Air-Set (No Bake)
Dry sand is combined with a liquid resin and hardener. The resulting mixture is then compacted around a pattern and left to set. When the mold halves are pulled apart, the pattern removed, and the mold is closed, molten metal is poured into the mold cavity, creating a casting. The air-set (no bake) process can handle larger and more complex castings than with the green sand molding process.
Equipment:
- 1 – 300-500 Castec Articulated Continuous Sand Mixer
- 1 – 50-150 Castec Continuous Sand Mixer
- 1 – Castec Vibratory Compaction Table
- 1 – Continuous Tube Sand Heater
- Flask Sizes: 8″x12″ up to 70″x112″
What is the “Air Set” Method?
The air set method uses dry sand bonded with materials other than clay, using a fast curing adhesive. The latter may also be referred to as no bake mold casting. When these are used, they are collectively called “air set” sand castings to distinguish them from “green sand” castings.
With both methods, the sand mixture is packed around a pattern, forming a mold cavity. If necessary, a temporary plug is placed in the sand and touching the pattern in order to later form a channel into which the casting fluid can be poured. Air-set molds are often formed with the help of a casting flask having a top and bottom part, termed the cope and drag. The sand mixture is tamped down as it is added around the pattern, and the final mold assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mold. Then the pattern is removed along with the channel plug, leaving the mold cavity. The casting liquid (typically molten metal) is then poured into the mold cavity. After the metal has solidified and cooled, the casting is separated from the sand mold. There is typically no mold release agent, and the mold is generally destroyed in the removal process.[5]
The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture to the surface, and this makes them easy to identify. Castings made from fine green sand can shine as cast but are limited by the depth to width ratio of pockets in the pattern. Air-set molds can produce castings with smoother surfaces than coarse green sand but this method is primarily chosen when deep narrow pockets in the pattern are necessary, due to the expense of the plastic used in the process. Air-set castings can typically be easily identified by the burnt color on the surface. The castings are typically shot blasted to remove that burnt color. Surfaces can also be later ground and polished, for example when making a large bell. After molding, the casting is covered with a residue of oxides, silicates and other compounds. This residue can be removed by various means, such as grinding, or shot blasting.
During casting, some of the components of the sand mixture are lost in the thermal casting process. Green sand can be reused after adjusting its composition to replenish the lost moisture and additives. The pattern itself can be reused indefinitely to produce new sand molds. The sand molding process has been used for many centuries to produce castings manually. Since 1950, partially automated casting processes have been developed for production lines.
Advantages
- Very good dimensional tolerances because the rigidity of the mold withstands the pressures exerted by the molten metal during casting;
- Compatability with most pattern materials including wood, plastic, metal, fiberglass and Styrofoam, allowing for inexpensive tooling options for casting runs as low as one.
- Design flexibility for intricate casting shapes. The rigidity and tensile strength of an Air Set mold allows for thin sections to be produced. In addition, mold strength allows for minimal draft and radii requirements in the casting design.
- Fine surface finish
- Low to medium volume production capability
