High-velocity melt

When an injection mold fills, the incoming high-velocity melt stream is resisted by, and must displace, the air in the feed system and cavities. Molders often rely on incidental air gaps between the parting faces and between the assembled parts of the core and cavity to provide a leakage path for air, but this is no substitute for properly engineered venting which should be... 

Fig. 4. Scanning electron micrograph of 5-p.m diameter Zn powder. Neck formation from localized melting is caused by high-velocity interparticle coUisions. Similar micrographs and elemental composition maps (by Auger electron spectroscopy) of mixed metal coUisions have also been made.

Collection. IDPs can be coUected in space although the high relative velocity makes nondestmctive capture difficult. Below 80 km altitude, IDPs have decelerated from cosmic velocity and coUection is not a problem however, particles that are large or enter a very high velocity are modified by heating. Typical 5-)J.m IDPs are heated to 400°C during atmospheric entry whereas most particles larger than 100 ]Am are heated above 1300°C, when they melt to form cosmic spherules (Pig. 6). 

In the spunbond process (Fig. 10), an aspiratory is used to draw the fibers in spinning and directiy deposit them as a web of continuous, randomly oriented filaments onto a moving conveyor belt. In the meltblown process (Fig. 11), high velocity air is used to draw the extmded melt into fine-denier fibers that are laid down in a continuous web on a collector dmm. 

Droplet Formation in Water Atomization. In water atomization of melts, liquid metal stream may be shattered by impact of water droplets, rather than by shear mechanism. When water droplets at high velocities strike the liquid metal stream, some liquid metal fragments are knocked out by the exploding steam packets originated from the water droplets and subsequently contract into spheroidal droplets under the effect of surface tension if spheroidization time is less than solidification time. It is assumed that each water droplet may be able to knock out one or more metal droplet. However, the actual number of metal droplets produced by each water droplet may vary, depending on operation conditions, material properties, and atomizer designs. 

One of the most interesting and fruitful approaches to the theoretical study of the non-isothermal flow of polymer melts has been suggested by Merzhanov and coworkers 82 84), which reduced the set of equations for the flow to the well-known equation of thermal explosion. The authors obtained the profiles of temperature and velocity for the case of stationary flow. However, the thermal explosion was founda5) to become feasible at too high velocities v lOm/s which could hardly be attained at low-pressure injection moulding of polymers. 

Table 24.3 shows an application of the stability criterion to three different types of melt. Completely different values are calculated for the ratio vq/j. It is an empirical fact that glass can be spun whereas metals cannot. Table 24.3 shows that unpractically high velocities would be necessary to stabilise a jet of liquid metal, i.e. in satisfying Eq. (24.8). Spinning of metals such as steel is possible in gaseous atmospheres where a reaction takes place on the surface of the metal jet (Monsanto, 1972). 

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