Investment Castings


When I first learned about investment casting I asked what does it have to do with investing?  Someone made a joke about the investment in tooling, another person thought it had something to do with having to make new patterns every time you wanted to make a mold.  While the first person was right that the cost of tooling is higher than for sand casting, the second person was a bit closer to the truth.  It turns out people used to say invested when referring to putting on clothing or when surrounding something.  In our case for investment casting, the investment refers to the ceramic shell that surrounds the wax or foam pattern.

A challenge I had was when foundries would ask whether I wanted castings made from the silica sol or water glass process.  What is the difference?  Without getting into detailed chemistry or material properties, silica sol (SiO2) molds will produce castings with a finer surface finish and better dimensional tolerances, roughly ±.005” per 1” of linear dimension, versus double that for water glass (sodium silicate).  The difference in price between the two processes is about 20%, but where machining is required some of those savings end up being used to remove more material. 

How the investment casting process works:

The lost-wax investment casting process starts with an aluminum mold which will create the wax patterns, each of which will produce a single finished casting.

Those patterns will be added to trees which will then be coated with the ceramic slurry.  The number of coats will depend on the alloy being poured and how strong the wall should be to hold the metal.

After cooling, the investment castings are removed from the ceramic shell, detached from the trees and the metal is recycled.

The chemical composition of raw material is retained on file, and any testing of raw castings, such as dye penetrant testing is performed.

After machining and quality control, the castings are ready for packing and dispatch.

The capability of consistently holding better ISO 8062 casting tolerances, much smoother surface finish, and comparable ease of casting complex geometry are the main reasons investment casting is chosen over sand casting.  For alloys that are more expensive and difficult to machine, like stainless steel, the reduction in material and machining costs outweigh the higher processing times and costs.  If you need any help determining the lowest total cost of ownership for your metal component parts, please reach out to us for a consultation and quote!

Sand Casting With Brukar


Did you know that sand casting is the most common casting process used today? The sand casting process is versatile and can produce parts ranging in size from a few ounces, to several tons, all with varying levels of complexity.  Sand casting is relatively inexpensive and the process is fairly simple.  It is commonly used to make parts out of gray and ductile iron, aluminum and zinc, but because sand can tolerate extremely high temperatures, sand casting is also often used for metals with high melting points such as steel, nickel, and brass.

Changing the sand casting method can yield parts of varying levels of dimensional accuracy and surface finish, but it does have its limitations.  The different levels of dimensional accuracy are detailed in ISO 8062, an internationally recognized standard that can be an invaluable guide to help determine whether or not sand casting is the most appropriate process for meeting the design requirements of your parts.

At Brukar, we can help you decide whether or not sand casting is the way to go to meet your needs.  We leverage the expertise is our supply base to try to determine different ways to reduce your part costs while maintaining your current design integrity and it may be as simple as changing the type of sand mold, or it might involve switching to a completely different casting process, such as investment casting.  conversely, it may be that changing from investment cast parts to sand cast parts is feasible, effectively resulting in lower costs.  It’s worth asking the question.  Let Brukar help you find the answers!

The Benefits of Sand Casting


I have often wondered if purchasing and engineering resources in our customer base have considered that various requirements for sand cast parts can be realized simply by changing the type of sand or sand mold used.

Using sand molds has several benefits.  Sand is inexpensive and can withstand high temperatures, thus allowing many metals with high melting temperatures to be cast using the sand casting process.  Sand casting also accommodates a large size range of parts.  The Sand, (typically silica sand), used to create the molds, is mixed with a “binder” to help maintain the shape of the mold cavity during the casting process.

Sand molds are prepared in different ways which can be categorized into 4 types:

  1. Greensand Mold – Greensand (sometimes called wet sand) molds use a mixture of sand, water, and a binder, usually called clay.  Typically composition of the mixture is 90% sand, 3% water, and 7% clay (or organic bonding compounds), but the percentage of clay can vary slightly depending upon surface finish, moldability, and degassing requirements specific to each part.  Greensand molds are the least expensive and as such, are the most commonly used sand molds.
  2. Skin-dried mold – A skin-dried mold starts out much the same as a greensand mold, but additional bonding materials are added and the internal surfaces of the mold are dried with a torch or a heating lamp which increases mold strength.  Skin-dried molds yield improved dimensional accuracy and surface finish, but usually at the expense of collapsibility.  Dry skin molds are more expensive and require more time, which equates to a lower production rate.
  3. Cold box mold – Cold box sand molds, or dry sand molds, use sand that is only mixed with an organic binder.  The mold is then baked in an oven to impart strength which results in a mold that produces a part with high dimensional accuracy.  The added preparation also adds cost thought, as the production rate is lower.
  4. No-bake mold – The no-bake  mold is one of the most advanced molding techniques used for sand casting.  Sand is mixed with a liquid resin and the mold sets relatively quickly at room temperature.  This process has excellent dimensional stability and produces parts with superior surface finishes when compared to other sand casting techniques/processes.

The quality of the sand that is used also greatly affects the quality of the casting and is usually described by the following five measures:

  1. Strength – Ability of the sand to maintain its shape.
  2. Permeability – Ability to allow venting of trapped gasses through the sand.  A higher permeability can reduce the porosity of the mold, but a lower permeability can result in a better surface finish.
  3. Thermal Stability – Ability to resist damage, such as cracking, from the heat of the molten metal
  4. Collapsibility – Ability of the sand to collapse, or more accurately compress, during solidification of the casting.  If the sand cannot compress, then the casting will not be able to shrink freely in the mold and can result in cracking
  5. Reusability – Ability of the sand to be reused for future sand molds

All of these options play a part in the sand casting process, and adjusting or changing these options can yield different results that may better suit a customers’ requirements.  An increase in the cost of the sand cast process may very well make economic sense if it can eliminate a subsequent machining or surface finishing process.  Or conversely, using a less expensive process may still yield a part that is perfectly functional if the current process is holding dimensional or surface finish characters “tighter” than what is actually needed.  Challenging the status quo and asking the right questions may very well result in a cost saving that’s been there all along.