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.

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

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