Thread: Working with Foam

Anyway to subscribe to a thread without replying? Oh well accidentally usubcribed so replying again.

That is pretty expensive foam lol

It's not free, I'll give you that. The thing to remember is that even that partial board, you could make patterns for a pair of custom speaker pods, a radio trim bezel custom knobs, levers, lids, containers... damn near anything you could think to do. And for only $130, it's all going to be completely stable. No worries about melting it, Shapes can be made incredibly accurate. Some of this stuff you just can't do with MDF or floral foam... Even if you did spend 14 times as long on it as I did. It's not a rip off, you just have to consider the all the hidden costs you can incur with other materials less suited for prototyping.

wow- i like that idea

Hey everyone! After that brief hiatus I have responded to some comments and queries in this thread and others about the uses of foam and casting techniques, and with the invaluable help of several members here (you know who you are) i give you my best efforts in demonstrating low-tech and effective mold making and casting techniques.

Rather than use my existing trim bezel, I wanted to make a few changes to hold closer tolerances to the screen itself and improve the fit and finish to some degree. So i remade the bezel out of RenShape (as described earlier) with the new form, and here it is in my car to see how it fits and looks:

The next step is to build a box around your part and block it out with cheap materials as necessary to reduce the amount of silicone you need for your mold.

You can see in this picture that i've filled the center of the box with a piece of MDF... there's no reason that needs to be solid silicone and so i save a little money.

For this particular mold which is a one part mold, all you need to do is ensure that your part sits perfectly flat on the bottom of the box and use some hot glue or other adhesive to make sure it stays in place.

Be sure to leave at LEAST 3/8" around thin sections on both sides. Any thinner than that and the silicone gets a little floppy.

This next step is to mix and pour the silicone. One very important thing you want to look for when purchasing a silicone is the viscosity. Lower viscosities are much easier to mix, not to mention far more forgiving when perfect surfaces are a must-have. Another important thing to check is whether it is a addition cure or condensation (tin-cure) silicone. Condensation cure silicones are almost inhibition proof which means they'll cure against most anything. Good safety factor there.

The method for pouring is a VERY clever method courtesy of TAP Plastics that they call the "Bombs-Away" method. If you are like me and just don't have $1500 to drop on a professional vacuum deairing station, this is a phenomenally cheap and easy way to ensure bubble free parts.

Here's how it goes:

1. You mix your silicone and prepare a dixie cup with a 1/2" hole in the bottom. Cover the hole with tape and place the cup at least 30" above your mold.

2. Pour an amount of silicone into the cup and pull the tape off. A glob of silicone will come out. I caught this in another cup and "followed" the stream down to the mold box so i didn't have a poorly aimed glob on the floor. Once your aim is right, let that thin stream start to fill the mold. Aim for a low spot in the mold and let it flow over and fill everything up to the top.

3. You can see in the cup itself the big bubbles that are trapped by the mixing process. As the silicone stretches out on its way down, the vast majority of those bubbles pop.

This is the worst part. Although there are some fast-curing silicones, this particular one made by Silicones-Inc (www.silicones-inc.com) is GI-1000 which is a lower viscosity condensation cure silicone that takes a good 24 hours to cure. So you just have to be patient and don't poke at it.

When it is ready though, break apart your box and gently peel the silicone from the part. If you designed enough draft into your part, it should come out very easily. One of the benefits of silicone though is that you can have minor undercuts and still be able to demold the completed part.

There may be some flash to trim off but if your part sat flat against the bottom and you did a good job in gluing/claying it, then it should be pretty clean. You can see that I did not!

Here's where we start to have fun. I was introduced to Alumilite by my local plastics store (plasticare for those of you in the denver area). This stuff is nuts. Two parts, 90 second open time, and demoldable in about 5 minutes. I do recommend leaving it in the mold until it has totally cooled though. This minimizes the possibility of shrinking or warping. This stuff is a little thinner than honey and mixes easily with a 1:1 ratio by volume. I just mix it and pour it on the edge of the mold. I let it roll down the side and fill the mold smoothly. This eliminates many of the surface bubbles that might otherwise occur.