Investment casting From Wikipedia

Investment casting, also called lost-wax casting, is one of the oldest known metal-forming techniques. From 5,000 years ago, when beeswax formed the pattern, to today’s high-technology waxes, refractory materials and specialist alloys, the castings allow the production of components with accuracy, repeatability, versatility and integrity in a variety of metals and high-performance alloys. Lost foam casting is a modern form of investment casting that eliminates certain steps in the process.


Inlet-outlet cover of a valve for a nuclear power station produced using investment castingThe process is generally used for small castings, but has produced complete aircraft door frames, steel castings of up to 300 kg and aluminium castings of up to 30 kg. It is generally more expensive per unit than die casting or sand casting but with lower equipment cost. It can produce complicated shapes that would be difficult or impossible with die casting, yet like that process require little surface finishing and only minor machining.

Cast iron

Cast iron production in addition to the proper choice of ingredients to be gifted to the organization, the heat treatment is further adjust and improve the matrix structure to enhance the performance of a cast iron important ways. Iron and steel heat treatment of the heat being buried in common, but also different. Cast iron heat treatment in general can not improve the original graphite in the shape and distribution. The grey cast iron, because flake graphite caused by the effects of stress concentration of iron play a leading role in the storm, the gray iron impose the enhanced effect of heat treatment are far less ductile iron and steel as significant. Guyou iron main heat treatment process for annealing, normalizing, and so on. For ductile iron, as a spherical graphite, the base of the fragmented role greatly reduced, through heat treatment will be based organizations to fully play its role, which can significantly improve the mechanical properties of the ball. Therefore, ductile iron like steel, its heat treatment process are annealed, normalizing, conditioning, multi-quenching, induction heating and hardening the surface, such as chemical heat treatment.

The cast iron heat treatment process:

1. Annealing elimination of stress due to uneven casting thickness, heating, cooling and phase transition process will produce stress and stress. Also in large parts of its processed within easy residual stress, all of which are necessary to eliminate stress. Annealing is usually to stress the heating temperature of 500 ~ 550 ?鏍╫lding time for 2 ~ 8 h, and then furnace cold (gray iron) or air-cooled (ductile iron). Using this technology can eliminate the stress of casting 90 to 95 percent, but the cast iron organization does not change. If the temperature exceeds 550 ?鏍痳 holding time is too long, it will cause graphite, to reduce the casting strength and hardness.

2. Elimination of the high-temperature casting white graphite annealing casting cooling, surface and thin cross sections, often produce white. White hard and brittle organizations, poor processing performance, easily peeling off. Annealing to be used (or normalizing) approach to the fight against white organizations. Annealing Process: heated to 550-950 ?鏍﹏sulation 2 ~ 5 h, then cold to furnace 500-550 ?鏍磑air-cooled. In high-temperature insulation, high outbound cementite and eutectic cementite divided into graphite and A, in the subsequent cooling process for the second cementite and eutectoid cementite also decomposition, in the process of graphite. As cementite of decomposition, resulting in hardness decline, thereby enhancing the cutting of.

3. Ductile iron is the iron fireballs normalizing the purpose is to be pearlite matrix organization, and refined grains, uniform organizations, to enhance the mechanical properties of casting. Sometimes the ball is normalizing iron surface hardening in the organization of preparation, normalizing at low temperature and high temperature normalizing normalizing. High-temperature normalizing temperature generally no more than 950 ~ 980 ?娆爈ow-temperature heat to a total of normalizing the general presentation of the temperature range 820 ~ 860 ?娈燗fter normalizing for the general need to deal with four to eliminate at normalizing the stress.

