Aluminum castings at low pressure

pressure versus gravity

An important factor in the classification of aluminum casting processes is that, whether the molten metal flows into the mold by gravity or is pushed into the mold cavity by pressure. Injection molding requires reusable molds, and it is commonly referred to as high pressure casting. The low pressure casting process is considered a variant of gravity casting into permanent molds [3].

Machine for low-pressure casting

Low pressure casting is a well-known casting process. In particular, most aluminum wheels is manufactured by injection molding low pressure. This method applies:

pressure about 2 have.

Fig. 1 – Scheme of low pressure aluminum casting technology [1]

Filling the mold is carried out by supplying compressed air to the holding furnace. The melt is fed upward along the tube profitable, which is submerged in the melt depth. This provides a smooth filling of the mold and the melt cleanliness. After that, as a mold filled, opposite the lucrative part starts solidifying metal. air pressure control in the holding furnace ensure complete filling of the mold. molding cycle is quite long due to the fact, that the casting during the entire solidification time is connected through a profitable pipe with a large volume of melt in the furnace [1].

Low pressure injection molding machine oriented vertically, which gives a horizontal parting line (Fig. 2). Molten metal is in a sealed vessel, which is then compressed by a gas (usually air), so that the metal is forced "up" into the mold cavity. It's profitable, as it leads to controlled form completion. This process is mainly used for the production of automobile wheels.. interesting to note, that every wheel is exposed to x-rays, as these are critical details [2].

Fig. 2 – [2]

The low pressure casting machine includes (Fig.. 3):

sealed holding furnace,
Equipment for feeding and compressed air control,
hydraulic crane for the installation and removal of molds.

Usually the machine for low-pressure casting works with one mold.

Fig. 3 - Machine for casting of aluminum alloys
under low pressure. At the bottom – holding oven [1]

Low pressure casting technology

Сasting cycle

In this process, permanent molds are installed over a sealed oven. The feed pipe exits the mold cavity into the molten metal below. By increasing the pressure in the furnace, the metal is pressed through the pipe into the cavity of the mold (Fig.. 4). When the metal hardens, relieve pressure, the mold is opened and the casting is removed, getting ready to repeat the cycle.

Main technological parameters

Process parameters include^

pressure application rate, which regulates the filling of the form
pressure, which is relatively unimportant once curing has begun
temperature gradients, which are necessary to ensure directional solidification.

Just like a regular permanent mold, these gradients are set by selecting and controlling the thickness of mold coatings and selective cooling of mold sections.

Most low pressure castings are made using only one metal entry point., and risers, usually necessary to prevent internal shrinkage voids, usually not used. therefore, the ratio of gross weight to net weight is low, cutting and finishing operations, associated with sprues, minimized.

Inclusion prevention

The low pressure casting cycle is caused by the solidification of the metal at the junction of the fill pipe and the mold cavity. While the countergravity flow of the metal into the mold cavity is at rest, the process poses a risk of contamination with impurities.

When opening the mold and removing the casting, vacuum sealing, existing at the liquid-solid interface, violated, and molten metal, left in the pipe, drops to the level of the furnace metal. The cyclic flow of metal in the vertical direction in the filling tube can lead to the accumulation of oxides on the internal surfaces of the filling tube., no matter, whether the tube is ceramic or coated metal. To keep it to a minimum, back pressure is maintained in the system, so that the molten metal is kept at an elevated level in the fill pipe at all times. In this case, it is possible to replenish the metal in the furnace at each cycle, not periodically., when the metal in the furnace is almost finished.

Metal filtration

Filtration of the metal at the point of entry into the mold is commonly used to prevent contamination of the casting by inclusions.. Filtration can consist of steel meshes or ceramic filters.

Fig. 4 – Low-pressure permanent mold [3]

Casting of automotive parts

Previously, most low pressure casting was limited to radially symmetrical castings.. So, almost all automobile wheels are cast with this method (Fig.. 5, 6). Currently, low-pressure casting is used to produce many non-symmetrical parts (Fig.. 7, 8, 9).

Fig. 5 - Aluminum company BBS Rim,
made by casting, low pressure

Fig. 6 – Alloy A356.0 alloy automotive wheels produced by low-pressure casting [3]

Fig. 7 - Aluminum cylinder head,
low pressure casting [1]

Fig. 8 - Aluminum V-shaped cylinder block,
cast by low pressure casting [1]

Fig. 9 – Variety of parts, including automotive pistons, metallurgically bonded diesel engine pistons,
compressor pistons, cylindrical and journal bearings, anodes, and cookware,
produced by the low-pressure casting process [3]

Low pressure casting and gravity casting

The mold usually has channels, through which is passed water or compressed air. This makes it possible to control the temperature of cooling conditions during solidification.

Directional solidification and pressurizing provide small volumes profitable part of the casting, a, so, less metal waste goes to remelting. In normal, gravity chill casting yield can be only 50 %, while for low-pressure casting it is about 90 %. This is shown in the drawings. 10 and 11: in injection molding, the profitable part is much smaller, than at chill casting.

Fig. 10 - aluminum wheels,
made by casting, low pressure.
A small feeder head is visible in the center [1].

Fig. 11 - aluminum wheels,
made by gravity casting.
Large feeder heads are visible on the hub and rim [1].

Source:

European Aluminium Association, 2002
TALAT 3201
Aluminum Alloy Castings Properties, Processes, and Applications /J. Gilbert Kaufman, Elwin L. Rooy – ASM International – 2004