Plastic-Integrated Magnets

The magnets are embedded directly in plastic injection molded parts (overmolding); compact, robust, precisely positioned and highly repeatable units are produced at low cost. They are common in sensor, actuator and dosing systems that require tight tolerances.

Embedding the magnet inside the part

In the conventional method the magnet is produced separately and then glued or screwed into a seat. This means both an extra assembly step and positional uncertainty. In the overmolding method the magnet is placed inside the plastic injection mold and the part is molded directly over the magnet. The result is a single-piece component with a fixed position that requires no assembly.

Three things gained

• Positional precision — because the magnet is positioned by the mold, it is in the same place in every part; in sensor applications this directly determines measurement repeatability
• Robustness — the plastic body protects the magnet from impact, moisture and chemicals; for brittle neodymium this protection is important
• Cost — the separate assembly step, adhesive and labor are eliminated; at high volumes the difference becomes marked

Points to watch on the process side

The magnet is briefly exposed to the melt temperature. The material grade must be chosen to withstand this temperature; otherwise the part comes out but the magnet has permanently lost part of its strength. In addition, the injection pressure must not move the magnet out of place, and the positioning inside the mold must be designed accordingly.

Where it is used

Position and speed sensors, actuators, dosing systems, household appliances and automotive components. When tight tolerances and high volumes are required together, it is the most economical solution.

The sampling stage

In overmolding applications the first mold trial is of critical importance: it must be measured whether the magnet's position shifts under injection pressure and whether it keeps its strength after being exposed to the melt temperature. Series production should not start before this verification has been done.