Die casting is frequently referred to as the fastest route between raw material and finished product.
Because of the differences in the melting temperatures of various die casting alloys, two methods of inserting the molten metal into the die cavities are used. These are referred to as hot chamber and cold chamber machines.
Hot chamber
Hot chamber or plunger machines are used mainly for zinc alloys. With modern technology, this process is increasingly being used for magnesium. The hot chamber process is a preferred die casting method due to its high rate of productivity. However, it cannot be used for some high melting point alloys or for those alloys which attack the steel working parts of the machine.
Operating sequence for the hot chamber die casting process:
1. Die is closed and gooseneck cylinder is filled with molten metal.
2. Plunger pushes molten metal through gooseneck passage and nozzle and into the die cavity. Metal is held under pressure until it solidifies.
3. Die opens and cores, if any, retract. Casting stays in ejector die. Plunger returns, pulling molten metal back through nozzle and gooseneck.
4. Ejector pins push casting out of ejector die. As plunger uncovers inlet hole, molten metal refills gooseneck cylinder
Cold chamber
Cold chamber machines minimize contact between the alloy to be cast and steel machine parts which allows the processing of high melting temperature alloys. Its primary use is for aluminum, brass, and larger magnesium die castings.
Operating sequence for the cold chamber die casting process:
1. Die is closed and molten metal is ladled into the cold chamber cylinder.
2. Plunger pushes molten metal into die cavity. the metal is held under high pressure until it solidifies.
3. Die opens and plunger follows to push the solidified slug from the cylinder. Cores, if any, retract.
4. Ejector pins push casting off ejector die and plunger returns to original position.
2008年2月21日星期四
2008年2月5日星期二
investment casting The process
A pattern of the component to be cast is produced by injection-moulding special waxes into a metal die. Pre-formed ceramic cores can be included in the wax pattern as it is moulded, which can create intricate hollows within the finished casting. As many as several hundred patterns may be assembled into a tree around a wax runner system (riser & sprue). Once a tree has been assembled, a pour cup is attached.
Fig 2. View of the ceramic impression in a turbocharger shell
Fig 2. View of the ceramic impression in a turbocharger shell
The completed tree is dipped, or invested, by hand or via robotic control into a ceramic slurry of ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying) or a hybrid of these controlled for pH and viscosity. A fine sand is applied to the invested tree in a fluidised bed, rain tower sander, or by hand. During the primary coat(s), the sand will typically be a zircon-based, as zirconium is less likely to react with the molten metal when poured into the shell. The stuccoed tree is then allowed to dry before re-dipping in slurry and applying secondary coats of mullite, Molochite, chamotte or fused silica refractory material. This process is repeated until the shell is thick enough to withstand the mechanical shock of receiving the molten metal. Dry times generally range from 24 to 48 hours, and total production from two days to one week.
Completed turbocharger
Completed turbocharger
After the shell (Fig 1.) has been constructed, the wax is removed in an autoclave or furnace (hence, the lost-wax process). Most shell failures occur at this point, as the fragile stuccoed shell is subjected to extremes of temperature and, in an autoclave, pressure. The shell is then fired at temperatures of around 1,100 degrees Celsius to induce chemical and physical changes in the set refractory materials forming a ceramic shell. This leaves a ceramic impression (Fig 2.) of the part to be cast. Most foundries remove the shells from the furnace while still hot and pour the molten metal into the ceramic shell. Various methods of pouring the molten metal include vacuum casting, anti-gravity casting, tilt casting, gravity pouring, pressure assisted pouring, centrifugal casting. After the molten metal cools, the shell is removed. This is generally done with waterjets, vibration, grit blasting or chemical dissolution. The cooled parts are removed from the tree by sawing them free or by dipping them in liquid nitrogen and breaking them off with a hammer and chisel. The parts are then finished. Many cast parts require grinding of the gate and runner bar attachments. Because molten metal cools slowly, it does not finish as hard as some forging and machining processes. Cast parts often are subsequently hardened by heat treatment, surface hardening, or HIP (Hot Isostatic Pressing) hardening (Known as HIPping). The parts are inspected by eye or in special cases by X-ray at the foundry or by specialty firms.
Fig 2. View of the ceramic impression in a turbocharger shell
Fig 2. View of the ceramic impression in a turbocharger shell
The completed tree is dipped, or invested, by hand or via robotic control into a ceramic slurry of ethyl silicate (alcohol-based and chemically set), colloidal silica (water-based, also known as silica sol, set by drying) or a hybrid of these controlled for pH and viscosity. A fine sand is applied to the invested tree in a fluidised bed, rain tower sander, or by hand. During the primary coat(s), the sand will typically be a zircon-based, as zirconium is less likely to react with the molten metal when poured into the shell. The stuccoed tree is then allowed to dry before re-dipping in slurry and applying secondary coats of mullite, Molochite, chamotte or fused silica refractory material. This process is repeated until the shell is thick enough to withstand the mechanical shock of receiving the molten metal. Dry times generally range from 24 to 48 hours, and total production from two days to one week.
Completed turbocharger
Completed turbocharger
After the shell (Fig 1.) has been constructed, the wax is removed in an autoclave or furnace (hence, the lost-wax process). Most shell failures occur at this point, as the fragile stuccoed shell is subjected to extremes of temperature and, in an autoclave, pressure. The shell is then fired at temperatures of around 1,100 degrees Celsius to induce chemical and physical changes in the set refractory materials forming a ceramic shell. This leaves a ceramic impression (Fig 2.) of the part to be cast. Most foundries remove the shells from the furnace while still hot and pour the molten metal into the ceramic shell. Various methods of pouring the molten metal include vacuum casting, anti-gravity casting, tilt casting, gravity pouring, pressure assisted pouring, centrifugal casting. After the molten metal cools, the shell is removed. This is generally done with waterjets, vibration, grit blasting or chemical dissolution. The cooled parts are removed from the tree by sawing them free or by dipping them in liquid nitrogen and breaking them off with a hammer and chisel. The parts are then finished. Many cast parts require grinding of the gate and runner bar attachments. Because molten metal cools slowly, it does not finish as hard as some forging and machining processes. Cast parts often are subsequently hardened by heat treatment, surface hardening, or HIP (Hot Isostatic Pressing) hardening (Known as HIPping). The parts are inspected by eye or in special cases by X-ray at the foundry or by specialty firms.
Investment Casting
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.
The 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 than die casting or sand casting, but can produce complicated shapes that require little rework or machining.
Investment casting offers high production rates, particularly for small or highly complex components, and extremely good surface finish (CT4-CT6 class accuracy and Ra1.6-6.3 surface roughness) with very little machining. The drawbacks include the specialized equipment, costly refractories and binders, many operations to make a mould, and occasional minute defects.
Investment casting is used in the aerospace and power generation industries to produce single-crystal turbine blades, which have more creep resistance than equiaxed castings. It is also widely used by Sturm, Ruger among other firearms manufacturers to fabricate firearm receivers, triggers, hammers, and other precision parts at low cost. Other industries that use standard investment-cast parts include military, medical, commercial and automotive
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