All About Casting — Sandcasting, that is!
Sandcasting Series Part 1
First off, let’s clear up what sandcasting is for those of you who are unfamiliar with this manufacturing process. Sandcasting is a method to create metal parts, or geometries, where one creates a mold using sand and fills in the negative space in the mold with metal.
Some commonly that you likely see or interact with on a daily basis include fire hydrants, sewer covers, car transmission housings, and cookware.
An important factor of sandcasting is what the mold is made out of. What makes a particular sand good for molding? How did we decide upon sand in the first place? I’ll be discussing one type of sand, “greensand”, in this post.
(Very) Brief History of Sandcasting
The oldest known casting is a copper frog from 3200BC Mesopotamia, likely made using a lost wax technique. Other molds (prior to true sandcasting) were typically made out of clay, while some molds were made out of stone when there was a simple object to be molded. Clay molds were usually made by mixing clay, plant ash, and water (a somewhat similar composition to today’s “greensand”).
Greensand, perhaps disappointingly, is not green but rather wet (like green wood). This type of sand consists of: silica, chromite, or zircon sand (75–85%), sometimes including an amount of olivine, staurolite, or graphite; bentonite (5–11%); water (2–4%); inert sludge (3–5%); and anthracite (0–1%).
Silica, SiO2, has a high melting point which enables it to be used as a main sandcasting ingredient. The high melting point allows the sand to come into contact with molten metal and not break down. In addition to the high melting point, silica also has a low thermal expansion rate. This means that the size of the silica particles changes very little as they are heated. It has uniform characteristics, particularly the sizing of the particles. The high melting point contributes to its desirability in sandcasting, as well as its low thermal expansion rate and high fusion point. Since the sands used are so chemically pure, it helps prevent chemical reactions with any other components in the sand mixture. This is important because if there were unknown chemical reactions happening in your mold, you probably wouldn’t be able to guarantee that your mold shape remained the same while casting, and your part might not come out as expected.
Silica sand available on Amazon
However, silica sand does have health risks associated with use. Silicosis is occupational lung disease caused by the inhalation of crystalline silica dust, resulting in inflammation and scarring in the upper lobes of the lungs. Deaths due to silicosis are down from 55,000 in 1990 to 46,000 in 2013 globally. OSHA (Occupational Safety and Health Administration) has set a number of safety measures to prevent silica-related diseases, including limiting exposure to silica dust, utilize engineering controls to clear the environment of silica dust, and providing medical examinations to the most exposed workers.
Chromite sand, FeCr2O4, has a high heat stability — it resists chemical and physical changes at a high heat (valuable when working with molten metal!). Chromite sand also offers a high sintering point, which means it is capable of compacting and forming a solid mass without melting, but will not do so until it reaches a sufficiently high temperature (1450 degrees Celsius-1500 degrees Celsius). Finally, it also has a high thermal conductivity. This thermal conductivity helps dissipate heat from the casting and plays a significant role in the overall cooling of the casting.
However, when chromite sand is not washed properly or is impure, it can lead to serious defects in both the molds (poor hardening in the case of resin molds) and the actual casting (having the molten metal penetrate the sand mold).
Zircon sand, ZrSiO4, is hard, durable, and chemically inert. It is a “highly refractory” sand, meaning that it is a strong sand at high temperatures. As a result, it is primarily used in steel casts with especially high temperatures.
Olivine is a magnesium iron silicate. It is incredibly resistant to thermal shock, and does not experience much physical change once stabilized. Olivine also improves with use, in direct contradiction to what happens when using silica sand. Its resistance to penetration by molten metal also means that one can use lower quality binders, therefore lowering costs. There are iron-rich olivine sands, and magnesium rich olivine sands. Olivine is one of the most common minerals on Earth by volume, although much of the magnesium rich olivine is inaccessible from the surface. Olivine sand is also, interestingly, no longer mined in the U.S. and there is a limited supply available. This limited supply has required foundry owners to be resourceful, and either import Norwegian sand or select a replacement: silica sand, Green Diamond aggregate, or Biasill aggregate. Olivine avoids the health issues that silica based sands have, as well as providing a finer surface finish.
All of these sand bases, of which at least one of the three is used as the primary ingredient in the sand, are generally chemically inert and can withstand high heat. This is desirable for obvious reasons — molten metal is incredibly hot, and one would not want the sand for casting to change that much during the casting process.
Bentonite is an aluminum phyllosilicate clay. With sandcasting, there are typically two types of bentonite clay used: calcium bentonite and sodium bentonite. Sodium bentonite is especially absorbent, and grows quite a bit when it absorbs water. It also has a high viscosity, which helps the mold hold together. The swell associated with water absorption helps to provide greater strength when the clay is dry or hot. Calcium bentonite, on the other hand, has less swell and a lower viscosity. It is easier to mix together with sand and is stronger when wet than sodium bentonite, but not as strong in dry or hot environments. Calcium bentonite is primarily used in nonferrous castings. These two types of bentonite are typically used as a mixture to maximize the positive properties in each for casting.
Sludge is a compound that forms in molten metal alloys containing silicon, which may form in the sand as a result of the high heats applied to silicon-containing compounds. Staurolite is also a type of silicate.
This wraps up the first installation in the sandcasting series, where we discussed greensand. Coming soon will be posts about sodium silicate (or water sand) and resin sand. Feel free to share your questions below!
This is from my blog, mechanicallydefined.com . Pop on over to check out what else I’m working on!