The process of dewaxing casting

Lost wax castingThere are two types: solid mold method and pottery box mold method. The latter is an improved method of the former. The solid mold method is to consider the condensation shrinkage of the wax mold, the heating expansion of the mold, and the condensation shrinkage of the molten metal to produce a mold similar in size to the casting. The dissolved wax is pledged and poured into a mold made of metal or silicon, and after the wax is taken out, it is immersed in a slurry made by mixing the micro-powder refractory material and the binder.

After dripping, sprinkle with coarse-grained refractory and dry. Put it into a casting frame, fill it with refractory pellets mixed with binder, and dry it. Then heating makes the wax melt and flow out and become a mold. Heat the mold at high temperature to burn off a small amount of remaining wax, improve strength, and inject molten metal.

The pottery box model method until the wax model is the same as the solid model method, except that the modelling operation is not carried out after one application and sanding (or floating sand), and the application and sanding are repeated until the predetermined shell thickness is reached. Drying, heating, and melting flow out of the wax, and the high-temperature heating method is widely used in the current dewaxing precision casting industry. Because it has the following advantages

(1) Good dimensional stability;

(2) The use of refractory materials is small

(3) Light weight, easy to handle, handle and manufacture large castings;

(4) The process can be partially automated, saving manpower and increasing production speed

(5) Low production cost;

(6) The pottery box mold is thin, the casting cooling rate is high and uniform after pouring, and the mechanical properties are good.

Dewaxing casting.

(a) Master model The shape of the master model is similar to that of the product, and the condensation of wax, the heating expansion of the model and the condensation of the cast metal must be contracted during production.

Reduction is under consideration. For example, when the cold shrinkage rate of the wax mold is 1.2%, the thermal expansion rate of the mold is 0.7%, and the cold shrinkage rate of the cast metal is 1.7%, the size increase rate of the main model is 2.2%. The commonly used materials are aluminum alloy, copper alloy, Stainless steel etc.

(b) Wax forming molds. Wax forming molds for mass production of mechanical parts are generally made by aluminum alloy machining methods, according to the customer's proposal.

The blueprint processing provided does not require the use of the master model. The aluminum alloy mold is composed of several pieces. It is manually assembled and locked before the wax injection, and manually disassembled after the wax injection is completed. The wax mold is taken out. Molded in plastic sheet metal.

(c) For people with a large solidification temperature range of waxes commonly used in waxing, press the semi-solid state inside the mold of the waxing machine, and the pressing pressure is high.

7~12kg/cm2, sometimes 50kg/cm2. When making wax molds, you must pay attention to the temperature of the metal mold, wax pressure, release time, and wax mold temperature to obtain dimensional stability. Therefore, the wax chamber must be kept at a constant temperature.

Sometimes plastics are used instead of wax. Polystyrene is used for small castings with large production volume and less demanding dimensional accuracy, and urea resin is used for large castings with thick sections (85% of urea particles, 10% of polyvinyl alcohol powder, and 5% of water). But the pressure reached 140kg/cm2.

(d) Hollow products with a simple heart shape, products with pores and deep grooves, and products with complex hollow parts, and products that do not adhere to glue.

A heart shape must be used. There are metal heart-shaped, soluble wax heart-shaped and ceramic heart-shaped types. The metal core method is to fix the metal core in the metal mold, inject the wax and solidify, and extract the metal core from the wax, which is limited to the middle.

Products with simple void shapes can be used. If the hollow part has a complicated shape or the inner hole is tortuous, the dissolving wax can be fixed in the metal mold as a heart shape. After the wax is injected and solidified, the wax is soaked in water to dissolve the heart shape.

However, since the heart shape made of dissolved wax has poor strength and is easy to break, the slender hollow part can be used for alumina powder, spinel powder, barium titanate powder, etc., and ethyl silicate, silica or resin. The mixture is mixed into a pottery heart shape, injected into the finished product, and after being suspected to be fixed, dissolve the pottery heart shape with a solvent. The hollow part of the turbine blade is made in this way.

(e) The wax patterns of the small wooden pieces must be manually welded on the watering guide one by one, and they are assembled into wax tree wax casting in the whole project.

A leading step that takes time is to use a flat-headed soldering iron to melt the trunk or substrate and wax-type bonding when composing the tree. The matters that should be paid attention to when assembling are as follows:

(1) Assemble as many wax patterns as possible

(2) When assembling, the wax pattern will easily flow out along the trunk or base when it melts

(3) The finished part of the assembly should be solidified first, and then solidified after the water tank

(4) After pouring and solidification, the workpiece is easy to cut off from the trunk or foundation

(F) Slurry and drenching sand. The combination tree first cleans the release agent, grease, etc. on the wax surface with acetone or alcohol, and then soaks the adhesive and the micro-powdered refractory material.

The slurry, drip the excess adhesive slurry, spray small particles of refractory material, and dry. The focus of this operation is to evenly spread on the wax surface without air bubbles.

Repeat many times until the predetermined thickness is obtained. The number of operations and the thickness of the ceramic box depend on the size and weight of the workpiece. The thickness must support the weight of the molten metal during pouring and gain pressure without breaking.

The main component of the slurry industry mostly uses silicic acid glue, and the filler uses zirconium powder with a weight ratio of about 1:35. The slurry must be added with an appropriate amount of wetting agent (to make the slurry easy to adhere to the wax surface) and defoaming Stir for 12 hours after soaking in the agent. The management items of the slurry have viscosity, specific gravity and PH value.

For high-viscosity ones with a large amount of filler added for the first layer, and low-viscosity ones with good permeability behind the second layer, keep 22~25℃ near the barrel to prevent rapid aging of the slurry.

Before the slurry drips and is not dried, sprinkle coarse refractory particles. The method of sprinkling generally uses a simple and effective sand-cleaning bucket. A few use sand spraying machines. The inner part of the coating uses fine particles of 50-100 mesh, and the outer thickness is 20. ~40 mesh, drying on a conveyor belt, in a forced air drying room with controlled temperature and humidity.

(g) The method used in the dewaxing and firing industry is to put the smeared wax-shaped tree in a high-pressure container and lock it, and add 150°C steam to heat it to make the wax fast.

The ground melts and flows out, and the purpose of firing is to completely burn the small amount of wax remaining on the surface of the mold to remove and improve the strength of the mold. The firing temperature of the mold is between 700 and 1100 ℃. Depending on the material of the cast metal, the time is 20~ After 60 minutes, the mold is taken out of the heating furnace and poured immediately