Hot plate 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.
After the parts are in full contact with the hot plate, the heating phase starts and pressure is reduced to a minimum.
The necessary welding pressure depends on the melt viscosity and wall thickness of the parts and usually ranges between 0.025 and 0.05 MPa.
Common variants of conventional hot plate welding include high-temperature and non-contact versions.
[1] The high temperature decreases the viscosity of the melt, so it can peel off from the hot plate when removing the parts.
[1] This can be accompanied during the change-over phase by rapid movement of the parts from the hot plate; this prevents stringing of the melted plastic due to its viscoelastic properties.
With the higher temperatures, the matching and heating phases are shortened from those of conventional hot plate welding.
In practice, non-contact hot plate welding is only used for small parts whose dimensions do not exceed 100 by 100 millimetres (3.9 by 3.9 in).
[1] An additional consideration is the stack effect when the hot plate is oriented vertically, which can cause uneven heating of the weld surfaces.
[1] With non-contact welding, the radiation heating depends not only on the temperature but also on the emissivity of the hot plate material.
The pressure during the matching phase removes warpage of the weld surfaces to ensure full contact with the hot plate without causing the parts to deform.
The change-over time determines the temperature of the melted material as welding begins and, therefore, should be as short as possible to minimize surface cooling.
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.
[1] Hot plate welding machines are generally operated by pneumatic, hydraulic or electromechanical controls.
High water content can lead to the formation of bubbles during heating and joining, reducing weld strength.
Common thermoplastic combinations include:[1] Hot plate welding is used for joining parts ranging from a few centimeters up to 1.6 meters.
[1] The cases and lids of automotive batteries are made of thin PP copolymers, which have low melt viscosity.
[1] Other automotive components welded by hot plate are carburetor floats, coolant and washer fluid reservoirs, and ventilation ducts.
[5] With this joint, the welding pressure is supplied by the interference fit of the pipe and socket, so these parts as well as the heating tools need to be within tolerance.
Visual inspection testing can only be used to detect flaws on the surface of the weld but is the least expensive method of NDT.
X-ray testing is a costly method of inspection; therefore, it is generally limited to pressure vessels and pipelines carrying hazardous materials.
[1] There are different variations of this test which are dependent on the type of medium (water, air, gas) used to pressurize the sample.
This test in performed by coating the weld with an electrically conductive substance such as a wire, fibers or coils.
This test is not well suited for polar thermoplastics such as PVC as they will generate heat leading to potential degradation of the weld.
[7] 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.
Current methods involve using practices that do not provide a complete analysis of a welded PE pipe.
[8] Using a butt joint configuration produces a small fusion zone and inspection is further complicated due to the high attenuation of PE.
The scanning system made specifically for this testing method carries the probe around the joint of the pipe during inspection.
[8] 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.
[10][11] A procedure has also been made for UT of butt fusion joints including but not limited to HDPE and medium-density polyethylene (MDPE).
