Additive manufacturing (AM), commonly known as 3D printing, is useful for its unique ability to produce complex shapes that can outperform conventionally manufactured items. There is no tooling required as with stamping, forging, and molding, nor is material wasted as with most subtractive processes. Many individual components can be printed as a single piece which simplifies production, assembly and inspection. Additively manufactured parts can be produced using a variety of materials that match the strength, durability, and performance of those that are traditionally manufactured.
Using a range of technologies from powder bed fusion to binder jetting to vat polymerization additive manufacturing builds up products layer by layer from polymers, resins, and metals among other materials. It has become a standard method for rapid prototyping because of its speed and relative simplicity compared to injection molding and CNC machining in low-volume and iterative design applications.
The past decade has seen AM begin to replace traditional manufacturing for full-scale production which brings new challenges to manufacturing professionals. One hurdle to greater adoption of AM in production manufacturing is quality control and part inspection for complex AM components.
Importance of Quality Control in Manufacturing
Quality control is vital for ensuring parts and products are made to specification and function as expected. It is also essential for maintaining consistency and repeatability in manufacturing operations.
Without dependable quality control measures manufacturers will be unwilling to commit to high-volume runs of additively produced parts that could put their reputation at risk. This is especially true in the industries where additive manufacturing is naturally applicable like aerospace, automotive, and medical. Each of these industries is highly regulated but also has the most to gain from efficient production of high-complexity, high-mix, and low-volume parts.
Precise quality control, however, depends on standardized ways to assess functionality, reliability, and safety amongst other factors. These standards are inherently difficult to establish for additive manufacturing because the process of manufacture varies greatly between AM technologies. Support structure and build plate removal, heat flux, trapped powder or resin, post processing and other factors all add new challenges for QC personnel.
In addition, rapid innovation in the AM space has regulators and standards organizations scrambling to keep on top of emerging materials and methods. The ASTM and ISO have teamed up on a variety of published standards covering AM applications, design, materials, processes, terminology and test methods. But in many cases, reverting to older standards developed before AM is still required.
Considering all of this, AM inspectors are left to analyze complex surfaces with GD&T’s profile callout which requires collecting a high density of surface measurements. Traditional inspection tools (contact/CMM) make this process very time consuming. But with 3D scanners capable of ever-increasing levels of accuracy, these measurements are much more convenient.
Why Traditional Quality Control Methods Pose Challenges in Additive Manufacturing
Inspection standards for AM are less established in newer applications and more fully fleshed out in older applications.
In healthcare, for example, inspection methods are more mature for parts that are custom 3D-printed for a specific patient, such as an implant or prosthesis, because these were among the first commercial AM applications. Parts with highly organic anatomic regions can be 3D scanned and compared to the build file where statistical methods combined with GD&T feature controls can be applied. Tolerances in the human body are also fairly loose compared with precision machining so the adoption of scanning happened earlier.
It is also important to understand why traditional measuring tools have been challenging to adopt in AM. These manual tools, like calipers and physical gauge kits, are very simple to use and can be very precise, but they only measure one geometric feature at a time (such as size, form, or angle) and they do not capture the data digitally. That makes them time-consuming and labor-intensive to use, and generally not a good fit for additive manufacturing that thrives on efficient digital workflows.
Another common method for inspection and quality assurance is destructive testing, which includes a wide range of fatigue, fracture, and mechanical tests. Destructive testing is expensive, slow, and wasteful. It may also not reveal the root cause of failure in an intricate 3D-printed part that involves multiple manufacturing steps.
For example, a metal 3D-printed part may involve printing, heat treating, support removal, plate removal, and finishing. Any one of these can change the shape and performance of the part. Unless you know where the issue is occurring a destructive mechanical test may cause you to waste a lot of time trying to adjust the printing process when the problem is the result of a later step. Destructive testing is useful for validating a machine/material combination with test coupons if necessary but should be avoided for end-use parts.
Why 3D Scanning Makes Sense For Additive Manufacturing
Additively manufactured parts often have a high degree of engineering complexity, whether it is intricate lattices, gyroids, infills, or topology optimization. This is the entire purpose of additive manufacturing — to make innovative use of these unconventional shapes and structures that can only be produced through additive means.
This is exactly why 3D scanning is the ideal quality control mechanism for 3D printing — because its capabilities align so well with the nature of additively manufactured parts. Some of these key applications include:
Full Shape Capture
Without 3D scanning, it is impossible to see the whole picture of a part or product. Compare the two images in Fig. 1, where it is obvious how 1,000 points of data (which would still take a relatively long time to acquire through traditional means) looks far less distinct than a full 3D scan with more than nine million points of data. There is simply no comparison in terms of accuracy to the high-fidelity data capture available with 3D scanning.
Many additively manufactured parts involve thin structures or soft surfaces that should not be touched, either because they may deform, or they have specific cleanliness requirements. In these cases, optical 3D scanning offers another advantage over other methods.
