What is investment casting

What is investment casting?

Investment casting, also known as wax loss casting, includes wax pressing, wax repair, tree formation, slurry dipping, wax melting, casting liquid metal and post-treatment. Lost wax casting is to use wax to make the wax mold of the parts to be cast, and then coat the wax mold with mud, which is the mud mold. After the clay mold is dried, put it into hot water to melt the internal wax mold. Take out the clay mold after melting the wax mold, and then bake it into a ceramic mold. Once roasted. Generally, the pouring port is left when making the mud mold, and then the molten metal is poured from the pouring port. After cooling, the required parts are made.

Introduction to investment casting

When making patterns with wax, investment casting is also called “lost wax casting”. Investment casting usually refers to a casting scheme in which a pattern is made of fusible materials, several layers of refractory materials are coated on the surface of the pattern to make a mold shell, and then the pattern is melted out of the mold shell, so as to obtain a mold without parting surface, which can be filled with sand after high-temperature roasting. Because the pattern is widely made of waxy materials, investment casting is often called “wax loss casting”.
The types of alloys that can be produced by investment casting include carbon steel, alloy steel, heat-resistant alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy and nodular cast iron.
The shape of investment castings is generally complex. The minimum diameter of the hole that can be cast on the casting can reach 0.5mm, and the minimum wall thickness of the casting is 0.3mm. In production, some parts originally composed of several parts can be designed into integral parts by changing the structure of the parts, and directly cast by investment casting, so as to save processing time and consumption of metal materials and make the structure of the parts more reasonable.
The weight of investment casting is mostly zero to tens of cattle (from a few grams to more than ten kilograms, generally no more than 25 kilograms). It is more troublesome to produce too heavy castings by investment casting.
The investment casting process is complex and difficult to control, and the materials used and consumed are expensive. Therefore, it is suitable for the production of small parts with complex shape, high precision requirements, or difficult to be processed, such as turbine engine blades.

Advantages of investment casting

The dimensional accuracy of investment casting is high, generally up to CT4-6 (CT10 – 13 for sand casting and CT5 – 7 for die casting). Of course, due to the complex process of investment casting, there are many factors affecting the dimensional accuracy of casting, such as shrinkage of mold, deformation of investment mold, linear change of mold shell during heating and cooling, shrinkage rate of alloy and deformation of casting during solidification, Therefore, although the dimensional accuracy of ordinary investment castings is high, its consistency still needs to be improved (the dimensional consistency of castings with medium and high temperature wax is much improved).
When pressing the investment mold, the pressure mold with high cavity surface finish is adopted, so the surface finish of the investment mold is also relatively high. In addition, the mold shell is made by coating the refractory coating prepared by high temperature resistant special binder and refractory materials on the investment mold, and the surface finish of the mold cavity in direct contact with the molten metal is high. Therefore, the surface finish of investment castings is higher than that of ordinary castings, generally up to RA. 1.6 – 3.2 μm.
The biggest advantage of investment casting is that because the investment casting has high dimensional accuracy and surface finish, it can reduce the machining work, just leave a little machining allowance on the parts with high requirements, and even some castings only leave grinding and polishing allowance, which can be used without machining. It can be seen that the investment casting method can greatly save machine tool equipment and processing hours, and greatly save metal raw materials.
Another advantage of investment casting method is that it can cast complex castings of various alloys, especially superalloy castings. For example, the streamlined outline and cooling inner cavity of the blade of jet engine can hardly be formed by machining process. The investment casting process can not only achieve mass production and ensure the consistency of castings, but also avoid the stress concentration of residual knife marks after machining.
Compared with other casting methods, the main advantages of investment casting are as follows:

