Injection Molded Impellers

Balance comes from mass being spread evenly around the axis. On a molded impeller that means the melt has to reach every vane at the same moment and pack each one the same amount. A feed point on the center of the hub does exactly that: the melt spreads outward in all directions and every vane fills from the same distance, so no vane is heavier or denser than its neighbors.

A feed point anywhere else breaks the symmetry. Vanes near the feed pack harder and shrink less, vanes on the far side arrive last and pack least, and the finished wheel has more mass on one side. At low speed nobody notices. At running speed the wheel shakes, the bearings take the load, and the noise gives it away.

Every vane also creates a joint. As melt spreads out from the center it splits around the root of each vane and closes again behind it, and along that closing line the plastic has less strength than the material around it. Those lines sit in a predictable place, so the design puts a generous fillet where each vane meets the disc and keeps the closing line out of the highest stress.

The hub is the last piece. A plastic hub pressed onto a metal shaft grips by interference, and plastic under constant strain slowly relaxes, so the grip fades over months. A key, a flat, a splined bore, or a metal insert molded into the hub carries the torque instead of relying on friction alone.