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