4. Ductile iron quenching and tempering in order to improve the mechanical properties of ductile iron, heated to Afc1 general casting more than 30 ~ 50 ?鏍ˋfc1 heating A representative of the end of a temperature), the insulation after quenching oil, be martensite . In order to appropriately lower after quenching the residual stress, the general should be quenched tempered, low-tempered organizations to increase tempering Markov for residual bainite with spherical graphite. This wear-resistance organizations, and demand for high wear resistance, high-strength parts. Tempering temperature in the temperature of 350-500 ?鏍ter tempering tempering troostite organizations to increase spherical graphite, applicable to the requirements of good wear resistance, a certain validity of the stability and flexibility in thick pieces. High-temperature tempering temperature of 500-60 D ?娆燼fter tempering tempering Soxhlet for organizations to increase spherical graphite, a good combination of strength and toughness of the comprehensive performance, wide application in production.

5. Ball and the use of multi-quenching ductile iron, such as the temperature can be quenched high-intensity, and both plastic and good toughness. Multi-quenching heating temperature of the main consideration so that the original choice of organizations of all A, non-residual F, but also to avoid A grain growth. The general Afc1 heating temperature above 30 ~ 50 ?娆爏uch as temperature temperature of 0 ~ 350 ?鏍磑 ensure access to a comprehensive mechanical properties of the bainite organizations. Mg-Al-iron, and other rare earth temperature quenched ?覡b = 1200 ~ 1400MPa, ??k = 3 ~ 3.6J/cm2, HRC = 47 ~ 51. It should be noted, such as temperature quenched with a tempering process.

6. Hardening the surface of certain items in order to improve the surface hardness, wear resistance and fatigue strength, the surface can be quenched. Ductile iron and gray iron castings to the surface can be quenched. The general high (in) frequency induction heating and hardening the surface, hardening the surface contact.

7. Chemical heat treatment requirements for wear-resistant surface or anti-oxidation, corrosion resistance of the castings, steel can be used similar to the chemical heat treatment process, such as soft chloride gas, chloride, boronizing, seepage, such as sulfur treatment.

About China Sand Casting

Sand casting is one of the most popular and simplest types of casting that has been used for centuries. Sand casting allows for smaller batches to be made compared to permanent mold casting and at a very reasonable cost. Not only does this method allow manufacturers to create products at a low cost, but there are other benefits to sand casting, such as very small size operations. From castings that fit in the palm of your hand to train beds (one casting can create the entire bed for one rail car). Sand casting also allows most metals to be cast depending on the type of sand used for the molds.

Sand casting requires a lead time of days for production at high output rates (1-20 pieces/hr-mold) and is unsurpassed for large-part production. Green (moist) sand has almost no part weight limit, whereas dry sand has a practical part mass limit of 2300-2700 kg. Minimum part weight ranges from 0.075-0.1 kg. The sand is bonded together using clays (as in green sand) or chemical binders, or polymerized oils (such as motor oil). Sand can be recycled many times in most operations and requires little additional input.

Aluminum Die Casting Technical Introduction

Die-casting is similar to Permanent mold castings except that the metal is injected into the mold under high pressure of 10-210Mpa (1,450-30,500) psi . This results in a more uniform part, generally good surface finish and good dimensional accuracy, as good as 0.2 % of casting dimension. For many parts, post-machining can be totally eliminated, or very light machining may be required to bring dimensions to size.

Die-casting can be done using a cold chamber or hot chamber process

1.In a cold chamber process, the molten metal is ladled into the cold chamber for each shot. There is less time exposure of the melt to the plunger walls or the plunger. This is particularly useful for metals such as Aluminum, and Copper (and its alloys) that alloy easily with Iron at the higher temperatures.

2.In a hot chamber process the pressure chamber is connected to the die cavity is immersed permanently in the molten metal. The inlet port of the pressurizing cylinder is uncovered as the plunger moves to the open (unpressurized) position. This allows a new charge of molten metal to fill the cavity and thus can fill the cavity faster than the cold chamber process. The hot chamber process is used for metals of low melting point and high fluidity such as tin, zinc, and lead that tend not to alloy easily with steel at their melt temperatures.

