Hot plate welding

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.

History

Hot plate welding was first used in the early 1930s for joining PVC.
It gained in popularity with the prevalence of polyolefins, which are difficult to adhesively bond.
By the 1960s, it was among the most widely used plastic welding methods.
Hot plate welding was used for pipelines and appliances as well as injection moldings. Numerous national and international associations for welding have specifications and guidelines for hot plate welding, including the Deutscher Verband fuer Schweissen in Germany, the American Welding Society in the United States, and the Comité Européen de Normalisation in Europe.

Process

Conventional hot plate welding

The hot plate welding process can be divided into four phases: matching, heating, change-over, and welding/forging.
The matching phase serves to match the geometry of the weld surfaces to the theoretical welding plane. The weld surfaces are heated through conduction by physical contact with the hot plate. The hot plate temperature range is above the melting temperature of the material, and a constant pressure between 0.2 and 0.5 MPa is applied against the hot plate. This causes the weld surfaces to conform to the hot plate, which has the desired weld geometry. This also removes surface irregularities that would increase thermal contact resistance. After the parts are in full contact with the hot plate, the heating phase starts and pressure is reduced to a minimum.
During the heating phase, the weld region is heated conductively until melted, without substantial displacement of the material. Pressure is maintained either at a minimum to keep the parts and the hot plate in contact or at zero with a preset displacement. The melt surface reaches approximately below the temperature of the hot plate. The viscosity of the melted material can be controlled through the temperature of the hot plate and the heating time. The surface of hot plate is often coated with PTFE to stop the molten plastic from sticking, which limits the hot plate temperature to.
The temperature of the parts during this phase can be modeled by assuming a constant temperature boundary condition and using the one-dimensional heat equation:
where θ is the temperature, x is the position, t is the time, θ_i is the initial temperature, θ_s is the constant surface temperature, κ is the thermal diffusivity, and erfc is the complementary error function. This model is valid for most cases, since the thermal contact resistance is low and the thermal mass of the hot tool is large compared to the plastic parts. For more precise predictions of the heat flow, the thermal contact resistance and the temperature dependence of the thermal properties of the plastic also need to be considered.
After sufficient heating time, the change-over phase begins. During this phase, the parts are retracted from the hot plate, the plate is quickly moved away, and the parts are brought together. The change-over should be as short as possible, because the melted region cools off during this time.
The welding/forging phase begins when the two molten surfaces are pressed together. This creates intermolecular diffusion of the plastic molecules according to reptation theory. Weld strength is provided by entanglement of the diffused plastic molecules. 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. This pressure is maintained while the melted material cools and resolidifies. During this, some plasticized material in the weld zone is squeezed out, forming flash. Mechanical stops may be used to limit the amount of squeezed out material in order to prevent a cold weld.

Variants

Common variants of conventional hot plate welding include high-temperature and non-contact versions. Both of these variants help with the problem of material sticking to the hot plate between weld cycles; the stuck material can degrade and transfer to subsequent welds, resulting in poor quality and aesthetically unappealing welds.
With high-temperature hot plate welding, an uncoated hot plate is heated to between, as the PTFE coating degrades at high temperatures. The high temperature decreases the viscosity of the melt, so it can peel off from the hot plate when removing the parts. 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. Any residual material on the hot plate's surface is usually either oxidized away or mechanically removed. With some thermoplastics, the residual material cannot be easily removed and accumulates over time. The hot plates may need to be removed and cleaned between cycles. With the higher temperatures, the matching and heating phases are shortened from those of conventional hot plate welding. However, reduced weld strength from thermal degradation of the plastic can still occur, though most of the degraded material is forced out by the flow of melted material. High-temperature hot plate welding is known to perform well for:

PP and PP copolymers
ABS and PMMA

With non-contact hot plate welding, the weld surfaces are melted without physical contact with the hot plate through convection and radiation heating. The hot plate temperature is between, and the weld surfaces are placed about from the hot plate. Heat input needs to be controlled to prevent thermal degradation while plasticizing the material. This variant has no matching phase, so part fit must be good prior to welding, with part deviation not exceeding. In practice, non-contact hot plate welding is only used for small parts whose dimensions do not exceed. An additional consideration is the stack effect when the hot plate is oriented vertically, which can cause uneven heating of the weld surfaces.
Another variant is hot wedge or hot shoe welding for joining thin sheets with lap seams. A heated wedge travels between the two sheets and melts the weld surfaces while wedge rollers apply light pressure to force intimate contact; drive rollers apply pressure at the tip of the wedge where the sheets converge to form a continuous seam.
Hot wedge welding can produce either single or dual seam joints. For dual seam joints, a split wedge that is unheated in the middle is used. This leaves an unwelded air pocket between the seams that can be pressurized to nondestructively test the joint integrity. With hot wedge welding, the speed of travel is an added parameter as the wedge unit is self-propelled by the rollers. The typical temperature range when welding high-density polyethylene is ; the travel speed is typically.

Parameters

Parameters used in hot plate welding are the hot plate temperature, the pressure during matching, the pressure during heating, the pressure and displacement during the weld phase, and the times for matching, heating, change-over and cooling. These parameters have an interdependent effect on the weld quality and cannot be set individually.
The hot plate temperature is taken at the surface of the plate. It is set based on the hot plate welding variant along with the properties of the material, including melting temperature, melt viscosity, and thermal degradation limits. Conventional hot plate welding uses temperatures above the melting temperature. The high-temperature variant uses temperatures above the material's degradation temperature, about above the melting point. The non-contact variant uses temperatures above the melting point. 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. During the heating phase, a minimum pressure is maintained to keep the parts in contact with the hot plate, as a larger pressure would squeeze out material. The welding pressure brings the molten weld surfaces into intimate contact and squeezes out entrapped air. Too high a pressure would squeeze out most of the hot material from the joint, leaving cooler material to form a cold weld. Too low a pressure limits intermolecular diffusion and produces a weak weld. A mechanical stop may be used in the welding phase to limit the amount of material squeeze out by varying the welding pressure.
The matching and heating times control the amount of heat input during those phases. The matching time is set so that surface irregularities are melted and removed. The heating time determines the melt layer thickness. Too thick a melt results in excess flash and unfavorable molecular orientation at the joint interface. Too thin a melt produces a brittle weld. 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. Typical change-over times are around 2 to 3 seconds, even for large parts. Cooling time refers to the time until the joined parts have solidified and can be removed from the machine. The welded part should not be stressed until it has further cooled until room temperature.