On Aug 6, 3:48*pm, "Existential Angst"
wrote:
Awl --

First, is "two-part epoxy" redundant? *Does "epoxy" nec. imply two parts?

Next, does the strength of a two-part epoxy vary with its setting time?
Intuitively, I would think the longer the stronger.

The classic is "5-minute epoxy", but I believe I have seen 60 sec epoxy, and
I have 90 minute epoxy.
I find 15 minutes to allow for a few parts without rushing, and without
waiting forever to cure. *Altho I have also found that the epoxy is not
really hard in that time.

The stuff I have now is Permatex (a 30 min epoxy), but DevCon and Loctite
are big players, with big-time industrial uses, such as epoxying *bearings,
instead of press-fitting them into their housings. *I believe DevCon stated
that epoxying bearings is in fact the preferred method, being stronger than
press fit.

The shear strengths are enormous, but require at least .010 on the radius
for good bonding. *So a slip-fit for epoxy would be counterproductive.

In some of my proposed assembly methods, I could use set screws, pins,
epoxied pins, or just epoxy -- typically nominal 7/8 alum round in 16 ga SS
tubing, or some such.

I thought epoxying would be slam dunk, and indeed it saves machining, but
goddamm, you gotta pick yer poison, it seems. *I could see arguments for
actually using a mechanical method AND epoxy, in some applications.

A feature of epoxy, which could be both good and bad, is its instability to
heat. *I think boiling water, or at least not that much hotter, can undo
epoxied parts.

Iny thoughts?
--
EA

The heat effects seem to vary a lot from one mfg. to another. All the
ones I examined give the maximum heat they will stand up to. I was
looking for an epoxy to use in assembling fiberglass pallets to carry
circuit boards through the solder reflow oven. The pallets are made
from circuit board strips salvaged from panels of curcuit boards. I
needed an epoxy to withstand 250-260 degree C. The only one available
at reasonable prices is JB weld.

It does turn dark over time - many months. A few times it has
eventually lost it's ability to hold onto the fiberglass board
material. The pallets have been through the oven perhaps a thousand
timed and still are ok. I recently had to rebuild some pallets because
the circuit board epoxy was delaminating the layers of FR4 material.
But the JB Weld was still holding and had to be cut away and sanded
down to the bare board material.

JB Weld takes up to 12 hours to cure. I speed it up with a heat gun.
Then about 1/2 hour.

Hope that helps some.

Paul