Lost wax casting \ Investment casting

February 25, 2018

Background

 

Lost wax casting, also called investment casting.

This casting technology has very ancient roots. Findings from the Chalcolithic period (the Stone and Bronze Age) found in the "Treasure Cave", which are known as the Nahal Mishmar Caves in the Judean Desert, were discovered in 1961 and contain copper artifacts that were produced using lost wax technology. Nests, heads of goddesses, decorated ornaments and various vessels. The treasure is dated to 3500-4500 BCE.

 

 

 

 

 

 

 

 

 

 

left side:some of the items from the "Nahal Mishmar" treasure,  right side: the Ibex Scepter

 

In many parts of the world, various objects were found cast  in this method: figurines from the region of India (2000-2500 BCE), objects from the tomb of Tutankhamun (1333-1324 BCE) and in other cultures from Mesopotamia, Central America, Africa And other cultures have found objects that were mold and casted with lost wax casting technique.

  

The stages of lost wax casting Process:

 

In the first stage, a master mold is to be created.

 

This mold will be used to create wax patterns. The master mold can be carved from wood, be plastic injected or be made from clay or metal.

In the industry the master mold is usually made of steel or aluminum.

With a combination of 3D printing technology, the master mold can be printed from various materials, a combination of printing technology with lost wax casting is a powerful tool that saves a lot of research and development work.

 

 

wax pattern inside the master mold

 

The main characteristic of a master mold is the ability to use it multiple times - such a mold will be consist of several parts that can be separated in order to retrieve the wax pattern  that we will create inside the mold.

 

In the second stage, a wax part is created: into the master mold, wax is inserter, either by pouring molten wax into the mold and filling it by using gravity or by injecting wax as a paste with pressure  into a metal mold.

After the wax is cooled and solidify the mold is opened and the wax part is removed from the master mold. Sometimes we will want to transfer the wax pattern in to a cooling device to prevent shrinkage of the wax during cooling. This will force the wax to maintain certain dimensions.

 

In the third stage, we attach the wax pattern to a cluster made up of wax sprues, funnel  and several patterns

 

The purpose of the sprues, and the funnel is to direct the melted metal flow during casting stage so  that it reaches all the wax patterns connected to the cluster, a correct design of a cluster will also consider the direction of solidification of the melted metal and design the cluster so that the patterns will solidify first and the sprues will solidify last (this prevents the formation of cavities within the final part)

In the industry field cluster design have to consider additional factors such as cutting of the metal parts from the sprues and the prevention of impurities and foreign parts contaminating the cast.

 

In the fourth stage, after the cluster is ready and clean, a ceramic material (high temperature resistant material) is built around it, plaster can be placed directly on the cluster manually (usually for models with simple geometry and no stringent quality requirements).

In the industry field the wax cluster is being dipped to various ceramic slurry’s either by hand or by using a robot. In some cases after the ceramic dipping a sand layer is added to increase strength to the cluster - depending on the final part requirements and various cluster parameters such as weight, geometry, etc.

 

The process of constructing the ceramic mold can consist of one ceramic layer  and up to 10 or more layers each layer with different purpose and properties.

A very important is the mold drying an emphasis should be drying rate and drying conditions: if the final part have high quality requirements humidity levels should be closely monitored.

Drying too quickly will cause the ceramic to crack and break during casting. Drying too slowly can cause the mold to become weak during casting.

 

The first layer of the mold is the most important because the ceramic material that  comes into contact with the wax part will ultimately determine the surface quality of the cast part.

 

In the fifth stage after finishing the preparation of the ceramic mold, the wax is removed from the mold. The mold can be turned upside down so that all the wax will drop out of the funnel. The industry uses more advanced equipment such as various controlled furnaces to prevent cracking of the ceramic mold and to prevent present of wax residue inside the mold.

 

The six stage is the cast  stage. There are several actions

 

that must be performed preliminary to the cast, such as burning the mold to remove moisture that remains in the ceramic, checking and repairing any cracks and defects in the mold, warping the mold with ceramic fiber to control the solidification direction. Finally, it is necessary to heat the ceramic mold before casting to a temperature close to the casting temperature in order to prevent breakage during casting as a result of thermal shock.

 

 

The cast part with sprues still connected to it

 

The casting itself is just pouring molten metal into the opening of the mold. It can be done in the open air or in a vacuum atmosphere if there is a quality requirement. If there are narrow areas in the mold it is advisable to heat the metal to a high temperature so the molten metal will be more liquid and will flow smoothly for a long time.

After casting, the mold is cooled. Here, too, the cooling rate is important. High a cooling rate can cause fractures in the metal due to thermal stress.

 

The seven stages is the breaking of the ceramic

 

mold and the extraction of the metal mold, the break is done by hand with a hammer or by machines. After mold break, the sprues are cut and the final parts are removed. Usually there will still be ceramic pieces on the metal parts and they need to be cleaned by chemical compounds or mechanical polishing, in addition it is necessary to sharpen and remove the areas where the parts were connected to the cluster.

 

Additional test should be made according to the quality requirements of the parts. In the industry usually tests are being made to see if there are defects on the surface or in the part bulk.

 

final part after sprues removal and cleaning

 

 

Advantages of lost wax casting

 

The major advantage of lost wax casting technology is the ability to produce parts of complex and complicated geometry in a relative simple process. Cast parts can be simple parts like jewelry, sculptures and artifacts to complex parts for jet engines such as blades and turbines.

Another advantage is the ability to produce materials with a wide range of melting temperatures, from aluminum to steel and nickel and Titanium super alloys.

 

The essence of the technology is the production of a wax part  that matches the dimensions required for metal - wax is a material  that is very easy to shape because of its properties.

Model of jet engine blade, right part wax and on the left a metal part after removal of sprues and cleaning


 

After the wax production stage, all other stages are preparation of the casting mold.

Fabrication of the same part by other methods will require the use of expensive processing equipment and will require considerable processing knowledge or alternatively require the use of cast molds from very expensive materials that withstand very high casting temperatures.

Aluminum parts can also be produced using other casting technologies such as sand casting, pressure casting but steel parts and super alloys can be produced almost exclusively through the technology of lost wax casting.

 

 

Disadvantages of  lost wax casting

 

The main drawback of this technology is the need to create a new mold for each part because the mold is being destroyed after each cast.

Another disadvantage is the long process and the use of a variety of materials such as wax, ceramics, metal, and a relatively long post processing of removing ceramic residues, cutting sprues and more ...

 

 

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