Large Scale Central

LSC Presents: Casting With RTV and Two Part Resin

Modeling
#1

Boomer,

I’ll add that when I cast I use the Allumilite regular as well. What I found out is that if you invest in some smaller bottles (say those 32 oz bottles from Micro-Mark) to pour the two parts into (seperately) and leave the 2 gallon jugs sealed well (some people have sprayed nitrogen? inside to pull the moisture out) the resin lasts a lot longer.

For the RTV I’ve used both the stuff from Micro-Mark, and the stuff from Tap Plastics. Both seem to work fine.

I will add that a really handy tool to add to the casting collection is a paint pot. It’s not required, but if you have a paint pot, you can cast under pressure, thus reducing and eliminating the air bubbles. A paint pot can be converted to a pressure pot with a few simple caps, and a valve. The quality and consistency of my casting improved quite a bit when I used a paint pot.

Craig

#2

Welll you REALLY don;t need two gallons of the stuff… Unless you are mass produciing a kit for retaiul sale or something…trust me a little goes a long way…I do piles of parts with the small orders from MM. Plus the stuff tends to go bad over long periods once its opened so its not like you can get a couple years supply at one go…

I’ve had bad experiences in the past with the Aeromarine stuff…(just sayin) and I find that the Micromark stuff (its actually made by Smooth-On) is the most forgiving to new inexperienced casters…Alumilite is good stuff too, and for rapid demold it can’t be beat! It sets up like a door slamming, you can retard this tendancy a bit by keeping it chilled in the fridge. However if there is smething that is going to need some working time, ya know undercuts air pockets needing to be thumped out, air channels and such you want something that will give you a little working time, thats another reason I prefer the MM stuff…

I actually have a couple of artivcles here on LSC already that the photos were lost from in the great hack a few years back, but there was never any interest in retaking them, should I? Part I:

http://www.largescalecentral.com/articles/12/Closet-Casting-Part-I:-Better-Living-Through-Chemistry

Part II

http://www.largescalecentral.com/articles/11/Closet-Casting-Part-II:-Casting-Off-Your-Inhibitions

Hey the pictures are there for the second part…and I can move forward with additional installments if interested…

#3

There are differences in each resin, Mostly the Hardness and durability of the finished casting, and the color when cured, from pure white to ultra black. Some will take colors added more readily then others. There are resins that redly take additives and others that will not. The Bottom line… Check with the manufacture and their tech sheets, to be sure that their product will meet your end results. Also… the resins and RTV has a shelf life, so buying in large quantities for price savings may not be cost effective if you don’t use it all before it goes bad.

#4

Bart,

If you put a smaller amount of resin in a squeeze bottle, then displace the air out of the 2 gallon container, the resin lasts a lot longer. I was told that the ‘shelf life’ was only 3-6 months. But I used my big gallon containers for well over a year (about a year and half) with out any problems. From what I’ve read it seems that resin doesn’t like moisture. I used about 3/4 of the 2 gallon stuff in that time frame. I was casting sections of building walls, and some other assorted parts.

Ordering directly from the manufacturer also allows you to order a ‘fresh’ batch of resin. Do you know how long that resin has been sitting on the shelf at the store?

Burl Rice did some excellent how to articles about resin casting in Garden Railways. He also has some posts on MLS about casting as well.

The other thing to think about with resin is the temperature deflection? of the resin. Some have a higher temp others have a lower temp. I think the Allumilite has one of the highests (200 degrees).

Craig

#5

Doing a fair amount of casting, There are a whole host of resins suppliers. Two that offer reasonable prices and technical support are Us Composites and SpecialityResins. Be sure to get polyurethane resins with UV protectors and the proper hardness. Some can get very soft and flexible in heat. The longer the open time the less bubbles. As this is a hobby, speed is not essential. RTV comes in diffrernt flexibilities also. For comes items such as people and animals the softest such as D100. but for buildings walls windows etc a more rigid RTV is better suited. Also there are diffreent open times. very fast 3-5 minutes with platinium initiator to the nickel salts with set demold times of 8-16-24 hours. with the slower ones, you can get away without degassing.

Again, it is best to discuss your needs with a supplier as the catch all resins such as Allumilte may not meet you needs, especially if outdoors in the hot southwest

#6

The articles that Burl Rice wrote in GR started with the June 2007 issue, until the June 2008. Comprehensive articles that cover all facets of casting. I would offer to scan and send a copy of the issue articles to you, but all my GR are 700 miles away in a storage unit… :frowning:

Craig

#7

Is it possible to cast multiple times for one part?

Such as a part with a deep overhang, cast some of the resin up to the overhang, let it start to set up and then pour more casting.

I was thinking this might help with the “gap” that might occur and air bubbles getting trapped under the overhang.

I’m sure there is issues with the resin not bonding to each other between different castings, but a dowel inserted into the resin should help???

#8

Jake, I’ve never tried it that way. I’ll bet it would hold, but not the same as one pour. I’d be more likely to let the 1st pour set, then scuff up the surfice deeply and then pour the second half.

On my more complex molds where I get a trapped bubble, I have made an escape route for the air. I have even drilled a hole thru the side of the mold, and when the resin starts to come out I plug it with a stick.

Using a water thin resin, with a long set time, allows you to turn and tap the mold to dislodge the air and get it to the contraction chamber, before it sets. Having an extra sprue to remove, is always better then a trapped bubble.

If it’s not a structural location and you get a bubble in the final, you can fill it with putty, if it’s going to be painted.

#9

I have done multiple pours on thicker material such as stone walls and stone trusses. I do this as there can be considerable heat build up derforming the pour in the flexible RTV molds. Just be sure there are no mold release agents present or if casting polyester there is no wax present. I also "weld’ my multipart objects such as buildings by adding talc or glass micro beads to the monomers to make a paste. The thicker material is necessary to hold it in place while it is setting up. reducing the activator “part B” can increase setup time giving more time for assembly.

#10

Thomas’s recommendations will work with certain resins but not all.

CAUTION: Not all resins can be mixed with less part “B” then recommended. Doing so may greatly decrease the strength and hardness properties of the cured resin, if it sets at all. See Manufictures recommendations and test for your self if the end results are acceptable for your end use.

Cleaning a complex mold of unset resin is a pain in the a$$.

#11

I do multiple poours all the time…basically cause its hard to mix up a small batch of resin, and it goes a long ways…so I keep a relatively large volume open faced mold out to take the leftovers in the cup…most of my truck sideframes are made from leftover resin…

#12

I hope I didn’t mislead anyone. I was not talking about reducing ‘part B’ for pouring molds. I do it only for making the paste for welding the parts together… As Dave pointed out experiment with mixture if modified on scrap prior to using it on your final project.

#13

I’ve been using Smooth-Cast 300, from Smooth-On, for several years. It currently lists on their website at $85.85 for the two-gallon kit (comparable to the sizes shown in your post). A “trial size” is also available for $25.96.

I haven’t tried any of those other brands so I don’t know how it compares with them as far as hardness, etc.

I have found that I can buy the two-gallon kit and make it last a very long time, simply by spraying some “canned air” dusting spray into each jug after use, just before screwing the cap back on. This forces out the air and humidity, which is what causes it to go bad.

For me it is easy to use and fairly forgiving. It is mixed by volume, not weight, and I usually just “eyeball it”.

It can accept dyes, fillers, and UV-inhibitors. I have used the first two but have never tried the UV-inhibitor, mainly because the inhibitor must be added by weight and requires a microgram scale.

I have done multiple pours with this product but found out the hard way that the previous pour must be fully hardened. If it’s only half set or still pliable, adding a new pour ruins the whole thing.