(Old)
The
first lasers were all solid state lasers. They used a ruby rod to
produce the laser. Today the most popular solid laser is the
neodymium-doped,yttrium aluminum garnet (YAG). This is a synthetic
crystal that when exposed to the intense light from the flash tubes can
produce high quantities of laser energy. The high powered laser lasers
have a problem because the internal temperatures of the laser rod
increase with operation time. These lasers are most often used with
low-power continuous or high powered pulse. The solid state laser is
capable of generating the highest powered laser pulses.
(Larry F. Jeffus, Welding: Principles and applications, Fourth edition, page 189 )
(New and better)
The active medium of a solid-state laser consists of a passive host material, typically a crystal, and the active ion, and it is these components that give the laser its name. An Nd:YAG laser, a very common solid-state laser for example, consists of a crystal of YAG with a small amount of Nd added as an impurity. YAG is short for yttrium aluminum garnet and has the chemical formula Y3AL5O12. The crystals are usually grown with the so-called Czochralski technique. The Nd ions, in the form of Nd2O3, are mixed into the host material, YAG. Then the two materials are melted together at the quite high temperature, and a single crystal is drawn out of the temperature controlled melt. The population inversion is created in the Nd ion and this ion generates the photon of the laser light. Typically the Nd ion concentration is about 0.1-1% as dense as the metal ions of the host crystal or glass.
Table lists the most important optically pumped solid-state laser systems.
Table lists the most important optically pumped solid-state laser systems.
2 - Forward Extrusion (Metal Forming Methods, Manufacturing Methods)
(Old)
A
metal billet is loaded into a container, and a ram compresses the
material, forcing it to flow through one or more openings in a die at
the opposite end of the container. As the ram approaches the die, a
small portion of the billet remains that cannot be forced through the
die opening. This extra portion, called the butt, is separated from the
product by cutting it just beyond the exit of the die.
One
of the problems in forward (direct) extrusion is the significant
friction that exists between the work surface and the walls of the
container as the billet is forced to slide toward the die opening. This
friction causes a substantial increase in the ram force required in
direct extrusion. In hot extrusion, the friction problem is aggravated
by the presence of an oxide layer on the surface of the billet. This
oxide layer can cause defects in the extruded product. To address these
problems, a dummy block is often used between the ram and the work
billet. The diameter of the dummy block is slightly smaller than the
billet diameter, so that a narrow ring of work metal (mostly the oxide
layer) is left in the container, leaving the final product free of
oxides.
(Groover M. P., Fundamentals of modern manufacturing: materials, processes and systems, p. 417)
(New and better)
In forward (direct) extrusion, the material flows in the same direction that the machine moves. (Fig. 1.a) . In this process, the billet is pushed through a container or die by means of a punch. Forward extrusion is the most commonly employed to produce shaft type components (rods and tubes), but it can also be used to form other shapes. In cold forward extrusion, the punch closely fits the die cavity to prevent backward flow of the metal.
(Tool Materials By ASM International. Handbook Committee, p.214)
3 - Processing Additives: (Plastics, Materials)
(Old)
Processing Additives are materials used at relatively low dosage levels, which improve processing characteristics without significantly affecting physical properties. In the early years, the classic additives such as mineral oil, bees wax, stearic acid, and palmitic acid, were used as a natural raw material, which did not have consistent composition and quality. In recent years, they have increased in quality through appropriate processes. The growth rate of additives such as homogenizers, peptizers, and lubricants increased about four times between 1980 and 1995. The chemical structures of processing aids have been classified as hydrocarbons, fatty acid derivatives, synthetic resins, low molecular weight polymers, and organic thio compounds. Fatty acid esters act as both lubricants and dispersing agents. The advantages of using processing aids are improved product surface, improved production rate, reduction of die build-up, and reduction of gel formation.
( J. L. White,K. Kim, Thermoplastic and rubber compounds: technology and physical chemistry, p. 57)
(New)
A link between the relatively good processability of low density polyethylene (LDPE) and the excellent toughness of linear low density polyethylene (LLDPE) has been a key subject for many researchers. Fluoropolymer-based additives provide this link. Today, the use of fluoropolymer processing additives has expanded greatly, from an early way of minimizing melt defects, to improving the throughput rates and properties of LLDPE. These additives now provide benefits in a host of polyolefin extrusion applications.
Fluoropolymers of the kind used as polymer processing additives (PPAs) are quite impervious to chemical attack and thermal degradation. They are of low surface energy, and are generally incompatible with other polymers.
Microscopic examination of a polyethylene containing PPA reveals discrete micron-sized droplet shaped particles of the fluoropolymer. Typical PPS use levels vary from 200 to 1000 ppm, depending on the application.
(Plastic Additives: An A-Z Reference, Geoffrey Pritchard, p.519)
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