From Wikipedia, the free encyclopedia. When complex, thin sections are involved, mould filling may be assisted by positive air pressure, tilt casting, vacuum casting or centrifugal casting process. A common form of this is called the Hitchiner process after the Hitchiner Manufacturing Company that invented the technique.
Investment casting process steps
Although the introduction of new materials and manufacturing cssting in the last few years has led to some changes and improvements, the investment casting process steps have largely remained unchanged. You can read about the advantages of using investment casting. In general, investment casting or sometimes referred to as lost wax casting can be summarised into the following 8 steps. A master pattern is created from wax, clay, wood, steel or plastic by either a specialised mould maker or sculpture artists as per final product requirement. Figure 1 below shows a wooden master pattern and its casting. Depending on the size, shape and material used, this process may take some trial and error to get the size and shape correct, therefore making these moulds expensive.
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Investment casting is an industrial process based on and also called lost-wax casting , one of the oldest known metal-forming techniques. Lost foam casting is a modern form of investment casting that eliminates certain steps in the process. It is generally more expensive per unit than die casting or sand casting , but has lower equipment costs. It can produce complicated shapes that would be difficult or impossible with die casting, yet like that process, it requires little surface finishing and only minor machining. Casts can be made of the wax model itself, the direct method; or of a wax copy of a model that need not be of wax, the indirect method. The following steps are for the indirect process which can take two days to one week to complete. The investment shell for casting a turbocharger rotor.
An introduction to the investment casting process
Although the introduction of new materials and manufacturing techniques in the last few years has led to some changes and improvements, the investment casting process steps have largely remained unchanged. You can read about the advantages of using investment casting.
In general, investment casting or sometimes referred to as lost wax casting can be summarised into the following 8 steps. A master pattern is created from wax, clay, wood, steel or plastic by either a specialised mould maker or sculpture artists as per final product requirement. Figure 1 below shows a wooden master pattern and its casting. Depending on the size, shape and material used, this process may take some trial and error to get the size and shape correct, therefore making these moulds expensive.
A mould, known as the master die to create wax patterns is made to fit the master pattern by either casting or machining. Technology advancement in the tooling and CNC machining has enabled machining a master die from a material such as steel and aluminium without the need for a master pattern as shown figure 2. If the master pattern is made out of steel then master die can be cast directly from the steel master pattern using metal with a lower melting point than steel.
Silicone rubber moulds are also used extensively these days due to its simplicity of using to cast a mould directly from a master pattern. Silicone rubber moulds would withstand casting temperature up to about o C o F.
The master pattern and master die must be designed taking shrinkage of pattern wax, refractory material and the metal cast material into consideration. Another important aspect of two-part master dies or moulds is mould alignment locators. Mould locators are critical in aligning the two halves to produce accurate parts without large parting lines or parting line shift. Figure 3.
The patterns are then made by injecting or pouring molten wax into the master die and allowing it to harden Figure 4. This coating is repeated until the desired pattern thickness is reached usually about 3 mm 0. Latter suitable only for simple parts. Sometimes patterns are also made using suitable polymers and frozen mercury. Recent advances in rapid prototyping have made the production of patterns directly from a CAD file possible, either as a master pattern or as wax patterns, consequently reducing the cost.
These rapid prototyping technologies use a layering technique in their creation so post-processing is required in most cases to ensure a good surface finish. Cores are generally made from soluble wax or ceramic where soluble wax cores are dissolved out of the patterns pre-processing, while the ceramic cores remain and are removed after solidification of the metal casting. The wax patterns are investment casting turned upside assembled onto a common wax sprue.
Sprue is a channel through which molten alloy can reach the mould in an investing ring after the wax has been eliminated. Using heated tools and melted wax, a number of wax patterns can be attached to a central sprue and runner system to create a pattern cluster or a tree.
Depending on the size of the parts, up to several hundred patterns can be assembled onto a tree increasing the productivity. Figure 5. Sprue will hold the wax pattern to avoid distortion during the casting process, creating a channel for the molten wax to escape from the mould and then allowing the molten material to flow into the mould. During this stage a very fine particle size is deposited to create a thin but very smooth layer of investment material, to ensure a smooth surface and intricate detailed final product.
Very fine silica and other binders, including water, ethyl silicate, and acids are commonly used as refractory materials. After the initial layer has dried, the pattern is dipped and coated repeatedly to increase its thickness for better strength.
Sometimes as an alternative method, the single-dipped pattern cluster is inserted into a permeable flask upside down and liquid investment material poured around it. The flask is then vibrated to remove entrapped air and ensure that the investment material surrounds all surfaces of the mould tree.
This is then left to dry completely, which can take between 16 to 48 hours. Drying can be accelerated by applying a vacuum or minimising environmental humidity. Once the refractory material mould has fully dried and cured, they are turned upside down and placed in an oven or special purpose de-waxing autoclavewhere the wax can melt and run out figure 7and any subsequent residue vaporized.
These simulation shows how specialised de-waxing machines work video 1 or video 2. The removal of pattern wax from the mould is an important stage in the investment casting process and when performed incorrectly will reduce the batch yield.
