Casting

Lost Wax Casting Process

• Casting can be done in many ways.

At TinkerMill we have the capability to cast with a vacuum caster and electro-melt furnace. We also have the ability to cast with a spin caster and dual-fuel fed torches. The process for both of these styles is similar in some ways but differ greatly in others. One similarity is the preparation needed to cast a wax model in metal.

• Currently, we offer a three day immersive class in Lost Wax Casting that covers the entire process from start to finish. Over the three days students learn how to carve or construct a wax model and connect it to the base of the flask, what sprueing is and how to create the best system of sprues for the model, investing the model in plaster, and finally casting molten metal into the baked plaster mold. The class also introduces other forms of model making, casting methods, and supply resources. See the TinkerMill meetup [1] channel for more information on this class. Below is an overview of the process including some vocabulary used during the process.

• • The Wax Model

First the wax model must be made. Wax is a suitable material for this process because it can be burned away leaving a cavity where the wax was in the plaster investment. This allows the metal to form a positive just like the wax carving. Waxes come in many different states of density, shapes and sizes. Once a model has been made it is necessary to build what is called the sprues.

• • Sprueing the Model

Next the flow system must be built for the metal to travel and air to escape during the casting process. These sprues attach directly to the wax model and are also built from wax. It is best to use perfectly round sprues to reduce resistance when metal is flowing through the plaster cavity. Once the model has adequate flow created it is sprued directly to a flask base. This base and sprueing system suspends the model in the flask making it possible for a solid mold to be created by the investment. It is at this point that the weight is calculated. *Hint: it is helpful to pre-weigh all of the flask bases and tape their weight to the bottom for quick calculations. Please see the calculating metal weight section for more info on how to find how much metal is needed for the cast.

• • Investing the model

Once the model has been secured to the base and all transitions (the sprue connections) have been assured smooth, it is time to invest the model in plaster. A fine dental plaster is used for this process, it can withstand very high heat for prolonged periods and is slow to cool. As with most processes of this nature there are safety concerns. At this point it is highly recommended that one employs the use of a respirator with fine particulate filtering. As mentioned the plaster used in this process is very fine and therefore aspirable. The plaster is mixed in specific ratios to water to create a batter like consistency. The flask is prepared dependent upon the style of casting, ie: spin casting or vacuum. These preparations vary due to the type of flask used for each process. The plaster is then carefully poured into the flask, it is best to have a vacuum table to release any air bubbles that could get trapped while pouring the investment, but the bubbles could also be vibrated out using a vibratory table. The plaster used in this process has a very short cure time. Approximately 9 minutes from the point at which the water touches the plaster powder. Some plasters are faster, somewhere around 7 minutes. When all bubbles have been released the flask is set to rest until fully cured (about 60 minutes)

• • Burnout

This is the process in which the wax is removed from the plaster. Using a kiln, the flask is fired for around 6 hours using two ramping cycles and a holding cycle. The first ramp removes the wax from the plaster and burns it away. The second cycle brings the flask and investment up to casting temperature and then holds it there for a set amount of time. It is recommended that to fully set the investment it must remain in the second ramp for a minimum of two hours. 3-4 hours is better. Burnout generally happens the day of the casting event. This can be programmed with a delay start the previous night, or started early in the morning for an afternoon casting. Depending on the equipment being used it may be recommended to assure the start of each ramp cycle.

• • Casting

The final phase of this process. Using the calculated amount of metal, it will either be melted in the electro melt furnace or with a dual fuel fed torch in a crucible. Once the metal is molten and free of debris it will be poured into the fresh from the kiln flask. It is important that the flask be at casting temperature when pouring the metal to ensure that there is very little difference in temperature between the two materials when contacting each other. Extreme differences can cause cracking or break down of the investment and failure in casting. It is highly recommended to cast with a partner, even if you use the electro melt and vacuum system. Having a casting buddy allows for back up assistance in the casting process and help if something goes wrong.

How To Calculate The Casting Metal Amount

The amount of metal needed to fill the mold = (W x SGm + 10%)

When:

• W = Grams of wax or other model medium.
  • SGm = Specific gravity of metal used for casting.

The following example uses wax as the model medium and sterling silver as the metal.

Let’s say that the weight of the sprue base, model, and sprues comes to 153 grams.

The base weighs 144 grams. 153 grams minus 144 grams equals 9 grams of wax model and sprues. Sterling silver has a specific gravity of 10.4. 

• • So, 9 grams x 10.4 = 93.6 grams of sterling silver.

To ensure there is adequate metal to fill the mold, add 10% to your total for the button.

• • 93.6 + 10%= 99.9 total grams of metal

Round it off to 99 grams of sterling to fill the flask. To calculate how many ounces that would be divide your number by 31, this weight is approximately 3.2 ounces of silver. This is handy when ordering your metal.