Die casting can be tricky to understand; additionally, the term has become something of a catch-all phrase for a production process that covers both low- and high-end technology. In this Original Heat Treat Today Technical Tuesday feature, come along with Martin Reeves, Owner of Fontec-Global, LLC, as he takes readers through the die casting process, giving helpful definitions of terms and easy-to-understand descriptions of processes.
The term "die cast" is one many will have heard referring to a particular part; sometimes as a recommendation, sometimes inferring a cheaper alternative, but always as a catch-all for a production process that encompasses a whole range of technologies from cheap and cheerful to the highest technology. So here is a quick overview of what can be termed as "die casting."
Die/permanent mold casting has become the dominant casting process for nonferrous alloys of aluminum, magnesium, and zinc, and with the growth of aluminum as a major structural component in automotive design, it is set to maintain that position in the future. This is an overview of the various ways and processes in which permanent metal molds are used today.
Typical metals used in die-casting are lower melting point alloys of aluminum, magnesium, and zinc. Lead and pewter are also cast in this way, and the original process was developed for creating movable type around 150 years ago for the fledgling printing industry. Copper and its alloys of brass and bronze can also be cast in this way, and even some cast irons have been cast in permanent molds with suitable mold coatings as the melting/casting temperatures approach that of the mold steels used.
Many of us will probably have used the process to make lead shot, fishing weights, or toy soldiers (showing my age now) in lead, which can be melted over a candle. Likewise, we see examples of die casting that can be produced in a vast range of sizes, and with high levels of surface finish and accuracy in our everyday life. It is this versatility to produce fine detail, accurate dimensions, and fine finishes that has driven the growth and development of the high pressure machines, which can produce these castings in high volumes (40 – more than 100/hr). This has ensured its continued popularity.
When we talk about die casting, it is normally the automated process using high pressure machines that force metal into a closed die; but there are several different processes that use permanent dies, and even within the high pressure die casting industry there are now different processes in use depending on the product, alloy, and industry. The majority of technological developments in the last 20 years have been in HPDC processes with bigger machines and computer controls.
Types of Die Casting Processes
The three principal (most popular) variations on the die casting process are:
- High Pressure Die Casting (HPDC)
- Low Pressure Die Casting (LPDC)
- Gravity or Permanent Mold Die Casting
High Pressure Die Casting (HPDC) – This is now the most common form of nonferrous casting production, in which the molten metal is forced into the cavity under very high pressures. This process utilizes two different means of introducing the metal. A hot chamber machine will have the holding furnace installed as an integral part of the die casting machine, and the metal pump is immersed in the metal and forces metal directly into the cavity. Bigger and more sophisticated castings and alloys use a cold chamber process where the metal is held in a separate furnace and transferred into a shot sleeve on the machine linked to the die. It is then forced directly into the cavity by a ram.
All of these processes have variations that make them uniquely suited to specific types of castings or alloys. The development of novel processes and controls has expanded the scope of HPDC in recent years to the extent that safety critical structural castings can be produced and heat treated.
A historical problem with conventional die castings was the turbulence of the injection process, which caused air and gases to become trapped in the solid metal. Attempts to heat treat these castings resulted in expansion of the gases, which created blistering and eruptions on the surface and a reduction in mechanical properties. New processes, new alloys, and improved controls on die casting machines have now eliminated these issues, and thin-walled structural parts for vehicle bodies and structures are now normal production.
The development of bigger die casting machines and the evolution of shot control has expanded the scope and size of parts that can be produced. Shot control now means that instead of the ram simply moving at a steady speed to push the metal into the cavity, the movement profile can be controlled to move the metal gently at first to avoid air entrapment, and then intensifying towards the end of the stroke to ensure complete filling and improvements in metal density. When this feature is combined with vacuum assistance in the die, then the casting quality is improved dramatically, and along with newly developed alloys, can be heat treated to a T6 level for optimum properties.
Cold Chamber Machines -- These allow for the metal to be dispensed into the shot sleeve (plunger cylinder) from an outside source. The transfer can be simply with a hand ladle, an automated ladle arm or robot, or via a pump or pressurized holding furnace to achieve an accurate and repeatable mass of metal.
