Metal powder production: quality control is crucial

Figure 1 shows the quality control (QC) points, the product movement from one stage to the next (arrows) and the scope of the production traceability system. QC points are used to decide whether or not the product complies and can move to the next stage. The product is diverted when it doesn’t comply. Managing non-conformities can significantly impact operational performance. As such, it’s important to determine acceptable limits and, in the case of a non-standard product, how to proceed: Rework it? Sell it as is? Destroy it? The riskiest elements are shown in red, and those with lesser risk are in orange and yellow. Each stage has different risks that must be properly identified to mitigate them and minimize any operational impacts.

Quality at the transformation steps

For each processing stage, there are three risk categories: inputs, equipment, and operations and maintenance. In general, input quality directly impacts product quality. As a result, after conducting a mass balance for each step, it’s necessary to develop QC points at the input and output. Equipment selection also affects quality, like when parts erode or when fluid leaks and comes into contact with the product. The equipment must also be cleaned to avoid cross-contamination.

In every case, the effect equipment use has on the product must be considered. Equipment selection can become more difficult for products that are more sensitive to contamination. Either existing equipment must be modified or new equipment must be designed. In the latter case, it’s important to properly document equipment design and conduct utilization tests to support performance.

Using the best equipment and raw materials doesn’t guarantee product quality. And poor quality powders can be produced if equipment isn’t used or maintained properly. To mitigate this risk, it’s necessary to know the key parameters, document work instructions and respect production and maintenance procedures for equipment and infrastructures.

Quality at the sampling step

After producing a quality powder, it needs to be qualified before proceeding to the next manufacturing step. This qualification is usually performed through product sampling. This is a critical QC step, because a very small amount of product is used to qualify a large quantity of powder. Therefore, the sample must represent the batch and not be altered before analysis. Deviation from a good batch caused by a non-representative sample is a very costly error.

To ensure a representative sample, refer to standardized methods, like the ASTM B215. Then, the equipment must be selected and a method adapted to the product and process developed to obtain this representative sample. The sample can then be moved to the QC location for analysis. Some analyses require less than a gram of product. As a result, there must be a method for homogenizing the production sample to ensure each analysis is representative of the batch produced. It may be tempting to oversample to increase result reliability. However, the more you sample, the more you increase operational costs. So, particular attention must be given to the sampling method and to protecting sample integrity.

Quality during handling

After completing the transformation step, the product moves on to the next step. Each move involves several contamination risks: intermediate container, powder transfer equipment, air exposure and human handling. For example, if an intermediate container is used to move the powder, it must be kept clean and prepared to prevent the product from being altered by the condition of the container. Was the product exposed when being transferred? Could the operator’s hair have fallen into the powder? To mitigate these risks, the causes must be determined and their potential impact on the product assessed. Since humans are generally one of the main sources of contamination, we can consider developing methods to limit product exposure to the environment or automating some handling.

The importance of traceability

A great deal of information must be recorded at each transformation stage, from handling to QC. Customers require much of this information. In such cases, orders must be traceable to one or more powder batches, raw materials, equipment used in manufacturing, production conditions, analytical devices used, people involved in production and containers used to ship the product. All production and QC data can be recorded using a manufacturing execution system or MES. This tool reduces the risk of losing information and adds value to the company.

Conclusion

When manufacturing powders for markets with high quality standards, the focus is on avoiding anything that could alter the product or delay delivery to the customer. Contamination can have catastrophic consequences for both powder manufacturers and customers in the aerospace and medical markets. For example, a contaminated powder used in 3D printing can cause an aircraft to crash. Or a contaminated powder used to manufacture a prosthesis can cause complications for the recipient.

With this in mind, any equipment that comes into contact with the product and any movement or material are risks. This approach requires significant thought to identify the right equipment and methods to produce a good product. In the context of Industry 4.0 and given the HSE risks discussed in a previous blog article, metal powder production is very complex and input from experts and engineers is required to develop a plan that respects quality, cost, efficiency and safety. For any projects related to metal powder production, contact BBA.