• 1) The casting has high dimensional accuracy and low surface roughness. It can pour castings with complex shapes. The general accuracy can reach grade 5 – 7 and the roughness can reach grade 25-6.3 μm；
• 2) Thin wall castings and castings with small weight can be cast. The minimum wall thickness of investment castings can reach 0.5mm and the weight can be as small as a few grams;
• It can cast fine patterns, characters, castings with fine grooves and curved pores;
• 3) The shape and inner cavity of investment casting are almost unlimited. It can produce parts with complex shapes that are difficult to be manufactured by sand casting, forging, cutting and other methods, and can make some assemblies and weldments directly cast into integral parts after slight structural improvement, so as to reduce the weight of parts and reduce the production cost;
• 4) There are almost no restrictions on the types of cast alloys, which are commonly used to cast alloy steel parts, carbon steel parts and heat-resistant alloy castings;
• 5) There is no limit on production batch, and mass production can be carried out from single piece to batch.
• The disadvantage of this casting method is that the process is complex and the production cycle is long. It is not suitable for producing castings with large outline dimensions.

Manufacturing of investment casting

The first process of investment casting production is to manufacture the investment mold. The investment mold is used to form the model of medium cavity of fire-resistant shell. Therefore, in order to obtain castings with high dimensional accuracy and surface finish, the investment mold itself should have high dimensional accuracy and surface finish. In addition, the performance of the investment mold itself should make the subsequent shell making process as simple as possible. In order to obtain the investment mold with the above high quality requirements, in addition to good molding (mold for pressing investment mold), it is also necessary to select appropriate molding materials (mold materials for short) and reasonable molding process.

(1) Classification of mold materials
With the development of investment casting technology, there are more and more kinds of mold materials with different composition. Usually according to the melting point of the mold material, it is divided into high temperature, medium temperature and low temperature mold material.

• The melting point of low-temperature molds is lower than 60 °C. At present, 50% of paraffin stearic acid molds widely used in China belong to this category;
• The melting point of high-temperature mold material is higher than 120 °C. the mold material composed of 50% rosin, 20% wax and 30% polystyrene is a typical high-temperature mold material.

The melting point of medium temperature molds is between the above two types of molds. The currently used medium temperature molds can be basically divided into rosin based and wax based molds.
(2) Basic requirements for die properties

• Thermophysical properties: appropriate melting temperature and solidification range, small thermal expansion and contraction, high heat resistance (softening point) and no precipitate in liquid state and no phase transformation in solid state;
• Mechanical properties: mainly strength, hardness, plasticity, flexibility, etc;
• Process performance: mainly including viscosity (or fluidity), ash content, coating, etc.

Mould material

The performance of molding materials should not only ensure that the investment mold with accurate size, high surface finish, good strength and light weight can be easily made, but also create conditions for the manufacturing of mold shell and obtaining good castings. Mold materials are generally prepared with wax, natural resin and plastic (synthetic resin). The mold materials mainly prepared with wax are called wax based mold materials, and their melting point is low, 60 ~ 70 ℃; The mold material mainly prepared with natural resin is called resin based mold material, with a slightly higher melting point of about 70 ~ 120 ℃.
Preparation and recovery of mold materials

Preparation of mold materials

The purpose of preparing mold material is to mix various raw materials constituting mold material into a uniform whole, and make the state of mold material meet the requirements of pressing investment mold.
During preparation, various raw materials are melted and mixed into one by heating, and then the mold material is stirred violently under cooling to make the mold material into paste state for pressing the investment mold. Sometimes, the mold material is melted into liquid and the investment mold is poured directly.

Recovery of mold materials

When using resin based mold materials, due to the high quality requirements of investment molds, most of them use new materials to prepare mold materials to press the investment molds of castings. The mold material recovered after demoulding is used to make the investment mold of the pouring and riser system after remelting and filtration.
When wax based molds are used, the molds obtained after demoulding can be recycled and used to make new investment molds. However, during recycling, the performance of the mold material will deteriorate, the brittleness will increase, the ash content will increase, the fluidity will decrease, the shrinkage will increase, and the color will change from white to brown. These are mainly related to the deterioration of stearic acid in the mold material. Therefore, in order to restore the original performance of the old mold as much as possible, the soap salt must be removed from the old mold. The common methods include hydrochloric acid (sulfuric acid) treatment, activated clay treatment and electrolytic recovery.

