Lost wax casting, with a long history over 5,000 years, has become one of the main casting types in the world. It has evolved to creat lots of complex metal parts used by manufacturers today.
Lost wax casting, also known as investment casting, is a process by which a metal casting is designed and created, and is especially known for its near net shape capabilities. The “lost wax” technique is so called because the wax model is destroyed in order to create the piece. When the piece is then fired in an oven, the wax sprues and model melt, leaving a mold in the investment and the openings through which the molten metal can be introduced.
Lost Wax Casting – The Process
Lost wax casting manufacturers use a specified casting technique known as a lost wax process, one of the oldest manufacturing techniques in the world. Many complicated metal components made from welding, forging or machining could be converted into lost wax casting process with a significant cost saving. A typical lost wax casting process involves:
The basic manufacturing stages of lost wax casting process are outlined below:
1. Wax Pattern Making – Wax replicas of the desired castings are produced by injection wax into a metal mold. These replicas are called “patterns”.
2. Wax Patterns Assembly – Wax patterns are then attached via a ‘gate’ to a central wax stick, referred to as a ‘tree’ or ‘sprue’, to form a casting cluster or assembly, and mounted on a pouring cup.
3. Shell Building – Dipping the assembly in a liquid ceramic slurry and a bed of extremely fine sand to build a shell. Several layers may be applied in this manner.
4. Dewaxing – Once the ceramic shell is dry, the wax is melted out in an autoclave, creating a negative impression of the assembly within the shell. The shell mould is then fired in a high temperature oven.
5. Pouring – The shell is filled with molten metal – metal injection moulding in a variety of materials, using various techniques and, as the metal cools, the parts, gates, tree and pouring cup become a solid casting.
6. Knock Out – When the metal has cooled and solidified, the ceramic shell is broken off by vibration or water blasting.
7. Cut Off – The parts are cut away from the central tree using a high speed friction saw.
8. Finishing – Minor finishing operations, such as fettling and grinding, are undertaken to produce a metal casting identical to the original wax pattern.9
9. Inspection – A specialist ensures the casted part is identical to the original design and contains no damage or weakness. Sinotech uses Magnetic Particle Inspection to look for inclusions and cracks. CMM confirmation and high-accuracy machining ensure your casting is dimensionally correct.
Lost wax Casting Metals
Lost wax casting uses a wide assortment of metals to create parts with exceptional accuracy and tolerances. The types of metal chosen for a casting depends on the requirements of the design and the type of part. One of the reasons that lost wax casting is so popular is the limitless number of metals that can be applied to the process.
The main alloys of stainless steel are chromium, nickel, and molybdenum, which determine the grain and mechanical properties of the casting. How the alloys are combined decides how the casting will be able to endure heat and resist corrosion. Since stainless steel is 10% chromium, it is resistant to liquid corrosive conditions and oxidation.
Carbon, low alloy, and tool steels are used in the aerospace, agricultural, medical, and firearm industries, to name a few. Steel is chosen for lost wax casting because it is economical, available in several grades, and can be heat treated to adjust its ductility.
Aluminum is an ideal metal for lost wax casting due to its machinability and corrosion resistance. When it is alloyed with other metals and heat treated, it develops exceptional strength that is comparable to low carbon steel. The fluid nature of aluminum alloys allows for parts to have thin walls.
The Advantages of Lost wax Casting
Since the molds from lost wax casting are made from ceramics, produced products are ready to ship or attach after being cast. The roughness average (RA) of a lost wax cast part averages around 125, which is the average of the peaks and valleys on the finished surface.
One of the critical factors in modern manufacturing is turnaround times since products have to be mass produced in a short time frame. Since lost wax cast parts do not require finishing, parts can be rapidly produced and shipped, which is further assisted by the reduction in handling and labor.
The initial expense for any manufacturing process is equipment and machinery. Lost wax casting uses less expensive equipment, which makes it less dangerous. The basic process employs the use of four machines though other equipment can be added.
The biggest advantage of lost wax casting is the tight and accurate tolerances that have a standard of ± 0.005. CAD computer designs are accurately and precisely recreated to exactly fit the final application.
Variety of Metals
There are very few limitations to the types and kinds of metals that can be used in lost wax casting. The types of metals include bronze, stainless steel, alloy steel, iron, aluminum, and copper, to name a few. The wide selection gives engineers a varied array of choices when designing parts. It also makes the process adaptable to any industry.
As there is little limitation on the types of metals used in lost wax casting, the same applies to the size of parts to be formed. The range of sizes starts with small dental implants up to complex aircraft engine parts weighing thousands of pounds. The size and weight of lost wax cast parts is dependent on the mold handling equipment.
Since lost wax casting requires less manpower, labor costs are significantly lower. Lost wax requires fewer resources and less time, which are major cost factors. The fact that parts do not need post processing further lowers the cost. These essential factors assist in keeping costs down for high quality parts.
Another factor that keeps the cost of lost wax casting down is the lack of waste. Most of the materials that are used in the process are saved and reused, which eliminates waste. In other production processes, post processing leads to waste of materials since the removed burrs and excess metals are unusable. Since lost wax casting does not require finish or post processing, waste is radically reduced.
Lost wax casting maximizes the use of resources and reuses the wax and ceramics for future projects. This eliminates waste materials being dumped into a landfill. Manufactures are able to reuse dies, molds, and wax multiple times without need for replacement.