Precision casting process analysis

  Precision casting method is the use of non-metallic mold, the dimensional accuracy of products is higher than the general term for ordinary sand mold casting of the casting, including dewaxing method or,
Plaster molding and ceramic molding three categories.
2 dewaxing precision casting
2-1 Features and Benefits
The maximum length limit (1) casting is 700mm, easy to make the length of 200mm or less. The maximum weight of about 100 kg of castings, general often
10 kg.
(2) the size of the casting tolerances 20mm ± 0.13mm, 100mm ± 0.30mm, 200mm ± 0.43mm, while the dimensional accuracy of the small pieces of hard-up
To within ± 0.10mm. Angle tolerance of ± 0.5 ~ ± 2.0 degrees, the minimum thickness of the casting 0.5 ~ 1.5mm. Casting surface roughness of about Rmax 4S ~ 12S.
(3) casting material almost without any restrictions, such as aluminum, magnesium alloy, titanium alloys, copper alloys, various steels, cobalt and nickel-based heat-resistant
Alloy, hard material.
(4) production complex shape of the workpiece, good dimensional accuracy and less cutting.
(5) saving waste materials, and mass production.
Dewaxing precision casting widely used in jet engines, gas turbines, steam turbines, aircraft parts, internal combustion engines, vehicles, food machinery, printing
Machinery, paper machinery, compressors, valves, pumps, measuring instruments, sewing machines, weapons, business machines, and other machine parts.
2-2 Process
Lost wax casting method has two categories: solid molding and ceramic shell mold method, the latter for the former modified method.
After molding the solid line in consideration of the amount of shrinkage wax condensate condensate shrinkage, thermal expansion of the mold and the amount of molten metal, and most production
Approximate dimensions of the final casting mold. After the melted wax infused into the pledge in this metal or made of silicone mold, remove wax, dip powder
Refractory slurry was mixed with a binder made of foam.
After dropping the net, spreading coarse refractory dried. Cast into the box, kneaded with an adhesive refractory grains to fill it, and then dry
dry. Followed by heating to melt the wax flow out, made the mold. The mold is heated to a high temperature, burning a small amount of wax residue and increase its strength, then
The injection of the molten metal.
Ceramic shell mold method until the wax-making related to the same solid molding, but a dip in the pulp and leaching sand (sand or floating), the package does not perform
Molding operation, but repeatedly carried out and stick pouring sand slurry until a predetermined thickness of the shell. Drying, heating, melting waxy flowing, high temperature
Heated pouring again, this method of lost wax casting is currently widely used by the industry, because it has the following advantages:
(1) better dimensional stability;
(2) use less refractory material;
(3) light weight, easy handling, treatment, production of large castings;
(4) can be partially automated process, in order to save manpower and increase production rate;
Lower (5) production costs;
(6) a thin ceramic shell mold, the cooling rate after casting casting high and uniform, so its mechanical properties better.
(A) the master model
Shape and size of the final product master model approximation, the production shall wax condensate shrink, condensing heat expansion of the mold and cast metal closed
Reduction into account. Such as wax-type shrink rate was 1.2%, the mold thermal expansion rate was 0.7%, the cast metal shrink rate is 1.7%, then the master model
The size of the rate of increase was 2.2%, commonly used materials are aluminum, copper alloy and stainless steel.
(B) forming a wax mold type
A large number of wax molding die with the production of mechanical parts are generally machined aluminum alloy is used in the method are made, then the system according to the customer mentioned
The blueprint for processing, without use the master model. Aluminum mold composed of several pieces composed by hand before shooting wax combination lock, and then shot after the completion of wax
Removal by hand, remove the wax, and made part of the Department of the forming die into the use of the master model or spraying low melting point alloy, or the master model squeeze trade deficit
Plastic and sheet metal forming.
(C) wax injection
Common system waxy solidification temperature range is greater, a semi-solid state charge into the mold mounted on the internal radio wax machine, charge injection pressure often
7 ~ 12kg / cm2, sometimes 50 kg / cm2, production of wax-type, should be noted that the mold temperature, the temperature of the wax into the charge, the charge pressure, the release time,
Wax temperature to obtain dimensional stability. So wax injection chamber shall be kept constant.
Sometimes the use of plastic replace wax, not very large production requirements of small size precision castings using a polystyrene, having a thickness of large sections
Member using a urea resin (urea granules 85%, 10% polyvinyl alcohol powder, 5% water) but the pressure at the exit charge up to 140 kg / cm2.
(D) Heart
Simple shape of the hollow article, elongated holes, article, or hollow part of the narrow groove shape complex and can not be attached slurry coated products are
Heart shall be used. Kind of heart-shaped metal heart-shaped, heart-shaped soluble wax and ceramic heart-shaped.
France is the first heart-shaped metal core metal fixed to the metal mold, solidified wax injection, the heart-shaped metal extracted from the wax type, which limited the
Empty part of simple shape products can be used. Hollow part of more complex shapes or bore tortuous, the available soluble wax made heart-shaped, fixed
A mold, after injection wax solidified, wax-soaked in water, the dissolution Heart.
