Learning to cast resin

[Rob de Bie]

Since you're such an enthousiastic audience, here are two freshly made photos. First a photo of the mold of the ALE-2 chaff pod, as shown earlier. I mainly want to show that there are no slits that run to the exterior of the mold. The product cavity only connects to what I call the 'resin chamber'.

 

 

Here's how I often place the filled molds (empty here) against each other to hold them in their original shape while the resin cures. The mold second from the right is discolored, a sign that it's at the end of its life. I pull just 20 castings from a mold, then a new one must be made. I think that's a pretty poor number, other materials may yield better numbers.

 

 

Rob

Edited March 21, 2021 by Rob de Bie

[arnobiz]

I also get about 20 castings out of my molds if that's any reassurance Rob 😉

 

Arnaud

[Rob de Bie]

39 minutes ago, arnobiz said:

I also get about 20 castings out of my molds if that's any reassurance Rob 😉

 

 

That's comforting indeed 🙂 My thinking is that a faster resin would give a longer life, since it seems that the total time of uncured (chemically active) resin determines mold life. Note that I did not investigate this scientifically, it's just a working thesis.

 

Strangely, my molds still release well after 20 castings, but the castings develop a scale-like skin. You can sand it off, but it's too much work, and you'll lose surface detail. I will make a photo of this phenomenem the next time, since I have never ever read about this.

 

Rob

Edited March 21, 2021 by Rob de Bie

[arnobiz]

haha I see we all pursue the same quest for a resin which remains super fluid for 10-15' as possible to enable maximal degasing, then sets super fast while shrinking very little 😄

[Rob de Bie]

Luckily, a long pot life goes together with small shrinkage. It's mainly because the curing resin does not get hot (or not as hot) as fast resins. The shrinkage mainly comes from the thermal expansion (contraction here) of the cured resin.

 

Of course the drawback is the long demolding time. But I can live with that, I'm not running an optimized production line 🙂

 

Rob

Edited March 21, 2021 by Rob de Bie

[arnobiz]

Indeed, I was just replying to your hypothesis that long pot life damages the molds 😉

[Rob de Bie]

Maybe one more comment: I never use release agents, in any stage. I really don't like the subsequent paint problems. And with my type of molds, you can't get release agent in every nook and cranny, so it's useless anyway.

Rob

[ElectroSoldier]

On 3/21/2021 at 7:39 PM, arnobiz said:

haha I see we all pursue the same quest for a resin which remains super fluid for 10-15' as possible to enable maximal degasing, then sets super fast while shrinking very little 😄

Yeah the real problem people come up to on casting is what resin to use because the options seem quite limited when you are on the amateur caster side.

[ElectroSoldier]

1 hour ago, Rob de Bie said:

Maybe one more comment: I never use release agents, in any stage. I really don't like the subsequent paint problems. And with my type of molds, you can't get release agent in every nook and cranny, so it's useless anyway.

Rob

It really depends on the release agent you use. And that depends on the resin you use.

[Rob de Bie]

26 minutes ago, ElectroSoldier said:

Yeah the real problem people come up to on casting is what resin to use because the options seem quite limited when you are on the amateur caster side.

 

So true. 10+ years ago, I made an overview of all Dutch suppliers of silicone rubber and PU casting resins, with all relevant properties, to help other Dutch modelers pick the right materials. But the editorial team of the IPMS-NL magazine did not accept the 5-page article, it was "too technical for the audience" because it included words like viscosity. I was quite disappointed by such stupidity. But I gave prints to some 15 IPMS-NL members who where interested, so it more or less served its purpose.

 

Rob

[crackerjazz]

Hi Rob, I have a question.   I missed the vacuum chamber that was for sale in the local ads.   I was having misgivings about the price and somone snapped it up, so I might have to do this with the simplest of tools.   I've cast some parts a long time ago similarly - just not sure why I can't seem to find the courage to do it again.    Anyway I have these parts I wanted to cast for converting my A-4E to an A-4C.   Sorry for using your thread, breadneck, but this might be a good practical example.

 

Can I just pour rubber onto these parts all together in one mold, instead of having a mold for each separately?   The parts aren't too big (compared to paint bottle).  I was planning to just glue them in this manner (with some separation between the pieces)  on a single piece of styrene styrene sheet, fence it up, and pour rubber.   

