PTB >> Laser Scanning Improves Dimensional Accuracy of Automotive Gas Tanks

A GKS technician inspects a new gas tank prototype using a laser scanning system.

In the past, one of the world's leading manufacturers of fuel tanks used coordinate measuring machines (CMMs) to inspect first articles. The geometry of the tanks is so complex, however, that it was difficult to fully inspect the surface one point at a time. Inergy Automotive Systems has achieved significant improvements in quality by switching to laser scanning, which "paints" the surface of the tank with a laser and then uses a sensor to capture all the points in the laser's path. The new approach generates a solid model of the as-built part that can be compared to the design intent to highlight the full extent of any differences between the two. As a result, Inergy can make required tooling changes with a higher level of confidence and ensure that production parts meet customer specifications.

In this three-dimensional perspective of a gas tank overlaid with CAD, green color indicates areas that are within tolerance.

An automotive fuel tank is one of the most difficult parts of a vehicle to inspect because of its complicated geometry. When designing the vehicle, fuel tanks are often one of the last parts to be designed because their functionality does not depend on their shape. So, after the other parts have all been designed, the fuel tank shape is designed around the other components to fit the available space. This explains why fuel tanks have few uniform features and instead incorporate many different contours that typically match whatever space is available around the other vehicle components. However, after the fuel tank has been designed, each of its many geometrical features must be positioned perfectly in order to avoid interfering with other components while at the same time ensuring that the tank provides the full stated capacity.

Blow Molding Process
Gas tanks are typically produced by a two-stage blow molding process. In the first stage the parison, a cylindrical tube of polymer, is extruded. Gravity and die head motion shape the parison thickness profile, which is usually nonuniform. Often several concentric parisons are extruded in order to provide multiple layers that help meet current and future EPA and California Air Regulatory Board (CARB) regulations. After extrusion, the parison is positioned between the two halves of a mold. Then, in the second stage, the parison is closed by the two mold halves and air is pumped in to expand it to fit the shape of the mold. When the two halves of the mold are opened, a completely formed tank is removed that has the precise shape of the mold.

Inergy Automotive Systems is a leading manufacturer of fuel systems. Headquartered in Paris, France, the company employs 4,500 people in 25 facilities located in 18 countries. In 2005 Inergy delivered more than 13.3 million fuel systems, generating revenues of $1.3 billion Euros. Inergy's plants in North America produce fuel tanks using multiple extruders to produce six different layers of material that are combined as they are extruded from the head of the blow mold machine. It's critical to try out the mold, accurately measure the geometry of the tanks that it produces, and make corrections when needed to the mold to ensure that the final tank geometry matches the customer's design.

Comparing the laser generated 3D image of a fuel tank prototype to the CAD model shows designers which areas need to be corrected

CMM and Laser Scanning Inspection Methods
In the past, first article inspection was done with a CMM. The operator moved the contact probe around the surface of the tank and the machine recorded the position of each point. The use of CMMs to inspect parts is time-consuming because of the need to manually move the machine probe into position for each individual point to be measured. As the geometrical complexity of the part increases, the number of points needed to fully characterize the geometry skyrockets. It takes many thousands of points to fully define a surface as complicated as a fuel tank, so it could take several days to measure one complex tank. Even if operators spend several days generating thousands of points, they can never be sure that they haven't missed a critical feature. It's always possible to miss a point that would have otherwise called out a difference between the original computer aided design (CAD) model and the prototype.

Laser scanning systems work by projecting a line of laser light onto surfaces while cameras continuously triangulate the changing distance and profile of the laser line as it sweeps along, enabling the object to be accurately replicated. The laser probe computer translates the video image of the line into 3D coordinates, providing real-time data renderings that give the operator immediate feedback on areas that might have been missed. Laser scanners are able to quickly measure large parts while generating far greater numbers of data points than probes without the need for templates or fixtures. Since there is no contact tip on a laser scanner that must physically touch the object, the problems of depressing soft objects, measuring small details, and capturing complex free form surfaces are eliminated.

Instead of collecting points one by one, the laser scanner picks up tens of thousands of points every second. This means that reverse engineering of the most complicated parts can often be accomplished in an hour or two. Laser scanning can reverse engineer parts that are so complex that they would be practically impossible to do one point at a time. Finally, the software provided with the scanner greatly simplifies the process of moving from point cloud to CAD model, making it possible in minimal time to generate a CAD model of the scanned part that faithfully duplicates the original part. Special software can be used to compare original design geometry to the actual physical part, generating an overall graduated color error plot that shows at a glance where and by how much surfaces deviate from the original design. This goes far beyond the dimensional checks that can be performed with touch probes on CMMs.

Working With a Service Bureau
"We considered purchasing a laser scanner but decided that both the initial investment and the cost of maintaining a trained operator on staff would be difficult to justify considering our relatively small volume of work," said Glenn VanVolkom, Validation Coordinator for Inergy Automotive. "Then we heard that GKS Inspection Services was operating a service bureau in the Detroit area that provides laser scanning services on a fast-turnaround basis at an economical cost. This seemed like a good opportunity to evaluate laser scanning without having to make a major investment.

The next time we produced a first article we sent it to GKS. They scanned the tank and then processed the resulting point cloud to provide a surface model of the as-built part. They sent the model back to us along with free viewer software. We were very impressed with how the model represented the complete geometry of the part rather than just the points that the operator had decided to measure. It took less than a week to get the model to us and since then GKS has demonstrated the ability to provide even faster turnaround when necessary."

As a next step, Inergy sent the CAD model representing the design intent to GKS. GKS then used Geomagic Qualify (Research Triangle Park, NC) software to superimpose the CAD and as-built models and call out the differences between them. The error plot highlighted several areas in the initial prototype that were too far from the target dimensions. Being able to view the design intent and as-built models on top of each other made it easy to determine what changes to the tooling and cooling fixtures were needed to correct the parts. Based on the success of the first project, Inergy now sends first articles to GKS on a regular basis.

"In the past all we had was dimensions of different points so it was difficult to determine exactly what changes needed to be made," VanVolkom said. "Often we had to change the tool several times because the first change did not completely fix the problem. Now we can see the complete surfaces rather than just a few points. This makes it possible to visualize the entire extent of the discrepancy so we can be much more precise in our tooling changes. We also save money on first article inspection because the cost of the service bureau is considerably less than what it cost to do the job internally on a CMM. The experience in working with GKS has been very good. They have demonstrated their commitment to meeting our requirements and they are very nice people to work with."

This article was written by Jim Andrews, Supervisor of Engineering and Laser Scanning Services, GKS Inspection Services Inc. (Detroit, MI). For more information, contact Mr. Andrews at jandrews@gks.com, or visit http://info.hotims.com/10976-201.