Precisely measuring and validating parts made from low-density materials such as plastic can be tricky and time-consuming. For example, silicone parts readily bend and flex, leading to inaccuracies. Fixturing, the traditional solution to immobilize and align parts for measurement, is a lengthy process involving engineering, construction and validation of the custom fixture. Another common challenge is inspecting a broken inhaler or other device for internal problems. It can be daunting, or even impossible, to correctly disassemble or precisely cut through the device. Still another issue is validation of micro-molded parts, which often calls for high-magnification microscopic measurements.
Although many designers and manufacturers rely on any or all of three traditional methods—laser scanning, coordinate measuring machines (CMMs) and optical metrology—these technologies come with significant drawbacks. Three-dimensional (3-D) X-ray computed tomography (CT) scanning, or metrotomography, addresses these deficiencies and provides a complete solution for industrial metrology and for validation of products made from plastics. This article provides a brief overview of all these technologies and describes the advantages of 3-D X-ray CT scanning in several use cases, including speed, cost-effectiveness and simplicity.
CMM. Coordinate measuring machines are comprised of three main components: the machine itself, the measuring probe and the control or computing system with appropriate measuring software. After placing a workpiece on the machine table, the operator uses a probe to measure different points on it by mapping the x, y and z coordinates. These points are uploaded to a computer interface where they can be analyzed using modeling software (such as CAD) and regression algorithms for further development. The probe can also be operated automatically via a control system. Traditional fixed CMMs carry out repeated measurements rapidly and accurately but are immobile, limited in measurement range and expensive for large applications.
Laser scanning. By covering both freeform surfaces and geometric features, laser scanners capture the entire part geometry. A 3-D scanner can be quickly and easily attached like a probe to a measuring arm, or can work as part of a laser tracker system or fixed CMM. The resulting point cloud can then be used to create a CAD model to reproduce, redesign, inspect or archive the part.
Laser scanners play a key role in quality assurance. The industry uses laser scanners primarily for quality control of geometries and surfaces, but also for reverse engineering, fit and finish, and assembly applications.
Optical metrology. Optical measurement systems are arm-free scanning systems that use optical triangulation methods and permit total freedom of movement around the object. Designed with compact linear or matrix array cameras, optical systems are smaller than CMMs with arms, have no wires and enable operators to take 3-D measurements of all types of objects located almost anywhere, including outside controlled environments. Optical metrology is appropriate for certain non-repetitive applications, such as reverse engineering, rapid prototyping and large-scale inspection of parts.
3-D X-ray CT scanning. CT scanning uses irradiation (usually X-rays) to produce 3-D representations of the scanned object. X-rays are directed at parts or assemblies to capture precise images of all internal and external components. The CT scanner rotates the article a full 360 degrees. During the scan, which typically takes 45 to 60 minutes, up to 1,200 2-D images are captured virtually. A detector and the computer linked to the detector then calculate volume and distances of the part. These 2-D images are constructed into a precise 3-D point cloud of the scanned part or assembly.
CT scanning is ideal for measuring complex, high-precision injection-molded parts, as well as assemblies with internal features that cannot be measured or analyzed without taking them apart.
Although all these methods provide accurate measurements, laser scanning, CMM and optical metrology have significant downsides related to speed, cost and complexity. By choosing 3-D X-ray CT scanning over these other technologies, organizations can move parts into production more quickly without sacrificing measurement precision. Following are the main differentiators for industrial CT scanning.
• No fixturing needed
Creating custom fixtures, as required for CMM, laser scanning and optical metrology, is complicated and demands time and expertise for engineering, construction and validation. Also, fixtures can cost thousands of dollars to produce. Industrial CT scanning avoids the need to immobilize a part or assembly with physical fixtures. Instead, parts are scanned in their free state on a platform of expanded polystyrene (PS). The low density of PS foam makes the platform virtually invisible. Once the 3-D data set is uploaded on the viewers, the part is easily analyzed, as it appears to be floating in midair.
Silicone parts and assemblies with silicone inserts provide a good illustration of the benefits of using a foam platform instead of fixturing. Silicone components, such as medical tubing, are very bendable and their free state can be easily affected during the fixturing process. Industrial CT scanning avoids the risk of inaccurate measurements from bending or flexing during fixturing.
