FACULITY OF ENGINEERING
DEPARTMENT OF MECHANICAL DESIGN AND PRODUCTION
SECOND YEAR MECHANICAL
Metal casting is the process of creating objects by filling a cavity with liquid metal and letting the metal cool. Metal casting is approximately 6000 years old, as the first cast objects appear to be copper arrowheads dated from approximately 4000 BC.
A. Sand casting
In spite of its innocuous name, sand casting is one of the most major industrial metal casting processes. Sand casting accounts for over 90% of all metal poured for casting.
The process of sand casting begins by fabricating a pattern for the final object. The pattern is often two piece due to the construction of the mold. The pattern can be made from virtually any substance including wood, foam, clay and plastic.
The mold which contains the sand is called a flask. It consists of two pieces, the top or cope and the bottom or drag. The centerline divides the cope from the drag. Holes called sprues feed molten metal into the flask and holes called risers allow air bubbles to escape.
To begin the casting process, the flask is broken into its two pieces. The pattern is inserted into the flask and the flask reassembled. Sand is packed tightly around the pattern. Then the flask is opened and the pattern removed. The sand imprint is checked carefully, and appropriate risers and sprues added (if not contained on the original pattern). Then the flask is closed and molten metal poured into the sprues until it emerges from the risers.
After the metal has cooled, the flask is broken open and the cast part removed. The sand is cleaned and recycled back for the next casting operation. The sprues and risers are removed and the part is cleaned.
Either "green" sand (actually black) or dry sand is used for casting. In green sand casting, the sand binder is kept moist with water. The part is cast as soon as possible after the pattern is removed. In dry sand casting, an organic binder is used -- and the mold is baked after the pattern is removed. Green sand casting is more economical, dry sand casting has better dimensional tolerances.
To create a hole in the middle of a casting, a baked sand part called a core is inserted in the mold after the pattern has been removed. The core will be removed destructively after the casting is complete -- leaving a hole in the middle of the part.
Polystyrene or Styrofoam can be used to create a one-time pattern for a specialty casting. In this process, the pattern is inserted into the flask and left there. When the molten metal is poured over the pattern, it vaporizes and the vapor escapes from the riser holes.
Any metal that can be melted. Common metals include cast iron, steel, brass, bronze, aluminum alloys, and magnesium alloys.
• Exceptionally economical
• Virtually no materials waste, as leftovers can be remelted and used again
• The castings can range from a few ounces to thousands of pounds
• The castings are isotropic
• Virtually unlimited freedom of shape
• Dimensional tolerances of 1/16" are typical -- this is large for many applications
• The castings have a work hardened (chilled) surface and cause significant tool wear in post casting machining
D. Permanent mold casting
(gravity die casting)
In gravity die casting, the metal is poured into the mold using only the force of gravity. The mold is typically a two piece mold clamped solidly together.
Molds for casting iron or steel are made of graphite or other refractories. Molds for casting aluminum, magnesium or copper alloy castings are made of iron or die-steel. Molds incorporate the necessary sprues and risers, and also typically have pins for ejecting the castings.
Materials best suited for gravity die castings are materials with relatively low melting points in order to be fluid in the mold. Typically iron, magnesium alloys, and aluminum alloys are cast using this process. Copper and zinc alloys tend to be insufficiently fluid. Steel and steel alloys require special molds.
Typical applications are:
• the mold can be chilled to speed cooling
• good surface finish
• good dimensional accuracy
• only one mold is required
• limited numbers of alloys can be used
• complex shapes cannot be cast
• mold production is time consuming and costly
• mold sizes are limited
E. Die casting
Die casting is one of the most common manufacturing processes. The basic idea is to force metal into a permanent mold using high pressure. The metal then cools (often assisted by water cooling of the die). The mold is then opened, and the casting ejected.
Molds for die casting are quite elaborate. They are usually constructed of alloy steel in two pieces (called the cover and the ejector). The die must withstand high temperature and pressure, so the die is typically made for chromium or tungsten steel alloys. In order to increase die life, and improve throughput, the die is usually cooled with water, air or nitrogen.
There are two major types of die casting machines. Hot chamber die casting machines are used for low melting point materials. A typical hot chamber machine is shown below. When the piston is raised, molten metal flows into the gooseneck. The molten metal is forced out of the gooseneck and into the die by the plunger. These machines are FAST typically operating at 150+ shots per minute.
The cold chamber die casting machine is shown below. Die castings of aluminum, magnesium, brass and bronze are all made on cold chamber machines. In a cold chamber, the metal is fed first from the holding furnace into a chamber. The plunger then takes that metal and forces it into the die. Cold chamber machines are typically a bit slower than hot chamber machines.
Materials best suited for die castings are zinc, aluminum, magnesium, copper, lead and tin. High pressure die casting is generally limited to non-ferrous metals because of the difficulty in making refractory molds capable of withstanding the high temperature and pressure.
- automotive parts
- office machines
- bathroom fixtures
- outboard motors
Hot chamber machines are:
- good for low temperature zinc alloys (approx. 400°C)
- faster than cold chamber machines
- cycle times must be short to minimize metal contamination
- metal starts in a heated cylinder
- a piston forces metal into the die
- the piston retracts, and draws metal in
Cold chamber machines:
- casts high melting point metals (>600°C)
- high pressures used
- metal is heated in a separate crucible
- metal is ladled into a cold chamber
- the metal is rapidly forced into the mold before it cools
• intricate parts possible
• short cycles
• inserts feasible
• cycles less than 1 minute
• minimum finishing operations
• thin sections, high tolerances, good surface finish
• metal die is costly
• porous parts
• not suited to large parts
• long setup times
• $5000-200,000 for machine
• metal melting point temperature must be lower than die