Since its first demonstration back in the 1930s, metal injection molding (MIM) has become an essential metal fabrication technique. MIM at a basic level is the formation of small metal shapes from a metal powder that is fused together. This enables highly complex shapes to be created with almost no waste – keeping costs and waste down. The final components are almost 100% dense, with higher strength than die-casting, better tolerances than investment or sand casting, and greater shape complexity than traditional machining techniques.
MIM has been a mainstream production technique since the 1970s, where it was used for dental orthodontic brackets, watch cases and firearms. Today, however, MIM technology is enabling the manufacture of complex components for high-performance applications, such as dental implants, artificial joints, pacemakers, and jet engines.
As with other metal-forming processes, the composition of the starting powder determines the composition and specification of the finished component. The performance and the longevity of the part is also affected by the presence of impurities, and the component must adhere to local, state, and federal statutory requirements. Chemical analysis plays a huge part in ensuring components meet customer specification, statutory requirements, and internal quality control.
In metal injection molding there are three main methods for alloying: elemental, pre-alloy, and master alloy. The type of starting powder chosen depends on the alloy method; either pre-mixed or manually blended to achieve the right composition.
For the elemental method, powders of individual elements must be blended in the right ratios to yield the correct composition after alloying.
The pre-alloy process can use a powder that precisely matches the composition of the final alloy specification.
The master alloy approach uses an elemental powder with the addition of specific alloying components. The majority of MIM stainless steel components and some low-alloy steel components are produced in this way. For example, a MIM part from 316L stainless steel is produced by combining one part of a 55Cr38Ni7Mo master alloy with two parts carbonyl iron powder.
It’s essential to verify the composition of raw powders before molding to ensure they meet the required specification for maximum yield and minimal scrap. Due to the complexity of the alloying process, the composition of the finished components must also be checked before shipping to ensure good quality and performance This is doubly important for components made for critical applications, such as medical implants or jet engine parts. This is where spark optical emission spectroscopy (OES) really delivers.
Optical emission spectroscopy is the ideal analysis technique for verifying the composition of MIM parts. It’s extremely accurate and precise, which is why OES has been used for decades for the most sensitive of applications, including melt control and tramp and trace element detection in metal fabrication facilities worldwide.
Spark OES spectrometers are used across the entire metal fabrication process and supply chain, starting with the analysis of trace elements in scrap metals, the control of incoming materials, metallurgical process control and quality control of finished goods.
A reliable OES measurement needs a clean, flat, planar surface at the point where the OES measurement head contacts the sample. The sample surface is ground or milled (depending on the material composition) just prior to taking a measurement.
As the dimensions of MIM fabricated components are typically quite small, accurate results are obtained by reducing the area of the measurement site with a special spark stand plate to reduce the size of the hole in the plate. You may have to use a special sample adaptor to ensure your samples are held correctly, especially if they are complex in shape.
Controlling the carbon content in metal components is also essential as even minute variations in carbon concentration can alter the microstructure and mechanical qualities of the finished part. It’s especially important to monitor the carbon content in MIM components as the binder is carbon-based and must be completely removed during the de-binding stage.
Our ground-breaking OE750 has been designed with a new optical concept that delivers results for all elements within metals, including gasses. This level of performance is usually only available from much more expensive instruments, yet innovations such as the use of dynamic CMOS detectors and direct coupling of the optics to the spark stand give the OE750 the optical resolution necessary for demanding metal injection molding applications.
The analytical performance of the OE750 offers an easy and affordable solution for carbon content control.
My colleague Maryam BeigMohamadi, Application Scientist – OES and I have put together an in-depth guide that provides insight on how we can help you to find an optimal solution for your metal injection molding process quality control.
In this guide we cover:
We’ve been working alongside metal fabrication businesses for almost 50 years, developing analytical techniques and solutions that support manufacturing processes, including quality and process control, as the industry specifications get more stringent. As a result, we’ve become experts in material analysis and today, we directly support our customers in finding the right analytical solution for their (often niche) processes.
If you have quality control issues with your raw powders before molding or finished components, we want to help you. By working together with our application team, we’ll be able to find the best solution for you.
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