De-waxing is also one of the main reasons for shell cracking. Shell cracking is the single most common investing casting defect.
Fundamentally there are two main methods for de-waxing, flash fire or autoclave steam. Steam autoclaves are the preferred choice due to its uniform heating which would reduce the stress on the material. The mould is heated 90 o C— o C and held in an inverted position for few hours about 4 — 12 hours to allow the wax to melt and run.
Pattern wax is usually recovered and recycled. The duration and the temperatures depend on the metal cast material. Most shell failures occur during the dewaxing stage as the waxes used have a greater thermal expansion coefficient than the refractory material.
Hence wax expands more than the investment material and introduces a possible stress fracture on the mould. To reduce this effect the wax can be heated rapidly so the outer wax melts quicker and allow the remaining wax to expand without stressing the mould.
Autoclaves shown in figure 8 are also used to heat reduce this effect as the heating cycles can be more uniform and controlled more accurately. Mould is then pre-heated in preparation for pouring. Preheating allows the metal to stay in the molten state longer so that it can better fill all mould details and increase dimensional accuracy. If the mould is left to cool, any cracks found can be repaired with ceramic slurry or special cement.
Mould heating also gives better dimensional control because the mould and the metal can shrink together during cooling. Molten metal up to o C is poured into the hollow mould and then left to cool. While gravity pouring is the simplest, other methods may be used to ensure complete filling of the mould. When complex, thin sections are involved, mould filling may be assisted by positive air pressure, tilt casting, vacuum casting or centrifugal casting process.
After the metal has solidified, techniques such as mechanical chipping, hammering, vibrating, high-pressure water jetting and media blasting are used to break the mould and remove the metal casting.
Then individual castings are cut away and excess metal is removed. The sprue is cut off and in most cases reused. The part is then post-processed as per the final product requirement such as post machining, heat treating, surface treatments, painting.
Sign in. Log into your account. Forgot your password? Privacy policy. Password recovery. Recover your password. How Investment casting works. Step 1 — Master pattern A master pattern is created from wax, clay, wood, steel or plastic by either a specialised mould maker or sculpture artists as per final product requirement. Figure 1 Master pattern source:garenphillips. Figure 2. Master die If the master pattern is made out of steel then master die can be cast directly from the steel master pattern using metal with a lower melting point than steel.
Investment casting die and cluster The master pattern and master die must be designed taking shrinkage of pattern wax, refractory material and the metal cast material into consideration.
Figure 3 Step 3 — Pattern The patterns are then made by injecting or pouring molten wax into the master die and allowing it to harden Figure 4. Figure 4. Pattern making Recent advances in rapid prototyping have made the production of patterns directly from a CAD file possible, either as a master pattern or as wax patterns, consequently reducing the cost. Step 4 — Pattern assembly The wax patterns are then assembled onto a common wax sprue. Investment casting cluster Using heated tools and melted wax, a number of wax patterns can be attached to a central sprue and runner system to create a pattern cluster or a tree.
Figure 5 Sprue will hold the wax pattern to avoid distortion during the casting process, creating a channel for the molten wax to escape from the mould and then allowing the molten material to flow into the mould.
Figure 6. Step 6 — De-waxing and firing Once the refractory material mould has fully dried and cured, they are turned upside down and placed in an oven or special purpose de-waxing autoclavewhere the wax can melt and run out figure 7and any subsequent residue vaporized. These simulation shows how specialised de-waxing machines work video 1 or video 2 Figure 7 Investment casting dewaxing The removal of pattern wax from the mould is an important stage in the investment casting process and when performed incorrectly will reduce the batch yield.
Figure 8. De-waxing autoclave Most shell failures occur during the dewaxing stage as the waxes used have a greater thermal expansion coefficient than the refractory material. Knock out casting removal step After the metal has solidified, techniques such as mechanical chipping, hammering, vibrating, high-pressure water jetting and media blasting are used to break the mould investment casting turned upside remove the metal casting.
References and recommended reading. Kalpakjian, S. London: Pearson. Black, J. London: John Wiley. Beeley, Peter, Foundry Technology Second ed. Oxford: Butterworth-Heinemann. Groover, P. Mikell, Fundamentals of modern manufacturing Fourth ed. Oxford: John Wiley. Investment casting. The Library of Manufacturing. Was this article helpful?
définition — Investment casting
Privacy policy. Parts manufactured by investment casting are normally small, but the process can be onvestment effectively for parts weighing 75 lbs or. Planetary Resources has used the technique to print the mold for a new small satellite investment casting turned upside, which is then dipped in ceramic to form the investment cast for a titanium space bus with integral propellant tank and embedded cable routing. For investment casting in art, see Lost-wax casting. Taggart of Chicago, whose paper described his development of a technique [ citation needed ]. Once the refractory coating reaches the desired thickness, it is allowed to dry and harden; the dried coating forms a ceramic shell around the patterns and gating. Using heated tools and melted wax, a number of wax patterns can be attached to a central sprue tjrned runner investmenr to create a pattern cluster or a tree. See more results
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