Alumilite handle - Cracked!

[HamTurns]

I cast this handle yesterday morning, then turned the handle last night and this morning I found it Cracked. Has anyone had this happen to them using Alumilite?

I glued the pizza cutter & ferrule with 2 part, 15 minute epoxy that I'd colored with Alumilite purple powder. Both the pizza cutter and ferrule went together by hand, with no pounding, I was able to insert both and give them a twist to smear the glue. I'd cut two glue grooves for the ferrule, and drilled about 1/4" deeper then needed for the pizza cutter so glue would have a home.

I took the picture of the non-cracked handle last night while the glue was still drying, I took the cracked handle picture this morning.


- Has anyone had this happen to them using Alumilite clear?
- Got any guesses as to why this happened (and how to avoid it)?
- Do you have any suggestions of how to fix it, or do I need to start over?

Thanks for any help with this.

Tom

 

[healeydays]

How long had the pour been curing before you turned it?
How long from turning it did you do the fitting?
How tight were the tolerances between the cutter and the handle?

 

[HamTurns]

How long had the pour been curing before you turned it?
How long from turning it did you do the fitting?
How tight were the tolerances between the cutter and the handle?

Mike - All good questions.
I cured the pour for an hour and 15 minutes under pressure. When I took it out of the pressure pot it was mostly cooled. Because I planned to turn it the same day I post cured it in a toaster oven for an hour @187F. I measured it with one of those fancy infra-red point and shoot thermometers. After that I took it out of the toaster oven, brought it upstairs into 70F and let it cool. Then photographed the blank, that was at 10:45am.

I put the blank on the lathe at about half way through the Ponderosa TV show, so about 2:30pm. My lovely wife came home early (because of the 16" snowfall), so I took a break from turning. By the time I took a break I'd drilled the 1/2" hole needed for the pizza cutter, and had not started on the tennon needed for the ferrule.

I got back to turning, finished finshing, then went and glued/fitted everything together at about 6pm. I ended up finishing it twice, so finishing was about 20 minutes. So 20 minutes after all the turning was done I was glueing/fitting it together.

I had about a .004" measured difference between the pizza cutter shaft and the drill for the hole. The drill was .004" smaller. Although I was able to fit the pizza cutter shaft into (and out of) the hole by hand, so the hole got opened up a bit during drilling, compared to absolute measurements.

The mix and pour of the Alumilite Clear went fine, no issues at all. Poured A&B by weight on the scale, started timer, Clearly mixed, then split full batch into 3 small cups and remixed with colors, poured, stirred, put in the BINKS @60lbs, cleaned up the bench, took off my gloves then the 7 minute (pot life timer)went off. :biggrin: I was a happy camper.

Now what?

 

[Sylvanite]

That's unusual. Alumilite Clear is a urethane resin, and is typically less brittle (and less prone to crack) than polyester resin or acrylic. I've noticed that Alumilite casts can feel a little rubbery at first, but harden over the course of a day or two. Your post-cure in the oven should have taken care of that though. Alumilite Clear doesn't shrink significantly when curing, so that's not a viable explanation. The crack looks like a stress fracture (typically from pressing in parts that expand the blank or from pressure using the cutter), but your assembly description doesn't account for that. Is it possible that the epoxy expanded while curing (also unlikely - most epoxies shrink slightly).

You can put a drop of thin CA glue on the crack, let it wick in, and wait for it to harden. Repeat until the crack is full and then hand-sand the spot back down. With luck, the fill won't be noticable.

Best wishes,
Eric

 

[healeydays]

I was thinking the same thing, but by accelerating the curing process in the oven I wonder if it made it more brittle than normal.

Other than that and noticing where it is cracked I was wondering about the turning of the tenon and somehow it was stressed at that point but didn't actually fully crack till later.

 

[HamTurns]

Is it possible that the epoxy expanded while curing (also unlikely - most epoxies shrink slightly).

Best wishes,
Eric

The epoxy I used has always shrunk in the past. I've had these epoxy bottles for at least a year, and have colored it before too. It's always shrunk, I thought of the epoxy expanding, but that would NOT follow suit compared to previous uses.

The only thing I can guess at is that the Alumilite wasn't fully cured, although it cut like normal. Nice long strings of plastic coming off of the tool.

I'll try some CA buildup on it and see how that works out.

 

[MesquiteMan]

Your post cure temp is fairly high for Alumilite. You really should keep it around 150° F or so. What you see could be thermal shear. Not saying it is, but it could be!

 

[HamTurns]

Thermal Shear?

I filled the crack with CA, I can still find it, but two other folks couldn't.

You tell me.

Good tip on how warm to post cure, 150 it is from now on.

Is thermal shear caused by different color mixes mixed together, when the two different colors cure at a different rate?

I had three different colors, black, purple and gold. I had a 6oz resin mix, poured 2oz into each color mixing cup. Then poured two colors at a time and alternated with the third color. Then gave it a swirl with a skewer after they were all poured into the 6" tall x1 1/2" pvc mold.

So if the colors could cure at a different rate I suppose the "line" between them could be a thermal shear.

Is that what you meant Curtis?