A screw and barrel overhaul, with parts cost, labor and days of downtime, is a serious bill. The cause is very often a single thing: a small piece of metal that got into the raw material. In this article we explain where metal enters the production line and how to stop it.
Where Does Metal Get into the Raw Material?
- Regrind: The biggest source. Pieces broken off granulator blades, inserts mixed in with parts sent for granulating, drill bits
- Big bags and octabins: Wire, staples, packaging residues
- Conveying line: Particles broken off pipes and elbows by wear
- The human factor: A dropped screw, washer, knife tip...
Even virgin raw material comes into contact with metal in the supply chain; on every line that uses regrind the risk multiplies.
The Cost of a Single Bolt
Metal that gets into the screw scores the screw flights, gouges the barrel surface and damages the check ring. The result is not visible at once — it starts insidiously as metering inconsistency, black specks and plasticizing problems. A particle that reaches the nozzle or hot runner clogs the gate channels. Worst of all: if it reaches the mold cavity, the damage is directly in the mold.
The comparison is simple: a magnetic separator at a critical point in the line costs a small percentage of a single screw overhaul.
Know Your Magnet Material
The strength of a separator is determined by the permanent magnet inside it:
- Neodymium (NdFeB): The highest magnetic strength — the standard in the plastics sector. With rods of up to 12,000 gauss, even weakly magnetic stainless particles are caught
- Samarium Cobalt (SmCo): Resistant up to 350 °C — hot process points
- Ferrite: Economical, corrosion-resistant — coarse pieces and pre-separation
- AlNiCo: Heat resistance up to 500 °C — special applications
Which Separator at Which Point?
The right strategy is not one big magnet but layered protection:
1. Granulator outlet / regrind line: Where the metal load is heaviest. A drawer separator — stacked layers of neodymium rods, easy cleaning. For dusty material that bridges, a rotating separator 2. Central conveying line: A bullet separator on the pneumatic line, or a hump separator on vertical lines — in-pipe separation without interrupting the flow 3. Silo/hopper openings: A magnetic grid — the last filter at the filling point 4. Machine inlet (last line of defense): A hopper magnet — the last checkpoint before the screw. It should be on every injection molding machine 5. High-capacity lines: A drum separator provides continuous, automatic separation; overband conveyor belt models are used in granulating and recycling lines
For periodic checks, raw material samples are inspected with magnetic inspection rods; on the workshop floor magnetic sweepers collect stray metal.
Four Critical Questions When Choosing
1. What is the flow type? Free fall, pressurized line, conveyor — each flow has its own separator family 2. How high is the throughput? Contact time with the magnet determines the capture rate; above its capacity a separator loses efficiency 3. Is the material dusty/sticky? If there is bridging, a rotating type is needed 4. How will cleaning be done? Easy-clean designs discharge the collected metal without stopping production
To identify the risk points in your line together and set up the right layered protection, contact us — as Mensink Magnetics' representative in Turkey, we also supply custom-sized separators.