casting failures

[BobL]

I have been turning for some years and decided to try turning pens. I am now an addict. Turning timber, and shop bought acrylic, blanks was no problem but i thought i would cast my own acrylic blanks. If i had any hair i would be pulling it out. Using polyurethane resin gives a soft blank which is easy to drill and turn but is very difficult to finish. Using polyester resin gives a blank which is brittle and cracks when drilling or turning. Polyester seems the way to go but i now have to wear safety goggles as the blank shatters and flies in all directions. Any info on what i am doing wrong would be appreciated. I have tried using from half a percent up to 4 percent catalyst to no avail.
Excuse the poor typing english is my second language i am scottish.

 

[Jim]

Hi Bob, Polyester Resin is widely known for its brittleness, if you are putting in the correct dosage of the catalyst then there is other reasons for this which could be the temperature of the place where you are mixing it, Polyester hates cold, another could be how you are turning it .. I make Polyester blanks also but have found going in to much with the gouge will rip it to bits, so what i have learnt is to use my 1" skew as a scraper sharpening very regular, once you get less than 50% on the blank it does start coming off in streams rather than bits ... Good luck and welcome to the forums ...

 

[BobL]

Thanks jim,
My workshop is unheated so this could be the problem. I usually dont get to the turning stage as the things crack on drilling. I will try adding heat to the mix and see what happens.

 

[Woody]

I find the wedge tool is the best one for cast polyester no snags shattering it just peals of in streams I use it on alternative ivory and the likes to make boxes and clocks I will see if I have time to make one tomorrow to shaw you what it is like they are very easy to make

 

[Jim]

Thanks jim,
My workshop is unheated so this could be the problem. I usually dont get to the turning stage as the things crack on drilling. I will try adding heat to the mix and see what happens.

If the blanks are shattering when drilling Bob, then i would go with the mix not been correct, to much catalyst!! Don't heat up the resin it is the workplace that needs to be at the right temperature .. :wink:

Also what speed are you drilling them at, and how are you drilling them Bob?

 

[BobL]

Drilling has been on the lathe at anything from 500rpm to 2000rpm. I also use a drill press which runs at 2400rpm. Cracking happens at all of them. I thought perhaps too little catalyst. The last batch was at half a percent by volume and the cracking on drilling was terrible but the temperature has dropped here the past couple of days so perhaps that has an effect. Will cast some more tomorrow at home in warmer temperatures and see what happens.

 

[Jim]

I wouldn't drill any faster than 500 RPM Bob ..

 

[turnaround360]

Bob, Try pre drilling with a smaller bit first take it slow on the second drill and pull out to clear often and see how that works Frankie.

 

[Doug]

Bob, Try pre drilling with a smaller bit first take it slow on the second drill and pull out to clear often and see how that works Frankie.

That`s good advice, I have the lathe speed down to 250-300, I also keep checking how warm the blank is getting they can get very hot mighty quick.

 

[BobL]

Well i cast a couple of blanks today, brought them into the house, and turned them this evening. They turned beautifully. I upped the catalyst to 2 percent and brought them into the heat. Thanks Jim, the heat really helped. I think the MEKP may be a bit high as they were really soft. They finished very well, giving a high gloss.
I drilled em slow and easy,with a pre drill, at 300rpm which went fine.

 

[Jim]

If the resin was really soft then you can still add a tad more catalyst to the mix Bob, i usually add between 12/15 drops from a syringe to 3oz of resin, this seems to work ok for me ... Took me a few goes to reach this mix due to similar problems that you have/had ...

It is good to hear that you made some turnable blanks Bob, now let us see the pens ...

 

[BobL]

OK ounces and drops :) u got me beffudled. I had trouble with ltrs and mlrs. I will mess around with the MEKP and see how it goes but your helpand advice is much appreciated.

 

[Jim]

I can make it easier for you Bob, do what i did, i bought some 3oz cups from ebay ... And the syringe that comes with the resin, just let it drip 12/15 drops into the cup ..