Optimizing the mold trial process using METROTOM tomography measurement technology

How does a set of injection molds achieve the transition from design to production? Most mold practitioners in the industry start with a product CAD model and eventually produce the corresponding mold. At this moment, the mold is installed on a precision injection molding machine, some plastic pellets are injected into the mold, and practitioners gather eagerly to see the plastic parts produced. The first part looks good but there is obviously still a lot of room for improvement. The injection molding engineer intervenes and uses his professional skills and experience to find the best parameter values ​​among the injection molding parameters. After the mold trial - including basic testing, you will get some pretty good parts - at least they look good.

Exploration - Weeks in the Quality Lab

Perhaps you are now focusing on the efforts to produce such plastic valve components and continuously optimizing the machine parameter settings using the design of experiments (DOE). To do this, you organize your technical think tank, study the key variables, set and standardize the DOE and let your injection molding engineer be responsible for the project implementation. Then many samples are produced - each one is produced based on different parameters, so each plastic part has different degrees of difference or change relative to other samples. The key is how much difference they have and where they have changed?

For the vast majority of mold industry practitioners, the necessary tools to discover such changes come from your materials and quality laboratory. You may use contact and non-contact coordinate measuring machines (CMMs) to measure visible or tactile features of samples. You perform various destructive tests through sealing, slicing and grinding, so that you can use microscopes, optical measuring instruments, CMMs and manual measurement tools to observe "hidden" features inside the material, or you may use traditional X-ray machines to detect the number of pores inside plastic parts and observe changes in wall thickness. The

common complaint of practitioners about destructive testing (sealing/grinding, etc.) is that the root problem of traditional laboratories is that the process of handling samples is usually "destructive".
Depending on the complexity of the sample/mold, the measurement cycle may take more than 3 weeks. If an expensive precision injection molding machine has to wait for the results of this experiment, or worse, it has been used for mass production of parts before the measurement results come out, we can't help but ask if there is a better solution?

Effective solution:

You may have heard of CAT (Computer-Assisted Tomography) and experienced its scanning process in the hospital. Its principle is to use X-rays to penetrate the human body and generate a three-dimensional image of the desired part. At the same time, there is a similar technology in the industrial field called CT (Computerized X-ray Tomography). The difference between the two lies in the energy of the X-rays, system parameters and image reconstruction algorithms.

For those in the mold industry who have never tried to use a CT system to scan plastic parts, you will see a dazzling image effect (Figure 1). Compared with traditional measuring instruments, this system is faster and does not require complex fixtures, thus eliminating the deformation of plastic parts caused by clamping force. [page]

METROTOM system

The new generation of CT measurement system from Zeiss in Germany has been fully promoted and widely used around the world. In view of its technological innovation, a new term "METROTOMOGRAPHY" was born to accurately describe this high-tech new technology. The METROTOM system is designed by the German Zeiss Industrial Measurement Department, including high-performance CMM technology, such as precision guides, drive systems, control systems, ultra-high precision turntables and the leading Calypso universal measurement software platform in the industrial field, which will give this system:

◆ The most important thing is: excellent measurement accuracy - for typical plastic parts, its measurement accuracy can start at 9 microns.
◆ The measurement results have excellent repeatability - the METROTOM system will pass the repeatability and reproducibility (GR&R) test.
◆ The equipped software platform can fully realize ANSI Y14.5/ISO GD&T geometric tolerance measurement and evaluation.
◆ Scanning can be completed within 60 minutes - if the sample volume is small enough (such as various medical plastic parts), more than one component can be scanned each time.
◆ This scanning process is non-destructive, and the sample can be retrieved after the scan is completed, and your sample is intact.

Typical applications

are based on the principle of X-rays, and the internal and external structures of the sample can achieve the best visualization effect. By scanning the entire workpiece once, 100% of the feature information can be accurately and quickly obtained, and more importantly, this information can be widely used in:

◆ Digital model comparison between samples and CAD models.
◆ Defect analysis: porosity and qualitative and quantitative analysis.
◆ Assembly inspection: internal assembly status analysis.
◆ Reverse engineering: generate STL for subsequent processing of CAD data.
◆ Rapid model repair based on digital model comparison.
◆ High-precision 3D geometric dimension measurement technology.

