I am interested in pouring 2-part urethane rigid foam in my molds instead of plaster and concrete. Anyone have experience with anything beyond fill-the-box urethane foams? I need to know about self-skinning action, and prepainting the mold and having it remain on the part as a primer/skin.
Dan The primary concern with self skinning foam is that you need to build a mold that can hold considerable pressure. It must be cleverly vented with very small vents that will allow the air to whistle out as the foam expands into all the mold's extremities, yet small enough that the viscous foam will not flow readily thru them- creating a back pressure. The higher the pressure the thicker and more uniform the skin it will form. You can get both rigid and elastomeric self skinning foams.
You can spray a barrier coat into the mold prior to casting the foam, and this will bond to the foam as it cures. You can order this barrier coat in whatever color you prefer so that is will act as the base color of the cast part- however, you will need a mold that can be laid open to allow even coating of all mold cavity surfaces. The very best barrier coat systems, which extended mold life dranmatically, were banned in the U>S> by the epa ten years ago- however- anyplace you can buy the foam chemicals should also sell or know of a local supplier of barriercoats.
Good luck Christopher
[I've done this with flexible but dense 'self-skinning' urethane foam. I wasn't very happy with the skin it formed- perhaps I didn't get enough pressure in my mold. I had better luck by pre-coating the mold with a compatible liquid urethane, then filling with the foam. I got the materials from BJB Enterprises: TC 281 skinning med density flex foam, 430 - 10 brushable urethane clr 6350 Industry Way Westminster CA 92683 714-5554-4640. Probably other manufacturers have similar products. When I've mixed the rigid urethane foam, the horrible fumes drove me out of the room, and the stuff stuck in my silicone mold. But you might have better luck with it- let us know. The flexible stuff wasn't bad to work with, but wasn't cheap either.]
Thanks! I figured trapped air was trouble. But high pressure should force foam through my pinholes. The architectural foamed parts I sell show no signs of little sprues. I wonder why on earth urethane foam should stick to silicone. I talked with a rep from Polytek today who said you need platinum-cure sils. for such molds. The manufacturing co. seen on This Old House used heavy silicone molds I think.
I looked at http://www.polytek.com/html/polyurethane_foam.html They seem to say the air vents are in the top sealing plate, not in the rubber mold itself. In plastic injection molding, the vents are just scores on the steel mold plate. Have you actually seen such molds? Where/how small were their vents?
Scoring or scratching a thermoplastic injection mold will not provide enough of a vent. Typically in plastic injection molds there is a .0003' to .0010' deep by 1/4' wide gap ground at the parting line across a .050' land. After this land the vent dumps into a much deeper passage way to the atmosphere. There is a large variation in these vents depending upon the material used.
There are plugs now available that are made of sintered metal that are porous and can be utilized to vent blind pockets. Some similar plugs look as if they have been pierced by a laser to achieve the same affect and look like a screen. These are easily clogged by material or residue from the escaping gases if they are not sized correctly or are too few.
What size of vents are needed?
The number of vents or length (1/4' in the above example) of vents would be based on the volume and pressure of the trapped gas and how quickly it needs to be vented. It is typical in injection molding to have the material burn due to the 'diesel' effect of gases self igniting from high pressure before they can escape.
Would layers of fine screen work for this application?
I have made such molds-
For example, when I did a mold on a headboard for a bed, it was a huge one piece rubber skin held in a robust plaster jacket that had been made dead level so that the rubber apron round the open cavity was absolutely parallel to the table it sat upon. Across the top of this wide openning we would strap a large aluminum plate that we would press down in a device that looked just like a large litho press. The plaster jacket would allow the rubber edge of the mold to be compressed maybe 1/8th of an inch ( that's a lot for a platinum rubber with a durometer over 40) Because we used the same plate and press for a wide variety of flat backed designs, we were loathe to cut the aluminum plate. Instead we would mold small grooves into the rubber apron that formed the pressure seal, located where they would best bleed air from the cavity. The vents were 3/16 inch deep grooves with a short section of only 1/16th of an inch.
We would pour resin into the mold and roll it under the press and clamp it down firmly and wait for foam to come shooting out of all the vents, then we would turn the press another half turn, shutting the vents completely by squeezing the 1/16th inch portion of the vent closed. This gave us a tremendous pressure in the mold and a good thick skin. Christopher
Dan- yes- Platinum cured molds are the best bet with high pressure foam. Tin cure systems are porous and the foam literally blows corrosives directly into the rubber structure- aging it very rapidly and often sticking to the rubber if it has lots of fillers in it.. A properly separated platinum mold can produce nigh onto a hundred foam castings.
Beware- platinum systems are notoriously finicky and easily inhibited- this can not be avoided by getting them drunk you must follow the recomended procedures very strictly to ensure a fully cured mold throughout. Christopher
The Vents are not operating at high pressure on a rubber mold filling with expanding foam. It is a slow process compared to injection molding, often taking a minute or more for the foam to inflate to full dimension. The air must be allowed to flow easily from the upper section of the mold with an eye towards the nature of the foam, which will occupy the bottom central mass first and then press outward and upward into the extremities. The ideal system allows for the vents to be closed off, or a sintered (and disposable) filter in the vent that will allow gases, but no foam, to pass. Only after the air has bled out and the vents are clogged or shut does the mold build any real pressure at all, as the self expanding foam tries to expand against a closed volume. This action causes foam cell structure to collapse on the inner surface of the mold and create a solid resin skin (with a self skinning foam)
Experimental casting may reveal that the still liquid foam must be apportioned within the mold cavity, or even swirled around with a brush to wet the surface, in order to achieve a reliably and repeatably good casting with any given mold.
I neglected to mention that S.I. also makes a tin cure silicone that is well suited to urethane foam production. It is their G.I. 1040 product and is supposedly almost as sulfur resistant as G.I. 1000. They claim it is the next best thing to platinum for any kind of high temp resin casting.
I love that one Christopher. Some of the pundits recommend a separate set of tools, ie spatulas, mixers, buckets, etc. set aside just for platinum cure - do you advocate the same?