what is investment casting?
Investment casting is have many nick name, such as lost wax casting(water glass casting); lost foam casting; stainless steel precision casting and Ceramic mold casting.
Usually applies several layers of refractory material on the surface of the wax mould. After it is hardened and dried, the wax mold is melted to form a shell, which is then calcined and then poured. till the Shell is strong enough, poured Ferrous Metal Liquid into the shell, break the shell and get casting parts, which is also called “casting precision casting” because of the high dimensional accuracy and good surface finish of the obtained casting parts.
The alloys that can be produced by investment casting are carbon steel, alloy steel, heat resistant alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy and ductile iron.
The shape of the investment casting is generally complicated, the minimum diameter of the castable hole on the casting can be up to 0.5 mm, and the minimum wall thickness of the casting is 0.3 mm. In the production, some parts that were originally composed of several parts can be directly cast by investment casting by changing the structure of the parts and designed as integral parts, so as to save processing time and metal material consumption, and make the part structure more. To be reasonable.
The weight of investment castings is mostly tens of cattle (ie tens of grams to several kilograms). Too heavy castings are more troublesome to produce by investment casting, but the current production of large investment castings has reached 800 cows.
The investment casting process is complicated, difficult to control, and expensive to use and consume. Therefore, it is suitable for producing small parts with complicated shapes, high precision requirements, or difficult to perform other processing, such as blades of turbine engines.
Investment Casting Process
The first step in investment casting production is to make a investment mold. The investment mold is used to form a cavity in the refractory shell. Therefore, in order to obtain a casting with high dimensional accuracy and high surface finish, the first mold itself should have a high size. Precision and surface finish. In addition, the performance of the investment mold itself should be as simple as possible for the subsequent process of forming the shell. In order to obtain the above-mentioned high-quality investment, in addition to a good press type (die for pressing the mold), it is necessary to select a suitable molding material (referred to as a molding material) and a reasonable molding process.
The performance of the molding material should not only ensure the convenient production of a coating with precise dimensions and high surface finish, good strength and light weight, but also create conditions for the manufacture of the shell and the obtaining of good castings. The molding material is generally formulated with a wax, a natural resin, and a plastic (synthetic resin). The molding materials mainly prepared with wax materials are called wax-based molding materials, and their melting points are relatively low, ranging from 60 to 70°c. The molding materials mainly prepared with natural resins are called resin-based molding materials, and the melting point is slightly higher, about 70 ～. 120°C.
Manufacturing of investment molds
In the production, most of the pressure is used to press the paste molding into the molding method to manufacture the welding mold. Prior to pressing the melt, a thin layer of release agent is applied to the profiled surface to remove the investment from the profile. When the wax-based molding material is pressed, the parting agent may be motor oil, turpentine oil or the like; when the resin-based molding material is pressed, a mixture of sesame oil and alcohol or silicone oil is commonly used as a parting agent. The thinner the release agent layer, the better the melt can replicate the surface of the profile and improve the surface finish of the investment. There are three methods for pressing the investment mold, the plunger pressurization method, the air pressure method, and the piston press method.
Assembly of the investment mold
The assembly of the investment mold is a combination of the investment mold forming the casting and the investment mold forming the pouring riser system. There are two main methods:
- Welding method A flaky soldering iron is used to melt the joint of the investment mold to weld the welding mold together. This method is more common.
- Mechanical assembly method In the mass production of small investment castings, the mechanical assembly method combination module has been widely used abroad. The use of such a module can greatly improve the module combination and efficiency, and the working conditions are also improved.
Manufacture of shells
Investment casting molds can be divided into two types: solid type and multi-layer type shell. Currently, multi-type shells are commonly used.
After dip coating the module with refractory paint, sprinkle the material refractory material, and then dry and harden it. This is repeated several times to make the refractory coating layer reach the required thickness, thus forming a multi-layer shell on the module. It is usually parked for a period of time to fully harden it, and then the module is melted to obtain a multi-layer shell.
Some of the multi-layer shells need to be packed with sand; some do not need to be cast directly after roasting.
When the melt-fuse is melted, the shell is subjected to the pressure of the molten mold which is increasing in volume; in the case of baking and injection, the various portions of the shell may have mutually contracted and uneven expansion shrinkage, and therefore, the metal may also A high temperature chemical reaction with the shell material. Therefore, there are certain performance requirements for the shell, such as small expansion ratio and shrinkage rate; high mechanical strength, thermal shock resistance, and chemical stability under fire and high temperature; the shell should also have a certain gas permeability. In order to allow the gas inside the shell to escape smoothly during pouring. These are all related to the refractory materials, binders and processes used in the manufacture of the shell.
Shell making process
The main processes and processes in the shell making process are:
1) Degreasing and degreasing of modules
In the case of a wax-based molding compound, in order to improve the ability of the coating to wet the surface of the module, the oil on the surface of the module needs to be removed.
2) Apply coating and sanding on the module
Before coating the coating, the coating should be evenly mixed, the precipitation of the refractory material in the coating barrel should be reduced as much as possible, and the viscosity or specific gravity of the coating should be adjusted so that the coating can well fill and wet the coating. The module is immersed in the paint and shakes left and right, so that the paint can wet the mold well and evenly cover the surface of the module. After the paint is applied, it can be used to make a noise.
