Hot-bar welding

Hot-bar welding is a technique for the sealing of films. A heated metal bar applies pressure to the films, softens the plastic at the joint and forms a weld. The equipment consists of one or two electrically heated bars, one of which is hinged for the insertion and removal of the films. The film is placed on the base bar, the upper bar is brought down and pressure is applied either mechanically by the operator, or pneumatically. After the required weld time, the bar is lifted to release the joined films. The bars are often PTFE-coated to prevent molten polymer from adhering to them. 

Hot-bar welding is a rapid process. Typical weld times for a 100-p,m thick sheet are in the range from 1-3 s. A reduction in welding time can be achieved by the use of two heated bars, one on either side of the films to be joined. The necessity for the heat to conduct through the film to the joint imposes a restriction, of approximately 1 mm, on the thickness of material that may be welded. 

Hot-bar welding is widely used in industry, mainly in the joining of thermoplastic films having a thickness of less than 0.5 nun. It could be used to weld thin composites and has the potential to tack prepregs. It is used extensively within the packaging industry for producing plastic bags. 

Impulse welding is an advanced form of hot-bar welding in which both the heating and cooling regimes are controlled while the joint is stUl under pressure. 

There are numerous extensions of these basic techniques, such as hot wire instead of a hot bar, pressure clamps instead of rollers, etc. Methods (a), (b) and (c) are applicable to sheet welding, whilst method (d) is more readily adaptable to mouldings and extrusions. 

Figure 16.1 shows part of a steel tank which came from a road tank vehicle. The tank consisted of a cylindrical shell about 6 m long. A hemispherical cap was welded to each end of the shell with a circumferential weld. The tank was used to transport liquid ammonia. In order to contain the liquid ammonia the pressure had to be equal to the saturation pressure (the pressure at which a mixture of liquid and vapour is in equilibrium). The saturation pressure increases rapidly with temperature at 20°C the absolute pressure is 8.57 bar at 50°C it is 20.33 bar. The gauge pressure at 50°C is 19.33 bar, or 1.9MN m . Because of this the tank had to function as a pressure vessel. The maximum operating pressure was 2.07 MN m" gauge. This allowed the tank to be used safely to 50°C, above the maximum temperature expected in even a hot climate. 

Metallic cannon were originally made by welding bars of iron longitudinally and binding them by rings, which were shrunk on over them while hot, — a plan which, with some modifications, has been revived of late years, and seems more feasible in the present state of the arts than it was 500 years ago. 

If welding is to take place inside the column, the officer responsible for safety must ensure absence of any flammable materials, and compliance with any additional requirements (e.g., presence of fire extinguishers). He should also ensure that personnel entry to areas affected by sparks is properly barred. In addition, he should spot-check the welding to ensure that smoke and fumes do not become too thick to be hazardous. When satisfied that all is safe, the officer can issue a written permit for hot work inside the column. The permit should be valid for no longer than one shift, or at most, one day. 

After performing the above described modifications the seal of the horizontal turboset flange as well as the front seal of the shaft seal carrier proved their reliability. The measured leak tightness of these flange joints was better than 10 Torr 1 per second. The weld lip seals of the hot gas duct were tight. Thus the helium losses against the ambient environment amounted to 10 - 20 NmVd at 51 bar (out of total helium inventory of about 8000 Nm ). 

Welding by Hot Gas A gun constructed like a welding hammer is used, through which flows a stream of electrically heated air or nitrogen. The temperature reaches 200 C-320°C. (It is mainly used for PE or PVC.) A welding bar, made of the identical polymer is commonly used to fill in the binding area. One must avoid degradation. 

Intrinsically conductive polyaniline (PANI) composite gaskets were used to microwave (2.45 GHz) weld high density polyethylene (HOPE) bars [ 148]. Two composite gaskets were made from a mixture of HDPE and PANI (50 and 60 wt%) powders in different proportions. The mixtures were compression molded in a hot (180 °C) press. Adiabatic heating experiments were used to estimate the internal... 

In the long-term tensile test on tensile test bars with dual hot wedge seams and extrusion fillet seams the plane of the seam is not perpendicular to the tensile direction, as in butt welded pipe seams, but parallel. The force flow in the loaded specimen is directed over the weld seam from one geomembrane into the other in an offset direction, therefore force flow lines get more dense in the boundary region of the weld seam and a stress concentration develops. In the boundary region there are also noteh effects induced by the transition from the welded material area to the base material. A eraek that develops there... 

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