Powder nozzle

Generating by laser cladding with a powder nozzle opens up the opportunity of combining different materials in order to optimize the thermal behavior of a tool. The so-called bimetal tools, for example, are made with a built-in copper zone that is applied during the AM process. 

Generating with a powder nozzle is used by Optomec (LENS 705, -800R) and POM (DMD 105D, DMD 44R/66R). Besides this, independent equipment manufacturer design customized machines. 

Direct Tooling preferably is applied to make cavities, sliders, and tool inserts from tool steel. Generating with a powder nozzle is mainly used for repair and design changes. 

Examples for the direct application using a powder nozzle and laser for repair and maintenance are the damaged texture on Fig. 10 and the buildup of a tool insert made from 1.2343 on a prefabricated substrate made from the same material. 

Rapid Tooling, Fig. 10 Repair of a textured surface, laser generating with powder nozzle (Source Trumpf)... 

Rapid Tooling, Fig. 11 Laser generating with powder nozzle. Tool insert, build-up on a prefabricated substrate, both made from 1.2343. Part as processed (left), after final machining (right) (Source Innoshape)... 

A variation of the sintering or melting process in the powder bed is the laser-based generation with the powder nozzle. The powder is, parallel to the laser beam, added in and then melted. The processes are suitable for larger layer thicknesses and are preferably used for repair purposes. 

By generating with the powder nozzle, molds can be built that are specifically mixing materials so that an optimum thermal efficiency can be realized. So-called bimetal molds are provided with, e.g., copper zones, which are embedded into the mold material and which are directly brought into the process. 

Examples for the application of generative processes in the repair and maintenance, the texturing (Figure 4.147), which is applied with the powder nozzle, and the construction of a mold insert made from 1.2343 on a prefabricated blank, are made from the same material. 

The AeroSizer, manufactured by Amherst Process Instmments Inc. (Hadley, Massachusetts), is equipped with a special device called the AeroDisperser for ensuring efficient dispersal of the powders to be inspected. The disperser and the measurement instmment are shown schematically in Figure 13. The aerosol particles to be characterized are sucked into the inspection zone which operates at a partial vacuum. As the air leaves the nozzle at near sonic velocities, the particles in the stream are accelerated across an inspection zone where they cross two laser beams. The time of flight between the two laser beams is used to deduce the size of the particles. The instmment is caUbrated with latex particles of known size. A stream of clean air confines the aerosol stream to the measurement zone. This technique is known as hydrodynamic focusing. A computer correlation estabUshes which peak in the second laser inspection matches the initiation of action from the first laser beam. The equipment can measure particles at a rate of 10,000/s. The output from the AeroSizer can either be displayed as a number count or a volume percentage count. 

An ingenious method to avoid or reduce segregation of alloying elements involves preparing small spheres of material by the atomization of a Hquid stream through a nozzle to produce a powder. This powder can be compacted, often hot and triaxially by gas pressure, to form a material where, on further heating, the residual pores close by diffusion to approach 100% density. 

A variation of the n on regen erabi e absorption is the spray dry process. Time slurry is sprayed through an atomizing nozzle into a tower where it countercurtendy contacts the flue gas. The sulfur dioxide is absorbed and water in the slurry evaporated as calcium sulfite-sulfate collects as a powder at the bottom of the tower. The process requires less capital investment, but is less efficient than regular scmbbing operations. 

The Majac jet pulverizer (Ho.sokawa Micron Powder Sy.stems Div.) is an opposed-jet type with a mechanical classifier (Fig. 20-55). Fineness is controlled primarily by the classifier speed and the amount of fan air dehvered to the classifier, but other effects can be achieved by variation of nozzle pressure, distance between the muzzles of the gun barrels, and position of the classifier disk. These pulverizers are available in 30 sizes, operated on quantities of compressed air ranging from approximately 0.6 to 13.0 mVmin (20 to 4500 ftV min). In most apphcations, the economics of the use of this type of jet pulverizer becomes attractive in the range of 98 percent through 200 mesh or finer. 

Fluidized-bed opposed-jet mills Hosokawa Micron Powder Systems Div.) differ from the Majac mill in that powder is not fed into the jets, but the jets impinge into a chamber mich contains suspended powder. The powder is entrained into the jets. This ehminates wear on the nozzles, and reduces contamination. Otherwise, construction and appheations are similar to the Majac mill. The fluidized-bed level is maintained a few inches above the jets. The Fluidized-bed mill is available in 13 sizes with air volumes ranging from. 50 to 11,000 mVh. One application is for toner grinding. 

The general arrangement of the apparatus is shown in Fig. 12.26. In recent years both the powder and wire processes have been automated for large projects. Mechanisation allows the use of large nozzles of special contours and the replacement of pneumatic drives by electric motors. Controls are usually from consoles by pneumatic or electronic signal systems. 

In all metal spraying processes the particles emerge from the nozzle in a conical stream, and although the particles near the centre are molten, those at the periphery have solidified. In the powder process there are in addition solid particles which have not melted. The solid particles tend to become entrapped in the coating, making it porous. The effect is more pronounced in the powder process owing to the larger number of solid particles present. 

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