Which of the following processes is not associated with the production of plasti

Question

Which of the following processes is not associated with the production of plastic sheet and film more than one)? (a) blown-film extrusion process, (b) calendering, (c) chill-roll extrusion, (d) doctor blade method, or (e) slit-die extrusion. In powder metallurgy, as particle size increases, interparticle friction (a) increases, or (b) decreases. A screen with 325 mesh count has wires with a diameter of 0.001377 in (a) the maximum particle size that will pass through the wire mesh is............. (b) the of the opening is (c) the percent open space is.........

Explanation / Answer

1) Doctor blade method

2)As particle size increases, interparticle friction decreases.

The particle shape is dendritic or fernlike and it has medium to high surface area. Due to fine grains, the interparticle contacts are enormous. This promotes subsequent sintering. But the dendritic shape of the grains increases the surface area of the powder and hence the interparticle friction is also increased. In turn, this affects the ease with which particles can move during powder flow, when settling in a die and when under pressure during compaction. Further the chemical reactivity of the powder is also increased due to increased particle surface area. This leads to greater adsorption of atmospheric gases and water vapour, resulting in the formation of oxides on the particle surfaces. After making the compacts, they are prefired in hydrogen atmosphere at 300°C. Metallic oxides are reduced into their metals and occluded oxygen is evolved as water vapour by this process. Since the fine and irregular grains are used, the apparent density of the powder is about 15% of the maximum density of copper and hence during compaction the reduction in volume of the powder is 80%. The flow rate of a powder is the ability of a metal powder to flow. The high flow rate of the powder is characterised by rapid and smooth movement of the powder particles leading to higher densification. The flow rate is reducedby interparticle friction. The flow rate is increased by adding lubricants which reduce the friction between the powder and the tooling surfaces. But the addition of lubricants like graphite, zinc stearate and lithium stearate is generally detrimental to compact quality and inconvenient in subsequent sintering 55 operations. The author has prepared the samples with and without addition of lubricants so as to evaluate the effects of lubricants in powder metallurgical compacts. Compressibility of the metal powder depends upon its initial density, particle size and shape. For the electrolytic powders the compressibility is high due to their irregular shape and low density.

3)

particle size PS = 1/MC - t

w= 1/325 - 0.001377

= 0.003077 - 0.001377 = 0.00170 in

(b) There are 325 x 325 = 105,625 openings in one square inch of the mesh. By inference from part

(a), each opening is 0.00170 inch on a side, thus each opening is (0.0017)2= 0.000002889 in2

. The

total open area in one square inch of mesh = 105,625(0.000002889 in2) = 0.30523 in2

. This is total

open space. Therefore, the percent open space in one square inch of mesh = 105,625(0.000002889 in2) = 0.30523in2

. This is totalopen space. Therefore, the percent open space in one square inch of mesh = 30.523%

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