3) Shell drying and hardening
After each layer of the shell is coated, it is dried and hardened, so that the binder in the coating is transformed from the sol to the gel and the gel, and the refractory is joined together.
4) Loss of investment in the self-shell
After the shell is completely hardened, the module needs to be melted from the shell. Since the module is usually made of a wax-based molding material, this process is also called dewaxing. Depending on the heating method, there are many, dewaxing methods, and the hot water method and the same pressure steam method are used more.
5) roasting shell
If it is necessary to mold (filling sand), before the roasting, the mold after the demoulding is buried in the sand in the box, and then calcined. If the shell has high temperature strength and does not need to be cast, the demoulded shell can be directly sent into the furnace for roasting. When roasting, gradually increase the furnace temperature, heat the shell to 8000-10000C, and keep it for a period of time before pouring.
Casting of investment castings
Common casting methods used in investment casting are:
1) There are several types of thermal gravity casting methods.
This is the most widely used form of casting, that is, after the shell is taken out of the baking furnace, it is poured at a high temperature. At this time, the metal is cooled slowly in the shell, and the mold can be filled in the case of high fluidity, so that the casting can well replicate the shape of the cavity and improve the precision of the casting. However, the slow cooling of the casting in the hot form causes the grains to be coarse, which reduces the mechanical properties of the casting. When casting carbon steel castings, the surface of the slower casting is also susceptible to oxidation and decarburization, thereby reducing the surface hardness, finish and dimensional accuracy of the casting.
2) Vacuum suction pouring
The shell is placed in a vacuum casting box, and the gas in the cavity is sucked through the tiny pores in the shell, so that the liquid metal can better fill the cavity, copy the shape of the cavity, improve the precision of the casting, and prevent the pores and pouring. Insufficient defects. The law has been applied abroad.
3) Crystallization under pressure
The shell is placed in a pressure tank for pouring. Immediately after the completion, the pressure tank is closed, 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 in foreign countries has reached 150atm.
4) Directional crystallization (directional solidification)
Some investment castings such as turbine blades, magnetic steel, etc., if their crystal structure is columnar crystals arranged in a certain direction, their working properties can be improved a lot, so investment casting directional crystallization technology is rapidly developing.
Lost Wax Investment Casting flowchart
Mold design – mold manufacturing – pressure wax (shoot wax wax mold) – repair wax – wax inspection – group tree (wax module tree) – shell (first paste, sand, re-slurry , the final mold shell air drying) – dewaxing (steam dewaxing) – mold shell roasting – chemical analysis – pouring (casting molten steel in the mold shell) – vibration shelling – casting and pouring rod cutting separation – – Grinding gate – initial inspection (blank inspection) – shot blasting – machining – polishing – finished product inspection – storage
The precision casting production process can be roughly divided into waxing, shelling, pouring, post-treatment and inspection.
Pressing wax includes waxing, waxing, and grouping.
The shell includes hanging sand, hanging pulp, and air drying.
Pouring includes roasting, chemical analysis (spectral spectroscopy), casting, shaking shell, gate cutting, and grinding gate.
Post-treatment includes sand blasting, shot blasting, correction, and pickling.
The inspection includes wax inspection, initial inspection, intermediate inspection and finished product inspection.
|Water Glass Investment Casting Process|
|Wax Shooting||Confirm the location and size of the product gate|
Confirm the group tree plan and the yield rate.
|Waxing parts cooling||1. Small waxy pieces of ice water;|
2. Plane requirements are strict product slabs
|Repairing and scraping the mold line|
|Inspection, inspection, appearance, size, flatness|
|Sealed iron piece|
|Counting wax skewers, handing over|
|Receiving wax string|
|Cleaning wax string|
|Secondary zircon sand with holes and deep grooves;|
|Two layers of paste|
|Three layers of paste||aluminum products three layers of sealing and sealing;|
|Four layers of copper paste||four layers of copper products and sealing;|
|Five layers of slurry||carbon steel, stainless steel five layers|
Add sealing, thick and thick six layers
|Dewaxing split mold product is immersed in a layer of final slurry|
|Placed (to be cast)|
|Check the shell model Check the quantity and material|
|Repair the shell mold, mix the glass with water and add the powder|
|Upper shell mold “to remove the raw edges around the cup mouth|
|Into the preheating furnace|
|Preheating shell mold|
Finishing the sand pit and cleaning the iron cover
|Deoxygenation twice deoxidation: pre-deoxidation, final deoxygenation|
|Pouring||“1. 304 without a cover, large pieces|
The riser can be added with a heat retaining agent;
2. A3 steel does not need to be covered, take
The mouth can be added with a heat retaining agent;
3. The rest of the lid is covered with a chaff seal.”
|Placed (to be shaken)|
|Concentrated shell model “Adjusted The vibration shell order according to production delivery schedule|
|Adjust the vibration of the vibration shell “View products, thin parts, small pieces|
|Vibrating shell “Some products should be thrown after the shell|
Clean the shell residue
|Cutting||“The die must be hanged and cleaned before it can be cut.”|
Send the casting back to the furnace. “
|Grinding gate reference gate sample|
|Drilling, drilling sand core||“The product with holes, try to drill first|
After the hole, coarsely throw and soak the acid sand.”
|Gross embryo rough surface|
|Acid acid, clear sand|
|“Reworked sour sand|
Or high-spraying sand”
|Renovation “All CNC products are not used|
|Final appearance inspection|
Lost wax casting youtube