Manufacturing and assembly of investment mold

Manufacturing of investment mold

In production, the paste mold material is mostly pressed into the mold by pressure to manufacture the investment mold. Before pressing the investment mold, a thin layer of parting agent shall be coated on the pressing surface to take out the investment mold from the pressing mold. When pressing wax based mold material, the parting agent can be engine oil, turpentine, etc; When pressing resin based molds, the mixture of sesame oil and alcohol or silicone oil is often used as parting agent. The thinner the parting agent layer, the better, so that the investment mold can better copy the pressed surface and improve the surface finish of the investment mold. There are three methods of pressing investment mold: plunger pressure method, air pressure method and piston pressure method.

Molding process

According to the specified composition and proportion of mold material, melt various raw materials into liquid, mix and stir evenly, filter out impurities and pour into paste mold material, that is, the investment mold can be pressed. Pressing investment mold generally adopts the method of pressing molding. This method allows the use of liquid, semi-liquid, solid and semi-solid molds. Liquid and semi-liquid molding materials are pressed and formed under low pressure, which is called pressure injection molding; Semi solid or solid die materials are pressed and formed under high pressure, which is called extrusion forming. Whether it is injection molding or extrusion molding, the advantages and disadvantages of filling and solidification must be considered.
(1) Injection molding
The wax injection temperature of pressure injection molding is mostly below the melting point. At this time, the mold material is slurry or paste with the coexistence of liquid and solid phases. In the slurry mold material, the amount of liquid phase significantly exceeds the amount of solid phase, so it still retains the fluidity of liquid. In this state, the surface of the investment mold has low roughness, and it is not easy to have surface defects caused by turbulence and splash. The temperature of paste mold material is lower than that of slurry mold material, which has lost fluidity. Although there are few surface defects, it has high surface roughness.
When the mold material is pressed and molded, the lowest mold material temperature and molding working temperature shall be used as far as possible under the condition of ensuring good filling. The selection of pressure is not the greater the better. Although the pressure is large and the shrinkage of the investment mold is small, the excessive pressure and injection speed will make the investment mold surface unsmooth and produce “bubbles” (bubble expansion under the investment mold skin). At the same time, it will make the mold splash and cold shut defects. In the process of molding, in order to avoid the adhesion of molding materials and improve the surface finish of investment mold, parting agent should be used, especially for rosin based molding materials.
(2) Extrusion molding
Extrusion molding extrudes the mold material in the low-temperature plastic state into the mold cavity and forms under high pressure to reduce and prevent the shrinkage of the investment mold. During extrusion molding, the die material is semi-solid or solid. The die material is relatively hard under normal conditions, but can flow under high pressure, which is characterized by high viscosity. Therefore, the pressure during extrusion depends on the viscosity of the die and the flow resistance in the injection hole and cavity. The greater the viscosity of the die, the smaller the injection aperture, the larger the cavity size, the smaller the cross-sectional area and the longer the die stroke, the greater the resistance of the die flow, so the higher the extrusion pressure is required. By using semi-solid die extrusion, the solidification time of investment mold is shortened, so the productivity is increased. It is especially suitable for producing castings with thick and large section.

