Molten surface

The laser spray process uses a high power carbon dioxide laser focused onto the surface of the part to be metallized. A carrier gas such as belium blows metal particles into the path of the laser and onto the part. The laser melted particles may fuse to the surface, or may be incorporated into an aHoy in a molten surface up to 1-mm thick. The laser can be used for selective aHoying of the surface, for production of amorphous coatings, or for laser hardening. 

The powdered raw material TiB, is deposited in a hot gas onto the molten surface of the crystal being grown. An arc is used instead of a combustion heat source. The growing crystal is shielded from air with Ar. 

The crystal grows when a supply of powdered raw material is dropped through the flame onto the molten surface. The powder is added to the flame from a screen, which is tapped intermittently. 

The time needed for polymer chains of amorphous thermoplastics above Tg and semicrystalline thermoplastics above Tm to diffuse across the interface and randomize is relatively short compared with the time needed for resin flow. It is believed, therefore, that diffusion bonding is completed immediately after the two molten surfaces merge, and that the microstructure of the contact zone is also assumed to be identical to that of the intraply sections [12,13],... 

Table 4. Chemical composition of molten and non-molten surface locations after two fluidisation runs.

When an optical pyrometer is sighted on a glowing material in the open it reads too low. Certain materials, important industrially, have a very high emissivity so that the corrections necessary to add to the observed temperatures to convert them into true temperatures are small. Thus with iron oxide the correction is only 10° at 1,200°C. The corrections are very large for clear molten metals, but are smaller for the oxides which soon form on the molten surface when exposed to the air. Table 12 shows the true temperatures corresponding to the temperatures observed when sighting on certain materials in the open. For temperature control it is unnecessary to apply these corrections. The observed temperatures, although known to be low, will be low by the same amount from time to time and hence will serve just as satisfac-... 

Thermal interlining can be made by flame bonding thin-sliced, low-density (1-1.5 Ib/ft ) polyester-based fabric. The flame melts about one-third of the foam thickness, and the molten surface adheres to the fabric. 

Oxides of iron, aluminum, copper, zinc, and glass often form on their molten surfaces, becoming inclusions in the final casting, probably causing it to be a reject. It is therefore desirable to minimize excess oxygen in contact with a molten metal bath thus, a quality air/fuel ratio controller can be a major help in controlling product quality. 

Some phosphorus compounds decompose in the condensed phase to form phosphoric or polyphosphoric acids. These can act as dehydration catalysts, reacting with cellulosics for example, to form a good char. Char yield is also increased with rigid polyurethanes. The polyphosphoric acid can also form a viscous molten surface layer or surface glass. This layer can shield the polymeric substrate from the flame (heat) and oxygen. Intumescence, which requires an acid such as phosphoric acid, results in a dense carbon char on the polymer surface protecting the substrate from heat and oxygen. 

Polystyrene modified polyphenylene oxide (PPO) or Noryl can be hot plate welded at 260 to 288°C and 20 to 30 s contact time. Unmodified PPO can be welded at hot plate temperatures of 343°C. Excellent spin welded bonds are possible with modified polyphenylene oxide (PPO), because the low thermal conductivity of the resin prevents heat dissipation from the bonding surfaces. Typical spin welding conditions are rotational speed of 40 to 50 ft/min and a pressure of 300 to 400 psi. Spin time should be sufficient to ensure molten surfaces. 

Fire polishing. In this techniques, discrete solid particles are heated to the softening or melting temperature of the material, usually between 1200 and 1650°C, while suspended and dispersed in a hot gaseous medium (e.g., fluidized bed). As particles become soft or molten, surface tension forms them into an ellipsoidal shape. If kept in suspension until cooled below softening temperature, the particles maybe recovered as spherical grains. 

From equation (9) we can derive the expression for the approximate evaluation of the equilibrium thickness of a molten surface layer as a function of the temperature... 

Fig. 2. Heterogeneous melting of lead the estimated dependency of the thickness of the molten surface layer on temperature.

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