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Discussion Starter #1
I created a mold using Alumilite's Quick-Set RTV silicone rubber. I prepared it for casting by dusting it with powder and heating it up before pouring in EasyCast Clear Casting Resin.

The cast looks beautiful. Perfectly clear with no bubbles in the body of the piece. I was very pleased.

48 hours later, I tried to remove the piece from the mold...and I can't. The resin is sticking to the rubber. The mold starts to tear when I try to remove it. Is there some technique I can use to remove the piece?

And, my second question, why did this happen? I have used this resin before without having it stick to the mold. Also, a second pour from the same mix of resin popped right out of another, older mold (this one was made from a different rubber, however).
 

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Dusting it with powder? That's the first time I've ever heard of that technique... it doesn't sound reasonable to me, though I suppose that someone may have done that before with success. I DO know that real molding applications never, ever use powder (which inevitably become embedded in the casting material).

If you do a search for "mold release agent" you'll find a lot of liquids, and for the sort of work you're trying to do, usually in a spray-can.

Here's one I've used before with good results.

http://www.smooth-on.com/Release-Agents-for/c1123_1226/index.html

(By the way, "Smooth-on" is a great source for some high-quality casting resins and mold-making materials as well.)
 

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Discussion Starter #3
Dusting it with powder? That's the first time I've ever heard of that technique... it doesn't sound reasonable to me, though I suppose that someone may have done that before with success. I DO know that real molding applications never, ever use powder (which inevitably become embedded in the casting material).

If you do a search for "mold release agent" you'll find a lot of liquids, and for the sort of work you're trying to do, usually in a spray-can.
Dusting the mold with powder helps prevent bubbles from forming on the surface of the piece being cast. The tiny amount of powder that is absorbed into the resin is harmless. It works quite well to reduce surface bubbles. Based on the number of times I have seen this recommended (including Fon Davis' "Professional Model Making" DVD) I believe it to be a fairly common practice.

Mold release would definitely have helped if I had applied some before pouring the mold, but I have never had a release problem before this pour so I didn't use any. Is there something I can do about it now, or is the mold a goner?
 

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Dusting the mold with powder helps prevent bubbles from forming on the surface of the piece being cast. The tiny amount of powder that is absorbed into the resin is harmless. It works quite well to reduce surface bubbles. Based on the number of times I have seen this recommended (including Fon Davis' "Professional Model Making" DVD) I believe it to be a fairly common practice.

Mold release would definitely have helped if I had applied some before pouring the mold, but I have never had a release problem before this pour so I didn't use any. Is there something I can do about it now, or is the mold a goner?
well, without knowing the geometry of the part and the mold, it's impossible to say for sure, but in general, if the silicone mold material "bonds" to the material cast inside of it, the bond is usually stronger than the mold material itself, so I strongly suspect that the mold is, as you say, a "goner."

What's the powder you used? If you used something like talc, there's no possibility. On the other hand, if the powder is a WAX of some sort, it may be possible to decrease the "bond strength" by heating the part.

In general, silicone makes for a very good adhesive. (In fact, Kapton tape, the clear, golden colored tape often used in professional electronics-manufacturing application, uses silicone as the adhesive.) If it's bonded, your only option is to try to work it free, slowly and carefully, and hope that you can manage to break the "bond" without ripping the mold material... which, as you said before, seems not to be very practical.
 

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I'm guessing that rubber and resin combo don't like each other. Easy cast is an epoxy, not urethane so that might be the issue. You might need to use some sort of barrier coat.

You are right about dusting the molds. I've been pouring resin for 20 years and have dusted every single pull with baby powder. It breaks the resins surface tension and sucks it right to the mold surface. It works great to make almost flawless castings. It does not harm the surface at all. I would not pour without it.

However, I don't work with clears, so that might make a difference.

Dave
 

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Discussion Starter #6
I'm guessing that rubber and resin combo don't like each other. Easy cast is an epoxy, not urethane so that might be the issue. You might need to use some sort of barrier coat.
You may be right, because the second part of the pour was into a different brand of rubber and I had no problem with that one at all.

I've never had this problem before but, then again, I haven't done a lot of clear casts either.
 

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I'm not at all sure, but I've had it happen in the past with Alumilite bonding to a latex mold and man, that problem sucked rocks.