Die casting molds (called dies in the industry) tend to be expensive as they are made from hardened steel-also the cycle time for building these tend to be long. Also the stronger and harder metals such as iron and steel cannot be die-cast

Common Alloys in Die Casting

Aluminum, Zinc and Copper alloys are the materials predominantly used in die-casting. On the other hand, pure Aluminum is rarely cast due to high shrinkage, and susceptibility to hot cracking. It is alloyed with Silicon, which increases melt fluidity, reduces machinability. Copper is another alloying element, which increases hardness, reduces ductility, and reduces corrosion resistance.

Aluminum is cast at a temperature of 650 ºC (1200 ºF). It is alloyed with Silicon 9% and Copper about 3.5% to form the Aluminum Association 380 alloy (UNS A03800). Silicon increases the melt fluidity, reduces machinability, Copper increases hardness and reduces the ductility. By greatly reducing the amount of Copper (less than 0.6%) the chemical resistance is improved; thus, AA 360 (UNS A03600) is formulated for use in marine environments. A high silicon alloy is used in automotive engines for cylinder castings, AA 390 (UNS A03900) with 17% Silicon for high wear resistance. Common aluminum alloys for die casting are summarized as follows:

Material	Silicon	Copper	Tensile Strength MPa (ksi)	Properties
AA 380 (UNS A03800)	8.5 %	3.5 %	324 (47)	Fair easy to fill
AA 384 (UNS A03840)	11 %	4 %	331 (48)	Easy to fill
AA 386 (UNS A03860)	9.5 %	0.6 %	317 (46)	Good corrosion resistance
AA 390 (UNS A03900)	17 %	4.5 %	283 (41)	Good wear resistance

Zinc can be made to close tolerances and with thinner walls than Aluminum die casting, due to its high melt fluidity. Zinc is alloyed with Aluminum (4%), which adds strength and hardness. The casting is done at a fairly low temperature of 425 ºC (800 ºF) so the part does not have to cool much before it can be ejected from the die. This, in combination with the fact that Zinc can be run using a hot chamber process allows for a fast fill, fast cooling (and ejection) and a short cycle time. Zinc alloys are used in making precision parts such as sprockets, gears, and connector housings.

Copper alloys are used in plumbing, electrical and marine applications where corrosion and wear resistance is important.

Minimum wall thicknesses and minimum draft angles for die casting are :

Material	Min. Thickness mm (in)	Min. Draft Angle (º)
Aluminum alloys	0.9 mm (0.035 in)	0.5
Zinc alloys	0.6 mm (0.025 in)	0.25
Copper alloys (Brass)	1.25 mm (0.050 in)	0.7

Die-castings are typically limited from 20 kg (55 lb) max. for Magnesium, to 35 kg (77 lb) max. for Zinc. Large castings tend to have greater porosity problems, due to entrapped air, and the melt solidifying before it gets to the furthest extremities of the die-cast cavity. The porosity problem can be somewhat overcome by vacuum die casting

From a design point of view, it is best to design parts with uniform wall thicknesses and cores of simple shapes. Heavy sections cause cooling problems, trapped gases causing porosity. All corners should be radiused generously to avoid stress concentration. Draft allowance should be provided to all for releasing the parts-these are typically 0.25º to 0.75º per side depending on the material.

Casting and other technology

p>Casting often use a variety of metal materials, coke, wood, plastic, gas and liquid fuels, such as modeling materials. Have the necessary equipment used in various metal smelting furnace, a Mixer with the various Mixer, a shape-made batteries used in various molding machine, machine-made batteries, cleaning up the castings used for loading sand, Shot Blasting Machine And so on. Casting is also available for special machinery and equipment, and many transportation and material handling equipment.

Casting and other technology have different characteristics, mainly wide adaptability, needed materials and equipment, pollution of the environment. Casting will have dust and harmful gases and noise pollution to the environment, compared to other machinery manufacturing processes to become more serious, need to take measures to control.

Casting product development requirements of the trend is casting a better overall performance, higher accuracy and less headroom and clean the surface. In addition, the requirements of energy-saving society and restore the natural environment of the call also getting higher and higher. To meet these requirements, the new cast alloy will be developed, the new smelting technology and new equipment will be corresponding.

