Meaning
Flow interruptions in gravity-fed resin hoppers occur when polymer pellets lock together to form a self-supporting arch above the discharge outlet. Known as hopper bridging, this phenomenon stops the continuous supply of material to the extruder screw. It happens when high humidity, excessive regrind fines, or elevated temperatures cause the pellets to stick or wedge together.
Molders experience short shots and production stoppages when the feed zone runs dry. The issue is localized to the hopper throat and does not involve the melt zone.
Blockage Mechanism
Interlocking occurs when the physical dimensions of the pellets or flakes are too large relative to the outlet size. Moisture or heat can create sticky surfaces that bind the particles into a cohesive mass. Once this mass forms an arch, the weight of the material above it reinforces the structure.
This prevents gravity from pulling the pellets down.
Process Interruption
As the extruder screw continues to rotate without a material feed, the barrel pressure drops to zero. This leads to empty mold cavities, generating defective parts that must be scrapped. Operatives must intervene to clear the block, which increases cycle time and operating costs.
It represents a major cause of downtime.
Remedial Action
Installing mechanical vibrators or pneumatic agitators on the hopper walls disrupts the arch. Controlling the feed area temperature also prevents pellets from softening and sticking. These measures maintain a continuous material feed.
