l Advantages and Disadvantages of Investment Casting;
l Raw Materials for Investment Casting;
l Application of Investment Castings;
l The Process Flow of Investment Casting.
Investment casting, also known as the lost wax casting, is making a wax mold, wrapping around the wax mold with adhesives and refractory materials to get a molded shell, and then smelting and recovering the wax mold at a high temperature to obtain fractions and the casting mold. Fire the casting mold and pour the molten metal into the casting mold to get the castings. This process of making castings is called investment casting (lost wax casting).
l The dimensional accuracy of investment castings is high, and the accuracy is generally up to CT4-6 (sand casting is CT10~13, die casting is CT5~7).
l The surface finish of investment castings is higher than that of general castings and can reach Ra.1.6~3.2μm normal. The main reason is: that when pressing the investment mold, make the mold with a high surface finish. The mold shell is also made of high temperature-resistant special binder and refractory coating made of refractory material to hang on the investment mold. The inner surface of the cavity in direct contact with the metal has a high surface finish.
l Using the investment casting for castings can save more machine tool equipment, processing time, and metal raw materials. The main reason is: that investment casting castings have high dimensional accuracy and surface finish, which can reduce machining work, only leave a little machining allowance on the parts with higher requirements, and even some castings only need grinding, polishing, and even also without machining.
l Can cast complex castings of various alloys, especially castings of superalloys. The blades of a jet engine have a streamlined profile and an inner cavity for cooling. It is almost impossible to complete the production of such castings using the machining process. However, the investment casting process can achieve mass production, ensure the castings can keep consistency, and avoid stress concentration of residual knife lines after machining.
l It is possible to cast castings with fine patterns, characters, grooves, and curved pores;
l There are almost no restrictions on the types of casting materials, and often used to cast copper, alloy steel, carbon steel, and heat-resistant alloy castings;
l There is no limit to the production batch, and it can be mass-produced from a single piece to a batch.
Although the dimensional accuracy of investment castings is high, its consistency still needs to be improved (the dimensional consistency of castings using medium and high-temperature waxes is much improved). Mainly because the process of investment casting is complex, and many factors affect the dimensional accuracy of castings, such as shrinkage of the mold material, deformation of the investment mold, changes in the amount of the mold shell during heating and cooling, shrinkage of the alloy and Deformation of castings during solidification, etc.
The weight of investment castings is mostly zero to dozens of Newtons (from a few grams to more than ten kilograms, generally not more than 25 kilograms), and it is more troublesome to use the investment casting method to produce too heavy castings.
The investment casting process is complex and difficult to control, and the materials used and consumed are expensive.
The investment casting process has many advantages. Castings of various raw materials can be processed. The raw materials of castings can be carbon steel, alloy steel, heat-resistant alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy, ductile iron, etc.
Investment casting is mainly used for casting small precision parts with complex shapes, mass production of precision castings with high melting point alloys, and small parts with complex shapes that are difficult to machine. Such as blades, impellers, guides, guide wheels and automobiles, tractors, agricultural machinery, pneumatic tools, small parts on machine tools, and handicrafts such as steam turbines, gas turbines, and hydraulic engines.
The mold material should not only ensure the convenient production of investment molds with precise dimensions, high surface finish, good strength, and lightweight but also create conditions for the manufacture of mold shells and good castings. Molding materials are generally formulated with waxes, natural resins, and plastics (synthetic resins). Two common mold materials:
Wax-based mold materials (prepared with wax materials) have a lower melting point of 60 to 70 °C;
Resin-based mold compounds (prepared with natural resin) have a slightly higher melting point, about 70 to 120°C.
The various raw materials are melted and mixed into one by heating, nest the mold material is vigorously stirred under the condition of cooling so that the mold material becomes a paste state for pressing and investment. (The purpose of preparing the mold material is to mix the various raw materials that make up the mold material into a uniform body, and to make the state of the mold material meet the requirements of the pressing investment)
In production, most of the investment molds are made by pressing the paste mold material into the mold.
Before pressing the investment mold, apply a thin layer of release agent to the embossing surface to remove the investment mold from the embossing easier. When pressing wax-based mold materials, the parting agent can be engine oil, turpentine, etc. When pressing resin-based mold materials, often use a mixture of sesame oil and alcohol or silicone oil as a parting agent. The thinner the parting agent layer, the better the investment molds will be. The thinner the parting agent layer, the better investment molds can replicate the embossing surface and improve the surface finish of the investment mold. There are three pressing investment methods:
Air pressure method
Piston pressurization method.
Wax repair: After completing the investment pressing, must repair the burrs.
Wax Test: Check the investment mold for pores, etc.
The investment mold assembly is to combine the investment mold that forms the casting and the investment mold that forms the gating riser system. There are two main methods:
1. Welding method Use a thin soldering iron to melt the joints of the investment to weld the investment together. This method is more common.
2. Mechanical assembly method When mass production of small investment castings, widely use the mechanical assembly method to complete the tree grouping. The use of this module can greatly improve the combination and efficiency of the module, and the working conditions are also improved.
To improve the ability of the coating wet the surface of the module needs to remove the oil stain on the surface of the module when using wax-based mold material to make the investment mold.
