Metal Casting Via Sand Molds

Sodium silicate or water glass is a popular compound that serves varied purposes. While it works as an effective sealer, adhesive and deflocculant, the most pervasive usage is in metal mold making in the manufacturing industry.

This process involves binding sand to form molds before pouring molten metal into them. It may sound convoluted at the outset, however a simple procedure emerges from the details.

Sodium carbonate and silicon dioxide react when molten to form sodium silicate and carbon dioxide. The aqueous solution of water glass is used to bind sand particles to form solid molds or cores. It works as a high strength binder and forms strong and resilient molds.

You need fine grained sand (about 100 mesh should do) for this task. Measure the amount of dry sand and add 3% to 5% sodium silicate by weight (depending on application and foundry conditions). Mix it thoroughly with hand (or a muller for large batches) till all the grains of the sand are properly coated.

Pack the treated sand into the core or mold box of the desired shape. Any excess mixed sand can always be stored in an airtight container for later use. Insert a rod or spike to vent out the center of the mold. Multiple vents can be required for bigger molds. Thin cores need to be supported with wires.

Now it is time to activate the water glass so that it can bind the sand particles together. For this, it has to be exposed to carbon dioxide gas or esters. In fact, the gas has to pass through the whole core and that’s the reason why we have made vents.

Apply carbon dioxide gas with a hose and nozzle or diffuser. Any low pressure source such as a standard brewery gas cylinder can be easily used for this purpose. Ensure that the gas passes through the entire core from one end to the other. For smaller molds you can also place the core mold in a plastic bag before filling it with the gas.

The required chemical reaction takes place and the sand cures almost instantaneously – within 5 to 20 seconds itself. The soldified core is now ready for use. Molten metal is later poured into these cores or molds. Once cured, the sand molds have to be broken apart (with heat or rods) to remove the metal casting.

This emerges as a very cost effective procedure of metal casting as almost any sand can be used for making the molds. It is generally washed, dried and sieved prior to use. Even sodium silicate is a low cost compound, thus adding to the gains. Moreover, it yields quick results and can be used at room temperature itself (no need for baking either). On the other hand, other binders like oil have to be heated while resins can prove more expensive.

Little wonder then that almost 70% of all metal castings are done in sand molds!


Post time: 03-08-2017