Research on Molding Process of Ceramic Spraying Method Based on Rapid Prototyping

Knowledge sharing by Guangdong Shunde Teamwork Model Technology Co., Ltd, whom with over 20 years rapid prototyping experience.

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The combination of ceramic casting and the recently developed rapid prototyping technology can quickly produce stamping dies, lost foam dies and other molds of different alloys 1 to 3. It combines the advantages of rapid prototyping and ceramic casting, and is used to Production 4 5. But there are also deficiencies, such as: the process is more complex, requiring the sand box to be turned over many times; due to the grouting easily involved in the air, affecting the surface roughness; large amount of ceramic slurry. For the lack of "grouting" process, the author proposed to change the "grouting" to "spray."

Using the most economical IDM prototype, the ceramic slurry was sprayed on the prototype and backfilled with sand to create a castable metal ceramic model.

In addition to the original advantages of ceramic type, it also has the following advantages. 1 The slurry is saved. The thickness of ceramic material sprayed on the model is about 2 mm. The sprayed ceramic shell is thin and uniform in thickness. 2 The spraying method has a smooth surface and there is no “grouting” method in which the slurry is entrained in the air. The formation of small bubbles on the surface; 3 spray molding suitable for the manufacture of large-scale molds.

The difficulty of this process is the slurry formulation and spray process.

1 Ceramic Spraying Modeling 1.1 Spray Painting Modeling Basic Requirements Spraying should pay attention to the following issues.

The spraying process is suitable for molds with little change in surface curvature and no deep cavity. The viscosity of the coating should be suitable.

1.2 Basic Properties of Spraying Ceramic Slurry The technological performance of spraying ceramic slurry refers to the process in which the coating is to meet operational requirements and ensure the main properties of the coating, such as viscosity, suspensibility, coatability, and fluidity. Wait. The ceramic slurry for spraying should have the following properties.

The paint should have good coatability. If the viscosity is too low, turbulence may occur and uniform coating cannot be formed; the viscosity is too high and is not conducive to spraying.

The coating should be able to be self-hardened and can be well combined with back-sand, and there is no separation of the paint layer from the backing sand when taking the mold.

2 Selection of main raw materials for coating The ratio of refractories and particle size is basically the same as that of “grouting method”. The hydrosilicic acid ethyl ester solution is used as a binder and carrier liquid for ceramic slurry.

2.1 Catalyst The ethyl silicate hydrolyzate is mixed with the refractory material and requires a very long gel time. In order to shorten the gelation time, a certain amount of catalyst must be added. The gelation rate of the ethyl silicate hydrolyzate depends on the temperature, the formulation of the hydrolysate and the pH (the pH of the coating should be controlled within the range of 1 to 2), which is mainly the pH value. The length of gelation time directly affects the quality of the ceramic type. If the gelling time is too long, large particles in the slurry will precipitate, and the surface roughness of the ceramics will increase, which will also prolong the production time. If the gelation time is too short, it will be unfavorable to the operation. Ceramic type strength. The study shows that the gelation time is shortened from 10 min to 1 min, the surface roughness Ra of the sample is increased from 1.1 Mm to 1.7 um, and the bending strength is reduced from 2.43 MPa to 0.88 MPa. Appropriate gelation time can make the colloid and The refractories are well mixed and the bond strength is high; when the gel time is only 1 minute, the strength is very low. In order to obtain a high quality ceramic type, it is necessary to select a suitable catalyst and amount to achieve the required gelation time.

Studies have shown that using powdered Ca(OH) and MgO as catalysts can also achieve better results. The powdered catalyst and the refractory powder are fully stirred and added to the hydrolyzed solution and stirred for 3 to 5 minutes, then poured into the spray gun for spraying.

2.2 Thickeners and Surfactants In order to improve the paint's coatability, thickeners and surfactants are added to the paint.

Surfactants can improve the wettability and penetration of the coating on the surface of the object. Alcohol-based paints have better surface penetration and penetration than water-based paints. The RP prototype used in this test has excellent wettability with alcohol-based paints and does not require the addition of surfactants.

