Investment Casting Basics
March 1, 2012
Investment casting, which is also known as “lost-wax casting” is one of the oldest manufacturing processes still in use today. Originally, beeswax was used to create the form around which a ceramic mold would be created, but today the technique has advanced far beyond as now “high-technology waxes, refractory materials, and specialist alloys” (Wikipedia) are used in modern day investment casting.
The mold is formed by using a wax pattern – a disposable piece in the shape of the desired part. The pattern is surrounded, or “invested”, into ceramic slurry that hardens into the mold. Investment casting is often referred to as “lost-wax casting” because the wax pattern is melted out of the mold after it has been formed. Lox-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes.
Source: Custom Part Net
China has a long history of investment casting production with examples of the technique being used as far as back as the Han Dynasty (209 BC – 9 AD). Today, as China has grown to be one of the most important suppliers of manufactured goods in the world, investment casting production has become a vital technology to the national economy and one that has been continuously developed and perfected.
I recently went on a factory visit to a foundry that used this ancient technique. Below is a more detailed explanation of the steps taken in the process as well as tips to look out for when performing your own factory audit of a supplier that uses this technique.
A tooling, also known as a die, first needs to be produced so as to create a mold for the wax. In China, most foundries actually sub-contract the tooling to somebody who is specialized in doing just that, and thus the foundries themselves normally do not do this on site. The main reason why there is such a separation in the production process is because it requires special expertise that a foundry often does not have. Moreover, the quality of the die used in investment casting is crucial to the success of the rest of the production as it will decide the ultimate shape of the final product. Each wax mold is made according to this die, thus if not done properly it could jeopardize the whole project.
Sometimes the die made into a shape known as a “tree” or “pattern tree”. This is because it is designed in a shape where many pieces are designed and attached to a central “wax gating system” and this final shape resembles a tree. However, sometimes this is done after the wax molds have been made (see below).
The tooling process normally takes about 20 -30 days depending on the tool suppliers’ current workload and the complexity of the product itself. Once produced and delivered, the foundry normally stores the tooling in a well-kept and organized warehouse. A foundry’s warehouse is usually a good indicator of the quality and seriousness of a producer and of how well your product will be taken care of. Checking the warehouse is an important step in a factory qualification process.
The next step is to install the tooling into the wax injection machine allowing for the machine to inject the wax automatically. This machine must be programmed with the proper specifications, after which the injection process is done within a couple of seconds. The worker will then let the wax sit and cool down and then remove the die, carefully extracting the wax parts that were just made, and then put the tooling back into the injection machine to repeat the process until the series is complete.
The wax parts are assembled into a tree-like shape by heating up the pieces just enough so that they will melt a little bit and stick together to form the “pattern tree”. This work is generally very labor-intensive especially in situations where the parts are very small as the workers have to work very carefully to ensure that none of the pieces are damaged in the process.
The finished wax “tree” will be hung up to allow the wax to cool down and be ready for the next step. In this next step the tree needs to be dipped into a tank where it will be ready for shelling. The shelling process is the most time intensive part of the entire manufacturing process. Several layers of ceramic are applied to the wax tree to ensure that the ceramic shell is suitably thick.
The process needs to be closely monitored, as each layer has to cool down for a precise amount of time and at a certain temperature. One layer usually requires about 24 hours to be done, which means that a shell of 7 layers will take about 7-10 days to complete, often longer.
After the shell has cooled down and become dry enough, the parts are put into the oven so as to melt the wax down and leave only the shell. The foundry will usually have a wax recycling system so as to save production material and costs.
Melting and Pouring:
The metal’s quality must be tightly controlled at the factory so as to ensure that its composition and purity match specifications. It is then melted at the appropriate temperature and carefully poured into the ceramic mold. The melting and pouring is normally conducted at night because the process requires a lot of electricity. Due to the lower demand for electricity in the middle of the night, melting and pouring the metal at this time can save on costs and can also ensure that it goes on uninterrupted by avoiding any power outages. The investment casting furnace is relatively small with a capacity of about 100 kg. An appropriate MOQ is therefore always required except for the situation when requesting a sample.
Removal and Cutting
Once the metal has cooled, the ceramic shell is broken (often using water jets) so as to remove the casting. Grinding is then used to smooth the part at the gates, where they are connected to the main “tree”. Heat treatment is also sometimes used to harden the final parts. Again this process requires great care and must be very carefully monitored.
Inspection & Straightening:
The parts need to be inspected before delivery. Inspection should also occur several times throughout the production process. Straightening is also a standard step in the investment casting process; after the last heating treatment, there will typically be shrinkage of the metal. Straightening is therefore often crucial to ensure that the shape and dimension of the ordered parts is respected.
Non-Destructive Testing (NDT):
This must be conducted in particular to make sure that the insides of the parts don’t have any defects.
Machining & Assembly
Some parts may not need machining but in many cases still will require assembly. It is extremely important to find a supplier who can perform these steps on-site properly so as to guarantee quality of the finished product.
Final inspection, Packing & Delivery
Delivery is as critical as production. Proper quality control systems should be in place and full inspections performed regularly to make sure that your shipment is sent out as ordered.
Investment casting, though a thousands of years old technology, is still a vital process in modern day manufacturing particularly in China. We can see from the above description with the use of the tree patterns, it is primarily used for the manufacturing of smaller parts (which are arranged as the “branches” of the tree). However, as explained on Wikipedia article on Investment Casting:
The process is generally used for small castings, used to produce complete aircraft door frames,steel castings of up to 300 kg (660 lbs) and aluminium castings of up to 30 kg (66 lbs). 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.
Thus having a good understanding of this process, its benefits and drawbacks and its step-by-step mechanics, is vital to maintaining quality production system.
If anyone has had any experience with investment casting, we’d love to hear your thoughts and experiences in the comments below.
- Jane Feng- Sourcing Team Leader