LOW PRESSURE PERMANENT MOLD CASTING

Low pressure permanent mold castingLow pressure permanent mold casting, also known as low pressure die casting, is a metal casting process that is mostly used in the production of parts made out of aluminum and magnesium alloys. A slower filling velocity with respect to high pressure die casting allows greater flexibility in design and the use of sand cores to cast the more intricate parts of the piece. The low pressure used results in a much lower occurrence of turbulence, substantially reducing the possibility of air entrapment. Low pressure permanent mold casting is especially suited for the production of numerous aluminum automotive parts such as wheels and cylinder heads. It is also used in the chemical and offshore oil industries for the production of security boxes. The molds are usually made out of aluminum (H11-H13) and are built with cooling channels that assist in a faster cooling of the cast piece resulting in greater productivity.

The Low Pressure Permanent Mold Casting Process

This casting process uses an air-tight chamber with a bath of molten alloy, which is located vertically below the mold. Molten alloy is pushed upwards through a ceramic refractory pouring tube into the mold cavity when pressurized air is released into the chamber. Following the filling phase, air pressure is increased to force molten metal up the tube and into the mold. This compensates for any shrinkage in the solidification phase, reducing or outright eliminating the problem of shrinkage porosity. Once solidification has occurred, the chamber is de-pressurized to allow the molten alloy in the tube to be released back into the alloy bath, minimizing metal wastage.

The main advantages of low pressure die casting are:

  •  high production yields
  •  excellent cast quality
  •  low per unit costs
  •  excellent dimensional stability and shape repeatability

The Numerical Simulation of Low Pressure Die Casting

The design phases of the mold and part to be cast are critical to its successful production. A thermo-fluid study that is able to produce an accurate virtual model of the part can help companies optimize their part quality while obtaining a higher production yield.

The virtual model allows engineers to evaluate the fluid-dynamic behavior of the molten at various stages and look for:

  • the effects of turbulence which can facilitate the formation of oxides and inclusions
  • vortices that cause entrained air
  • sudden drops in temperature in the flow front causing the appearance of discontinuities

It is furthermore possible to analyze the dynamics of the solidification process associated with the transient heat retained by the mold with respect to:

  • shrinkage porosity
  • overheating of the molds which is a cause for dimensional instability, difficulties in ejecting the part and reducing the fatigue life of the mold
  • the efficiency of the cooling channel as it affects the directional solidification in order to optimize the timing used to activate and deactivate each individual circuit
  • the time taken by each step in the cycle in order to maximize productivity
  • dimensional quality
  • uniformity of the microstructure and mechanical properties

Using non-ferrous micro structural models that are specific to aluminum it is also possible to control:

  • phase transformations during solidification to predict the secondary dendrite spacing (SDAS)
  • the microstructure and local mechanical properties for each phase

The Advantage of Engineering Simulation in Low Pressure Permanent Mold Casting

The virtual simulation of the production process allows foundries to reduce waste and to optimize the molds to achieve more efficient filling, solidification and cooling phases. The advantages of using virtual simulation during the design phase are numerous and include:

  • waste reduction
  • a reduced number of physical prototypes needed
  • fewer design changes after the initial production
  • increased productivity
  • reduced time to market
  • reduction in raw materials used and in the labor and energy needed
  • the ability to have a complete and integrated virtual model for the design and production of a part where information derived from the production process (stress state, microstructure and mechanical properties of the piece) is passed to the FEM code to test for structural integrity