Process control of silica sol casting

Silica sol casting refers to the casting method that is made of fusible material, coated with several layers of refractory material to make molded shell, melted out of shape after high temperature roasting, and can be cast.

The process of silica sol casting:

1. Mold Manufacturing

A die is a mold used to press the appearance of a work piece.In order to ensure the quality of the wax mold, the mold must have high dimensional accuracy and small surface roughness.When the casting precision is not high or the production batch is not large, fusible alloy, epoxy resin, gypsum can be directly poured to the mother mold;When the casting precision is high or mass production, the pressure is generally made of steel, copper alloy and aluminum alloy by cutting, and the general enterprise uses more aluminum alloy molds.

2. Pressing wax mold

Push the processed wax into the mold cavity, take it out after the ice water condenses in the ice water machine, put it into the pool for a period of time, and get a single wax mold after dressing and testing.A wax mold can be understood as a pressure cast part.

2.1 Process parameters of wax pressing

Room temperature: 25℃±2℃, wax cylinder temperature: 56~60℃, wax nozzle temperature: 56~60℃, wax injection pressure: 2.5~15MPa(the specific should be adjusted according to the size of the workpiece wall thickness), pressure holding time: a few seconds to dozens of seconds (according to the size of the workpiece wall thickness adjustment), cooling water temperature: 9℃±2℃.

2.2 Precautions for operation

Before pressing the wax, check the mold model, open the mold to clean and wipe the mold cavity, and remove the residual wax on the mold. Only bamboo knife can be used, and metal objects are not allowed to contact with it to prevent damage to the mold surface.Clean the mold cavity with compressed air and spray with release agent to close the mold.The pressing parameters cannot be easily changed in the wax molding process.For the pressed wax mold such as local bulge, wax mold has a lack of Angle, deformation can not be repaired and the size of the problem of the wax mold should be reported as waste treatment.

3. Repairing mold

The stand or fall of wax pattern, surface quality, deformation degree determines the product of some important parameters, so the wax mold workshop need to temperature control, minimize the deformation of wax pattern, at the same time for each inspection, on the surface of the wax models also need to have flow mark should be washed, and repair the mold to remove the wax a burr, flash and moulding line, with trace defects should be repaired.

4. Combination wax mold

After mold repair, for qualified single wax mold, in order to improve productivity, the single wax mold is fused to the pre-made wax public gating system to form wax mold.Usually 2 ~ 80 wax molds can be welded on a wax module.The group tree scheme determines the yield of products and the utilization rate of molten steel. The group tree scheme should be carefully considered in the design, and whether it is convenient for wax recovery and reuse should also be considered.

5. Clean the wax mold

After standing for more than 45min, the wax modules after the assembly can be cleaned. Before shell making and slurry hanging, the wax modules after the assembly number must be cleaned to remove the wax debris and grease on the surface and improve the adhesion of shell making.Dip the module into the cleaning solution and repeat for three times, about 4S in total. After taking out, blow dry the module with compressed air, and then sizing and shell making can be done.

6. Manufacture shell

After the coating is dipped outside the wax mold, it is put into the shell to dry. The surface layer is dried naturally, and the transition layer and the back layer are dried by rotating air. The coating is repeated for 5 ~ 7 times (5 ~ 6 layers for small castings and 6 ~ 9 layers for large castings).The surface layer is made of fine sand, the transition layer is slightly coarse, and the sand of the later layers is coarse, until the coating is formed into a hard shell of 7 ~ 15mm. This kind of hard shell casting with sufficient strength is called the shell.

6.1 The ratio

According to different products should use different viscosity value, complex shape structure, paint should be sparse, simple shape has a large area of plane products, paint to take some thicker.

6.2 Control requirements

The temperature between shell making is 25℃±2℃, the humidity between surface layer is 55%~70%, the transition layer is 50%~65%, and the back layer is 40%~60%.When the surface coating is not new material, stirring time ≥24h, part of the new material stirring time ≥12h, the transition layer and the back layer of new material, stirring time ≥12h, part of the new material stirring time ≥6h.

6.3 Main points of sizing operation

Will be cleaned, module with certain Angle slowly into the bucket, slightly rotate, take out the module at slightly faster speed, except redundant paint, use compressed air blowing broken attached to the top of air bubbles (for font and grooves have not hang pulp in local repair) with a writing brush besmear to brush, constantly rotating module, in the rotation into the spray sand machine, let the modules on the surface after sand evenly and then hang on the turnover of the car and dry.The same method is used to sand the transition layer and the back layer. After the last layer of the back layer is dried, the final slurry drying is carried out.

6.4 Shell drying time control

The drying time of the surface layer is 6~8h, the drying time of the transition layer is ≥10h, the drying time of the back layer is ≥12h, and the drying time of the sealing layer is ≥14h. Different sand drying time is also different, and appropriate adjustment can be made according to the drying situation. The phenomenon of sand loss and crack indicates that the shell is too dry, so the drying time should be shortened appropriately.

7. Dewaxing and wax treatment

The shell backwards into the steam dewaxing axe, melt wax models, and outflow from below, by the water separator separation, charging liquid wax in except the bucket further separation and remove the impurity in the wax wax water, 12 hours after wax liquid through a filter to be included in the static barrels 6 ~ 8 hours, finally will wax liquid into the insulation heat preservation box for the use of again.After the wax mold flows out of the shell is the cast mold.

Note:(1) The time from the shell to the dewaxing axe should not exceed 1.5 minutes, and it is best to control within 1 minute;(2) after entering the dewaxing axe, the pressure in the dewaxing axe should reach a minimum of 0.6Mpa within 13 seconds;(3) In addition to the mixing temperature of the bucket 110~120℃, the temperature of the standing bucket < 90℃, the wax is easy to age at a high temperature, and the temperature of the incubator is controlled at 54~62℃.

8. Shell is roasted, poured, sanded and cleaned

After dewaxing, the shell should be roasted during casting. The shell should be roasted in an oil or electric furnace at 1100℃ for 50 minutes. The main purpose is to remove the residual wax liquid after dewaxing and improve the strength and permeability of the shell, so as to prevent the deformation or rupture of the shell during casting.Another extremely important factor is to reduce the temperature difference between molten steel and molded shell to reduce shrinkage of the workpiece.When the molten steel can be poured by melting, heat preservation and calming, the molded shell that reaches the roasting heat preservation time can be poured by taking out of the roasting furnace.After the casting condenses, the mold is destroyed, the gating system is removed, the burr is cleaned and shot blasting is carried out to obtain the target casting.The surface of stainless steel castings is usually pickled and passivated.

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Post time: Apr-08-2021