1. The hopper purpose is to transfer a polymer into the extruder barrel, where it is compressed. This compressed forces the air out from between interstices of resin pellets or rubber chunks (air is expelled back through the hopper).
2. Flow in the hoppers depends on their type (gravitational or forced), bulk density of the material, the shape and location of the feed inlet (direct or tangential feed, straight or with the chamfer on the down going side), as well on the type of screw and the barrel.
3. The solid conveying in the feed hopper usually depends on the gravity flow of particulate polymeric material moves down the hopper by its own weight.
4. Some materials have poor flow characteristics and may block the flow (know as arching or bridging).
5. This is more likely to occur when the particle shape is irregylar, the particles are small, and the internal friction on the bulk material is high.
6. Highly compressible bulk material are also susceptible to solids conveying instabilities.
7. Eliminate these problems, a vibrating pad may be attached to the hopper to dislodge any bridges that may be from, stirrers can be incorporated into the hopper to prevent material from the feed hopper into the extruder.