after sintering at 850°C and visually judged whether or not the mold collapsed due to the water. The test material made of the comparison material collapsed when fired at 850°C.
The results show that the test material containing a higher proportion of sodium sulfate have better mold collapsibility.
This means that conventional inorganic molds consisting mainly of water glass do not soften even when water is added. In comparison, the proposed molds collapsed easily, suggesting that better productivity can be expected.
3.4 Ratio of thermal expansion of moldTable 3 shows the measurement results of the ratio of thermal expansion of the molds. Usually, molds have low ratio of thermal expansion. However, the inorganic molds in this study showed a higher ratio of thermal expansion than the furan molds, and the thermal expansion ratio tended to increase as the ratio of sodium sulfate increased. This is presumably due to the fact that the thermal expansion ratio of sodium sulfate, which forms the adhesive layer, is larger than that of polymerized sodium phosphate. However, veining defects in high-melting-point castings such as iron castings are more likely to occur when the thermal expansion ratio exceeds 0.9%.15) Inorganic molds made by this technique have a low thermal expansion ratio of 0.9% or less, suggesting that thermal expansion ratio is effective for reducing veining defects.
Figure 9 shows the appearance and dimensions of the inorganic molds obtained from the 3D AM sand molds made with furan binder. There is almost no dimensional change in the in organic molds made by 3D AM sand mold using inorganic binder.
Photos of sand mold additive manufacturing technology.
Casting odor due to thermal decomposition of the hardened binder was observed from the 3D AM sand molds made with furan binder, but no odor was detected from the molds made with inorganic binders.
As for the collapsibility of the molds after casting, the mass ratio of sodium dihydrogen phosphate to sodium sulfate (15:85) was excellent, and the molds could be collapsed by adding water.
All the comparison core and test core had excellent casting surface and no casting gas loss occurred.
The following results were obtained from the study.
