Crammer feeders

Solids conveying is carried out in two steps the feed hopper and the back (entrance) portion of the screw. The feed hopper is an inverted cone or pyramid, in which solid pellets or powder flow downward from the force of gravity. If they flow poorly ( arcing or bridging ), the problem may be solved by installing a vibrator or a stirrer ( crammer/feeder ) in the hopper, or by coating the particles with a small amount of a chemical that acts as an external lubricant. ... 

Large rectangular standard feed opening (round with lining, optional, for use with crammer feeders). 

Bulk density is the measure of the density of the feed material, including the air between particles. The bulk density value can provide one measure of the solid s ability to flow freely. Generally, materials with higher bulk density flow more freely than those with low bulk density. Pellets resembling ball bearings that flow easily may have a bulk density of about 0.7 g/cnf. Very fluffy regrind may have a bulk density as low as 0.3 g/cm and is more difficult to feed, perhaps requiring special hardware such as a crammer feeder. 

Figure 3.1 A crammer feeder for pellets and fluff (Foremost Machine Builders, Inc.)...

Crammer-feeder (force feeder) n. A device fitted to the inlet port of an extruder that precompacts a low-density feedstock and propels it into the feed section of the extruder screw. As originally produced by Prodex Corp (now a division of HPM), the crammer-feeder consisted of a conical shell within which turned a decreasing-diameter screw driven independently of the extruder. 

Figure 5.12 Hopper designs (a) standard hopper with gravimetric feed, (b) spiral hopper, (c) crammer feeder, and (d) standard hopper with metered feed. ...

Crammer feeders are used for bulk materials that are very difficult to handle. Other materials, particularly those with low bulk density, tend to entrap air. If the air cannot escape through the feed hopper, it will be carried with the polymer and eventually appear at the die exit. In most cases, this will cause surface imperfections of the ex-trudate. In some cases, it causes small explosions when the air escapes from the die. 

Sion. Low bulk density materials (Pb < 0.2 g/cc) tend to cause solids conveying problems, either in the feed hopper or in the feed section of the extruder. Materials with irregularly shaped particles tend to have a low bulk density examples are fiber scrap or film scrap (flakes). When the bulk density is low, the mass flow rate will be low as well. Thus, the solids conveying rate may be insufficient to supply the downstream zones (plasticating and melt conveying) with enough material. Special devices and special extruders have been designed to deal with these low bulk density materials. A crammer feeder, as shown in Fig. 6.1, is a device used to improve the solids transport from the feed hopper into the extruder barrel. 

Some feed hoppers are equipped with crammer feeders see Fig. 7.3. 

The most important component of the feed system in a flood-fed extruder is the feed hopper with possible stirrer and/or discharge screw. A mechanical malfunction of this system can be determined by visual Inspection. If the feed hopper is equipped with a discharge screw (crammer feed), constancy of the drive should be checked. For proper functioning of a crammer feeder, the drive of the crammer feeder should have a torque feedback control to ensure constant feeding and to avoid overfeeding. 

Some bulk materials are very difficult to handle for instance, post consumer reclaim (PCR) usually has a wide particle size distribution and a low bulk density. As a result, special extruders have been developed to handle this bulk material efficiently. These machines can have larger diameters in the feed section than in the metering section they may also use crammer feeders to achieve steady flow of material into the extruder. In some cases, a grooved feed section is used to improve the feeding of low bulk density material. 

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