Manufacturing of shell

The mold of investment casting can be divided into solid mold and multi-layer mold shell.
After dipping the module with refractory coating, sprinkle the material refractory, and then dry and harden it for many times to make the refractory coating layer reach the required thickness. In this way, a multi-layer shell is formed on the module. Usually, it is parked for a period of time to fully harden it, and then the module is melted to obtain a multi-layer shell.
Some multi-layer shells need to be packed and filled with sand; Others do not need to be poured directly after roasting.
When the mold is lost, the shell will be under the pressure of the increasing volume of molten mold material; During roasting and pouring, each part of the mold shell will produce mutually restrained and uneven expansion and contraction. Therefore, the metal may also have high-temperature chemical reaction with the mold shell material. Therefore, there are certain performance requirements for the shell, such as small expansion rate and shrinkage rate; High mechanical strength, thermal shock resistance, fire resistance and chemical stability at high temperature; The mold shell shall also have certain air permeability, so that the gas in the mold shell can escape smoothly during pouring. These are related to the refractory, binder and process used in the manufacture of mold shell.
Material Science
There are two types of materials used for manufacturing mold shell. One is used to directly form mold shell, such as refractory, binder, etc; The other is to obtain high-quality shell, simplify operation and improve process materials, such as flux, hardener, surfactant, etc.
Refractory
The refractories used in investment casting are mainly quartz and corundum, as well as aluminum silicate refractories, such as refractory clay, bauxite, pyrogem, etc. Sometimes zircon and magnesia (MgO) are also used.
Binder
The most common binder used in investment casting is silicic acid colloidal solution (silicic acid sol), such as ethyl silicate hydrolysate, water glass and silica sol. They are mainly composed of silicic acid (H2SiO3) and solvent, and sometimes stabilizers, such as NaOH in silica sol.
Ethyl silicate hydrolysate is the earliest and most common binder used in mold casting of silicic acid sol obtained by hydrolysis of ethyl silicate; The water glass shell mold is easy to deform and crack, and the dimensional accuracy and surface finish of the castings poured with it are poor. However, in China, water glass is still widely used in the production of carbon steel castings with low precision and non-ferrous alloy castings with low melting point; The silica sol has good stability and can be stored for a long time. No special hardener is required for shell making, but the wetting of silica sol to the investment mold is slightly poor. The shell hardening process is a drying process and takes a long time.
Making process of shell
The main processes and processes in the shell making process are:
1) Degreasing and degreasing of modules
In order to improve the ability of coating to wet the module surface, the oil stain on the module surface needs to be removed when wax based mold material is used to make investment mold.
2) Paint and sand the module
Before coating, the coating shall be stirred evenly to reduce the precipitation of refractory materials in the coating barrel as much as possible, and the viscosity or specific gravity of the coating shall be adjusted to make the coating fill and wet the investment mold well. When hanging the coating, soak the module in the coating and shake it up and down from left to right, so that the coating can wet the investment mold well and cover the surface of the module evenly. After coating, sand can be sprinkled.
3) Shell drying and hardening
After each coating is coated with a layer of shell, it must be dried and hardened, so that the binder in the coating will be changed from sol to gel and gel, and the refractories will be linked together.
4) Loss of fusion mold in self formed shell
After the mold shell is completely hardened, the module needs to be melted from the mold shell. Because the module is usually made of wax based mold material, this process is also called dewaxing. According to different heating methods, there are many dewaxing methods, most of which are hot water method and CO pressure steam method.
5) Roasted shell
If molding (sand filling) pouring is required, before roasting, the demoulded mold shell shall be buried in the sand in the box, and then loaded into the furnace for roasting. If the mold shell has high high temperature strength and does not need molding and pouring, the demoulded mold shell can be directly sent to the furnace for roasting. During roasting, gradually increase the furnace temperature, heat the shell to 800-1000 ℃ and keep it warm for a period of time before pouring.

Pouring of investment mold

Common pouring methods in investment casting are:
1) There are several methods of thermal gravity pouring.
This is the most widely used pouring form, that is, after the shell is taken out of the roasting furnace, it is poured at high temperature. At this time, the metal cools slowly in the mold shell and can fill the mold under the condition of high fluidity, so the casting can copy the shape of the mold cavity well and improve the accuracy of the casting. However, the slow cooling of the casting in the hot mold will make the grain coarser, which reduces the mechanical properties of the casting. When pouring carbon steel castings, the casting surface with slow cooling is also easy to oxidize and decarburize, which reduces the surface hardness, finish and dimensional accuracy of castings.
2) Vacuum suction pouring
The mold shell is placed in the vacuum pouring box, and the gas in the mold cavity is absorbed through the small 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, and prevent the defects of pores and insufficient pouring. The method has been applied abroad.
3) Crystallization under pressure
Place the mold shell in the pressure tank for pouring. After that, immediately close the pressure tank and inject high-pressure air or inert gas into the tank to solidify the casting under pressure, so as to increase the density of the casting. The maximum pressure abroad has reached 150atm.
4) Directional crystallization (directional solidification)
Some investment castings, such as turbine blades and magnetic steel, can improve their working performance if their crystal structure is columnar crystals arranged in a certain direction. Therefore, the directional crystallization technology of investment casting is developing rapidly.