Investment casting process
Before painting and hanging the coat, should stire coat evenly to reduce the precipitation of refractory materials in the paint bucket as much as possible, and the viscosity or specific gravity of the coating should be adjusted so that the coating can fill and wet the investment mold well. The module is immersed in the coat and shakes up and down so that the coating can cover the investment mold and evenly cover the surface of the module. After the paint is applied, carry out stickying sand.
After coating each layer of the shell, dry and harden it, so that the binder in the coating changes from sol-like to jelly or gelatinous, and connects the refractory materials.
After hardening the shell completely, should take out the wax mold from the mold shell. Because the wax mold is usually made of wax-based mold material, this process is also called dewaxing. According to the different heating methods, there are many dewaxing methods, and the hot water method and the same pressure steam method are used more.
If molding and pouring, it is necessary to bury the mold shell into the sand in the box before firing, and then load it into the furnace for firing. If the mold shell has high strength under high-temperature conditions, no casting is required, and the mold shell can be directly sent to the furnace for roasting. During roasting, gradually increase the furnace temperature. Heat the mold shell to 800-1000 ℃ and keep it for some time, then take them out for use.
Should Pay Attention To These Problems In Mold Shell Manufacturing:
There are two types of investment casting molds: solid type and multi-layer mold shell.
After dipping the wax mold tree with refractory paint, stick the refractory material in the form of material, and then dry and harden it. Repeat the above steps several times until the refractory coating reaches the required thickness. After Making the mold shell, it usually needs to be parked for some time to fully harden (normally dried with a blower), and then melt the wax mold away to obtain a multi-layer shell.
When melting the investment mold, the shell will be under the pressure of the increasing volume of the molten mold material; during firing and pouring, the various parts of the mold shell will have mutually restrained and uneven expansion and contraction. Therefore, the metal may also High-temperature chemical reaction with mold shell material. Therefore, there are specific performance requirements for the shell, such as low expansion rate and shrinkage rate; high mechanical strength, thermal shock resistance, refractoriness, and chemical stability at high temperatures; the mold shell should also have specific air permeability, So that the gas in the mold shell can escape smoothly when pouring. These are all related to the refractory materials, binders, and processes used to manufacture the mold shell.
The materials used to make the mold shell have two types: one is used to directly form the mold shell, such as refractory materials, binders, etc.; the other is to obtain high-quality mold shells, simplify operations and improve processes, such as fluxes, hardeners, surfactants, etc.
The refractory materials used in investment casting are mainly quartz and corundum and aluminum silicate refractories, such as refractory clay, bauxite, coke gemstone, etc. Sometimes zircon, and magnesia (MgO), are also used.
The most common binder used in investment casting is the silicic acid colloid solution (also coaled silicic acid sol), such as ethyl silicate hydrolyzate, water glass, and silica sol. The components that make up the adhesive are generally silicic acid (H2SiO3), other solvents, and sometimes stabilizers such as NaOH.
Ethyl silicate hydrolyzate is the earliest and most common binder used in silicic acid sol mold casting obtained by hydrolysis of ethyl silicate; the water glass shell is easily deformed and cracked, and the dimensional accuracy and surface finish are poor. However, in our country, when the production precision requires carbon steel castings and non-ferrous alloy castings with a lower melting point, the water glass is still widely used in production; silica sol has good stability, can be stored for a long time, and does not need to be used when making shells. Special hardener, but the wetting of the investment mold by silica sol is slightly poor, and the shell hardening process is a drying process, which takes a long time.
Commonly used pouring methods in investment casting are:
The most widely used form of casting is after taking the mold shell out from the roaster and poured at a high temperature. At this time, the metal cools slowly in the mold shell and can fill the mold shell with high fluidity, so the casting can replicate the shape of the cavity better to improve the accuracy of the casting. However, the slow cooling of the casting in the hot mold will cause coarse grains, which reduces the mechanical properties of the casting. When casting carbon steel castings, the surface of the casting that cools more slowly is also prone to oxidation and decarburization, which reduces the surface hardness, finish, and dimensional accuracy of the casting.
Put the mold shell in the vacuum casting box, and suck the gas in the mold cavity through the tiny pores in the mold shell, so that the liquid metal can better fill the mold cavity, copy the shape of the mold cavity, improve the casting accuracy, prevent pores, pouring Insufficient defects.
Put the shell in the pressure tank for pouring. After the end, the pressure tank is closed immediately, and high-pressure air or inert gas is introduced into the tank to solidify the casting under pressure to increase the density of the casting. The maximum pressure abroad has reached 150atm.
Some investment castings, such as turbine blades, magnetic steel, etc., if their crystalline structure is columnar crystals arranged in a certain direction, working performance can be better. So the directional crystallization technology of investment casting is rapidly developed.
The main contents of investment casting cleaning are as follows:
(1) Remove the mold shell from the casting;
(2) Remove the casting from the pouring system (Using the cutting machine);
(3) Remove the shell refractory adhering to the casting (Using the shot blasting machine);
(4) Cleaning Investment castings after heat treatment, such as descaling, edge and cutting gate residues, etc. (Using heat treatment furnace).
Carry out polishing, machining, powder coating, galvanizing, titanium plating, and other surface treatments according to specific needs. Or welded or assembled with other accessories.
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