Feed screw design

Miller noted that the total compaction power requirement is the sum of the power required for the feed screw(s) and the roll drive (13). Feed screws not only convey the powder material from the compactor storage hopper but they also help deaerate the powder in the process. The deaeration of the powder acts as a minicompactor by precompacting the material just prior to roll compaction. Optimum compaction pressures and feed screw designs vary widely for different powder material properties. Changes in bulk powder density and feed screw speeds will affect the roll gap, the compaction pressure, the throughput, and the quality of the compact (13). 

Weggel indicated that feed screw torque varies directly with the precompaction pressure (32). He suggested that by maintaining a constant feed screw pressure, the compactor operator can control the compact quality. Variations in precompaction pressure and in the compacting pressures are directly related to feed screw amperages and the roll drive motor. The compaction pressures are dependent on a continuous flow of powder into the feed screw area. If the powder feed flow is intermittent or 

Fast turning vertical screw for removing air and force feeding at rate provided by horizontal screws 

Arch-breaking Diamondback Hopper for cohesive feed 

RoHer Compactor Feed Screw Des ns Twin feed screw. Single feed screw 

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A history of hopper and feed screw designs showed that each design evolved to facilitate and improve powder flow to the compactor feed screw conveyance system. Feed hopper designs depicted in Fig. 4 show designs incorporated in older compactor models [examples (A)-(C)]. Dehont et al. described the powder compaction feeding systems that were in use up to 1989. ... 

Fig. 12 Roller compaction compacts. Roller compactor feed screw designs twin feed screw and single feed screw. (Courtesy of R. W. Miller, personal files.)...

Miller indicated that a multiple horizontal feed screw design system provided a more uniform powder distribution across the rolls in area and volume than a single vertical feed screw system. This concept applied to either the vertical or the horizontal single feed screw systems. ... 

The compaction rolls operate at different speeds and are supported on heavy-duty bearings in such a way that the lower roll is fixed and the upper roll is slightly movable in the vertical plane. The deaerated material passes through the roll pair, which is under infinitely variable hydraulic pressure. The deaeration, auger feed screws design and speed, roll speed, and hydraulic roll pressures are the main factors in producing a compact with specified properties. 

A specially designed compression feed screw and upper arm assembly are shaped to closely conform to the inner wall of the feed hopper. The feed screw (usually non-cylindrical) has uniquely pitched edges. The feed screw design features and gravity aid to drive the powder-stock into the compaction area. There is no vacuum deaeration system. Predensifica-tion and deaeration occur in the screw feed region during operation. 

As with other unit operations, the scale-up success for roller compaction is dependent on understanding and identifying the process parameters that will influence the quality of the final product. The typical parameters that can influence compacted ribbon characteristics and the resultant final product qualities are hopper design, feed screw design, and roll configuration. 

The feed-screw design can vary (i) vertical straight and lightly tapered screw feeder (ii) slight inclined screw feeder (iii) tapered vertical screw feeder and (iv) horizontal straight feeder screw. The screw design requirements will depend on the flow characteristics of the precompacted material. Powder feed variability can lead to undesirable outcomes, namely excessive fines and irregularly compacted ribbons that can lead to broad particle size distributions. 

The work by Guigon and Simon provided new meaning to the importance of the feed screw pushing powder into the rolls and the feed screw design s influence on compact formation. The author of this chapter raises some questions on how a multiple feed screw system under vacuum deaeration environment would affect compact formation. No information has been published on investigating these aspects. 

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