However, poorly soluble wax made heart-shaped strength, easy to break, therefore, an elongated hollow part, could use aluminum oxide, spinels
Binder mixed powder, barium titanate powder and ethyl silicate, silica gel or resin made heart-shaped pottery, the finished casting, solid suspect, and then the solvent
The heart-shaped pottery dissolution. That is part of the turbine blades into the air made using this method.
(E) group tree
Wax small parts shall be artificial way piece by piece welded to stick to the water flow guide above, assembled into a wax tree, which is lost wax casting Dui most-wide
A time-consuming step guide, system with a flat head type electric iron melt or trunk group when the substrate and the wax tree, and then be bonded. Assembly should pay attention
The issues are as follows:
(1) Try to assemble as a group of a plurality of wax;
(2) should be assembled so easy to wax flows along the trunk or on the base melted;
(3) assembly of finished products should be part of the first solidified casting pool final solidification;
(4) casting after solidification, the work is easy to cut from the trunk or base;
(F) dip pulp and leaching sand.
Group tree first washed with acetone or alcohol wax surface release agent, grease, etc., then soak into the binder and the refractory powder into the bubble
Slurry, drip off excess slurry adhered to, then sprinkle cream refractory pellets, to be dried, the focus of the operation is that the paste evenly on wax table
Surface, and the bubbles do not adhere residue.
So repeatedly, until a predetermined thickness, and the number of operating system depends on the thickness of the ceramic shell size and weight of the workpiece may be, its thick
When the degree shall be capable of supporting the weight of the molten metal pouring and earn pressure without rupture.
The main components of the industry to use more silicic acid gel slurry, zirconium powder filler is used, the weight ratio of about 1:35, add the appropriate amount of slurry to be run
Aerosol (the slurry is easy to wax adhering surface) was stirred subject to twelve hours before use defoamers and foam slurry, slurry management project
Viscosity, specific gravity and PH.
The first batter layer of filler is used to add large amounts of high-viscosity who, after the second layer of low viscosity slurry back and permeability are preferred,
Nearby pulp barrels should be kept 22 ~ 25 ℃, in order to prevent rapid aging of the slurry.
Before completion of the slurry drops dry, coarse refractory grains spreading, spreading methods are generally easy to use and better results net sand pail, a few
Leaching using sand, inner part of the coating layer using fine particles of 50 to 100 mesh, to the outside of the enlargement is from 20 to 40 mesh, and dried in the conveyor belt, control
Temperature and humidity forced air drying indoors.
(G) dewaxing and firing
The method used in the industry is a wax-coated group tree placed in a pressure vessel locking, steaming heating 150 ℃, so rapidly and waxy
Melt, flow, it aims to enable the firing residue attached to the mold surface a small amount of wax complete combustion, removing, and improve the strength of the mold, the mold
The firing temperature is between 700 ~ 1100 ℃, depending on the material of the metal casting time of 20 to 60 minutes, remove the heating furnace of the mold, then
Be cast to make a good cast microstructure.
3 plaster molding
Chemical composition of gypsum to calcium sulfate as the main body, by way of crystal water and into anhydrite (CaSO4), hemihydrate
(CaSO4, 1 / 2H2O, calcined gypsum) and gypsum (CaSO4,2H2O) are three limit plaster molding for the production of non-ferrous alloy castings, has
The following characteristics:
(1) adapted to non-production of precision casting, high mold costs;
(2) casting a smooth surface, high dimensional precision, complex shape, fine uneven surface (e.g., of 12 m) can be cast;
(3) Due to the small cooling rate can be cast thin-walled castings, such as 0.6 ~ 1.5mm;
(4) casting strain is small, uniform microstructure and mechanical properties;
High (5) manufacturing costs for a sand mold casting 5 to 10-fold, from modeling to casting dried, it takes 2 to 3 days;
(6) Thermal conductivity of the plaster mold is poor, prone to shrinkage defects in castings low yield;
(7) the permeability of the gypsum mold poor pouring required pressurization or depressurization.
Plaster mold Department gypsum as a binder, with a variety of additives to improve the refractoriness, reduce shrinkage stress when drying and heating
Expansion of the firing stress.
1 Engineering jobs
(1) model with release agent
Model material can be used metal, rubber, wood, etc., using a wooden model must first be coated to prevent infiltration sticking, release agent
There stearic acid is dissolved in the kerosene, petroleum jelly, silicone grease, etc. Unicorn.
(2) mixing with water and stirred
Use pure water or water of a similar nature who, between the amount of water mixed with half water gypsum powder 30 to 35%, a spiral stirring blade evenly in the container, into
Creamy slurry in the slurry have fitted into the molding box model sufficient liquidity period, slightly subjected to vibration or subjected to 10 ~ 10-1 torr
The vacuum degassing to avoid the buildup of air bubbles in the surface of the model, then the hemihydrate gypsum and water to produce the reaction, the slurry viscosity increases, loss of mobility,
Condensation and hardening to form gypsum, crystal growth with fever, swelling caused by condensation, calendar dozens of hours before the end of the expansion.
