Question for Vinny

[awemawson]

Vinny,

What was the pourable resin that you used for your circular LED array ?

Can you give us the low down on moulding and machining it please.

[Vinny]

It's SC300 from Smooth-On. I buy it by the gallon for about $100. You actually get two gallons (1 resin, 1 hardener) it's mixed 1:1.

In this case the mold was a piece of 4" PVC pipe. I sprayed it with mold release (although I usually forget).

It takes about 15 minutes to go from mix to full cure. You can also use a special dye to color it. The other day I made a gray cap for the power feed I just got. It covers the gear on the bottom.

Usually if it's something I'm going to machine, I just use PVC for the mold. If it's something more involved, like the afore mentioned cap, I took a piece of PVC and machined it to fit the power feed. Now the piece of PVC is the model for the final product.

I found something slightly larger than the model, put it on a somewhat level rface, the crockpot full of wax was already plugged in and the wax melted and I poured some wax in the bottom. Since it's liquid it self levels and hardens. After it's hardened I put the model on it - sometimes adding a couple drops of wax to keep it from moving. In this case I didn't bother. As I mentioned above this is to be a cap so it needs an end. So I poured more wax in the middle of the model to make the outside walls and inside different levels. When making a mold you have to think in negatives.

For making a mold, unless I'm using a clear or water clear resin I use a tin based silicone material called Mold Max 14NV. It sets up fast, not as thick as some of the others and the NV means No Vacuuming which saves time and I can't reach my pressure pot right now anyway (long story). This mold material is mixed 100:10 (or 10:1) by weight so I use a high tech scale I got from harbor freight (ok, not so high tech).

After it's mixed, slowly pour from about 12" above the mold box to limit the air bubbles. Leave it alone and in about 4 hours it's ready to come out. One of them thin knifelike things you would frost a cake with comes in handy for getting the mold out of the box. In this case I used a crappy sauce pan my wife was tossing a few years ago.

If you just want something to machine, we'll do round for this example, find a piece of PVC about 1/4-1/2" larger in diameter for a mold and of course long enough. You want to plug one end, I usually use modeling clay for that. You don't want to use wax for this 'cuze the resin gets up to about 180 degrees F while curing and it'll melt the wax. Don't ask how this minor detail came to reality for me! Mix the plastic resin thoroughly (note: it's thin like water and splashes easy. Wear an apron or something - I use one of those HF machinist aprons). Once mixed, pour it in the mold and wait. SC300 will be clear and start turning white as it cures. It's done when it's cooled to room temp. Probably sooner but I always wait, besides you probably got some on your hands and need to wash them anyway.

Machine it like a soft piece of metal BUT don't go too fast or there will be too much friction and you'll melt the plastic in places. You can mill it, drill it, bore it, turn it down, whatever you want. Depending on the operation the chips will be alot like machining metal. Carbide, HSS, doesn't matter. The really cheap Chinese cutters even work well.

Some things you'll need to do any of this, most are available at the grocery store, Meijer, or WalMart:

4oz paper bathroom cups
9oz plastic cups (the clear ones)
16-20oz plastic beer cups
wooden popsicle sticks (you can get a box of 1000 for just a few bucks in the project section of the store)
nitrile gloves (optional)
a curing oven

The curing oven is any kind of oven that will go down to 150-175. In a pinch you can put it in your kitchen oven on low when your wife's not looking! I built one out of plywood, fiberglass insulation, a PID controller, some aluminum flashing
and a 150 watt incadescent bulb. This one will do 212 degrees F and the door doesn't even close that tight. The outside of the box doesn't even get warm after 6-8 hours.

After making a mold out of silicone rubber, you want to put it in the oven for at least 2-4 hours at 150F/65C to cook out any moisture that got caught inside the mold. After machining your plastic you want to put it in at 150F/65C for 2 hours to finish curing. Let it cool slowly. I just shut it the oven off and come back later.

Hope that made sense!
Vince.

[f350ca]

Thanks Vinny. That opens some possibilities. Checked and its only $121 in the land of high prices and Justin Trudeau.

[Vinny]

If/when you buy some, get a can or two of ER200. It's the mold release. When you don't use it (like I tend to forget to on many occasions) you'll have to use a hammer to get the plastic out of the PVC. If you're using a rubber mold, you don't really need it but they say it'll reduce the life of the mold. They're also talking about production use so you probably won't see and degradation.

[pepi]

Vinny,


Thanks for the write up, scooped it to a word doc, for the future. Process was very well explained.

[awemawson]

Many thanks Vinny, sounds a useful addition to the workshop 'toolkit' of ways and means :)

[Sunset Machine]

I was watching PBS (public broadcasting) yesterday and they were using a big tub of UV resin and a 3D printer to make archaeology models with. It used UV light for a print head.

Thanks for the write-up Vinny!

[TomG]

Nice write up Vinny.

Smooth-On makes a good product. We use their platinum cure silicone at work to make vibration isolators, along with Slacker to fine tune the viscosity and pigments to color it.

Tom

[Vinny]

I use the platinum when I'm making lenses and other clear things. Dragon Skin is really stretchy and forgiving.