With some costume details I've used a RTV mold and cheap common 2 part epoxy. I paint the mold cavity with acrylic paint as a release agent and plop the mixed epoxy right in. Haven't had a problem with this specific mold for OMG 20 years. (seriously?! I've been casting these stupid pins for 20 years? man I need to get a life. :) )

I'm not at all sure what to do with something you're trying to keep water clear, or at least the same translucent index. Liquid soap? WD-40?
 

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I found this in an EasyCast instruction pdf

MOLDS:
Polypropylene or polyethylene resin molds are designed for use with Polyester Casting Resin or Clear Casting Epoxy due to their self releasing characteristics. Rubber molds made from latex, urethane or silicone can be used if treated with Castin'Craft Mold Release / Conditioner. WARNING: Due to the strong bonding properties of EasyCast, only use plastic molds designed for resin casting. Other molds such as candy, soap or candle molds are generally not useable even with a good mold release. If you're not sure, test a spot on the mold, such as the back side.

Dave
 

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I found this in an EasyCast instruction pdf

MOLDS:
Polypropylene or polyethylene resin molds are designed for use with Polyester Casting Resin or Clear Casting Epoxy due to their self releasing characteristics. Rubber molds made from latex, urethane or silicone can be used if treated with Castin'Craft Mold Release / Conditioner. WARNING: Due to the strong bonding properties of EasyCast, only use plastic molds designed for resin casting. Other molds such as candy, soap or candle molds are generally not useable even with a good mold release. If you're not sure, test a spot on the mold, such as the back side.

Dave
Yep... sounds about right.

And the stuff I linked to, above, is a really good mold-release agent, and is among the most popular ones out there.

Mold-release agent is very, very common. Every injection-molded product you've ever received uses something like this (albeit not the material I linked to). It gets sprayed onto the mold surface periodically... sometimes every shot, sometimes every five shots or so, and this is determined pretty much through trial and error (and varies from mold to mold, and also varies by "shot temperature" and cycle time and the like).

This is why you need to wash model parts (usually with a "mild detergent" like dishwashing soap) in order to get the thing to glue, accept paint, etc, effectively. Because there's always some residual mold release agent on the parts.

For resin parts, this is far, far more common, and for most cast resin parts I've ever dealt with, they've been downright greasy when I got them. (Which infers overuse of mold release. And if too much of this stuff is present, it can actually marr the casting surface, by preventing the fill material from getting all the way up against the mold!) You'll also frequently see this in vac-form parts, but whether or not the former uses something like this depends, largely, on how much draft various features in the formed sheet have.

I've never heard of using "baby powder" (talc, basically fine-milled mineral calcium carbonate) for this purpose. I guess it might work if the material goes in under very very low pressure, is "mostly solid" at fill time, and is very, very flexible... AND is chemically suited to the mold being used. But I'd never risk that, myself. Mold release agents are inexpensive and work wonderfully and reliably.

Basically, mold-release agents are very thin oils which form a very thin fluid layer over the mold surface which prevent the fill material from coming into physical contact with the mold entirely. Because, as a general rule, plastics really, really like to bond to plastics.

If properly applied (ie, sparingly and uniformly), mold-release agents like the Mann stuff I linked to make demolding parts very, very simple and have no deleterious effect on the part. You just have to wash the demolded parts to remove the oily surface coating before working with them.
 

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I've never heard of using "baby powder" (talc, basically fine-milled mineral calcium carbonate) for this purpose. I guess it might work if the material goes in under very very low pressure, is "mostly solid" at fill time, and is very, very flexible... AND is chemically suited to the mold being used. But I'd never risk that, myself. Mold release agents are inexpensive and work wonderfully and reliably.
I do use a mold release. The talc is dusted on after the mold is sprayed. I don't pressure cast, so the addition of the talc really improves the quality of the part. I'm not sure what you mean by ""mostly solid" at fill time"

Of course, I am talking about pouring urethane into silicone molds. I can't imagine powdering regular metal injection molds.

It's always worked for me.

Dave
 

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I do use a mold release. The talc is dusted on after the mold is sprayed. I don't pressure cast, so the addition of the talc really improves the quality of the part. I'm not sure what you mean by ""mostly solid" at fill time"

It's always worked for me.

Dave
When I say "mostly solid at fill time" I'm talking about, for example, low-temperature-melt rubber materials. They flow at a pretty low temperature, so the energy in the material when "flowing" is low enough that I can imagine them not bonding to the mold material... that's all.

