What Is Ceramic Mold Casting?

Ceramic mold casting refers to a casting method in which a ceramic slurry is used to produce a casting mold.

The ceramic slurry is a mixture of ethyl silicate hydrolysate and fine refractory sand with pure texture and high thermal stability, such as fused quartz, zircon, corundum, etc. In order to make the ceramic slurry gel in a short time, calcium hydroxide or magnesium oxide is often added as a catalyst. Due to the composition and appearance of the refractory used

There are two types of ceramic type: The ceramic type is all made of ceramic slurry. The production process is first

At present, the most widely used in production is CO2 water glass sand bottom cover ceramic type. CO2 water glass sleeve has the advantages of high strength, good air permeability and simple production. The next process is shown in the figure. The special feature is that two models must be prepared in advance, one for the ceramic mold casting mold (A mold), and the other for the manufacture of the bottom sleeve (B mold), which has a larger size than the (A mold).

1. The surface finish of the casting is high;

2. High dimensional accuracy of castings;

3 Can cast large precision castings

Although investment casting can cast precise size and high-quality castings, due to the limitations of its own technology, the weight of the casting is generally small, the largest piece is only tens of kilograms; while the ceramic castings can be up to a dozen tons.

4 Less investment, fast production and short production preparation cycle.

The disadvantage is that the raw materials are expensive, and because of the grouting process, it is not suitable for casting castings with large batches, light weight, and complex shapes, and it is difficult to realize mechanization and automation of the production process. ^[1]

Ceramic casting is a new process developed on the basis of sand investment casting. The ceramic type is made of a refractory material with a relatively pure texture and high thermal stability as a modeling material; an ethyl silicate hydrolysate is used as a binder, and under the action of a catalyst, through the steps of grouting, gluing, mold-making, and baking And made.

Castings cast by this casting method have high dimensional accuracy and surface finish, so this method is also called ceramic precision casting.

The manufacturing methods of ceramic molds can be divided into two categories: one is to use all ceramic slurry to manufacture the casting method; the other is to use the surface of the bottom sleeve (equivalent to the back sand layer of the mold) to refill the ceramic slurry to make ceramic Type method. The bottom cover is divided into sand cover and metal bottom cover;

Small ceramic mold castings often use ceramic molds made entirely of ceramic slurry, and the modeling process is shown in the figure. First fix the model to the template

(A), put the sand box on

(B), then pour the pre-adjusted ceramic slurry into the sand box

(C) Scrape the upper surface and wait for the glue to harden

(D) Once the slurry has elasticity, it can be removed from the mold.

(E). Immediate ignition and combustion (combustion with compressed air)

(F) After the fire is extinguished, move it into a high-temperature furnace and spray it to form the desired ceramic type.

The other type is a ceramic type with a bottom sleeve. In the production, a compound mold with a bottom sleeve is often used, that is, the surface layer directly in contact with the liquid metal is filled with ceramic slurry, and the rest is replaced by a sand sleeve or a metal sleeve. ^[2]

(A), modeling materials used in ceramics

The molding materials used in the ceramic type include refractories, binders, catalysts, release agents, breathers, and the like.

1. Refractory materials, ceramic type refractory materials require less impurities, high melting point, and small high-temperature thermal expansion coefficient. Ceramic refractory materials include corundum powder, bauxite, silicon carbide and zirconium sand powder.

2. Binder, ceramic type commonly used binder is ethyl silicate hydrolysate.

3 Catalysts, catalysts used in ceramic slurry include calcium hydroxide, magnesium oxide, sodium hydroxide and calcium oxide.

4 Release agent, commonly used release agents are polishing wax, transformer oil, motor oil, silicone oil and vaseline.

5. Breathing agent, adding a breather to the ceramic slurry to improve the permeability of the ceramic type. The commonly used air permeable agent is hydrogen peroxide, and the added amount is 0.2 to 0.3% by weight of the refractory powder.

(II) Preparation and grouting of ceramic slurry

1. Selection of refractory materials

When casting alloy steel forging dies, die casting molds, glassware molds with high requirements on casting size and finish, high temperature resistant, thermally stable corundum powder, sand blasting powder or silicon carbide should be used as the refractory material of ceramic slurry. For cast iron or aluminum castings, less expensive bauxite or quartz powder can be used as the refractory material, and the particle size composition of the refractory material should be selected correctly.

2. Preparation and grouting of slurry

The ratio of the refractory material to the silicate ethanol hydrolyzate should be appropriate, and the ratio of the hydrolyzate required by different refractories is also different. The time to start gelling and the time to grout have a great influence on the quality of the ceramic mold. Catalyst content and room temperature also have an effect on the gel time. It is not good to grout too late. The time to start gelling can be determined from a pre-made sample.

During the grouting, the slurry should not be poured in too fast to prevent air from being drawn in. It is best to vibrate while grouting to exclude the gas in the slurry.

(Three) mold lift

After grouting, the ceramic type is cemented and cured. However, it is not possible to lift the mold immediately after the cementing has started. Generally, 5 to 15 minutes after cementing, the effect of the mold is the best when the ceramic layer has elasticity. When ejecting the mold, first take out the pouring system and aeration rod, and then take out the master mold vertically (do not knock). After the mold is ejected, it should be fired and fired immediately, otherwise cracks are likely to occur.

(4) Burning

After the mold is lifted, it should be ignited immediately and blown with compressed air to spray, so that the surface of the ceramic type has a certain strength and hardness.

(E) roasting

The purpose of firing is to burn off the ethanol, water and a small amount of organic substances remaining in the ceramic type, and increase the strength of the ceramic layer.

The ceramic type is all cast by ceramic slurry. The firing temperature can be as high as 800 , the firing time is 2 ~ 3h, and the temperature should be below 250 to prevent cracks.

Ceramic type with water glass jacket, firing temperature (drying) is 350 ~ 550 .

(6) Pouring and cleaning

Attention must be paid to slag blocking during ceramic casting. The pouring temperature and pouring speed can be slightly higher than the sand castings of the same type. Ceramic castings are best boxed when they are cooled to room temperature to prevent cracking and deformation of the castings.

Gong Keqiang, chief editor: Special Casting, Machinery Industry Press, Beijing, 1982.