Assembly of investment mold

The assembly of investment mold is to combine the investment mold forming the casting and the investment mold forming the gating and riser system. There are two main methods:

• 1. Welding method use a sheet-shaped soldering iron to melt the connecting parts of the investment mold and weld the investment mold together. This method is more common.
• 2. Mechanical assembly method in mass production of small investment castings, the combination module of mechanical assembly method has been widely adopted abroad. Using this module can greatly improve the module combination and efficiency, and improve the working conditions.

Cleaning of castings

The main contents of investment casting cleaning are:

• (1) Remove the mold shell from the casting;
• (2) Remove the casting from the pouring system;
• (3) Remove the mold shell refractory adhered to the casting;
• (4) Cleaning of castings after heat treatment, such as removing oxide scale, trimming and cutting gate residue, etc.

The task of investment casting process design is:

• (1) Analyze the manufacturability of casting structure;
• (2) Select reasonable process scheme, determine relevant casting process parameters, and draw casting drawing on the above basis;
• (3) Design the pouring system and determine the module structure.

It should be noted that when considering the above three aspects, the main basis is still the basic principles of the general casting process, especially when determining the process scheme and process parameters (such as casting fillet, machining allowance, process reinforcement, etc.), except that the specific data are slightly different due to the process characteristics of investment casting, the design principle is exactly the same as that of sand casting.
Task and importance of investment casting process design
Investment casting is a casting process with little or no cutting, which was also called wax loss casting before. Because the castings produced by investment casting process are higher in dimensional accuracy and surface quality than those cast by other casting methods, in addition, investment casting method can complete some castings with high complexity and difficult to process, Therefore, it is deeply loved by enterprises.
As for the investment casting process, the investment casting process design is one of the most important links in the whole investment casting process. The technical difficulty coefficient of this link is large, which plays a very important role in the whole investment casting process, which is also the reason why this link is mostly composed of some professional and technical people.
Like other casting process design, the design tasks of investment casting process are: A. analyze the manufacturability of casting structure according to specific requirements; b. Plan reasonable process scheme, determine relevant casting process parameters, and draw casting drawing on this basis; c. Design the pouring system and determine the module structure. Except that the specific data are slightly different due to the process characteristics of investment casting, the design principle is exactly the same as that of sand casting.
Because the investment casting process design link is of great significance to the whole casting process, and this link has very high requirements for the quality of designers, enterprises should pay considerable attention to this link.

Defects and prevention methods of investment castings

• The defects of investment castings are divided into surface and internal defects and out of tolerance of size and roughness.
• Surface and internal defects refer to under casting, cold shut, shrinkage porosity, porosity, slag inclusion, hot crack, cold crack, etc.
• Out of tolerance of dimension and roughness mainly includes elongation and deformation of castings.
• The surface and internal defects are mainly related to the pouring temperature of alloy liquid, the baking temperature and preparation process of mold shell, the design of gating system and casting structure.
• The main reasons for the out of tolerance of casting size and roughness are the design and use wear of die, the casting structure, the baking and strength of mold shell, the cleaning of casting and so on.

For example, when the investment casting is under cast, the reason may be that the low pouring temperature and mold shell temperature reduce the fluidity of liquid metal, the casting wall is too thin, the design of gating system is unreasonable, the baking of mold shell is insufficient or poor air permeability, the pouring speed is too slow, and the pouring time is insufficient. At this time, the problems should be solved according to the specific structure of the casting and relevant processes involved, Eliminate defects.

Source: China Butt Weld Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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