Butt Fusion Welding HDPE Pipe
"The fundamental of heat fusion welding is to heat two hdpe pipe surfaces to an appropriate temperature, changing the resin's molecular structure to an amorphous (pliable state, and then fuse them together by application of prescribed force until cooling occurs, returning the material to a crystalline state and creating one homogeneous pipe."
"When fusion pressure is applied at the designated temperature and prescribed force, the molecules from each pipe surface end mix. As the joint cools, the molecules return to their crystalline form, the original interfaces have been removed, and the two pipes have become one continuous length. The end result is a fusion joint that is as strong or stronger than the pipe itself, and this creates the leak-free joint that is one of the amazing strengths of HDPE pipe."
Generally speaking, HDPE pipe is Butt fused together using a "fusion welder". Welding machines vary depending on the Outside Diameter (OD) of the pipe to be welded. The pipe pieces are held axially by a clamping device to allow subsequent operations to take place. Large diameter pipes may require hoisting assistance such as an excavator or crane. Once the pipe is clamped, the pipe ends are "faced" with a machining tool to establish clean, parallel mating surfaces, perpendicular to the centerline of each pipe. A heating element or heating plate is inserted in between the faced ends, and the pipe is drawn together against the heating plate. A melt pattern that penetrates into the pipe ends is formed around both pipe ends. Once the correct melt temperature is reached, the heating plate is quickly removed, and the melt ends are drawn together with a specified force. The specified force on the joint must be continuous, and held until the joint cools. A small melt bead forms at the joint. At completion, the fused pipe is removed from the welding machine."
Recent US Patents
12/21/2010
7,854,246
Method and device for forming a weld joint
Tesch of Urban GmbH & Co. Maschinenbau, Germany developed a welding method for thermoplastics based on synchronous movement of the parts being welded along with the heating element. (RDC 6/13/2011)
Recent Journal Articles
Analysis of Effect of Pressure and Heat on Mechanical Characteristics of Butt Fusion Welding of Polyethylene Pipes
(907 – 915) Polymer - Plastics Technology and Engineering 50 #9 (2011)
Riahi, Kooshayan and Ghanati of the Iran University of Science and Technology, Iran, conducted 9 different experiments with different heat and pressure conditions and equal timing for each heating stage. Numerical modeling of Finite Element Method (FEM) of the heating process was carried out in the computer software considering governing physical conditions in the weld to determine heat distribution as well as primary bead geometry. From mechanical strength perspective, the weld formed in 210°C and 0.18 MPa pressure demonstrated higher values. However, impact strength in the weld formed in 180°C was higher. By reducing pressure in a specified heating process, value of impact energy required to break the piece was reduced. Mechanical strength in heating condition 240°C was lower than all other cases. As for geometry of the formed bead; its size in 180°C and 0.03 MPa pressure in the first stage of the heating process was smaller than other cases. (RDC 6/9/2011)