Case analysis:

Place the sample on a rotating worktable, and after precise rotation (30-60 minutes), complete 3D data can be generated. The CT 3D voxel metadata and the CAD model initially have different coordinate origins and are located in different spatial positions.

Unlike traditional industrial CT systems, the system does not have traditional data processing and scaling errors. In any case, the METROTOM system can provide MPE-E (measurement error) starting from 9+L/50 microns. Its excellent measurement performance covers the characteristic elements inside and outside the workpiece. MPE-E is the acronym of the ISO international standard for "maximum permissible indication error" in geometric measurement, and is used to describe the traceable system measurement error (precision). The

Calypso measurement software platform uses a professional mathematical reference system (DRF) to quickly align the CAD model data (Figure 2). Once the alignment is completed, the measurement data and the CAD model are perfectly overlapped, and the CT 3D voxel data set becomes the "boundary" of the CAD surface, which reveals the basic geometry of the sample. This realistic image effect helps engineers quickly identify the characteristic elements of small plastic parts, which may usually be overlooked due to their size.

The Calypso software platform is characterized by not measuring based on STL data files - the STL format is traditionally used for visualization applications, but it cannot accurately define the 3D geometric dimensions of the parts. However, Calypso software can directly measure with high precision based on the original CT 3D voxel images.

Once the data set is aligned with the CAD model, the digital model analysis can begin, and the deviation status of all internal and external surfaces relative to the CAD model can be displayed graphically (through color gradients), and even the deeper hidden positions inside the part can be observed. The non-destructive analysis software tool can perform cross-section analysis of the plastic part from any direction. The sectioning tool can also reveal the distribution of pores inside the material. The

scanning and analysis of the plastic part can be completed in a few hours. Usually, the deviation spectrum analysis diagram helps to quickly analyze the product quality, but if you really need a complete first-article measurement report, you can use the powerful functions of the Calypso measurement software platform to provide in-depth sectioning visualization tools to achieve rapid offline measurement plan preparation, for example, such as fluid valve plastic parts. Through a powerful geometric dimensioning and tolerancing (GD&T) mathematical operation engine, the Calypso measurement software platform can process and calculate all dimensional features and provide inspection reports in a variety of formats. Figure 3 shows a measurement report in Excel format, while Figure 4 shows a visual report of the measurement value of the measured feature element with an arrow pointing to it. The measurement report can not only show whether the measurement value is qualified or out of tolerance in color, but also show the percentage of the corresponding tolerance occupied by this measurement value.

METROTOMOGRAPHY: Several points of accuracy that need attention

: Resolution and measurement accuracy are not the same concept - this is particularly important. For example, high resolution can never compensate for the accuracy of a mechanical system. The key is that the results of the measurement of this system must meet the corresponding international standards. You can ask the equipment manufacturer to explain to you the measurement accuracy of their X-ray tomography system. [page]

Repeatability: If you are evaluating a new METROTOM measurement system, consider whether the measurement system can provide repeatable and operator-independent measurement results. A good rule of thumb for judging the overall performance of a measurement system is the measurement repeatability and reproducibility test (GR&R).

A good rule of thumb is that your measurement results must be repeatable to 3% of the feature tolerance and achieve a GR&R of 10%. This type of comprehensive performance test will undoubtedly take a long time - sometimes 3 to 4 days to complete a full GR&R. However, it is an excellent rule for discovering measurement problems and ultimately avoiding critical errors.

Speed: If you measure, you must do it quickly - remember that the goal is to complete the trial verification of a new mold in one day - not a week or more. The METROTOM system can scan a plastic part (including image reconstruction) in less than an hour.

Software: Even a fast, accurate and repeatable system will not be the best system solution for the measurement object if it does not provide accurate measurement results. The measurement software platform must support various CAD data file formats, have a powerful GD&T computing engine, and provide functions such as fast programming and program editing, report output, and most importantly, support the measurement of CT volume element data sets.

Summary: When you see the digital images obtained by tomography, don't just believe what your eyes see, you need other information - especially if you want to use the data to make important decisions. Usually image processing involves complex processing algorithms, but often you only need to load a two-dimensional image into any image editing software to understand the ability of image operations to process data. By changing parameters such as contrast, different image effects can be generated and completely different results can be obtained. The clear images provided by the software are a good starting point, but if you want to achieve precise measurements or reverse engineering based on this data, this is just the beginning. (end)