 

 

 

 

[arnobiz]

Doing a single mold will be fine, the drawback is that you will use a lot of silicon for the thin parts: I tend to group parts by height to minimize silicon waste.

 

Don't worry about the vacuum chamber, I use a stainless steel pot (about 15 euros at IKEA), a silicon seal and a very smooth (varnished) plank and it works perfectly. Of course you cannot see inside but it's not really an issue for me.

 

Arnaud

[crackerjazz]

Thanks, Arnaud.  What's the silicon seal for, and how do you vacuum the air out on your setup?

[Rob de Bie]

2 hours ago, crackerjazz said:

Hi Rob, I have a question.   I missed the vacuum chamber that was for sale in the local ads.   I was having misgivings about the price and somone snapped it up, so I might have to do this with the simplest of tools.   I've cast some parts a long time ago similarly - just not sure why I can't seem to find the courage to do it again.    Anyway I have these parts I wanted to cast for converting my A-4E to an A-4C.   Sorry for using your thread, breadneck, but this might be a good practical example.

 

Can I just pour rubber onto these parts all together in one mold, instead of having a mold for each separately?   The parts aren't too big (compared to paint bottle).  I was planning to just glue them in this manner (with some separation between the pieces)  on a single piece of styrene styrene sheet, fence it up, and pour rubber.   

 

Basically yes. But I add one more 'ingredient': I put the parts on blocks of metal or plastic, to create a 'resin chamber'. Nearly all commercial parts have a block of resin attached, and that's the excess resin that cures in the 'resin chamber'. In vacuum casting you need it because the resin bubbles and foams, and you would lose most of the resin. But if you don't use a vacuum chamber, I guess you can leave out the resin chamber.

 

You can see the resin chambers in all molds shown in the photo below. Sometimes I group several parts on one resin chamber, like in the mold on the left, the front resin chamber.

 

 

Lastly, play it safe with buying a vacuum chamber. I remember working with a large glass one, but there was a steel wire cage around it. A steel one will be safe, but the plastic lid could fail under load. I used to think that an implosion was harmless, because all shards will move to the center. But they will continue their path, pass the center, and then move away from the implosion, just like in an explosion. Therefore I would not easily buy a second-hand one, of which I do not know the history. I discarded my first plastic vacuum chamber when I saw that the polycarbonate lid had lots of tiny spider cracks in it. I may have caused that damage myself by using a solvent in the vacuum chamber one time.

 

Rob

[ChernayaAkula]

Some additional cents from a resin beginner (who isn't vacuum casting (yet)):

I guess it's absolutely possible to put all the parts in one mould. As a beginner, I would do separate moulds anyway.

Reason #1: If it works according to plan you will not save much silicone. If it doesn't work according to plan, you'll waste much more. Ask me how I know....

Reason #2: it'll be simpler to cast individual replacements should a part or two come out wonky. Say you get good casts of the nose and the intake walls, but a bad one on the intake lips. It'll be easier to cast extra intake lips with separate moulds. Had that happen with some 1/72 pilots I tried to cast for my MiGs. The bodies were always usable, but I did need additional arms because the mould wouldn't fill properly at times. Had the first set of moulds in one big block, but the second set used individual moulds.

 

I'd put a spacer (gate?) between the styrene sheet and the parts themselves. Should be much easier to get the parts off the pour stub after the cast this way. Not a big deal for the nose and intake walls, but the intake lips in particular are gonna be a b*****d to remove from a stub that's end right with the part.

I'd also go with the resin reservoirs/chambers Rob talks about even if you're not going to vacuum cast. For a little extra "wasted" silicone and resin you get a much more comfortable casting process with much reduced chance of spilling resin.

 

The following is a bit of speculation/ loud thinking:

Regarding the nose: is a solid nose from a single-piece mould running the risk of noticeable shrinkage? I mean, the Skyhawk nose isn't that big, but ... maybe? If shrinkage is indeed a problem, here's my idea: to get thinner walls (and thus less shrinkage), cast a silicone plug of the inside of the nose with a screw or nose embedded in the silicone. Using the screw, you could suspend this silicone plug in the single-piece mould to displace the resin that would otherwise form a solid nose. You'd basically get a resin nose with the wall thickness of the plastic original. The resin of the gate/spacer should also be easier to trim and sand than the - comparatively - huge stub of a solid nose.

