LESS COMMON THAN GRAVITY SAND CASTING AND LOW-PRESSURE PERMANENT MOLD CASTING, LOW-PRESSURE SAND MOLDING HOLDS A DISTINCT COMBINATION OF ADVANTAGES FOR LARGE ALUMINUM CASTINGS.
Originally published in the January/February 2016 issue of Metal Casting Design & Purchasing by Franco Chiesa and Jocelyn Baril
A majority of aluminum castings are produced via sand or permanent mold casting, but for large precision components, another viable option is low-pressure sand casting, which uses principles from both low-pressure permanent mold (LPPM) and gravity pour sand casting.
Low-pressure sand casting marries the use of bottom pouring for tranquil filling of the mold (which avoids metal oxidation) with the flexibility to make larger parts. The capable process can be ideal when producing large “top quality” aluminum castings. The process also can be considered when walls are too thin (such as 0.1 in. [2.5 mm]) to be obtained by gravity casting.
LPPM casting is a common process producing high-quality castings due to tranquil filling of the mold and the application of pressure to fill the mold efficiently and cleanly.
The two main characteristics of the LPPM process are:
1. The filling from the bottom of the mold is perfectly controlled compared to the turbulent flow associated with gravity casting. Also, the liquid metal is drawn from under the melt surface, preventing dross entrainment into the mold cavity.
2. Efficient feeding from the bottom injection pipe occurs through pressure applied to the melt during solidification, eliminating the need for risers. The resulting yield is high: typically 80-90% versus 50-60% for gravity permanent mold casting. However, not all casting geometries are amenable to the LPPM process.
Figure 1: Low pressure sand mold
In low-pressure sand casting, the sand mold rests on top of a pressurized enclosure as shown in Figures 1 and 2. The similarity between LPPM and low-pressure sand casting is in the controlled tranquil filling of the mold with a dross-free melt. Both processes also share the ability to produce thinner walls than gravity pouring would.
However, in contrast with the LPPM process, in low-pressure sand casting, no excess pressure is applied at the end of filling. Feeding from the bottom is interrupted early and long before the casting is fully solidified, so risers are necessary, just as in gravity casting.
Figure 2: The principle of the low-pressure sand casting process is illustrated
Low-pressure sand casting eliminates liquid metal handling, so the process is also advantageous over gravity sand casting when pouring large parts.
Size, quality and wall thickness will be the primary considerations when deciding between LPSM and gravity sand casting.
Compared to gravity sand casting, the low-pressure sand molding process simplifies the filling of the mold. A single operator can repeatedly fill the mold for a 600-lb. casting at the push of a button, compared to the manpower necessary to fill the mold by gravity through multiple sprues. The filling metal is also cleaner.
Solidification times are typically five times longer in sand casting than in permanent mold. This is why the low-pressure sand molding is no match to LPPM when castings are small enough to be produced on an LPPM press. Since the majority of aluminum castings are relatively small, the LPPM process is much more widely used than low-pressure sand casting. But when the dimensions are too large for LPPM, low-pressure sand casting is a viable option. Dozens of casting facilities are capable of this process and often cater to the aerospace industry.
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