Mold Making 101

[Grandpa]

After spending the last 2 week making the master patterns for the Chief Hook(er) to make the molds I learned a valuable lesson. Never change materials that have worked in the past together! The process I use is to make the master pattern out of 1/8" acrylic and use Bondo and spot glaze and wet sand it with 800 grit sandpaper to make it glass smooth. The next step is to use "LACQUER" primmer, wet sand it to 1200 grit and then buff it on a wheel it to a mirror finish. I then make the mold box with parting lines etc and pour the silicone rubber and pressure cast. After 2 days I pour the second half of the mold and after 2 more days of cure the molds are done.

To make a long story short, after demolding the master patterns I found they had a "crinkle finish" in them. The auto store was out of lacquer primmer and I thought it would be alright to use "ENAMEL" primmer........WRONG!!

Now it is back to stripping the master patterns and starting over and use "LACQUER" primmer.

[robothut]

I hate it when that happens! But have you thought about offering a textured Hook Robot? sort of a radicon hook type look. you might be able to get back some of your time and money by offering a textured version.

[laserman]

Grandpa, I never had that problem. I have had problems with the primer wrinkling due to spraying it during periods of excess humidity. I just refinished the master and sprayed under better conditions.

[Grandpa]

John,

I like your idea, but it's not uniform enough to make that work.

Bob,

The humidity was low when I sprayed it and I post baked it before polishing it. It had to of been a chemical reaction with the silicon rubber that made it lift.

[robothut]

I supose you could just sand with 80 grit to knock off the high points then paint with wrinkle finnish paint and bake, this should cover any non uniform finnish areas. But I guess since you are remaking the mold you might just as well use the new good mold for texture finnish robots and save the sanding stage.

I have never had the silicon rubber react with my primer coats in the 20 years I have been casting. It might be a good idea to post the name brand of the paint and so on so that we all can be sure to steer clear of the problem as well.

[Grandpa]

John,

The primer I used was Dupli-Color Enamel Filler Primer High Build Formula. Needless to say I won't be using that again.

[neuston]

Hum....might be a stupid question. I guess that you are talking about mold making of robot (is it right?), but don't understand what you are talking. Too much professional words I don't understand. However, I guess that I should be serious interested in it. I am a quite fresh graduate of an art school, major in sculpture. Thus, I had learnt mold making such as resin casting, but just cannot follow you. (I must be a lazy and stupid student :P )

I really want to learn how to make a robot. If you are talking about molding of robot's form, can you teach me or tell me the ways to learn? Any tutorials on the web? Thanks a lot.

[robothut]

Neuston if your looking for videos on how to make masters and silicon rubber molds then there are lots of books and videos out there, almost every supplyer of casting materials has there own line of books and videos. Here is a link to Silpak for example. http://www.silpak.com/video.php

Mode Making Videos

Silpak Mold Making Training Video: Nothing teaches like seeing it done by the experts! We make molds of different types right before your eyes, explaining as we go. Save hours of frustration and wasted material by learning how, and doing later!

Take the mystery out of molding.

Molding and Casting Hand Book: It describes various rubbers, resins and misc. materials used to create props in the entertainment industry.

Great hand book of molding and casting materials.

Monster Mask Book: Discover the secrets of making latex mask and decorating.

Grandpa here is apicture of the primer I think you are talking about. If this is the same primer that you used then I do not think this was the problem as I have been useing this brand of high build primer for over 20 years. It used to be called Krylon high build Auto primer but when Krylon split off in to diffewrent divisions it became there Plastic coat brand. Any way every single project that I have posted here on Alphadrome in the past 6 years has used this primer. I am not trying to sell you on this stuff. I totaly agree with your origional statment. " Never change materials that have worked in the past together!" I think you should always stay with methods and material that work for you. The only time I try different materials and methods is when I am not trying to turn out product and just want to experiment.

I can not think what when wrong for you, the only thing I can come up with is that the Curative that you add to the silicon rubber (10% by weight with the stuff I use) it does settle in the bottle so I always make sure I have mixed it up well and I know you do as well, I state this for the other guys that want too learn from our posts. Also the curative has as I recall has a hot thinner in it ,MEK I think. So if for any reason the curative does not get mixed 100% with the silicon rubber , say you do have a uniform color mix but there is a very small pocket of the curative in the rubber then this pocket of unmixed curative could wringle the primer. Again I do not think this was the case with your mold as thr wringled paint area or mold area would have been darker in color (what ever color your additive is) normaly they are either blue or green.

Maybe when your ready to make the new mold you could try a test as well? just spray some of the high build primer on a scrap of something and poor a little of the mixed silicon rubber on it just to see if it has the same reaction again.

[Grandpa]

John,

The primer in your picture is what I normally use. The stuff that gave me the problems has a blue label and does say Enamel on it, I tossed it so I can't take a picture of it. I called smooth-on and they have seen the same problem when using the enamel primer with the platinum curative silicon rubber, the curative does have a MEK base that affects the primer. I learned a long time ago that when you think you have it mixed, keep mixing to make sure it's mixed :) I should have the master plugs cleaned up and ready to start over making the molds on Monday, I'll keep you informed.

[neuston]

Thank you for your link, robothut. It does help me :)

[robothut]

Also check out Smooth-On web site, they have a news letter and seminars. http://www.smooth-on.com/

here is there how to section http://www.smoothon.com/howto.htm

then there is SYNAIR CORP. http://www.synair.com/

The sell threw dealers. Por-A-Kast® line http://www.sunbeltmaterials.com/por_a_kast_chart.htm

Ask Dr. Silicone

If you have questions about our products for a specific application we can help. Dr. Silicone will offer expert advice to assist in your project. We recommend trying any suggestions on a small scale before implementing in production.

drsilicone@sunbeltmaterials.com

[designprof]

Hi Guys and Gals...I'm a newbie here.

Interesting discussion about paint and I have some strong opinions on that subject!

Always use Lacquer paint for your restoration projects...never Enamel.

I strictly use Martin-Senour paint...available at your local NAPA auto store in spray-cans.

I use their sanding lacquer primer for raw plastic or metal surfaces. Dries quickly and can be sanded to an extreme smoothness (wet-dry). Can also be used to fill slight imperfections, scratches, etc. Comes in white, gray and black.

Then I follow up with any of the Martin-Senour lacquer auto colors...multiple fine coats (no sanding between!). When the last coat looks perfect and has dried for a few days, the finish can be hand-buffed or polished with good old auto paste wax.

By using lacquer paint from the same source, you minimize chemical differences that might cause crazing or crackling of the finish. Martin-Senour lacquers may be a bit more expensive than your dollar-store enamel, but what is your Robot worth to you?