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Archive for October 10th, 2017

Glass vs. Epoxy: Divot!

The WS2812 RGB LED atop the Bowl of Fire Floodlight

Reflector floodlight - purple phase

Reflector floodlight – purple phase

failed in the usual way after a bit over a year of constant use.

I’d done an unusually good job of epoxying the ersatz heatsink in place:

Reflector floodlight - finned LED holder

Reflector floodlight – finned LED holder

I wrapped the bulb in a towel with only the heatsink sticking out, whacked the side of the heatsink parallel to the glass with a plastic-face hammer, and it popped right off:

Epoxy vs glass - divot

Epoxy vs glass – divot

The missing piece of the epoxy ring turned out to be a divot ripped out of the glass, which I didn’t notice until I’d chipped the fragment off the aluminum, so no pictures.

Given the relative strengths of epoxy and glass, pulling a divot seems impossible, but folks doing 3D printing on glass platforms have been reporting exactly that failure for years. The prevailing theory seems to involve small scratches and defects in the glass surface, with subsequent weakening, and stresses applied to the epoxy perpendicular to the glass surface pulling the cracks apart.

Replacing the RGB LED will require drilling it out and probably a complete rewiring, because I seem to have made liberal use of epoxy inside the heatsink and brass tube.

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