... now i understand the need for a metal heatsink.
YES, good point ... so better not to exaggerate with resin impregnation to make the thing IP69!!!
Here below:
Picture 1 shows Voltage and Frequency at no-load conditions at 20 km/h on a 28" wheel.
Picture 2 shows Voltage and Frequency on a 12 Ohm resistor at 20 km/h on a 28" wheel.
Below data are referred to Shimano DH-3D32-QR hub dynamo.