(3) sulfate
Drying plaster molding the whole project is the most time consuming and most important step, emergency heat can cause rupture of the mold, and therefore should be set to the number of segments dry
Drying temperature was gradually raised, heating, final drying temperature was 200 ~ 230 ℃, the mold reaction, water formed to anhydrite.
Drying apparatus is best to use automatic temperature control hot air forced circulation Exhaust electrical drying furnace, furnace temperature evenly distributed to shorten dry
Drying time, which directly set the final drying temperature, mold rarely produce the case of rupture.
2 use plaster mold of the lost wax casting method
Dental, rings, jewelry, ornaments and small crafts are made using this method, as the gypsum binder, the silicon body is white
Stone refractories.
The finished metal production of the original model, placed between two sheets of rubber, put in a hot press heated and pressurized, so that the rubber permanent deformation, and
The formation of air cavity model compound. The two rubber on the joint, locking, so close to the inlet spout of wax injection machine, to 0.2 ~ 0.5 kg / cm2 of
Lili will melt the wax filled rubber mold.
After opening the condensed rubber mold, remove wax, use of low-power soldering iron localized melting wax, it will be welded to act as a flow path of the wax dry,
Conduct operations group tree, the tree will be set into the stainless steel ring, into the gypsum slurry, then vibration and vacuum degassing.
After hardening the cast at room temperature for more than 12 hours using an electric furnace (120 ~ 150 ℃) or a pressure sodium vapor dewaxing, mold dewaxed
Slowly warming up to 650 ~ 800 ℃ baking the plaster seepage attached to the surface of the wax is completely burnt. Waxy residue incomplete combustion, carbon residue become
In the casting surface defects.
In the firing temperature for a period of time, so that the mold inside and outside temperature uniformity in the furnace slowly adapted to the Shi pouring cold temperature, then remove the stone
Paste into a vacuum mold casting or centrifugal casting machine casting. Jewelry such as rings, brooches, and so the shape is very complex, with subtle texture uneven
The finished products are centrifugally cast into the legal system.
Casting machine has attached high frequency heating device to melt in the crucible of the precious metal, and then by the centrifugal force (400 ~ 1000rpm) molten metal injection
Full plaster mold, after condensation hammering stainless steel ring, remove the plaster mold, then water cannon to break up the plaster, remove the casting process.
Such as vases, statues or other large pieces of handicrafts using a vacuum casting mechanism.
4 ceramic shell mold method
Ceramic shell mold casting, mostly low-volume production, easy mechanization and automation, high cost, and therefore, its scope is rather limited, mainly
Like there:
(1) The person is welded integrally cast
(2) poor cutting material can be greatly reduced the amount of the fine processing
(3) arts and crafts complex shape by
Comparison with the lost wax casting, casting can be made large or thick pieces of meat or in the number of lost wax casting method too few persons. Applicable products include:
(1) all kinds of mold: die casting, plastic molding (injection molding, thermoforming) glass products, rubber products, forging
Made with;
(2) Blade: large steam turbine, a variety of moving impeller;
(3) parts: electronic waveguide, aircraft ballast valve body;
Ceramic shell mold to law silicate melt glue binding agent is mixed refractory filler powder and gel-based pro.
Into the agent made a slurry. There will be sufficient liquidity to cover the model surface water slurry, slurry to be coagulated, after stripping to 700 ℃ dehydration,
Dried, fired to 1000 ℃ to improve strength, made of ceramic shell mold. The thickness of the ceramic shell mold is 3 ~ 20mm, when casting large castings, the back lining shall
Pad to prop supporting material to support the weight of the molten metal.
When less material is used in the model surface-treated material, a metal or a resin mass production, the use of fillers of high purity, high melting
Refractory point to prevent the molten metal and react according to the material type of metal casting, depending on the particle size distribution is large refractory grain
Powder mixed with a small sub-particles obtained with the surface of the appropriate strength, gel-promoting agent is aimed at accelerating the curing binding agent,
Typically using a basic substance such as ammonium carbonate.
Adjust the percentage by weight fillers and binders, the paste can control liquidity, the strength of the ceramic shell mold and breathability. The use of fillers
So that the amount of foam into the slurry volume of 1.5 to 2.5 times the volume of the caking agent.
Ceramic shell mold of the back support bracket mode can be used dried mold, oil sand mold, carbon dioxide mode, you can also use permanent mold. Ceramic shell mold in
Be preheated prior to casting, casting steel material is 500 deg.] C, the low melting metal was 200 deg.] C, after casting or casting, preferably in an inert gas
Performed in order to prevent oxidation or decarbonization of.http://www.cnwxyszj.com

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