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Precision Casting

Date:10/12/2013 1:53:49 PM

1 introduction
Precision Casting method is refers to the use of non-metallic Mold, product size accuracy is higher than ordinary sand Casting of the floorboard of the Casting process, including the dewaxing method (Lost idea for Casting or Investment Casting), Plaster molding (Plaster Mold Casting) and Ceramic Mold (Ceramic Mold Casting) three categories.
2 precision casting dewaxing method
2-1 features and advantages
(1) of the casting of maximum line length is 700 mm, easy to make the length of below 200 mm. Casting about 100 kilograms, maximum weight of generally used for 10 kg.
(2) the size tolerance of casting 20 mm + / - 0.13 mm, 100 mm + / - 0.30 mm, 200 mm + / - 0.43 mm, and small dimension precision is not easy to achieve within + / - 0.10 mm. Angle tolerance is + / - 0.5 ~ + / - 2.0 degrees, casting the minimum thickness of 0.5 ~ 1.5 mm. The casting surface roughness about Rmax 4 s ~ 12 s.
(3) a few without any limitation, casting material such as aluminum alloy, magnesium alloy, titanium alloy, copper alloy, various kinds of steel, cobalt and nickel based heat resistant alloy, hard materials.
(4) to make complex shape of the workpiece, good dimensional accuracy, less machining.
(5) save material waste and can be mass production.
Lost wax precision casting is widely used in jet engines, gas turbine, steam turbine, aircraft parts, internal combustion engines, vehicles, food machinery, printing machinery, paper machinery, compressor, valves, pumps, measuring instrument, sewing machines, weapons, office machinery, and other machine parts.
2-2 process
Lost wax casting method has two kinds: Solid model (Solid Mold) method and the Ceramic Shell Mold (Ceramic Shell Mold) method, the improved method of the latter to the former.
Solid model law considering wax type condensing shrinkage, mould heating expansion and the condensing shrinkage of molten metal, production and final approximation of mould casting size. Will melt wax infuse the pledge to within the mold of metal or silicon, remove the wax type, immersed in micro powder, refractory materials and binder bubble and into the slurry. After the net drop, and coarse cloth, refractory dry. Cast into the box, to practice a mixed binder of refractory grain filling, then dry. Then heating melt wax out, make molds. With high temperature heating mold, burning a small amount of residual wax and increase its strength, and then into the molten metal.
Ceramic shell mould method with solid type until the wax production mode method is the same, just in a moistening slurry and spray sand (or float sand), not BaoMo operations, but is repeatedly and moistening slurry spray sand, until get the shell thickness. Dry flow, heating, melting wax, high temperature heating again pouring, the method for the lost wax precision casting industry is widely used, because it has the following advantages:
(1) better dimensional stability
(2) the fire-proof material usage is less
(3) light weight, easy handling, processing, production of large castings
(4) process can be partial automation to save manpower, increase production rate
(5) the production cost is low
(6) with thin ceramic shell mould, pouring castings after cooling rate is higher and evenly, so its better mechanical properties.
(A) Master model (Master Pattern)
The appearance of master model approximation and the final products, should be the wax of condensing shrinkage when producing, mold heat expansion and condensing shrinkage of casting metal into consideration. Wax type cold shrink rate is 1.2%, for example, mould thermal expansion rate is 0.7%, casting metal cold shrink rate is 1.7%, the size of the master model to increase the rate of 2.2%, commonly used materials such as aluminum alloy, copper alloy and stainless steel.
(B) type wax forming die
Mass production machinery parts with wax type forming mode of general use aluminum alloy is made by the method of machining, which provided according to the customer's blueprint processing at this time, do not need to use the master model. Aluminium alloy die is composed of several, shooting wax by hand before combination, lock, wax pattern is completed to remove by hand, remove wax type, part of the forming die is using the master model into or spraying low melting point alloy and consummate, or the master model surpluses plastic extrusion sheet metal forming.
(C) wax pattern
Commonly used wax is supplied by setting temperature range, with a solid state YaRu in wax machine mould inside, and radiation pressure of 7 ~ 12 kg/cm2, often sometimes 50 kg/cm2, wax type, must pay attention to the temperature of the metal mould, wax YaRu YaRu pressure, temperature, before it's time to die, the temperature of the wax type for dimensional stability. So the wax pattern room must maintain constant temperature.
Sometimes use plastic instead of wax, production requirement for dimensional precision and not very small castings using polystyrene, with thick section of bulk urea resin is used (urea 85%, polyvinyl alcohol powder 10%, water 5%) but he shot pressure must be up to 140 kg/cm2
(D), heart-shaped
Shape simple hollow products, with elongated holes, narrow deep groove of the products or the shape of the hollow part of the complex and coating slurry cannot attach products are to be used, heart-shaped (Core). The types of heart shape metal heart shape, soluble wax heart and contented heart.
