Cleanliness Inspection of Fasteners | FASTENER EURASIA MAGAZINE
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How Cleanliness Inspection of Fasteners Is Made Easy for Novice Operators

Christina Hesseling 
Product Manager, Industrial Equipment, Olympus

Fasteners are essential parts in production machinery and goods in major industries, such as automotive, oil and lubricant manufacturing, aerospace, and medical device equipment. One major challenge for fastener manufacturers, especially when the components are used to manufacture precision instruments, is particle contamination. Since every industry has defined acceptance criteria for residual particle contamination of their products to maximize product lifetime and safety, fastener manufacturers face many technical cleanliness demands. 

This article will explore the technical cleanliness workflow for fasteners, its challenges, and how automated systems can simplify it—even for new and inexperienced operators. 

The Typical Technical Cleanliness Workflow for Fastener Manufacturing
Microparticles can cause the malfunction of technical products, such as gearboxes, turbines, and engines, or medical devices, such as syringes and filters. Consequently, the technical cleanliness of each component is an essential part of quality control and assurance. Specific inspection processes and standards are defined for each application.

A common process to count and measure contaminants on the surfaces of parts like bolts, screws, or other fasteners is to wash the part and then filter the rinse fluid through a filter membrane. Particles larger than the filter’s mesh size deposit on the membrane. After drying the filter membrane, a comparison of its weight without and with contaminants yields a first rough estimate on the amount of contamination on the investigated part. 

Next, a microscopic inspection reveals the size and shape of the microparticles. Cleanliness standards define size classes and particle types to which each particulate must be assigned. If the number of particulates in a size class exceeds an acceptable limit, the inspected part will fail the technical cleanliness inspection. This often means a change is needed in the production process or handling of the part. Also, microscopic inspections often reveal characteristics that signify a potential problem. For instance, while smoother particles may have slowly abraded over time, edgy particles are often caused by a new problem in a system that must be dealt with promptly.

In addition to measuring the size and shape of particulates, identifying metallic particles is important. Due to their hardness and electric conductivity, metallic particles are potentially more harmful to mechanical and electrical systems. The demand to identify metallic particles has become even more important with an increasing market for electric mobility. 

The Challenges of Technical Cleanliness Inspections
Even knowing the importance of technical cleanliness inspection, the workflow can still be difficult to perform.

In a conventional cleanliness inspection workflow, manually operated microscopes are used and contaminants on filter membranes are classified individually and counted by the operator. Yet, this is a time-consuming process that strongly depends on the operator’s experience level and judgment. On top of that, the intensive manual work poses the risk of contaminating the specimen during the investigation. 

Fast decision-making is essential in technical cleanliness inspection to give direct feedback to quality control. Repeatable, comparable results are also indispensable for consistent quality control across different production facilities. The results must be presented in compliant, professional reports. 

Today, modern microscopes for technical cleanliness inspection are fast, reliable, and easy-to-use with consistent results independent of the operator. 

Easy Specimen Positioning and Maximized Automation
Systems that maximize automation can help simplify cleanliness inspections from start to finish. For example, modern solutions like the OLYMPUS CIX100 technical cleanliness system only require a simple specimen positioning step before automation takes over. 
In the workflow, a prepared specimen is placed on the filter holder and secured with a screwed piece that covers the filter’s edge to avoid stretching or twisting the membrane. Then, the holder is clicked into place using a bayonet lock on the microscope stage. No tools are needed to fix the membrane.

Once the specimen is mounted, the whole system is completely hands free and software controlled. Some operators even use the option to enclose the system to protect it from environmental influences like contamination and background light.

Intuitive Software
A confusing software program can slow down cleanliness inspections of fasteners and increase the risk of user errors. Fortunately, modern inspection systems are equipped with intuitive software so users of all experience levels can perform the particle analysis workflow. Here are some software capabilities that can simplify inspections: 

Tidy interface with a guided workflow: Look for an organized interface with a simple, guided workflow. For instance, with the OLYMPUS CIX100 system, the user follows just three steps: inspect sample, review results, and create report. The buttons for the steps are large and easy to press on the touch screen monitor. 

One-click reporting: By using software with a selection of predefined customizable standards and report templates, users can define their standard workflows and produce compliant reports with just one click. An option to directly export the reports to Microsoft Word or PDF further simplifies this step. 