• No cutting or disassembly required
One of the major disadvantages of CMMs, laser scanning and optical metrology becomes obvious when you need to analyze the interior of a part or assembly. Because these methods are limited to the exterior features of a part, the only way to visualize or measure interior features is to cut, break or disassemble it. Cutting or disassembly can ruin a valuable prototype, and may damage or alter interior geometries, thereby affecting metrology results.
In contrast, 3-D CT scanning uses X-rays and geometric analysis software to virtually cross-section the part. By “seeing” through walls, this technology makes it easy to inspect seals for leaks; measure features, volumes and diameters; highlight voids, inclusions or foreign material; analyze wall thickness; and show how parts fit together for optimal functionality.
• Faster results
Designers and manufacturers are always under pressure to get parts validated and into production as quickly as possible. Speed is another benefit of 3-D X-ray CT scanning over competitive technologies. Besides eliminating long delays caused by the fixturing process, CT scanning itself is highly efficient. One 3-D CT scan captures 800 to 1,200 images and integrates them into 3-D data sets—in just one hour. A complex part or assembly is easily analyzed in a fraction of the time it would take with traditional methods such as CMM and optical metrology.
Using CT scanning, inspection reports can be completed within one to two days of receiving the item. This is because the part can be programmed ahead of time using a CAD file and 2-D drawing. The part itself is not required for programming because there are no fixtures that might need adjustment.
• Better process accuracy
While all methods offer comparable measurement accuracy, industrial CT scanning can improve the overall accuracy of the process in several ways. First, the CT scanner does not touch the part or assembly, avoiding the risk of impact from a CMM probe or laser scanner arm that can occur if the part was not measured correctly before the fixture was engineered. Impact is especially common with flexible silicone parts: a CMM or any other contact measurement device could easily bend the item. Even if the part is moved by only a few microns, inaccurate measurements are possible.
Another scenario is verification and validation of dimensions for micro-molded parts. These parts have traditionally required high-magnification microscopic measurements and visual measurement devices, which need fixtures for proper part alignment. With an accuracy of 5-7 microns and no fixturing, industrial CT scanning is ideal for inspecting micro-molded parts. For greater efficiency, multiple parts can be scanned at one time.
Finally, CT scanning produces a 3-D visualization, rather than data points in an Excel spreadsheet, which is the output from a CMM. A 3-D visualization can reveal unexpected anomalies in the part.
Popular uses for CT scanning
The following challenges illustrate the power of industrial CT scanning:
- Validating assembly fit. In the past, companies that design and build cap closures had to saw through the cap and bottle assembly to determine how well the components fit together. Industrial CT scanning provides a nondestructive alternative. In this scenario, CT scanning is done three times: once with the cap fully assembled, once with the cap half assembled and once with the cap loosely assembled. This makes it quick and easy to take internal measurements and inspect fit without changing the assembly’s free state.
- Correcting defective sealing. Locating leaks in cap closures is very frustrating because they are usually invisible to the naked eye. With micron resolution and advanced geometry processing software, industrial CT scanning can identify hard-to-find leaks and allow inspection of defective seals. Virtual, 360-degree cross-section images of the assembly are taken without cutting or disassembly. To measure internal dimensions, the system software gathers data in minutes. These files can be saved and recalled at any point in the future.
- Qualifying the mold. Mold qualifications for complex plastic parts can take can weeks or even months. Industrial CT scanning saves hundreds of hours during qualification. Before going into a full first article inspection, the best parts from the best processes are scanned. Creating part-to-CAD overlay comparisons can quickly determine which process yields parts closest to the intended design. As shown in Figure 1, CT scanning can cut the mold qualification timeline in half compared to traditional metrology.
Industrial CT scanning is a fast, accurate, non-invasive technology that can save a great deal of time and avoid the expense of constructing custom fixtures. Unlike laser scanning, CMMs and optical metrology, CT scanning provides a detailed view of internal part or assembly features without cutting or disassembly, preserving valuable prototypes. Choosing this technology over traditional approaches can help manufacturers accelerate and simplify the process of moving a part into production.