Yuan Da, male, born in 1948, Senior Engineer, Laser Rapid Prototyping Center, Department of Mechanical Engineering, Tsinghua University, Beijing (100084), Phone: 010--Cement-based spray-on-mold modeling process for ethyl silicate hydrolyzate in plastic The viscosity before condensation is very small and does not have a very high viscosity when sprayed. In order to make the paint have a good coatability when spraying, a certain amount of thickener must be added.

(1) Organic bentonite: Organic bentonite is an ideal thickener for alcohol-based coatings. It can swell in alcohol, increase the viscosity of the coating and have a certain thixotropy. The coating prepared with it is easy to establish the coating thickness and make the ceramic surface smooth.

Fine, can be used as a thickener for alcohol-based coatings. Its combination with phenolic resin can make the coating bond strength higher. And can make the coating have a high thixotropy, but the amount can not be added too much.

3 Experimental study of thickeners for coatings It can be seen from the above analysis that the selection of thickeners is the key to the spray process of ceramic slurries.

3.1 Effect of PVB on coating process performance Adding different amounts of PVB to test and analyze the viscosity, coating thickness, and coating relative strength. The test data is shown in Table 1. Table 1 Effect of PVB on coating process performance No. - Viscosity/s Coating The relative strength of the thickness/cm coating is shown in Table 1. After PVB was added, the coating viscosity and coating thickness increased. In some tests, pits appeared on the surface. The main reason was that the viscosity increased after PVB was added. The droplets ejected were large and did not form a mist. Therefore, nozzles with a small diameter should be used as far as possible without affecting the spraying to make the atomization more thorough.

3.2 Effect of organic bentonite on the performance of spray coating Add different amount of organic bentonite, test the viscosity, coating thickness, surface strength, test data, see Table 2 The data shows that organic bentonite helps increase the viscosity and build the coating thickness, but There is no effect on the relative strength of the coating.

Table 2 Effect of organic bentonite on the coating process performance Numbered addition/g Viscosity/s Coated coating Relative mullite powder Hydrolysis organic bentonite Thickness/cm Intensity 3.3 Simultaneous addition of organic bentonite and PVB Effect on spray coating process performance Based on the proposed basic raw materials, different amounts of organic bentonite and PVB were added at the same time to conduct tests. The viscosity, coating thickness, and relative strength of the coating were tested and analyzed. The test data is shown in Table 3. There was no specific change in the data. Organic bentonite cannot increase the relative strength of the coating.

Table 3 Effect of organic bentonite and PVB on the coating process performance Numbered Addition/g. Viscosity/s Coating Coating Relative Mullite Powder Hydrolysis Solution Organic Bentonite PVB Thickness/cm Intensity 3.4 PVB and Organic Bentonite Bending Strength When pouring, the ceramic type is squeezed by the metal liquid and it is necessary to test the bending strength. The sample is 40mmX20mmX3mm. The sample is placed on the fulcrum of the bending instrument and uniformly loaded with iron pellets until the specimen breaks. The mass of the added iron pellet is weighed and the bending strength is calculated.

It can be known from the test that adding a suitable amount of PVB results in a higher flexural strength of the shell samples. When the amount of PVB added is 1% (mass fraction), the shells have a high resistance within the range of 200~1000°C. Bend strength.

4 mold casting process with a good ceramic powder, hydrolyzate and additives.

Apply a splitting agent to the original RP. The coated parting agent should be smooth and without streaks so as not to affect the surface quality.

After mixing the powder with the additive, the mixture is added to the ethyl silicate hydrolyzate and stirred for 3 to 5 minutes so that the refractory material is sufficiently wetted.

The slurry was sprayed on the model and a uniform layer of approximately 2 mm thick was formed on the model.

(6) After blown sand, 0 gas is blown to accelerate hardening. After the coating layer is hardened, it can be moulded and immediately burned to stabilize the ceramic size.