One of the numerous benefits that the investment castings process offers is a fine 3.2μm Ra surface finish, which is often emphasised as an advantage by investment casting manufacturers. But why is this point such a relevant USP, and just how big of an advantage can the 3.2μm Ra surface finish of investment castings offer?
The favourable surface finish is the result of the care taken to manufacture wax patterns and the high grade of refractory material used in the early stages of the manufacturing process.
“The surface finish derives from coating a smooth wax pattern with a fine primary coat of high-grade refractory material prior to dewaxing and casting,” Curley begins.
This creates the basis of a ceramic shell around the wax pattern with a very smooth interior wall. It’s the smooth surface of the interior of this shell that is directly responsible for the fine surface finish of the castings that it will eventually produce.
A great deal of care goes in to the production of the wax patterns at PI Castings. From using state-of-the-art Aquaflame torches to smooth the surface of wax patterns, to meticulous cleaning and degreasing, there are several steps taken to ensure that a smooth wax is produced. There are also similar controls in place in the Shelling department. The ceramic baths used to shell wax patterns are closely monitored to ensure that the optimum grade of zircon sand is being used to ‘invest’ wax pattern assemblies. The particle size in the PI Castings primary coat is subject to rigorous inspection to ensure that no oversized particles can affect the finish of the interior wall.
Material gains
After explaining why investment casting is able to produce castings with an incredibly fine surface finish, Curley suggests why this quality is so advantageous: “The exceptional surface finish that can be achieved permits a major advantage in terms of dimensional accuracy. Investment castings can be produced with higher dimensional accuracy than castings manufactured using other processes, making it a more appropriate production method for components that need to be produced with great accuracy. Tolerances of ±0.125mm in ferrous and non-ferrous materials can be achieved at PI Castings, allowing them to meet the demands of most of our customers with casting alone.”
The 3.2μm Ra surface finish that investment casting can achieve aids with the creation of fine details such as embossed text and logos. This not only allows for a greater freedom of design, but can save having to involve other identification processes which can cost time and money.
“It can even make such processes easier when they are required, as investment castings provides a suitable surface for subsequent identing methods, such as engraving, dot peening and laser marking.”
Reduced machining
Curley adds that while investment castings may appear more expensive than other castings, the higher grade of surface finish can remove secondary costs such as additional machining to achieve the required finish. This can result in the need for lengthy and expensive processes being reduced if not removed, resulting in cost savings when comparing the prices of fully finished components.
As well as being expensive, in some instances, carrying out machining operations can be impractical. When the design of a component involves complex shapes and complex internal features, investment casting not only realises the design, but also ensures that parts can be manufactured to the necessary specification, without compromising design intent when machining is altogether not permitted.
As an aerospace-approved manufacturing company, PI Castings is able to manufacture and supply components to Nadcap and AS9100 standards. This requires compliance with strict manufacturing procedures and the use of only approved service providers. For foundries and machine shops that aren’t approved like PI Castings.
“Being able to produce components that have a desirable surface finish without the need for machining can avoid the cost and trouble associated with finding an approved machinist,” he notes.
Meanwhile, a decreased requirement for machining can be advantageous when working to just-in-time schedules.
“The reduced need for lengthy secondary processes means that we can save a lot of time in production and better meet the short lead-times requested by some of our customers.”
As a supplier to many demanding industries, PI Castings benefits from this advantage in particular. Many companies trust PI Castings with their projects, due to its ability to manufacture components to the required timescales; the fine surface finish that its casting process achieves is a key benefit to their production lead-times and as such, a major advantage of this quality.
Curley reckons that the process’ inherently fine surface finish grants PI Castings – and consequently its customers – some great advantages when certain treatments are required, adding that the fine surface finish offers a good starting point to ensure that components react well to operations such as painting and coating.
“The investment casting surface finish provides enough friction to guarantee good adhesion, but smooth enough to facilitate easy and even paint application. Investment castings react to surface treatments, such as electro-polishing better than sand castings for instance, due to having a finer surface. Similarly, minimal polishing preparation is required when compared with sand casting for electro-plating or mechanical polishing processes.”
Control your quality
PI Castings inspects its products at every stage of the manufacturing process. Being heavily involved in quality control, Curley has first-hand experience in how the finer surface finish makes visible defects easier to detect and rectify where necessary. This benefit can prevent time and money being wasted as a result of products being rejecting, ensuring that components can be manufactured to the required standards efficiently and with minimal waste.
In summary, the benefits Curley shares clearly show why the fine surface finish that the investment casting process can achieve offers a great advantage to investment casting manufacturers and their customers. When you take into consideration that this is just one of a number of benefits that investment casting offers, it’s easy to see why the process is requested by many companies in various industries.