Digital Retention and Archiving
Full shape capture via 3D scanning works well with the digital workflows common in additive manufacturing. It allows manufacturers to retain a complete digital copy of the inspected part for as long as necessary, even after it is shipped and used. This can pay off for future understanding of part wear and fatigue, for example.
Total shape capture with a 3D CT scan allows designers and engineers to examine whether a jetting application for a casting pattern, for example, can not only build internal channels and pintle nozzles but also clean those passages out. Destructive testing would be an option in this case too, but any single cross-section chosen could render a false positive or negative. 3D scanning with a CT system, on the other hand, provides access to all cross-sections in the part.
With access to a 3D CT scan, designers can perform lattice inspection to make sure nodes and straps are correctly placed, that lattices are in the correct areas of the part, and that mating surfaces interact correctly for the application.
Build Volume Inspection
There are many situations in which measuring any single product in a build may give you a different impression of what happens during that build. Build volume inspection with 3D scanning allows you to examine all of the parts to understand what parts have the highest deviation and risk of warp. From this point you can modify the build style for the parts accordingly. 3D scans can also archive your part at each step of the post-processing journey to track all geometric changes along the way.
Quality Control and Inspection Solutions for Additive Manufacturing
Implementing 3D scanning for quality control and inspection in additive manufacturing requires two components, the scanner and a software application capable of managing the data.
When it comes to optical 3D scanning technologies, the two types primarily used in AM inspection are laser line probes and structured light scanners.
Laser triangulation captures an object by measuring the deformation of a laser beam projected onto its surface. One or more laser sources project very small points onto the surface and a camera, or cameras, record the positions of those points.
The angle between the laser line points and a camera is pre-determined, allowing for the 3D triangulation to be calculated. On-the-fly calculations record these points in 3D space as the laser line(s) move across the object’s surface. This approach produces accurate, high-resolution scans, and is the most capable when objects are reflective and/or dark and shiny.
It is common to either mount the laser on a mobile platform (traditional or portable CMM) tracked by an external system (laser or optical tracker) or keep the laser fixed and have the part move underneath it on a conveyor or turntable.
Structured Light Scanning
Structured light scanning generally uses one light projector and multiple cameras. It is similar to laser triangulation, but it measures the deformation of a fringe pattern or grid that is projected over the surface.
Typically, a blue LED or laser illuminates a DLP chip to create alternating light and dark regions known as fringe patterns. As the pattern shifts across the surface, cameras collect data about the variations in the known pattern and triangulate the distances to create a point cloud. Structured light creates dense point clouds with great detail on sharp edges, given the resolution of the on-board cameras often utilized.
Computed tomography (CT) scanners are a third option for 3D scanning. CT scans generate a complete view of a product’s internal details, and the data is digital, both of which can be helpful for 3D-printed parts. Specifically, CT scans enable channel design and lattice inspection, both of which require data about the interior of the part. Historically, CT scanners have been very expensive, although some models have been coming down in price.
Quality Control and Inspection Software for Additive Manufacturing
Data collection with a scanner, however, is only the first step. Software is vital to process the data in a meaningful and reliable way. Scanning software overlays the point cloud onto the CAD or STL reference data to extract comparison points and measurements.
Options for scanning software include:
➤ Traditional contact inspection software that is designed to work with CMMs and other contact methods, in which non-contact scanning was an afterthought
➤ Scan-native inspection software that is specifically designed for 3D scanning
Scan-native software provides some important advantages compared to legacy CMM software because it is purpose-built for modern, high-resolution 3D scanning. These platforms allow you to:
➤ Handle large data sets faster and more easily
➤ Achieve high precision with scan-native algorithms for noise filtering and geometry fitting
➤ Detect deviations with cross-section, boundary, curve, silhouette and virtual edge comparisons
➤ Accelerate GD&T calculation by automatically identifying features in a 3D scan and filtering data intelligently
➤ Create digital archives with a full history of measurements and their trends
Geomagic Control X, for example, is Oqton’s scan-native metrology software. It integrates with any optical scanner or portable CMM arm, collects and analyzes very large data sets, and automates repetitive and complex tasks so users with little to no training can still get accurate results. For many additive manufacturers, it is an ideal starting point for 3D scanning in quality control and inspection.
Preparing For Efficient Quality Control and Inspection for AM
Regardless of what path you take, 3D scanning for quality control and inspection is a critical step on the way from prototyping to full-scale production in additive manufacturing. By simplifying the inspection process 3D scanning helps manufacturers save time, perform advanced analysis more easily, and detect issues with a higher degree of accuracy.
Most importantly, high-fidelity 3D inspection gives additive manufacturers the data to establish quality control standards for complex parts, and to ensure those parts meet the performance and compliance expectations of even the most demanding and highly regulated industries.
For more information: www.oqton.com
Author: Adam Waraksa