Casting the mechanized production continuously improve the degree of automation in the same time, to be more flexible production development, to expand the volume and variety of different production adaptability. Conserve energy and raw materials of new technologies will be giving priority to the development, have little or no pollution of the new equipment, new technology will be the first attention. Quality control technology in all stages of testing and nondestructive testing, stress determination, there will be new development.

Die casting materials

The main die casting alloys are: zinc, aluminium, magnesium, copper, lead, and tin. Specific dies casting alloys include: ZAMAK, zinc aluminum, AA 380, AA 384, AA 386, AA 390, and AZ91D magnesium. The following is a summary of the advantages of each alloy:

Zinc: the easiest alloy to cast; high ductility; high impact strength; easily plated; economical for small parts; promotes long die life.

Aluminum: lightweight; high dimensional stability for complex shapes and thin walls; good corrosion resistance; good mechanical properties; high thermal and electrical conductivity; retains strength at high temperatures.

Magnesium: the easiest alloy to machine; excellent strength-to-weight ratio; lightest alloy commonly die cast.

Copper: high hardness; high corrosion resistance; highest mechanical properties of alloys die cast; excellent wear resistance; excellent dimensional stability; strength approaching that of steel parts.

Lead and Tin: high density; extremely close dimensional accuracy; used for special forms of corrosion resistance.

Maximum weight limits for aluminium, brass, magnesium, and zinc castings are approximately 70 pounds (32 kg), 10 lb (5 kg), 44 lb (20 kg), and 75 lb (34 kg), respectively.

The material used defines the minimum section thickness and minimum draft required for a casting as outlined in the table below.

What is Investment Casting?

Investment casting / precision casting or the “lost wax” process has been in use since the construction of the first pyramid. The Egyptians and Chinese used the process in their early history to make jewellery and statues. The investment casting method was largely ignored as an industrial process for the fabrication of parts until the demand for rapidly finished parts during World War II created the need for “near net-shape” components that could readily be put into their final form. At this time new inorganic high temperature ceramic mold binders were developed to industrialize the process applications to include high strength and corrosion resistant materials such as low to high carbon alloy steel, tool steel, stainless steel, and nickel and cobalt base alloys. Aluminum and brass alloys are available also.

It is a process capable of producing intricate shapes weighing from a small fraction of an grams up to 20 Kgs. or more. Some examples would be: aircraft structural parts, components for the automotive industry, military weaponry, jet engines, machinery components, dental appliances, jewelry and many others.

In general, an injection molded wax pattern is used for each part produced which is then encased in multiple layers of ceramic material. The wax pattern is then removed from the ceramic shell mold. The mold is fired in an oven and then molten metal is poured into the cavities left by the evacuated wax pattern. Upon cooling, the resulting precision casting are cleaned and subjected to further processing such as heat treatment. At this point, many parts are in their final form and are ready for use while others may require a small amount of further processing such as machining before reaching their final form.

Why Investment Casting?

FLEXIBILITY IN DESIGNING

Investment casting produces near-net-shape configurations, offering engineers and designers freedom of design in a wide range of alloys. The process is capable of producing precise detail and dimensional accuracy in parts weighing many Kgs or just a few grams.

WIDE CHOICE OF ALLOYS

More than 125 ferrous and nonferrous metals are routinely cast.

TOOLING SET-UP IS ELIMINATED

By offering near-net-shape configuration, fixturing costs are substantially reduced or eliminated.

REDUCE PRODUCTION COSTS & ENHANCE PROFITABILITY

Costly machining operations are reduced and often eliminated. No capital equipment investment is needed to produce parts in-house.

CUT ASSEMBLY OPERATIONS

Several parts can be made as one casting, reducing handling, assembly and inspection costs.

REPRODUCE FINE DETAILS

Splines, bosses, serrations, lettering, holes and even some threads can be cast.