Problem scribing lines on resin casting

Hobbit 77 · Post by **Hobbit 77** » Sun Jun 19, 2005 2:53 am

Hi,
I made rubber molds of a 20mm ball bearing. Molds came out just fine. I poured resin (por-a-cast) into the molds and the casting come out fine also. What I need to do is scribe some lines on these casting but when I attempt this I run into small air bubbles just under the surface of the resin which ruins the line I'm trying to scribe. Any ideas as to how to fix this problem?
Also I've been thinking of just scribing these lines on the ball bearing itself. Any thoughts of what tool to accomplish this? Don't think a simple modelers scribing tool could do this.

Thanks!

Troll 7

Jason Abbadon · Post by **Jason Abbadon** » Sun Jun 19, 2005 6:37 am

Hmmm....scribe the lines on a larger master of the sphere you want- there is a "reducing" technique for making a mold proportionally smaller.
PM Nova- he was telling me about it.

Mr. Badwrench · Post by **Mr. Badwrench** » Mon Jun 20, 2005 2:22 am

Ball bearings are harder than just about any tool you could use for scribing. Files, saw blades, any sort of knife, machine tool blanks, drill bits, all are softer than bearings. It is possible to cut bearing steel with carbide or ceramic inserts, but don't expect to scribe any sort of accurate line with them.

Other options: fill in the exposed pores of your casting with CA or putty. Maybe someone with more casting experience can help you with getting rid of the porosity in the first place. Make your master out of something besides 90 roc. hardened chrome steel. Plastruct has a wide variety of acrylic spheres and hemispheres, which could be easily scribed before casting them.

Mark Yungblut · Post by **Mark Yungblut** » Mon Jun 20, 2005 12:33 pm

My recommendation would be to fill the bubbles with epoxy putty. The putty will have the best chance of having an equal hardness as the resin. While you can fill the holes with super glue, you may find that it sets up harder than the resin and that can cause problems when scribing.

Vince Hoffmann · Post by **Vince Hoffmann** » Mon Jun 20, 2005 3:12 pm

Moving to the construction forum.

USSARCADIA · Post by **USSARCADIA** » Mon Jun 20, 2005 6:53 pm

I'm having a similar problem with a scratchbuild. I built up the surface with homemade putty and am finding bubbles as I scribe. I'm doing just what Mark said, filling the grooves with epoxy putty to rescribe into. Haven't done the rescribing yet(too tired) so can't tell ya more.

jack wendt · Post by **jack wendt** » Mon Jun 20, 2005 7:45 pm

Troll 7 wrote:Hi,
I made rubber molds of a 20mm ball bearing. Molds came out just fine. I poured resin (por-a-cast) into the molds and the casting come out fine also. What I need to do is scribe some lines on these casting but when I attempt this I run into small air bubbles just under the surface of the resin which ruins the line I'm trying to scribe. Any ideas as to how to fix this problem?
Also I've been thinking of just scribing these lines on the ball bearing itself. Any thoughts of what tool to accomplish this? Don't think a simple modelers scribing tool could do this.

Thanks!

Troll 7

have you thought of buying a wooden ball from say hobby lobby, scribing, sealing it, then pouring your rubber?

Hobbit 77 · Post by **Hobbit 77** » Tue Jun 21, 2005 2:01 am

I have tried super glue to fill the imperfections but didn't work as well as I had hoped. I have used putty but constantly have to reputty areas to get it right. Haven't tried the epoxy and gotten wooden balls from Hobby Lobby. Didn't like the results. Will give the epoxy a try.

Thanks!

Troll 7

Shinnentai · Post by **Shinnentai** » Tue Jun 21, 2005 5:49 am

Etching rather than scribing is another possible solution. Give the ball bearing a good spray bomb coat, then scribe your lines in the paint. Immerse the ball bearing in a glass or plastic jar filled with etchant (more on this below), and agitate while vegging in front of the TV. After it's been etched to a suitable depth the paint can be cleaned right off, leaving you with a verrry cool piece of metal.

There are a few different etchants out there. The most commonly cited is ferric chloride, which is sold at electronics stores (Radio shack, Fry's, etc) for making DIY circuit boards. I've used ferric chloride sucessfully on steel before, but it's unpredictable: one kind of stainless steel will etch in twenty minutes, another will take days. Not sure what the determening factor is there (I suspect it to be chromium content, if such is the case you might not want to use ferric chloride). Sodium Persulfate is another, also sold for making circuit boards. Never tried it on steel myself, so it might work. The third is a hydrogen peroxide and hydrochloric acid solution you can mix yourself (drugstore peroxide, and hyrochloric acid is sold at pool care places under the name "muriatic acid"). I haven't tried that either, but have heard very good things.

I've only dabbled in etching so far. I know there are others here who have done it to a much more in depth degree, and can comment further on the above. For hand etching (as described above) though this would be easy-peasy, just a matter of figuring out the right mordent/etchant. I've done the above a few times to make one-off pieces, and it produces very nice results.

Blappy · Post by **Blappy** » Tue Jun 21, 2005 4:53 pm

To eliminate the bubbles you really need to pressurize your castings. And for that you need to degas you rubber when making your molds. Its a nasty cycle I know. If you knwo someone who can do this for you you might be in luck.

Starship Modeler Discussion Forums

Problem scribing lines on resin casting

Problem scribing lines on resin casting

Re: Problem scribing lines on resin casting