Casting processes - Low Pressure Die

Brief description

A metal die is mounted above a sealed furnace containing molten metal. A refractory-lined tube, called a riser tube or stalk, extends from the bottom of the die into the molten metal. When air is introduced into the furnace under low pressure (15 - 100 kPa, 2- 15 psi), the molten metal rises up the tube to enter the die cavity with low turbulence.

When the metal has solidified, the air pressure is released allowing the still-molten metal in the riser tube to fall back into the furnace. After a further cooling time the die is opened and the casting extracted. With correct die design, directional freezing of the casting is achieved so eliminating the need for risers, the casting being filled and fed from the bottom. Because there is usually only one ingate and no feeders, casting yield is exceptionally high, generally over 90%.

Details of the process

A mould or die, having a horizontal parting line, is mounted on a holding furnace and is connected to the molten metal by a feed tube or stalk. The furnace is pressurised by the introduction of air above the surface of the molten metal causing it to rise steadily in the stalk and quietly fill the mould. The air in the mould cavity is expelled through suitably positioned vents in the die and when the cavity is filled, solidification commences.

Directional solidification, commencing at the extremities and terminating at the sprue, is effected by correct die design and eliminates the need for conventional feeding systems. When the metal has solidified as far back into the sprue as is required, pressure is released in the furnace and the molten metal left in the stalk returns to the holding furnace.

A further short cooling period is allowed to ensure that all sections of the casting are solid, the mould is opened and the casting removed. The molten metal is contained in a plumbago crucible heated by resistance windings or by induction. The capacity of the furnace is usually sufficient to make around 10 castings before refilling is necessary.

The crucible can be topped up with molten metal as necessary via a filler port. The whole furnace is contained in a pressure vessel sealed with a gasket by a top plate. The riser tube is suspended from the top plate by a riser cap or nozzle. The riser tube is immersed into the molten metal nearly to the bottom of the crucible. The riser tube may be made of ceramic but is more commonly made of cast iron coated with a refractory wash to prevent attack by the molten aluminium. A cast iron stalk, properly coated with the correct refractory dressing at regular and frequent intervals (usually once per shift) has a life of around 6 months.

The lower fixed die-half, mounted on a base plate, is fitted to the furnace top-plate. The base-plate carries 4 corner tie-bars on which slides the moving platten carrying the upper die-half.

A hydraulic cylinder mounted on a fixed top platten opens and closes the die. The die itself may carry hydraulically operated core-pulls. The whole unit is designed in such a way that the top plate carrying the die and its opening mechanism can be swung away to permit easy access to the riser tube and furnace.

When the holding furnace is at temperature, a little above the melting point of the alloy being used, it is filled by way of the filler port, which is then sealed. When the metal temperature has stabilised to the required value and the die has been pre-heated to its operating temperature (250-400ºC) and closed, the inlet valve is opened and dry compressed air is allowed to fill the sealed furnace to a controlled pressure causing the aluminium to rise in the transfer tube and fill the die. With the furnace remaining under pressure, the casting solidifies quickly, the direction of the freezing following a downward path with the sprue section being the last to solidify.

When the metal in the nozzle has frozen, the pressure is released allowing the still-molten metal in the riser tube to fall back into the furnace. A further short time is allowed to ensure complete solidification of the casting, the die is then opened and the casting released into the upper die-half from which it is retrieved, usually mechanically. Once the sequence has been established, it can be controlled automatically using temperature and pressure controllers to supervise more than one diecasting machine.

Typical applications

• Aluminium automotive parts: wheels, cylinder heads, blocks, manifolds and housings.
  
  
• Critical aerospace castings
  
  
• Electric motor housings
  
  
• Domestic kitchen ware such as pressure cookers

Large castings up to 150kg (A1) can be made but can only be justified in special cases because of high die costs.

Courtesy of the Institute of Cast Metals Engineers.