Flow of powders

Process variables that must be controlled include the power level, pressure, and flow of the arc gases, and the rate of flow of powder and carrier gas. The spray gun position and gun to substrate distance are usually preset. Substrate temperature can be controlled by preheating and by limiting temperature increase during spraying by periodic intermptions of the spray. 

A powder was emptied down a metal duct into a plant vessel. The duct was replaced by a rubber hose, as shown in Figure 15-2. The flow of powder down the hose caused a charge to collect on it. Although the hose was reinforced with metal wire and was therefore conducting, it was connected to the plant at each end by short polypropylene pipes that were nonconducting. A charge therefore accumulated on the hose, a spark occurred, the dust exploded, and a man was killed. 

Figure 15-2. The flow of powder caused a static charge to collect on the insulated hose.

Into this plasma of ionized gas is introduced an accurately metered flow of powder material suspended in a carrier gas. As the powder particles are fed into the high velocity/high temperature plasma, they are rapidly and thoroughly heated and accelerated to the surface to be coated. 

During the filling of a hopper an active state of stress exists since the powder tends to compress vertically. This case is also referred to as the static state of stress since the powder is at rest. On the other hand, during discharging from the hopper, the powder tends to expand in the vertical direction. In that case, the horizontal stress becomes the major principal stress, leading to a state of passive stress. The passive state of stress in this case may also be referred to as the dynamic state of stress because of the flow of powder. 

Some of the operating variables are characteristic of the equipment (i.c., pressure, rhythm, and frequency of vibration to accelerate the flow of powder into the die), while others depend on the charge and, for a given powder, on the granulometry, the kind and concentration of binders, and the lubricants. The ratio of the diameter of the powder particles to that of the final granules should fall between 1/20 and 1/50. 

Lubricant The use of a lubricant is essential to increase the free flow of powders and to prevent manufacturing disorders in the tablet production. The type and amount of lubricant are cautiously selected in the formulation. The order of addition and mixing time is also considered in the tableting process. 

Weight and hardness variation are common problems experienced when tableting. Tablet weight is mainly affected by factors such as powder variation, tablet press condition and tooling, and flow of powder on the tablet press. 

The coefficient of pressure at rest is a descriptive index used in soil mechanics. It shows that the flow of powder packings cannot be regarded as analogous to the behavior of liquids. Let us consider a die with a large square cross-section. They axis points vertically downward. Let be the horizontal and Tyy the vertical compressive stress at any given depth acting edong the x and y axes, respectively. For reasons of symmetry, these stresses are also principal stresses. The coefficient of pressure at rest is = r hyy. 

For additional comments and conclusions see Section 4.2.2.3.5 Segregation during feeding and filling also leads to density variations due to local changes in size distribution and, in the case of mixtures, to differences in the plasticity and/or friability of the component materials. Since there is evidence that macroscopic flow of powder during compaction is negligible. 

Computational mechanics, and in particular the DE method, may be successfully used to analyze a wide range of operations involving powders and granular materials. Of particular relevance in a pharmaceutical context are applications related to flow of powders in hoppers (66), fluidized beds (67), and mixers (68,69). The method has been applied in the analysis of ball mills (70) and may be used to look more deeply into certain processes such as vibration-induced size segregation of granular materials (71) and packing (72). 

The vibration pressing method with wet powders was used for the forming of specimens in this investigation. This is a new plastic pressing method with vibration,and it could reduce to applied pressures for compacting in contrast to a conventional die pressing method with dry powders,because the binder content was sufficient to allow the freedom of the flow of powders. 

The three failure properties above are those related directly to flow of powders as required in hopper design, for example. There are another two properties related to powder flow ... 

Therefore, using moisture sorption, microcalorimetric, IGC, molecular modelling and other techniques, the consequences of the particle size reduction process can be assessed. Moreover, surface energetics can be measured directly and predictions made about the nature of the surface, which ultimately could affect properties such as the flow of powders or adhesion of particles (Podczeck et al. 1996b). 

Freely flowing Improves flow of powders by increasing the particle size. 

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