Powder compaction feeding systems

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. ... 

The most common form of dry granulation is roller compaction. A roller compaction system consists of a feed system which conveys powder between two counterrotating rolls. The powder is drawn between the rolls where a specified force is applied causing the powder to compact into a briquette or a continuous ribbon. The compacted material can then be milled in-fine or collected and milled off-line in a separate processing step (i.e., screening mill). 

Ultimately, the feed system is not only responsible for conveying powder to the rolls, but also aiding in an initial deaeration of the blend. Weggel7 found that the torque on the feed system screw(s) is directly related to the precompaction pressure. He concluded that by maintaining a constant back pressure, the resulting compact quality can be better controlled. 

An additional general method to reduce bypass is through vacuum deaeration. Miller9 performed experiments on a low-density active blend with and without vacuum deaeration, while holding all other parameters constant, and quantified the bypass for each trial. With the vacuum deaeration engaged, the bypass rate was 2% as compared to the control rate of over 20%. He also observed that the powder feed was uneven and the compact quality was not uniform. Most units are equipped with this capability and it should be considered to minimize bypass for low-dose products. However, vacuum deaeration can cause potency loss, unless the collected material is recirculated to the feed system. 

As a dry granulation technique, roller compaction has to be considered as a simple continuous process able to convert powder into granules (Fig. 6). The system comprises powder hoppers, feeding screws, a roller press, a mill, and, optionally, a recycling unit. 

In several experiments, Miller studied the effects of using horizontal twin auger feed screws under partial vacuum (Alexanderwerk Inc., Horsham, Pennsylvania, model 50/75 compactor). The feed powder was vacuum deaerated just before roller compaction. The experimental design showed that the compactor s deaeration feed system significantly increased compaction output and minimized powder leakage when compacting very low density blends (<0.35 gcm ). ... 

Guigon P Simon O. Roll press design-influence of force feed systems on compaction. Powder Technol 2003 130 41-8. 

Miller s experiments evaluated the effects of powder density screw feed speed, roll speed, roll pressure, vacuum deaeration pressure, compaction rate, and the compaction leakage rate. Test results demonstrated that the first compactor s deaeration and feed system designs significantly increased compaction output. The new equip-... 

Miller RW. Using vacuum-deaeration feed system to minimize powder leakage during roll compaction. Powder Bulk Eng 1997 ll(2) 71-75. 

The equipment feed design consisted of a funnel-shaped powder hopper (with no vacuum deaeration system) that was located directly above the rolls. The powder hopper was retrofitted with vacuum deaeration capability. A high compression feed screw, fitted inside the funnel hopper, fed the powder directly into knurled rolls. The compaction trials were conducted with and without vacuum deaeration. The compact was carefully collected directly on a 10-mesh screen. Powder particles, that were not compacted (i.e., those particles which were not attached to the compact, for example, fines bypassing roll compaction and the non-adhering compacted powder particles) were weighed and separated. The compact was not milled. The parameters are noted in Table 4. 

The consistency and evenness of the powder feed into a roll pair determines to a large extent, how complete a compact is made and ultimately the success of a compaction process. Most roller compacting systems suffer the disadvantage of leakage, i.e., 20%-30% powder particles (depending on the formulation) are not compacted. This primarily occurs because of uneven powder feed and powder slippage between individual... 

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. 

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