brittle blanks

[Califo]

Hi everyone, recently I casted my first PR acrylic. Nothing special about it, used 2 colors. The main idea behind it was to get a feel of casting your own blank. Anyway, I used PVC molds, as many have used and cured it for about 48 hours before turning it. The ratio between PR and catalyst was 6 drops per ounce. What I found was that the blank was brittle. This thing was chipping when turning and not the smooth acrylic strings you normally get. I used a roughing gouge and a skew, no difference. I was able to turn it into a pen but I wouldn't call it a great pen.

Any ideas what may have happened? Does it matter that I used an acrylic based paint opposed to the Pearl-ex I hear? Would different base pigments change how the blank turns out?


Thanks.

 

[Majorbdk]

I see that you live in a cold climate. PR blanks are effected by cold. If I'm going to turn blanks in cold weather, I bring them in the house so they aren't brittle. Just a thought.

 

[brownsfn2]

I find that 3 to 3.5 drops per oz are the most I can do before they start to become brittle. I would not recommend going higher than that. It might take more time to cure but the results are much better. Make sure you do not let them cure in the cold. The need a room temp place to cure.

 

[BSea]

I find that 3 to 3.5 drops per oz are the most I can do before they start to become brittle. I would not recommend going higher than that. It might take more time to cure but the results are much better. Make sure you do not let them cure in the cold. The need a room temp place to cure.

I agree. 6 drops is just too much catalyst for pen blanks. This time of year, after the blanks are set (about 1 to 2 hours after pouring), I put them in a cardboard box about 18" from a space heater. I then cover the box with a towel. The next morning they are cured enough to take out of the pvc. It helps to let them cool to room temp before removing them from the pvc.

 

[Califo]

I will try that but would too much heat applied at once make them brittle and potentially crack?

I will give it another go at this.

Thanks

 

[Ed McDonnell]

Marco - Before you start adjusting your MEKP, try casting a plain resin blank with no additives. That will tell you what effect (if any) your acrylic paint is having.

If your plain clear blank is still "brittle" then you can try adjusting your MEKP, but if it turns acceptably then you know the acrylic paint is likely the problem (assuming sharp tools and good technique).

Ed

 

[Pjohnson]

Marco - my opinion and each turner has a method that works for them.

I live just south of Boston and pour strictly PR in PVC and flat molds. PR is more brittle than Alumilite - resulting in more chipping. Once down to the final stages you should be getting ribbons.

Your ratio of PR to MEK is on par with mine. The one difference is that I do not use acrylic for colors. My choices are pearlex or silmar pigments that were purchased. The suggestion of trying a pour with no color is a good one. The PR:MEK ration has worked for me for 3+ years.

Also, for PR blanks my success improved when I switched to carbide as opposed to HSS for my turning chisels. PM me if you have any questions.

PJ

 

[BSea]

I put them in a cardboard box about 18" from a space heater. I then cover the box with a towel.

I will try that but would too much heat applied at once make them brittle and potentially crack?

I will give it another go at this.

Thanks

I'm guessing that the box stays about 90 degrees. I've had no problems doing it this way.

I didn't read your original post to the end. I'm going to agree with the others about trying a pure resin blank to see if that changes things. I only use MICA powders and Dye sold by US composites. The paint may be most if not all of the problem.

 

[79spitfire]

I've used more hardener than that and not had the blanks turn 'brittle'. I pushed it to 15 drops per oz and all was still good. (well as far as brittle, it set fast enough to trap more airbubbles than I'm used to...)

I agree with trying a clear blank.

Another point worth considering, I have found that different brands of PR behave differently. The most brittle blank I had was with Castn-Craft brand and had about 5 drops per oz. I had similar results with TAP plastics casting resin. I used US Composites (Simlar 41, I believe) and it seems the least brittle.

And one more point....

I've had much more success with all brands of PR when I turned the lathe speed up and made sure my tools were 'shave with it sharp', and took light cuts.

 

[Califo]

I think it may be the brand of the PR. I bought it from AC Moore, 55% off, couldn't resist trying. I tried the clear resin cast and the results were better but still brittle. I also found that when I casted with 2 colors, Blue and Yellow, the areas with the blue was more britle than the yellow. The blue was also on the bottom of the pcv mold. I will try a square mold next and lay it flat opposed to an upright pcv and see if there is a difference.

 

[ofd8001]

I put them in a cardboard box about 18" from a space heater. I then cover the box with a towel.

A space heater 18" from a cardboard box is way too close. :frown:

In the 39 years I spent in the fire service, I've seen this very scenario cause an awful lot of fires.

Given all the flammable/combustible "stuff" in a workshop, any fire is going to get real big, real fast.

 

[79spitfire]

I put them in a cardboard box about 18" from a space heater. I then cover the box with a towel.

A space heater 18" from a cardboard box is way too close. :frown:

In the 39 years I spent in the fire service, I've seen this very scenario cause an awful lot of fires.

Given all the flammable/combustible "stuff" in a workshop, any fire is going to get real big, real fast.

Agreed.... Painting a cardboard box black and setting it in the sun might work just as well, and not be an invitation to making your shop into a Roman Candle...

 

[ofd8001]

I had a pretty good "casting session" today. While my garage/shop has heat, it still doesn't get "toasty" enough for resin to set-up. Plus keeping a window open for "well ventilated" purposes doesn't help much.

So I tried setting up quartz lights on a tripod I had for a house painting project. They generated enough warmth for improving set-up time and gave me a little extra light to work with.

After 2-3 hours of the molds being exposed to this light/heat, I was able to de-mold the blanks and was happy with the results.

I was there to keep an eye on things to avoid an unpleasant surprise.

 

[Andybear]

Try putting the blank in some warm to hot water before you start. This has worked for me when I have a brittle blank.