Direct feedback: Consider using software with live analytics to save time. For example, when the CIX1000 system scans the filter membrane, it automatically classifies detected particles and clearly shows a NOK (not ok) sign if the contamination exceeds a set maximum value. This direct feedback during the membrane scan saves time in this potentially critical situation.

User rights management: With this feature, administrators can limit access for the inspection technicians to the most basic, easy-to-follow workflow. Access to system calibration and modification of standards and report templates can be limited as well to minimize user error. 

An Easy and Fast Method to Distinguish Metallic from Nonmetallic Particles
The process to distinguish metallic from nonmetallic particles in cleanliness inspections typically requires the specimen to be imaged twice—but new technical innovations in modern microscopes are simplifying the process. 
Metallic particles generally have a higher reflectivity than non-metallic ones, and this principle is used to discriminate between the two types. When illuminated by linearly polarized light, the light reflected from the surface will preserve its polarization if the surface is reflective. 

If the surface is non-reflective (i.e., the illuminated particle is non-metallic), the polarization is not preserved. By analyzing the polarization of the light scattered from a filter membrane with particulates, the system can identify reflecting particles and classify them as metallic.

In the past, this meant the filter membrane was illuminated by linearly polarized light and two images had to be taken. An analyzer (i.e., a polarizer with a known polarization orientation in relation to the incident light), is placed between the filter membrane and camera. If the analyzer is oriented parallel to the incident polarization, reflective particles will show up bright on the camera image. As soon as the analyzer is rotated at a 90° angle, the same particle will appear dark in the camera image. 

Comparing the two images shows which particles are reflective. A particle that changes from bright to dark when the analyzer is rotated is classified as metallic. While this method works to distinguish the particles, the process is tedious and requires careful adjustments. Recording two images of the membrane and checking the image alignment increases the inspection time. The required rotation of an optical component in the light path poses additional risks on the calibration stability

These risks are eliminated using a color-based discrimination method on the OLYMPUS CIX100 cleanliness inspector. It uses an innovative and patented illumination and recording system that omits any moving parts from the optical light path. This reduces wear and tear on the instrument and minimizes the risk of adjustment errors. A cover also protects the camera and optical components to stabilize the system and system calibration.

The color-based discrimination technology works by illuminating the specimen with linearly polarized white light, and the polarization of its blue content is rotated in relation to the other colors. The analyzer is positioned parallel to the polarization orientation of the blue component. The result is that reflective sections appear blue on the camera image. This blue coloring is then used to identify metallic particles. When the incident light is scattered by the particle, the polarization is not preserved, so the blue color is no longer dominant in the image. This innovative method only requires one image to distinguish between the particles, helping to simplify and speed up the process. 

One-Click Review of Particles
A simple particle review process can help operators of all experience levels analyze and revise the data, images, and results. 
For instance, the CIX100 system automatically classifies detected particles according to the selected standard, sorts them in classification tables and, if needed, retrieves the component cleanliness code. All information is organized in one view with particle image thumbnails sorted from largest to smallest. 
For easy review, the system directly drives to a particle’s recording position as soon as the user selects the respective thumbnail. The user can also easily perform a closer investigation, merge, split, and delete particles, and take height measurements. 

For particles that cannot be recorded in focus at a single position of the objective, all-in-focus images can be recorded. The system automatically combines multiple images recorded at different distances of the objective to the particle, resulting in an image with all particle heights in focus.

Pre-Calibrated and Pre-Configured System
Pre-calibrated and pre-configured systems are ready for technical cleanliness inspection so that the workflow can be performed quickly and reliably. Modern systems will even regularly remind users to perform automated system self-checks with a particle standard device to verify the system’s functionality. 

Many fastener manufacturers have realized that an automated inspection system that can readily adapt to the requirements of multiple industries helps improve efficiency and productivity as well as product quality. When results can be processed for multiple standards with simple operation steps, even novice operators can generate reliable technical cleanliness inspection data quickly and easily.

Christina Hesseling 
Product Manager, Industrial Equipment, Olympus
Christina Hesseling joined the product management team at Olympus Soft Imaging Solutions in Germany at the beginning of 2020. She is experienced in product management of system and software solutions involving applied optics, microscopy, computer science, and robotics. She enjoys working with researchers and engineers in academia and industry from around the world.