It might work just fine, especially if you aren't going into some large scale production.
http://plastics.dupont.com/NASApp/myplastics/Mediator?id=0The standard plastic paint types are Acrylo-nitrile, Catalyzed Acrylic Enamel, NitroCellular, Lacquer - catalyzed or not, and of course Poly Urethane {Imron etc.}
There are also some clear Poly's at hardware stores as sealants, they look the same but don't need catalysts to harden. Probably cheaper, but they're colorless. Ask the manufactuer for the appropriate catalyst as it will be like rubber without it.
They will have a wide selection of Poly's at any auto paint shop along with additives. Retarders and reducers {same effects in this use} might raise elasticity but take substantially longer to harden. They would be vital with Lacquers though as the resin of Lacquers are brittle. Use a fiberglass catalyst in this case. {Use any thinners sparingly, we generally aren't spraying the plastic we're pouring it.}
Hardeners with Urethanes can make it stronger setting and helps when you pull the part from the molds sometimes.
Unless this is a one-off pull, you will need to make a prototype piece. Use a wood or other easliy cut mockup. Dip it in a plaster filled paper {or something you don't care about ruining} container . Then fill the interior up too and allow it to dry. This is a messy job so think it though a little so you don't make a big mess.
When the plaster has sat a day or two to harden, cut it in half cleanly and straight down the longest line across the mold as possible. Pull your mockup out and away carefully, then separate the other half of the mold from the mockup half. Do the same with the other half of plaster.
Take those four pieces and reassemble the two interior pieces. If you used plaster it's like spackling a wall crack and the joints reattach easily with some new plaster. Worry about the other two after completing the rest of the preparation.
If we're talking huge pieces here, chafmer the two big outside pieces along the 'internal' edge. Look the both of them over for any tiny defects and fill them with more plaster.
Then spray it down with multiple {3-4} coats of catalyzed {preferably Black} Lacquer black paint and when it's dry, wet sand it smooth. This is called 'tooling' Wax it a few coats with a hard pasty wax formulation by hand. Perhaps a little ingenuity will be necessary to make your two piece mold work easily, but the importance of blow release holes drilled into the center of the pieces is sometimes an important add-on. You will also need a fill hole to pour the liquid into from the bottom of the piece so it should be within the crack somewhere, possibly in more than one place. A rat tail file makes these half-holes nicely. They should be painted and sanded as well. Remeber that if you knick the inside of this mold somehow while handling them, you can use a paper cupfull of More Black paint to fill these tiny holes and scratches, then sand and wax them smooth.
I'm sure you can visualize the best arrangement for using {and casting/making} your mold. But you'll need a water-based prep paint for a light dusting before proceeding shortly to pouring your liquid plastic. It should be watery and barely "dust" your mold halves faces to function right.
Air pressure can push the cured Urethane {etc.} pieces out but can also break a soft newly-made piece. So try without them unless it's really big.
That's probably more help than you asked for. But others may be interested too someday. You can swap an expanding foam for the plaster in one or more steps if you want. The treatment and prep is the same. Clamps of some type will be needed to hold the stuff together while you fill it with plastic. A rope wrapped around it or wood clamps may work. Putty is somethimes used to seal the crack of the bottom mold halves, work it smooth with water before spraying the water-based {Release agent} paint. That children's putty in the cardboard can works fine.
The coloring agents themselves add integrity to the plastic, they're copolymers themselves. And of course you can add fiber or fabric if the need dictates it for your desired product.
The people at Dupont Chemical, Dow Chemical, and any other Plastic or Paint Industrial supply manufacturer really are into these kinds of ideas and can be a really great source of information. They'll offer free phone call assistance with your project. So whether it's one-time special purpose job or a possible pre-production idea you should probably inform them of that point. They can help you make time-saving decisions and really have a love of their job.
Check the linkup above on this post for some ideas you may not have thought of for additives.
Ask away.
Andy :2guns:
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Topic: Polyurethane for making strong plastic (Read 14619 times)