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= Expand/Improve Article on Hot Plate Welding =

Compare with current Wikipedia article on Hot_plate_welding

Wikipedia page will include a subheading 2.4 Equipment under the heading  Process 

A brief description of various types of joint configurations is included under the heading  Applications 

Added a new heading Non-Destructive Testing of Pipe Welds

Equipment
Typical hot plate equipment consists of two main components which include the clamping fixtures and hot plates. The primary function of the fixtures is to provide support during the welding process. Conventional or standard machines have fixtures that may be changed out to appropriately fit to the parts that are to be welded, whereas custom machines are made for welding one specific component. Hot plate welding machines are usually operated by pneumatic, hydraulic or electromechanical controls. Machines are capable of welding in both the vertical and horizontal position, where longer components are more commonly welded with the latter. PID controllers also assist in maintaining desired temperatures during each process.

Applications
Hot plate welding is used for joining parts ranging from a few centimeters up to 1.6 meters. It is also used for making continuous welds in lining membranes. Its usage can be divided into two main categories, namely production applications and pipe welding. These differ in their equipment and joint designs. One of the common joint designs is a simple butt joint but other joint configurations include the following listed below.


 * Enlarged Joining Surface - Using fillers in welds lowers the overall strength and this can be compensated for by enlarging the joint surface.


 * Butt Joint Flash Trap (Internal) - The bead is covered by a rib or flash trap.


 * Butt Joint Flash Trap (External) - This type of joint hides ides flash on outside surface. Used to reduce noise in surrounding areas.
 * Butt Joint Flash Trap (Double) - Makes surface more visually appealing by hiding the flash on both sides of the weld.

Non-Destructive Testing of Pipe Welds
Polyethylene pipes are desirable over other conventional materials for the transportation of fluids due to their resistance to corrosion leading to a longer lifespan. They are however, limited from being used in nuclear power plants due unreliable NDT methods. Current methods involve using practices that do not provide a complete analysis of a welded PE pipe.

Butt fusion welding produces a small fusion zone and inspection is further complicated due to the high attenuation of PE. Probe placement is also limited during inspection due to the presence of the weld bead. Phased array ultrasonic testing (PAUT) has become a method of conducting non-destructive testing of PE pipes. The PAUT system is primarily made of five primary components. These components are the phased array probe, probe wedge, probe holder, scanner and flaw detector. A minimum of four phased array probes are required for this testing in order for the ultrasonic signal to detect a flaw. The membrane water wedge transmits the ultrasound from the probes into the pipe while minimizing energy loss. The probe holder is in charge of making sure there is proper contact between the wedge and pipe. The scanning system, made specifically for this testing method carries the probe around the joint of the pipe during inspection. Lastly, the flaw detector analyzes the signal from the probe. This method is specifically designed for  inspection of electrofusion and butt fusion welds of various sized pipes ranging from a thickness of 8-65mm and  a diameter of 90-800mm. PAUT is well suited for the detection of:


 * Planar flaws- This may be caused by the welding surface being covered in moisture.


 * Contaminants - Arid and windy environments may lead to particles attaching to the surface of the pipe.


 * Cold welds - This is caused by incomplete or partial intermolecular diffusion which results in a brittle failure. Induced by cold temperatures or if there is a large gap between the pipe and fitting.


 * Under penetration - This type of defect is caused by clamps not being properly secured during the welding process.

Two ISO reports are under development and being reviewed by working group TC 138 (Plastic pipes, fittings and valves for the transport of fluids) to include PAUT as a method of volumetric NDT of PE pipes. A procedure has also been made for UT of butt fusion joints including but not limited to HDPE and MDPE. The ISO and ASME standards are listed as:


 * ISO/DTS 16943 - Thermoplastic pipes for the conveyance of fluids - Inspection of Polyethylene  electrofusion socket joints using the phased array ultrasonic testing method


 * ISO/DTS 22499 - Thermoplastic pipes for the conveyance of fluids - Inspection of Polyethylene  butt fusion joints using the phased array ultrasonic testing method


 * ASME E3044/E3044M1 − 16e1 Standard Practice for Ultrasonic Testing of Polyethylene Butt Fusion Joints