I can't imagine wanting to do this with a chemically-curing material, unless I knew a LOT about the chemistry of both materials, and was absolutely certain that there was almost no chance of the mers in one bonding to the mers in the other during cure.
 

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I've never done clear parts (would like to though) but from using scrap mold material to mix epoxy on, that is surely the issue. Hard to believe that anything will grab onto silicone rubber but epoxy and CA glue do fairly well at it. Alumilite QuickSet rubber is not my favorite as it tends to tear easily and is pretty rigid, but softer rubbers may not fare any better when used with epoxy.

I used powder for a while and it did help prevent some bubbles. As you know though it is not a release agent. I've never felt the need for release agent casting polyurethane resin. The heat of casting seems to release some silicone fluid from the mold which "lubes" the part right out. Evidently, epoxy is a very different beast.

You say your second pour in different rubber went better. What type of rubber was this?
 

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Discussion Starter #13
You say your second pour in different rubber went better. What type of rubber was this?
That was Smooth-On Tin-Cure Silicone Rubber. I picked some of that up today and will try this whole procedure again using that. I have used it before to cast clear parts (anyone remember my shuttlecraft diorama with the "crystalized redshirt"?) without any problems.
 

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Sounds good. Should be a better all around rubber, especially for complex parts. I guess QuickSet is useful....IF the part is simple and you need the mold really fast. I'm never in that big a hurry.

I'm no Trekkie but the dio sounds pretty cool. Have to look it up.
 

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I've never felt the need for release agent casting polyurethane resin. The heat of casting seems to release some silicone fluid from the mold which "lubes" the part right out.
If there's stuff being drawn out of your mold material, that sound like mold deterioration is next. A release agent, while not needed to facilitate release, may act as a barrier and extend the life of the mold if there is something going on.
 

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The problem originally described here sounds like the rubber was not silicone, but urethane. All the rubbers we use, no matter what the base, are known generically as RTV. In that category you will have tin and platinum cured silicones, and urethanes. The one thing you do not want to do with a urethane rubber is pour a urethane resin into it. They will bond.

According to the Alumilite site, their rubber is silicone, not urethane. However, I have to wonder if they are using 'silicone' as a generic term and are actually packaging urethane rubber. I have no way of knowing this, but when a urethane resin bonds to a mold, the chances are the mold is not silicone.

The use of talcum powder has been SOP for years and works quite well-- though the viscosity of the resin also plays a part as to whether or not parts come out without bubbles. The thinner the resin, the better.

Over time, many mold releases have come and gone. Practically all of them are useless when used with silicone rubber. I won't go into why, it would take too long. Just know my personal search for a silicone-compatible mold release went on for years and I finally discovered Price-Driscoll Corp. Ultra 4 mold release. I have used this on molds that were completely shot and new castings came out of the mold as if it was new. Additionally, this mold release MUST be applied in the thinnest possible coating (it is in a spray can). Actually, you'd have to really screw up to get too much on the mold, which I have done, but the first casting takes out the excess. And the best thing, when applied correctly, the part is paintable as it comes out of the mold. This mold release is actually urethane-based, so the compatibility with our resins is assured. You can't see it or feel it, other than the rubber feels a bit 'slicker'. No oil or grease. And yes, I talc after applying the release. I only use the release if a mold is feeling dry and it does extend the mold's life for another 10 castings or so. I order my Ultra 4 release from Glenmarc Industries in Chicago. They also make the resin I use.

Scott
 

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War stories. When I worked at Streamline Pictures Modelworks, we always sprayed the molds with mold release, which was important because we had to get the most pulls from a single mold. We didn't use powder since we pressure-cast. And there's a story...

One afternoon, Carl Macek wanted us to crank out a boatload of a particular kit. (You may remember, it was based on a cover of Heavy Metal magazine featuring a robot, ummmm, "romancing" a female bikini-clad welder.) That kit required three large molds, and we sprayed them, placed them in the pressure-pots (which were pressure cooking pots you can get at a restaurant-supply store), filled them with resin (which we "shot" into the mold with a resin gun hooked up to two 55-gallon drums filled with resin and catalyst), then seated a rubber "O" ring before capping the pot. One time, the O ring wasn't seated properly by the person doing the seating. (I should explain, we were doing these castings as a team; one person shooting the resin, one person seating the O rings, and two people capping the pots. I was one of the crew capping the pots.) we capped one pot, and I grabbed the compressor hose, and started filling to 160 PSI. At EXACTLY 150 PSI, the O ring let go, and I was sprayed with resin. (I should also mention that Carl ordered us to start immediately, and not to grab our safety goggles and NIOSH masks. I grabbed my goggles anyway.) My t-shirt was covered in resin, and since resin cures by exo-thermic reaction (it heats up), I couldn't take it off over my head, so a couple crewmates and I were holding the shirt out and away from my skin until the resin cured. Then Carl walks in, sees what happened, and I was all, like, "So. Carl. Didn't want us to grab our safety equipment, huh?"
 

