Hopper Magnet

It is mounted in the feed hopper or throat of the injection/extrusion machine; by catching ferrous contamination at the last moment, it protects the screw, the mold and the product. It is easily removed and cleaned.

The last line of defense at the machine inlet

The hopper magnet is placed at the point the material passes just before entering the processing unit. However much protection there is at the start of the line, metal that gets mixed in later from the granulator, the conveying line or operator intervention can only be caught here. That is why the presence of other separators does not make the hopper magnet unnecessary.

Why this point is so critical

A metal piece that gets past this magnet goes straight to the screw. Screw and barrel wear cannot be reversed; shot weight starts to fluctuate, scrap increases and the repair cost rises many times above that of a separator. The subject is covered in detail in the screw and barrel wear article.

Mounting and access

The hopper magnet should be mounted so that it can be easily removed from and refitted to the hopper; a position that is hard to reach leads to cleaning being postponed and therefore to the magnet becoming ineffective. It is also important that it does not narrow the material flow — if bridging occurs, the rod spacing should be reviewed.

Cleaning discipline

A full hopper magnet does not hold new particles, and on top of that the collected metal can be carried down with the flow. The cleaning frequency should be determined according to how contaminated the raw material is and tied to the shift routine.

How the cleaning frequency is determined

At first installation the magnet is checked daily and the amount of metal collected is recorded. After a few weeks this record is enough to set a realistic cleaning interval. When the raw material or supplier changes, the interval should be reviewed.