One of the biggest challenges in plastic injection molding is getting parts through first article inspection and into successful production. Delays in mold verification and performance qualification while waiting for first article approval often hold up the assembly, delivery, and sale of finished products, a major reason to verify molds and molding processes as quickly as possible. CT x-ray scanning is an invaluable tool for quickly getting parts approved for production, troubleshooting production problems faster, and eliminating guess work.
First Article Inspection
The first part out of a mold is the critical test of the process and the mold itself. First article inspection (FAI) is a process of carefully measuring the test part for conformance, in both shape and size, to the original CAD model. This process can involve multiple pieces of measurement equipment including CMMs and vision systems and take dozens, even hundreds of hours when checking the geometry of multi-cavity molds.
The entire process of developing satisfactory molds and processes can take weeks or months and strain resources. When simple parts require just a few measurements to be made, CT scanning may not be necessary. But more complex parts have a lot more features to be measured, and this is where CT x-ray scanning really pays off. The more complex the part, the more time CT will save. FAI using CT looks at all prior setups and operations of both critical and non-critical dimensions without adding inspection time.Imagine performing first article inspection of a complex part requiring over 1,500 features to be measured and doing it in less than one hour!
Computed Tomography (CT) as a Measurement Tool
CT scanning is a powerful tool for mold designers and for production and quality departments. In medicine, it can be used to find abnormalities in the brain or heart or look for fractures in a bone. In industrial process, it isused to find voids and other flaws inside castings and for other kinds of troubleshooting as well.
An item to be evaluated is placed on a rotary table and turned as multiple two-dimensional x-ray images are made. The computer joins these images into a detailed three-dimensional image. Traditionally that 3D CT image has been used to identify the presence or absence of specific features or flaws, but not to perform measurement. CT was essentially a pass/fail imaging process, and for this reason it has never focused on extreme dimensional accuracy.Today, however, enhanced technology allows the capabilities of CT to be combined with the precision of metrology to not just find flaws, but to measure the dimensions, both internal and external, of parts and even assemblies. Sophisticated software combines the functions of CT and CMM and lets actual measurements be overlaid with CAD models to see where the two differ, how they differ, and by exactly how much they differ. The enhanced measurement capabilities of CT scanning provide the information designers and engineers need to revise processes or remake molds. It allows faster more accurate evaluation of molded parts and provides more exact information for revising production processes or re-cutting molds.
With CT scanning the total time required to evaluate the shape and size of test parts can be slashed from weeks to hours. As a result, the speedy redesign of processes and molds can be based less on intuition and more on hard data and traceable dimensions. The number of iterations goes down along with the drain on resources and the cost of final mold production. Molded parts are available sooner – allowing products to go to market faster.
How Injection Molding Benefits from Enhanced CT Scanning
FAI of injection molded parts is an ideal application for CT scanning. In molding, each iteration of measurement and mold modification imposes large costs and significant delay, so anything that increases accuracy and speeds up the process is valuable. Second, plastic of almost any sort is an ideal material to image with x-rays. And with suitable hardware and software, it is equally well-suited when the plastic includes metal inserts.
Enhanced CT scanning is like having x-ray vision making it ideal for non-destructive testing (NDT). It provides the ability to see into a part, and provides dimensional measurement of features and structure without destroying the part. Where CMM touch probing, microscopes, and optical checks are currently utilized, accurate measurements can be taken simultaneously with CT, all in one go.
CT quickly provides the ability to overlap actual scan data with that of the original CAD model – the “perfect part” – showing user-defined pass/fail criteria, comparing actuals to nominals, and graphically displaying out-of-tolerance variances in easy to see colors. The process can be used to reverse engineer existing products, generating files that can, in turn, generate tool-paths for mold, die, or fixture tooling.
One of the most significant benefits of CT for plastic injection parts is it easily tells how much shrinkage has occurred throughout the part, and graphically illustrates precisely where.It can provide complete data on wall thickness and draft angles to help prevent problems in part ejection. The process will identify part warp, voids, and porosity to help detect structural integrity issues and possible points of failure. It will quickly find missing features, identify human error, and help troubleshoot assembly issues.
CT gives the ability to measure mold wear by easily comparing the CAD model or first article inspection image with that of the latest production run to determine the degree of mold wear over the course of the product life-cycle. Images of parts produced late in a production run can be graphically compared to graphical displays of earlier measurements
CT scanning is ideal for analyzing very small parts and very small features that would be difficult or impossible to measure using other technologies. It can be used to scan “micro molds” used for extremely small parts. CT imaging makes small parts look big – easy to see and analyze – where CMM’s are unable to gain access to small features and where the probe styli might be bigger than the feature or part itself. The process can often replace vision systems or microscopy. And the entire process is simplified by eliminating the need for traditional fixturing. Parts can be simply set into a piece of Styrofoam.
Investigative & Failure Analysis
CT X-ray scanning is also ideal for use in failure analysis of systems. It can actually see how assemblies fit together and how their plastic, rubber, and metal parts function together, without the need for disassembling or destroying the object.This allows identification of problems that could not be identified by examining disassembled individual parts.
CT can be used to perform permeability testing to predict vapor or moisture pass-through and measure loss-of-content and shelf-life.This is particularly important in applications like container manufacturing. As container walls get thinner it is important to ensure that they are not so thin that contents are lost through leakage or evaporation, reducing shelf life. Those manufacturing plastic bottle tops for liquids,who need to test for sealing and leakage,can also benefit significantly by using CT scanning.
CT can help deliver cost savings by helping by determining the amount of plastic material being consumed. The thinner the bottle, without leaks yet durable enough for the application, the greater the savings in material. CT provides the ability to look at overall wall thicknesses of a container, all in one scan. Today, more plastics recycling is being done to save on costs, and CT can be used to analyze how many times plastic can be recycled without losing its integrity.
Meeting FDA Requirements
To deal with possible litigation or other regulatory matters, the FDA requires producers of medical products to hold a “first-shot sample” for seven years. The problem is that storing physical parts can require large amounts of space and create a variety of logistical problems. In addition, saved parts can change shape over time. CT scanning provides an unchanging “thumb print” at the time of the molding and processing of the part. Currently, there are discussions with the FDA about the benefits of saving data files with complete part information rather than the parts themselves. In addition, a modern CT scanning system can generate a variety of FDA-required paperwork including data on system installation, qualification, and calibration.
CT scanning offers a variety of benefits to producers of molded plastic parts, often saving hundreds of hours during the overall plastic injection process of mold building, mold tryout, through first article approval. By quickly providing thorough inspection and measurement of first article parts it frees up professional resources, maximizes accuracy, reduces the number of mold remakes, documents all measurements, and helps speed products to market. Modern CT scanning equipment can potentially replace multiple inspection systems and even reduce total equipment cost. It allows actual measurements to be directly compared to references, whether the original CAD model or a previous set of measurements, and graphically highlights variance. It allows inspection of smaller parts than CMM alone and of multi-part assemblies, which CMM simply cannot inspect. And it combines internal and external measurement. In all these ways, it does what no other system, either CMM or ordinary CT, can.
This article was provided by Zeiss. For more information: www.zeiss.com