Traceability is important. It’s why manufacturing standards like ISO exist, so that customers can have assurance that accredited companies meet minimum quality standards. In a global economy where parts can come from suppliers located around the globe, traceability is more important than ever. The quality standards of one manufacturer in one country may be quite different from that of another manufacturer in another country. Without global standards, buying parts can be like spinning a roulette wheel. With the advent of conflict resources such as “blood diamond” and conflict minerals such as cassiterite (for tin), wolframite (for tungsten) or coltan (for tantalum) entering the marketplace, traceability becomes a necessity for political and ethical reasons as well. And with the growing importance of sustainability, new standards that measure ecological footprint, circular economy and sustainable growing and harvesting are already beginning to appear such as the Forest Stewardship Council’s FSC certification for sustainably harvested timber or the cradle-to-cradle C2C certification. From illegal logging to pesticide laden food products, knowing what resources came from where, and how they were extracted is becoming an increasingly important supply chain management issue in an age of social and ecological upheaval. We specialize in manufacturing of industrial luminaires and has learned important lessons of traceability in the lighting industry. Manufacturers have to play a proactive, pre-emptive role in the quality assurance of their products or risk products falling out of specification, brand damage, expensive retrofit solutions and costly manufacturer product recalls.
In the early days of our company’s entry into the global marketplace, a number of scenarios emerged that highlighted the need for traceability and spot checks to ensure that specified materials and material properties of components were being met. In the course of vetting new oversea vendors, our processes exposed a number of suppliers who misrepresented themselves. However, even amongst the honest vendors that are shortlisted, their manufacturing processes can suffer poor quality control issues, resulting in end products that do not meet their original specifications.
A Case Study
A case study with die casting illustrates the real world complexities in our industry. We make extensive use of aluminum die casting in producing the housings for our luminaires. These housings have unique industrial designs to maximize heat dissipation and manage heat, curved surfaces designed to minimize packaging debris collection or accumulation, and many details for ease of assembly, installation, or sealing purposes. The package design is critical to meet our more demanding industrial applications in marine, hazardous locations, and challenging outdoor environments.
With metal castings, quality issues can show up in simple ways such as a bad batch of material or can appear in more complex ways, such as when components preferentially diffuse out during a production run so that initial production parts have materially different composition than latter ones. In plastics, improper injection temperatures can degrade the components, and improper mold design can lead to bubbles, porosity, poor fill and warping of the finished part. An experienced vendor should be able to manage most of these issues and prevent them from occurring, but independent inspection and verification is required to verify their process is in control and capable of producing good components reliably.
For this case study, a vendor was not able to provide timely or cost effective certificate of analysis from an ISO accredited test laboratory fora critical safety component of one of our hazardous location luminaires. Without a traceable set of documents to confirm that the product met the required specifications, due diligence compelled us to initiate destructive testing of a set of samples to validate the physical properties and composition of the aluminum alloys was within our specification. As one set of tests revealed a certain flaw, it led to another test. After a series of consecutive tests, our engineering department was able to characterize the extent of the problem. While the cost of destructive testing isn’t cheap, the added time and substantial delay in the product development was an even greater expense. Three samples of two different components representing different production runs were submitted for tensile strength testing, and one sample of each component was also submitted for compositional analysis. Additionally, two fully assembled sample cases were also submitted for radiographic evaluation.
Test results for aluminum die castings exposed major issues with composition and physical properties. The chemical composition of two of the smaller components were discovered to be out of spec. Testing showed that the correct alloy was used in the larger component casting, but was slightly out of spec for one element, indicating inadequate control over composition. The test results for the smaller component revealed something far more serious, however; it was cast from a completely different alloy than specified. Alloy composition has a major influence on the physical and corrosion properties of a metal, where changes in elemental composition by fractions of a percent can lead to dramatic shifts in properties and material failure. Safety in hazardous locations can be compromised if incorrect materials are used, so further testing was required to track down the problem.
Two physical properties were evaluated as representative of the strength and functionality of the components: Ultimate Tensile Strength (UTS) and yield strength. Both of these values are obtained from a destructive tensile test on a sample with precisely defined geometry. UTS is the stress (force divided by the sample cross section, measured in MPa or psi) an object can withstand at the moment of breakage or rupture. This is most relevant when parts could be subject to breakage, but is also a standard measurement that is easily compared against published standards. A more useful measurement in cases where deformation or bending of components would lead to a failure is material yield strength. Yield strength is the stress required to cause a plastic deformation of 0.2% – effectively the limit at which a material experiences a permanent change in shape.
In general, we expect that as-cast samples will have slightly lower strengths than the “informational” tensile strength values provided in alloy specification standards. This is due to casting fill and porosity differences between thin and thick sections in a casting. Material near the surface of a casting will have improved density, grain structure, and generally fewer casting defects resulting in higher physical strength compared to thick sections. The cast geometry and location in a casting that test samples are machined from can have a significant impact on the physical properties. As can be seen from the table of measurements for our test samples, there was a very wide % variation over the range of the samples tested for UTS. In one set of components the average UTS was 150 Mpa, 30% lower than the nominal value for the alloy. Some variation in test results between samples is expected, but the values our testing revealed an abnormally wide range, from 16600 psi to 31900 psi. The test result variation of the Yield Strength and UTS was far wider than expected. A wide variance in measured values can be caused by excessive variation in the test procedure or it can be representative of the actual range of strengths of the samples. In this case, an inspection of the cross sections that samples were machined from showed the cause to be variation in the casting quality. Excessive variation from part to part for strength, substantially lower strengths than expected, and out-of-spec composition proved that the supplier was not delivering what it had stated, and led us to the search for an alternate vendor since trust was irrevocably lost.