Metal heart shape method is to fix the metal heart metal mould, injection wax after solidification, the metal heart shape out from the wax type, this limited Yu Zhongkong part shape simple products can be used. Hollow part shape more complex or inner hole twists and turns, available soluble wax block type, fixed in the metal mould, injection wax after solidification, the wax type submerged, dissolution, heart-shaped. Soluble wax into heart shape, however, strength, and is easy to be broken, therefore, the hollow part of the elderly, can use alumina powder, spinel powder, barium titanate powder rubber etc with ethyl silicate, silica or resin binder blended into tao, heart-shaped, finished products pouring, solid, suspected to solvent type will be contented heart dissolution. Turbine blades (turbine blade) empty part of the use of this method is made.
(E) the tree
Small artifacts of wax type must be one by one in the form of manual welding stick on water flow guide above, assembled into wax tree, this is the lost wax casting  whole project, one of the most time consuming guide steps in the set is the tree trunk with flat type electric hot melt or substrate and wax type, glue again. Assembly time matters that should be paid attention to are as follows:
(1) multiple wax type assembled into a group of as much as possible
(2) assembly should be easy in molten wax type along the trunk or base flow
(3) part assembling finished products should be the first solidification, casting pool last set
(4) after pouring and solidification, the workpiece is easy to be cut off from the trunk or base
(F) on plasma and spray sand
Group tree first washed with acetone or alcohol type wax on the surface of the mold release agent, grease, etc., and then dipped into the binder and the powder of refractory bubble into slurry, drop all excess attached slurry, then pour sa refractory small grain, dry, this operation is focused on uniform paste wax mould surface, and do not make the bubble adhesion, residual. So repeatedly, until a predetermined thickness, the number of operations and the thickness of the ceramic shell is depending on the size and weight of the workpiece, its thickness needed to support the weight of the pouring molten metal and make pressure without breaking up.
Slurry more ingredient industry using silica gel, colloidal silica), use the zirconium powder filler, its weight ratio is about rising, slurry must add the right amount of wetting agent (the grout to adhere to the wax surface) and defoaming agent bubble slurry must be approved by 12 hours after mixing, use slurry management project has the viscosity, density and PH value. MianJiang use filler added quantity of the first layer of high viscosity, more back after the second slurry with low viscosity and good permeability, near hods must keep 22 ~ 25°C, in order to prevent slurry rapidly aging.
Before serous drops of bi did not work, and bulky refractory materials, and methods of general use effect is better and easy clean sand barrels, spray sand machine, use a few coating layer of the medial part of using fine 50 ~ 100 purpose, to the lateral enlargement is 20 ~ 40 mesh, dry on the conveyor, forced air supply control temperature and humidity in drying chamber.
(G) dewaxing and firing
Industrial used method is to coat of wax tree in a high-pressure container locking type group, going into the 150°C steam heating, the dispatch of wax melting, flow velocity, burning to make residue with a small amount of wax on the surface of the mold complete combustion, remove, and improve the strength of the mold, mold the firing temperature is between 700 ~ 1100°C, depending on the casting metal materials, time of 20 ~ 60 minutes, molds out of furnace, pouring immediately, in order to make the casting with good microstructure.
3 plaster molding
Plaster the chemical composition of calcium sulfate as the main body, according to the way of crystallization water into anhydrous gypsum (CaSO4), half water gypsum (CaSO4, 1/2 h2o, plaster) and dihydrate gypsum (CaSO4, 2 h2o) three, gypsum molding limit for the production of nonferrous alloy casting, have the following characteristics:
(1) suitable for the production of precision casting, the high cost of mold.
(2) the casting surface smooth, high dimensional precision, complex shape, fine surface has concave and convex (such as 12 microns) can also be cast out.
(3) due to the cooling rate is small, can be cast out of thin wall castings, such as 0.6 ~ 1.5 mm.
(4) casting strain is small, uniform microstructure and mechanical properties.
(5) the manufacturing cost is high, 5 ~ 10 times of the sand mold casting, from dry to injection mold, take 2 ~ 3 days.
(6) the thermal conductivity of plaster mould, casting the shrinkage defects, the yield is low.
(7) the permeability of plaster mold is bad, compression or decompression during pouring.
Casting in plaster gypsum as binder, cooperate with various additives, to improve its refractoriness, reduce dry shrinkage stress and swelling stress of heat when burnt.
1 job engineering
(1) model and mold release agent
Model of the material can use metal, rubber, wood, etc, use wooden model must be coated to prevent to produce glue phenomenon, mold release agent has the stearic acid of dissolved in oil, petroleum jelly, silicon likang grease, etc.
