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Copied from Hot plate welding

The following has been added to the Hot Plate Welding Wikipedia page

Equipment and Joint types are sub-headings under the process section, included a section on NDT. Added sentence to lead section to include a brief statement on NDE techniques.

Hot plate welding, also called heated tool welding, is a thermal welding technique for joining thermoplastics. A heated tool is placed against or near the two surfaces to be joined in order to melt them. Then, the heat source is removed, and the surfaces are brought together under pressure. Hot plate welding has relatively long cycle times, ranging from 10 seconds to minutes, compared to vibration or ultrasonic welding. However, its simplicity and ability to produce strong joints in almost all thermoplastics make it widely used in mass production and for large structures, like large-diameter plastic pipes. Different inspection techniques are implemented in order to identify various discontinuities or cracks.

Equipment

Hot plate welding equipment consists of two main components, a clamping fixture and one or multiple hot plates. The primary function of the fixture is to provide support during the welding process to prevent deformation under welding pressure. Conventional machines have fixtures that fully conform to the parts being welded and allow for flexibility in production by accepting different fixture configurations. Custom machines may be configured to weld a specific component and do not provide as much flexibility as standard machines.

Hot plates are generally designed for specific working temperatures. Hot plates for conventional hot plate welding have a working temperature of at least 270 °C and are made from aluminum alloys. The hot plates may also be coated in Polytetrafluoroethylene (PTFE) to prevent sticking of the polymer to the hot plate. Caution should be taken as the PTFE coatings degrade over time and sets of interchangeable fixtures should be available during continuous operation. Hot plates for high-temperature hot plate welding have a maximum working temperature of 430 °C and are made from aluminum bronze alloys. Due to the lower thermal conductivity of these alloys precaution must be taken to ensure that there is uniform heating along the hot plate surface. PTFE has a maximum working temperature of 270 °C therefore, non-stick coatings cannot be used for this type of operation. Lastly, hot plates for non-contact hot plate welding are used for temperatures up to 550 °C are made from either aluminum bronze or stainless steels.

Hot plate welding machines are generally operated by pneumatic, hydraulic or electromechanical controls. Machines can be configured to perform welds with the faying surface in either the horizontal or vertical position. Longer components such as pipes are more commonly welded in the horizontal position whereas moldings with internal fittings such as a starter battery are welded in the vertical position. A proportional-integral-derivative (PID)  controller also assists in maintaining desired temperatures during each process.

Joint Type

Thermoplastic Joint Designs

While there are various joint configurations, a butt joint in which the two joining materials are aligned along the same plane is one of the most common joint designs for thermoplastics. There are various modifications of this joint that are implemented for different applications which include the following listed below.

• Enlarged Joining Surface - Using filler materials 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 flash on the outside surface and is generally used to reduce noise in surrounding areas.
• Butt Joint Flash Trap (Double) - Makes the surface more visually appealing by hiding the flash on both sides of the weld.

Non-destructive Testing (NDT)

There are two methods of testing including non-destructive and destructive testing. While the quality of a weld can only be determined through destructive means, NDT allows for the determination of defects in the welded region. The following section will highlight some of the non-destructive methods used in welding of plastics.

Visual inspection

Visual inspection testing can only be used to detect flaws on the surface of the weld but is the least expensive method of NDT. This method of inspection may be performed both during and post welding. During welding the operator is inspecting for discoloration, misalignment, notches and other surface discontinuities. Post welding inspection allows the operator to inspect for microstructural features which may be detrimental to the welded part.

X-ray testing

X-ray testing is a costly method of inspection; therefore, it is generally limited to pressure vessels and pipelines carrying hazardous materials. This method is most effective when the densities of the imperfection and the plastic have a substantial difference and is used for detection of voids, inclusions and other imperfections. A disadvantage to this method is that microstructural defects cannot be determined through this method of testing.

Leak-tightness test

This method of testing is most often used for welded pipes and other closed containers. There are different variations of this test which are dependent on the type of medium (water,air,gas) used to pressurize the sample. It is common to conduct this test in vacuum conditions.

High-voltage test

High voltage testing known as “spark test” is an alternative to the leak-tightness test. This test in performed by coating the weld with an electrically conductive substance such as a wire, fibers or coils. When a voltage is applied an arc will form, showing the presence of a leak. This test is not well suited for polar thermoplastics such as PVC as they will generate heat leading to potential degradation of the weld.

Ultrasonic testing

Ultrasonic testing uses high frequency waves that travel through the welded regions. These waves are able to detect defects based on the different densities between the imperfection and plastic part. There are two primary methods of conducting ultrasonic testing and that is by using a transmitter and receiver in conjunction or by using an ultrasonic transducer. These conventional methods similar to X-ray testing are not able to detect microstructural changes in the weld. Advanced ultrasonic testing such as phased array ultrasonics (PAUT) is currently being developed for inspection of hot plate and electrofusion joints.

Polyethylene (PE) pipes are desirable over other materials such as metals for the transportation of fluids due to their resistance to corrosion leading longer lifespans. 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.

Using a butt joint configuration produces a small fusion zone and inspection is further complicated due to the high attenuation of PE. Proper probe placement is also limited during inspection due to interference with the weld bead. The PAUT system has 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 the ultrasonic signal to detect a flaw. The membrane water wedge transmits the ultrasound from the probes into the pipe while minimizing energy loss and the probe holder ensures 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 technical committee (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 medium-density polyethylene (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

References

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I made an attempt to do research on joining/welding methods of SMP but unfortunately I could not find much/any work on this. Some suggestions include FSW but I could not find any concrete research on this.

I know we discussed a couple of other topics such as SLA 3D printing of photopolymers. I was also thinking of expanding on shape memory polymers and their use for space application. If there is any other suggestions you recommend

please let me know.

Instructor Comments

All of your topics are already covered fairly well in Wikipedia. Therefore, I assume that you would like to improve or expand these articles. I will go ahead and assign to you expanding the Shape Memory Polymer article to include welding/joining, but please let mw know if you would prefer another topic.

Topics for Wikipedia:

• Shape Memory Polymers
• Polyether Ether Ketone (PEEK)

• Stereolithography (photopolymers)
• Selective Laser Sintering
• Composite Materials (aerospace applications)