Then again, this might be total overkill. What do you say, Rob?

[crackerjazz]

Thanks, Rob!  Great idea, Moritz, thanks, will give it a try : )

[arnobiz]

22 hours ago, crackerjazz said:

Thanks, Arnaud.  What's the silicon seal for, and how do you vacuum the air out on your setup?

I got a good vacuum pump 🙂

[Rob de Bie]

On 4/26/2021 at 2:41 AM, ChernayaAkula said:

Some additional cents from a resin beginner (who isn't vacuum casting (yet)):

I guess it's absolutely possible to put all the parts in one mould. As a beginner, I would do separate moulds anyway.

Reason #1: If it works according to plan you will not save much silicone. If it doesn't work according to plan, you'll waste much more. Ask me how I know....

Reason #2: it'll be simpler to cast individual replacements should a part or two come out wonky. Say you get good casts of the nose and the intake walls, but a bad one on the intake lips. It'll be easier to cast extra intake lips with separate moulds. Had that happen with some 1/72 pilots I tried to cast for my MiGs. The bodies were always usable, but I did need additional arms because the mould wouldn't fill properly at times. Had the first set of moulds in one big block, but the second set used individual moulds.

 

I'd put a spacer (gate?) between the styrene sheet and the parts themselves. Should be much easier to get the parts off the pour stub after the cast this way. Not a big deal for the nose and intake walls, but the intake lips in particular are gonna be a b*****d to remove from a stub that's end right with the part.

I'd also go with the resin reservoirs/chambers Rob talks about even if you're not going to vacuum cast. For a little extra "wasted" silicone and resin you get a much more comfortable casting process with much reduced chance of spilling resin.

 

The following is a bit of speculation/ loud thinking:

Regarding the nose: is a solid nose from a single-piece mould running the risk of noticeable shrinkage? I mean, the Skyhawk nose isn't that big, but ... maybe? If shrinkage is indeed a problem, here's my idea: to get thinner walls (and thus less shrinkage), cast a silicone plug of the inside of the nose with a screw or nose embedded in the silicone. Using the screw, you could suspend this silicone plug in the single-piece mould to displace the resin that would otherwise form a solid nose. You'd basically get a resin nose with the wall thickness of the plastic original. The resin of the gate/spacer should also be easier to trim and sand than the - comparatively - huge stub of a solid nose.

Then again, this might be total overkill. What do you say, Rob?

 

Agreed on the spacer / gate, I forgot to mention that. You can do without, but then you would have to grind away the whole resin block. With a spacer / gate you can use a saw to separate the part and the block.

 

The solid nose could indeed lead to the resin getting (very) hot during curing. It all depends on your resin. The slower the resin, the less heat-up, and the less shrinkage. The shrinkage comes mostly from the resin curing while hot, and then cooling down to room temperature, and thus shrinking. I posted this small table earlier in a Britmodeller thread, that shows a clear trend for Smooth Cast resins. I mostly use 305, and I think the A-4 nose would cure without heating up much. But with a fast resin, it could be quite different.

 

Type                           potlife            demould     viscosity        shrinkage

 

Biresin G26                 3-4 min         30 min         70 mPa.s     ?
Biresin G27                 2 min            15 min         30 mPa.s      ?
Smooth Cast 300Q     0,5 min          4-5 min       80 mPa.s     1%
Smooth Cast 300        3 min            10 min         80 mPa.s     0.5%
Smooth Cast 305        7 min            30 min         80 mPa.s     0.35%
Smooth Cast 310        15-20 min    2-4h             80 mPa.s     0.025%

 

Rob

Edited April 27, 2021 by Rob de Bie

[crackerjazz]

1 hour ago, arnobiz said:

I got a good vacuum pump 🙂

Did you have to cut a hole through the stainless steel pot to attach fittings for the vacuum pump?   Just found a supplier of vacuum chambers with pump included -- will check out what they have.

 

3 minutes ago, Rob de Bie said:

Agreed on the spacer / gate...

Thanks, will fashion one from strip styrene.  I hope I'm visualizing this correctly.   And thanks for pointing out the danger of implosions.   I'm guilty of having that misconception about implosions being safe as they are directed toward a center.  

 

[arnobiz]

No, I drilled a hole through the wooden base, much easier 🙂 I then glued a 6mm hose adapter with epoxy glue