‘A Comparative Study of the Integrity of Joints Between Multilayer Fuel Line Constructions and Different Connector ‘Barb’ Designs

With the advent of low evaporative emission requirements there has been the rapid adoption of multilayer extrusion technology into the production of Fuel and Vapour tubing used on Fuel systems on automobiles. Multilayer extrusion technology enables a manufacturer of Fuel and Vapour tubing to simultaneously co-extrude dissimilar thermoplastic materials in tubular form. This allows the manufacturer to combine expensive and brittle high performance evaporative emission ‘barrier’ polymers with lower cost engineering polymers.

However, it is a well-known characteristic of these multilayer tube constructions that the joints between them and connector ‘barbs’ have lower joint integrity. Joint integrity is most often quantified by ‘Pull-off’ and leakage tests. Recent developments in LEV-II requirements for 2004 and beyond indicate that joint integrity will become a focus area for study and improvement.

The authors set out to study by comparison the joint integrity of a number of different connector ‘barb’ designs in relation to monolayer ‘flexible’ Pa12 tubing (currently the industry standard) and two multilayer constructions.

Comparative ‘Pull-off’ and leakage test data was obtained and a comparative study made of the effects of temperature on the ‘Pull-off’ loads obtained. In addition a comparative study was made of the following in relation to the comparative test data:

The dimensional differences between the different ‘barb’ designs.

• The Burst Values of the different tubes - a measure of the ‘Hoop Stress’ applied by the tubes at the joints.
• The Hardness of the different materials in contact at the joints.

An explanation was made of the observed differences with recommendations for possible new areas for study