Over the last 20 years or so, the size of die casting machines has increased dramatically to accommodate both heavier and larger castings. The capacity of a machine is defined as the locking force that can be applied to the die platens, and this has now reached over 5,000 tons of force. This allows for larger, thin-walled parts, such as complete door frames to be cast as a single piece.
Hot Chamber Machines -- These, on the other hand, have the holding furnace as an integral part of the die casting machine, and metal is transferred via a pump that is permanently immersed in the metal. A plunger is adjusted to push a defined amount of metal into the die for each cycle.
Because of the limitations of the furnace and immersed pump, this process is confined to smaller die casting machines and parts. Because of aluminum’s aggressive affinity for ferrous metals, this process is more often used for zinc and magnesium casting.
Vertical High Pressure Machines -- These use a similar technology to conventional horizontal machines, but the shot sleeve is vertical and is filled completely in a vertical orientation before injection, allowing a less turbulent flow of metal into the die.
Vertical machines can also use a vacuum and siphon tube system to fill the shot sleeve where the metal is sucked from the holding furnace by a vacuum in the die cavity. Like the low pressure process, it has the advantage of creating a smooth metal flow into the die while the vacuum, strong enough to suck the metal, has the added advantage of a partial degassing effect and gives better quality castings.
There are variations of High Pressure Die Casting that generally have well defined niche markets:
Semi-solid casting, also known as Thixotropic or Rheocasting, uses a similar machine; but instead of molten metal, a billet of semi-solid metal is inserted into the shot sleeve before injection into the die.
The process uses a property of aluminum alloys to be solidified in a way that retains approximately 40-60% liquid, is stable enough to be handled, but can be cut with a knife. The advantage of this process is in creating a casting with similar properties to a forging that can be heat treated and polished. The downside has been the overall cost compared to conventional castings.
Metal powder injection uses fine metal powders with a binder material instead of molten metal, and is poured into the shot sleeve in measured amounts and then introduced into the die and compacted by the plunger. The part is then sintered to remove the binder and consolidate the part. This is used for small and intricate parts where very specific alloy mixes can be created without having to melt an alloy. The process is also used for some magnesium parts overcoming the dangers associated with processing molten magnesium.
Low Pressure Die Casting (LP casting) -- This is used almost exclusively for aluminum road wheels as well as other high integrity and safety critical parts where heat treatment is also a prerequisite to achieve mechanical properties; in this process, the molten metal is forced into the die cavity by pressure in the holding furnace below the die, which raises the molten metal into the die where it is held until the narrow inlet area solidifies and the pressure is released. The smooth flow characteristics provide a high quality casting that can be heat treated.
Gravity (Permanent Mold) Die Casting is maybe the simplest of the die casting processes and can vary from a simple mold for lead shot or fishing where the molten metal is poured directly into either static or tilting molds and flows by gravity into the cavity.
Static molds are filled directly by hand or auto ladle into a split metal die. After solidification, the die is opened and the casting, with runner system, are removed.
Tilting dies are mounted on a tilting mechanism, and the metal is poured into a tundish attached to the die. After filling, the die is tilted to allow the metal to run from the tundish into the die cavity. This enables large castings to be made as the tundish can be filled fully before tilting. Castings of 100 to 300 lbs. are not uncommon, and the limit is normally the size of the die block.
An advantage of this process is the ability to use sand cores to create hollow internal shapes in castings. In gravity and low pressure die casting, the metal flow is gentle enough that sand cores can be used without breaking up and producing hard inclusions in the casting. In HPDC, the speed and turbulence of the metal entering the die is violent enough to break conventional sand cores. There are some foundries using salt cores, which are much harder, and must be dissolved from the casting. They can also create corrosion problems if not done correctly.
Centrifugal casting is used primarily in the casting of pipes and tubes in a variety of ferrous and nonferrous alloys. Molten metal is poured into a spinning tubular mold where centrifugal forces force it into an even coating on the wall of the mold, which is then cooled to create the pipe. The process is predominantly for ductile iron water distribution piping as well as engineering pipe and tube stock.