(2) mix water and stir
Water use pure water or similar nature, half water gypsum powder mix water between 30 to 35%, with spiral blades in the container and a creamy paste, during the period of slurry has sufficient liquidity into a model of the forming frame, slightly to vibration or to 10 ~ 10-1 torr decompression with deaeration, cleave to avoid bubbles model surface, the half water gypsum to produce water and reaction, the slurry viscosity rise, loss of liquidity, setting and hardening, dihydrate gypsum formation, with crystal growth fever, cause condensation expansion, through dozens of hours to the end of the expansion.
(3) the dry
Dry plaster mould method is the most time consuming in the whole project, which is the most important steps, thermal shock will cause the mold is broken, and therefore should be set several period of drying temperature, slowly heating, heating, ultimately drying temperature is 200 ~ 230°C, mold reaction, to water and anhydrite. It is best to use the automatic temperature control of hot air drying device forced circulation exhaust type electric kiln, uniform temperature distribution in the furnace, so as to shorten the drying time, namely direct set final drying temperature, molding and rarely produce rupture.
2 using the method of lost wax casting gypsum mould
Dental, rings, jewelry, ornaments and small arts and crafts are applying this method to finish, gypsum used as binder, the main body of white silica refractory, etc.
The metal to produce the finished product of the original model, placed between two pieces of rubber, and the pressure on the hot pressing machine heating, make the rubber to produce permanent deformation, to form a model of empty cavity. The two pieces of rubber involution, lock, make note of injection closemouthed on in wax injection machine exports, with the power of 0.2 ~ 0.5 kg/cm2 force will melt wax filled with rubber mold.
Condensation after open the rubber mold, remove the wax type, using low power of the soldering iron partial melting wax type, welding for a port of wax dry, group tree operations, the group tree into the stainless steel ring, pour into the gypsum slurry, then the vibration and vacuum deaeration.
Of plaster mould hardening at room temperature after 12 hours, using electric furnace (120 ~ 120°C) or sodium pressure steam dewaxing, slowly lost wax mold after burn into heating to 650 ~ 800°C, which is attached on the surface of the gypsum wax burn completely. Residual wax, incomplete combustion can become carbon residue formed in the casting surface defects.
In the sintering temperature to maintain a period of time, make the mold inside and outside temperature uniformity, in cold to fit for pouring furnace temperature, and then remove the plaster mould in a vacuum casting machine or within the centrifugal casting machine casting. Jewelry such as rings, brooches shape is complex, slightly concave and convex texture of the finished product in centrifugal casting. Casting machine for high frequency heating equipment to melt in the crucible of precious metals, (400 ~ 1000 RPM) again by centrifugal force, the molten metal filling of plaster mould cooling after hammering stainless steel ring, remove the plaster mould, again with water gun, gypsum, flushes out of casting processing.
Such as Buddha vase or other larger handicraft, use vacuum casting system.
4 ceramic shell mold
Ceramic shell mould castings are mostly a small amount of production, is not easy to mechanization and automation, the cost is high, thus, its scope of application is quite limited, main to like:
(1) this is welded together can be a body cast
(2) the poor machinability of materials can be fine processing capacity
(3) the arts and crafts complex shape
Compare with lost wax casting, casting or thick meat products can be made large or on the number of too little lost wax casting method. Applicable products include:
(1) all kinds of metal mold, die casting, plastic forming, injection forming, extrusion forming glass products, rubber products, forging.
(2) the blade: large steam turbines, various impeller wheel
(3) parts: electronic wave guide tube, ballast for aircraft use, body, etc
Ceramic shell mould law with silica gel solution as binder, mixed refractory filler powder and gel to promote
Into the agent, made into slurry. Will be sufficient liquidity of slurry water the cover on the model surface, slurry condensation, after demoulding, with 700°C dehydration, drying, with 1000°C sintering in order to improve the strength, make ceramic shell mold. The thickness of the ceramic shell mold for 3 ~ 20 mm, pouring large castings, it must be on the back pad, to support materials to the weight of the bearing body of the molten metal.
Model material used in small amount of surface treatment materials and mass production with a metal or resin, filler with high purity, high melting point of refractory material in order to prevent and respond to molten metal, its material depends on the kind of casting metal, particle size distribution of refractory material is big particles mixed with small particle powders, moderate intensity can be obtained with surface, gel used for the purpose of promoting agent is to accelerate the curing of bond, the general use of alkaline substances such as ammonium carbonate, etc.
Adjust the weight percentage of filler and binder, controllable pulping of liquidity, strength and permeability of the ceramic shell mold. Filler usage of the bubble into the volume of slurry to knot agent volume of 1.5 ~ 2.5 times.
Ceramic shell mold of back brace mode can be used to dry mode, sand mould, carbon dioxide, mould, also can use permanent metal mould. Ceramic shell mould should be preheated before casting, casting iron and steel materials is 500°C, low melting point is 200°C, metal casting or casting, preferably in an inert gas to prevent the occurrence of oxidation and decarburization.

 
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