Finally figured out how to shrink a 2 piece mold

[Fomen38]

To whom it may concern, I figured out how to perfectly make/shrink a 2 piece mold. On the makelure page on Youtube, Mike emphasizes the difficulty of making a 2 piece silicone mold when adding odorless mineral spirits 3 to 10 ratio to the silicon and catalyst mix. After letting the mold air dry for a week, the mineral spirits will evaporate off leaving you with a mold that's about 20-25% smaller than the original. It's hard to make a 2 piece mold because you can't ever ensure that both pieces will shrink/cure to the same shape/quantities. So here's what I did......

I took my original 2 piece silicone mold (that I wanted to downsize) and poured a casting into it with Alumilite super plastic casting resin. Then I repeated the process. I now have 2 identical castings. I place one casting in each half of the mold and create a mold box around each. Then I cover both halves (super plastic cast pieces in place) with rubber to rubber mold release. Then I mixed enough silicone with the odorless mineral spirits to cover both pieces and let cure. When cured, I removed both pieces of silicone with the mineral spirits in them and let them air dry. Because they were mixed from the same batch, and because they covered identical mold halves and cast pieces, they shrink uniformly. Voila! A perfectly shrunken, mirror image of the original.

I would like to note that I wouldn't recommend this process for the HS3 silicone rubber, as it's already really soft and may warp a tiny but during the evaporative process (if it's a thin piece). But if you're making a 2 piece mold you're more than likely not using the HS3 rubber anyways, since the purpose of it is to be able to cast pieces with deep undercuts without needing a 2 piece mold. The more rigid silicone is better for this process.

Also- on another note, I figured out how to make a 2 piece mold for casting larger lead pieces (like 12+ ounce sinkers, or custom sinker shapes). This is more for the guys who use heavy weights for deep, saltwater applications. You basically use the same process as the aforementioned with the original silicone mold and the 2 identical casting pieces. You want to cover the cast pieces (not necessary with the silicone mold) with petroleum jelly. If you have any alignment holes on the mold, I recommend using some small metal springs (approximately the same diameter of the alignment holes) to place inside of them before pouring. You're going to be pouring plaster of paris, so you will want the springs in the alignment pins to add rigidity to them.

Now don't hate on me because I'm recommending using plaster of paris instead of high temp silicone. I tried the high temp silicone route. It works great for about 6 pours, then the silicone just starts to deteriorate. The larger the lead casting, the faster the mold will deteriorate. The HT silicone is expensive, and (IMHO) you don't get enough pours out of it to justify the cost.

You're also going to want to go to the local hardware store and buy some fiberglass mat. Shred it up into thin strips and set it aside. So once you have your mold box set up around your silicone mold (with sidewalls high enough and sealed up to contain wet plaster of paris), place your springs into your alignment holes (on the 1/2 of the mold that has the alignment indentions of course), and mix up your plaster of paris. Pour a thin coating of the plaster of paris over the cast piece and silicone mold, and spread it around with a paint brush to make sure it's evenly coated. Then take some of your shredded fiberglass mat and sprinkle it around over the wet plaster. Push it down into the wet plaster. Repeat this process alternatively 3 or 4 times, until all of the plaster of paris and shredded fiberglass has been used up. Then let it set. The instructions say you can demold in 30-60 minutes, but I would wait at least 4 or 5 hours just to be safe. The fiberglass gives the mold rigidity throughout, and helps hold it together when dry. Also, it's glass, and it can stand up to 2000 degrees.

CAREFULLY extract the mold, and CAREFULLY separate the plaster from the silicone. Once you have the 2 pieces of your mold pulled apart, let them air dry for 24 hours. Then pop them into a 350 to 400 degree oven for 4 or 5 hours. You will know when they are done because no more steam will rise off of them, and they will be about 65-70% lighter then they were when you first placed them in there. You DO NOT want to pour molten lead into a plaster mold that has any moisture left in it. The moisture will flash into steam on contact with the hot lead, and it will erupt in your face like a molten volcano.

At this point, they are VERY delicate, so handle them gingerly. This next step is crucial. Buy some high temperature spray paint, and cover both halves of the mold inside and out. This serves 3 purposes. 1- It seals the mold and prevents any moisture from penetrating back into the mold, 2- It adds strength to the exterior of the mold so it doesn't crumble apart on you, 3- It smooths out any imperfections in the casting. High temp spray paint can withstand up to 1200 degrees, so 700 degree lead is nothing for it to handle. I recommend 3 to 4 coats.

The mold is still pretty delicate, but if treated properly it can last for a long time. Always warm the mold up to about 400 degrees before casting lead into it because you don't want the hot lead to shock the mold and crack it. Also, if any of the plaster chips off, just spritz it with some more spray paint. As long as the piece doesn't have any deep undercuts, chipping shouldn't be too much of an issue.

The whole reason I posted this is because I made a custom lead core that gets encapsulated with Flex70. It's a very unique shape, and it weighs about 16oz. I could go to a machine shop and have them make me a 2 piece mold from aluminum, but considering they would have to 3D scan my insert, and then mill it out, the price is STEEP! Believe me, I've called around.

So if that instruction is beneficial to anyone, I'm happy to share!

[dahlberg]

Great info, thanks for sharing!
Advice to any and all pouring lead items ESPECIALLY in the half pound and up category to be really, really careful and make sure everything is well clamped and secured before you pour! And be sure to heed the advice and warning Fomen38 has mentioned about the moisture in the mold and the volcano effect!
L

[Mike - Alumilite]

I've never poured that much lead at one time before and learned something new tonight. Thanks for the post and explanations!

If you want, please feel free to post some pics on the makelure.com lure gallery. Would love to see them.
thx,
Mike

[Fomen38]

I plan on making another mold this weekend. I will take pictures of the process and post them up. Also, I'm teachable. If someone out there knows a way to better my process, please chime in.

[aka anglinarcher]

I found another way to shrink items or molds some time ago, but I have not spend the money yet to try it.

http://www.sculpt.com/catalog_98/Castin ... Shrink.htm

It appears that through observation and timing any size reduction could be captured down to 1/2, then you could do it again for even more.....

Have not tried it yet.