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The proverbial sticky situation. :p Glad no one got an eyeful.

I actually wondered about pressure cookers when setting up a pressure pot but wasn't sure that they could be made to seal well enough. They normally cook at pretty low pressures but evidently they'll stand a lot more.
 

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In general, silicone makes for a very good adhesive. (In fact, Kapton tape, the clear, golden colored tape often used in professional electronics-manufacturing application, uses silicone as the adhesive.)
Wrong. Silicone is NEVER a good adhesive! By itself it doesn't have the ahdesive properties you described. Manufacturers use it because it's soft, and liquid enough to be put into a tube, is plyable, and can be used for applications where epoxy, or other glues can't absorb shock from vibration. There's an additive added to it to make it tacky enough to stick to most surfaces, but it won't stay permanently if pulled on hard. Usually a styrene based monomer is used to give adhesion to most resins used over the counter for general public use. An alcohol based additive is added to silicone to allow it to stick, or adhere to surfaces that it wouldn't normally. It is also why it is perfect for moulding as it doesn't stick to anything, except clothing, fabrics, or porous surfaces. The silicone we use isn't formulated as a water barrier, but general adhesive type silicone made for sealing is. This is generally used for water use areas that can be affected by moisture erosion, mildew or decay. The adhesion is added to help keep it in place once dry.

The tin based, or platinum based silicones we use for modeling is made to release as it doesn't bond to its master forms. There's no adhesive type additive added to the silicone mix, so it won't stick to hard non-porous surfaces. It's designed for this use alone. I don't normally use talc for moulding, except for ornate, or complex shapes that need the resin to flow into that wouldn't otherwise. I brush in on lightly to keep it from forming blobs inside the mould where the resin can't get into. This makes the surface dry, and somewhat rough looking. It works so I'm not going to knock it. I just don't recommend it if you're using a low viscosity resin. I also don't recommend using a mould release either as this can make voids in your parts if used to heavily, or if it gathers in a low part of the mould. This is something that I would only use if you're going to use a mould only a few times, then retire it for a few months without use. This is a storage method to ensure the physical properties of the silicone (durometer, and elongation) so that they will perform as intended.

If your rubber mould is sticking to the part, it's more than likely that it's ruined - permanently! If you're using a lot of resin that generates a lot of exothermic heat, this can cause the resin to adhere to the mould if there's no mould release agent. This is especially if you're using URETHANE rubber, and POLYURETHANE resin. You can even use petroleum jelly lightly brushed into the mould to keep this from happening. Also, the non-stick properties of silicone don't have this disposition whereas urethane rubbers will. They always require some type of a release agent which is why I chose the casting materials I use. I prefer Tin-sil 7025 from www.uscomposites.com and the color-pro polyurethane resin from www.specialtyresin.com My moulds are tiny which require a low viscosity resin to aid in the flow into the mould recesses. While no release agent is required, if rubbed with alcohol, or acetone which is a petroleum based alcohol that flashes off very quickly, it WILL destroy the non-stick properties of your moulds' rubber surface. I found this out with a waste mould that I was experimenting with. The only way to save it is to cut it up, and strategically place it in another mould, and pour new rubber around it. This is how I recycle silicone rubber that I only used a few times. It will keep the hardness (durometer), and stretchyness - plyability without deforming (elongation) that it was originally designed for it even in thick walled moulds. I'm afraid that you'll have to slowly remove the part while wiping it down along the mould edge with some type of oil, or mould release as you do so. Canola oil can help if you soak the part in it so that it can get into the mould line between the two surfaces. This may take a while with poking and prodding. If nothing else works - hug your teddy bear and hope for the best! :confused: I hope that you'll be able to figure out something either way.

~ Chris​
 
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