There are two basic processes used industrially--horizontally spun molds and vertically spun molds.
The horizontal process is used for longer pipes--up to 6 m (20 ft) long--with molten metal being introduced simply at one end and allowed to flow or progress along the mold length (DeLavaud process) to provide a more accurate wall thickness in the finished pipe.
The vertical process is used for smaller cylindrical parts, such as brake drums and other parts where a more dense, defect free metal structure is required. The process is confined to simpler geometries where no or simple cores can be used. Brake drums and discs would be typical parts in grey cast iron.
Continuous casting is a process used to continuously cast various alloys into billets, Direct Chill Casting (DCC), for further processing such as for extrusion, or to provide a continuous supply of form to be cut off and processed. The molten metal is poured into a tundish, which provides a continuous and controlled flow of metal into a water-cooled die, which moves up and down as the metal solidifies and keeps the solid metal moving downwards, making more room for new metal being poured.
Slush casting is not such a common process and is used where a hollow part is required with simple geometry and a hollow shape without the use of cores. Molten metal is poured into a mold and given enough time for a solid skin to form before any excess metal is poured out of the mold. The result is a hollow casting. Used traditionally in the art world for hollow sculptures, decorative pieces, etc., it can be used with most alloys but more often with pewter and precious metals.
Counter pressure casting (CPC) uses two separate pressure chambers, creating pressure in the holding furnace above the metal surface while equalizing this with a vacuum above the furnace and in the mold cavity. The result is to draw the molten metal into the mold cavity with the minimum of disturbance and turbulence, thus ensuring a sound casting. The metal is encouraged to solidify with water or air cooling at the in-gate area before pressures are released.
Terminology – some common terms used in the foundry industry:
- Die or Mold: the main component of die casting. The terms can vary depending on location and industry.
- Clamping force: the force exerted between the two halves of the die (the platens), which resist the hydrostatic force from the metal entering the cavity at high pressure.
- Platen: the main carrying plate on the die casting machine to which one half of the die is attached.
- Ram: the piston or plunger that is used to force the metal into the die.
- Tundish: a simple bowl arrangement that allows for metal to be accumulated before or during a casting process.
- Cores: used to create internal cavities in castings. They can be re-useable mechanical components of the die, or more often in LP and gravity they will be expendable pieces made from a resin-bonded sand.
- Holding furnace: used to hold the metal at the chosen casting temperature during casting operations. This is refilled periodically to maintain consistent production.
- Shot sleeve: connected to one half of the die and allows metal to be introduced and held prior to being injected into the die.
- Gooseneck: named for its similarity to a goose's neck, this is the pump unit that sits in the metal and supplies metal in a hot chamber machine.
Die Casting Defects – some terms that refer to common defects on die castings:
- Mis-Runs: where the metal has failed to completely fill the die cavity.
- Porosity: internal voids created by gas trapment or evolution during the solidifying process.
- Blow holes: produced as a result of moisture or contamination on the die or core surface when metal is introduced and caused by the rapid expansion of moisture.
- Inclusions: foreign matter that has become trapped in the casting, such as core sand, oxide particles, etc.
- Hot tears: when the casting strength is insufficient to withstand cooling contraction during solidification.
- Cold shuts: created when the metal entering the die is too cold and solidifies prematurely in some positions while metal is still entering the die.
- Die soldering: aluminum has an aggressive appetite for iron, and certain alloys will attack the steel dies even in the short time for die casting injection, creating adhesion of the casting.
- Heat checking: a form of thermal fatigue cracking where temperature cycling at the die surface creates fine cracks that encourage sticking and degrade the surface finish of the casting.
- Shrinkage: found in castings with heavy sections with poor connections to the main feed path of metal. The contraction to the solid state can create shrinkage depressions at the surface or internal voids.
- Laminations: where cold dies allow rapid solidification of a thin skin before final solidification of the main mass.
