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 casting advice?

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casting advice?
« on: Nov 23, 2019, 10:08 AM »
so ive just started molding and casting for the first(relatively successfull) time, but i'm still running into a few big issues. one of them is that i find it very difficult to get a consistent thickness for the helmet, and mine has turned out very thin in comparison to other resin cast helmets i've bought.

the walls are roughly 2-3mm thick,whereas something like my headprops helmet is about 5-6. would that just be a matter of using more resin? i've also found it very difficult to get the thickness consistent with slush casting. some parts will end up very heavyset, especially towards the dome, while other sections, towards the bottom, tend to get rough or spill. for example, you can see light shining through the cheek on one side

lastly, i noticed a lot of dots, divets, and warping  on certain parts of the helmet. some i was able to reshape by heating, but others just...sunk in like sratches. i believe it's because my outer mold shell might not have been rigid enough. i made the outer shell from plaster cloth, layered up, but the texture may have caused issues in the soft rubber mold. what matterial is ideal for an outer mold, and how thick should it be?
lastly, if it makes a difference, my i'm using rebound 25 for my mold, and smooth-cast 65D for my resin. thank you for any advice! i'd like to be able to get these casts up to sellable quality!

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Bowfanny


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Re: casting advice?
« Reply #1 on: Nov 23, 2019, 01:54 PM »
Mother Moulds (outer shell) need to be very rigid.
Ive used fiberglass cloth and resin to make my mother moulds before.
As for your other question on amount of resin. How much are you pouring in at once. Is it the whole helmets worth or are you doing multiple pours and rotating each until theyre set?

When I used to cast helmets I would pour four separate layers. When one set Id mex the next batch that was already premeasuref. Basically I measured out both halves and had them in different color cups. Part A was in one color and part B was in another. If you slush cast like this you get better consistency because you can control where the layers go better.

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QMU#084
Re: casting advice?
« Reply #2 on: Nov 23, 2019, 03:30 PM »
I was doing it by layers (adding 3 medicine cups each of part A and B) and then letting it set, but i found it would set inconsistantly, some layers taking 5-6 minutes to set, but others only 2-3. Ill try remaking the mother mold. I had tried fiberglass on a previous attempt, but found the cloth did not form well enough. Should i make a layer of just resin first, and then add the cloth?
Also, is it possible to repair torn molds? Ive already found a tear after the first use.

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Bowfanny


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Re: casting advice?
« Reply #3 on: Nov 23, 2019, 08:43 PM »
I was doing it by layers (adding 3 medicine cups each of part A and B) and then letting it set, but i found it would set inconsistantly, some layers taking 5-6 minutes to set, but others only 2-3. Ill try remaking the mother mold. I had tried fiberglass on a previous attempt, but found the cloth did not form well enough. Should i make a layer of just resin first, and then add the cloth?
Also, is it possible to repair torn molds? Ive already found a tear after the first use.
I would up your amount of resin per layer. I was using about 4-6oz of part A and another 4-6oz of part B per layer. You have to keep rotating. The more you add the faster the layers set also so layer one sets slowest and the second and the rest of the layers set faster since the resin heats up to set.

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QMU#084

Lane

Re: casting advice?
« Reply #4 on: Nov 26, 2019, 10:48 PM »

lastly, i noticed a lot of dots, divets, and warping  on certain parts of the helmet. some i was able to reshape by heating, but others just...sunk in like sratches. i believe it's because my outer mold shell might not have been rigid enough. i made the outer shell from plaster cloth, layered up, but the texture may have caused issues in the soft rubber mold. what matterial is ideal for an outer mold, and how thick should it be?

The divets are most likely sub surface bubbles in the silicone mold. The resin heats up as it cures which heats the air in the bubble causing a bulge in the mold.

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Grimstuff


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Re: casting advice?
« Reply #5 on: Dec 01, 2019, 03:41 PM »
Keep the mold in rotational motion constantly, you want to prevent areas pooling which creates globby areas that once they start setting are unable to flow and just end up curing to thick ridgy globby areas. If an area is pooling, rotate it 180 and let some of it drip to the opposite side then resume rotating. If you find yourself unable to prevent pooling entirely, you're probably using too much resin per layer. Also use mold release inside the mold to reduce surface tension and allow smoother flow. Also try to reduce humidity as much as possible in your casting environment, the worst casts I get are always when it's humid.

Also it's a little more expensive, but consider investing in Smooth-Cast ONYX Fast for all the later layers. It goes on much more thickly and cures faster, which makes it work out really well for rotocasting. Still use the 65D for the first coat or two, as it works great as a microdetail print coat, but yeah ONYX makes a fantastic reinforcement coat. It's also much quicker to work with which makes layering less tedious of a process.

And yeah as a whole you need to do quite a few layers and get it fairly thick, target at least 4mm. Especially if you're not using onyx as the 65D series has a very low melting point and is very delicate to warping. Personally I use 321 + Onyx and avoid 65D entirely.

It's also something that just takes some time and experimentation to get right, you will get better at it the more you do it.

« Last Edit: Dec 01, 2019, 03:45 PM by Grimstuff » Logged

Ori Dart


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Re: casting advice?
« Reply #6 on: Dec 03, 2019, 03:29 PM »
Some great info above so I'll only add this bit:

Also use mold release inside the mold to reduce surface tension and allow smoother flow.

Don't waste money on expensive mold release. A really cheap, effective and safe thing to use is liquid dish soap. A buddy of mine who used to do this stuff professionally (mass producing BBQ parts, amongst other things) used Sunlight dish soap. I've used it too and it makes life so much easier in that you just need to run the pull under some water to clean it.

Just brush thin layers of the soap all over the mold until it dries up, pour your stuff and when you need to de-mold, you're good to go. Also really useful when doing 2-part molds as a barrier between the 2 parts.

I've also used cornstarch as a mold release and it's super cheap and effective - just make sure you don't let it clump in the mold.

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Oya!

Andry         ; )

Bowfanny


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Re: casting advice?
« Reply #7 on: Dec 03, 2019, 05:38 PM »
Baby powder works effectively too.

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QMU#084
Re: casting advice?
« Reply #8 on: Dec 04, 2019, 09:01 PM »
The divets are most likely sub surface bubbles in the silicone mold. The resin heats up as it cures which heats the air in the bubble causing a bulge in the mold.
Huh, i never considered that! Is there a good way to prevent it? :o
All of this is super helpful! Im going to invest in some onyx after christmas! Im really hoping this mold can still be salvaged!

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Grimstuff


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Re: casting advice?
« Reply #9 on: Dec 04, 2019, 10:10 PM »
Huh, i never considered that! Is there a good way to prevent it? :o
All of this is super helpful! Im going to invest in some onyx after christmas! Im really hoping this mold can still be salvaged!

Unfortunately not really if that is the cause, just something you'll have to sand off each casting. I guess technically you could try injecting silicone into the bubble cavities or something, but that might be more trouble than it solves. Though with luck you might find it to just been part of the poor casting and the mold is fine, fingers crossed.

And yeah onyx fast will help a ton with the internal smoothness, do be aware that you have to work really quick with it though. It says in the literature you have a minute of mixing time, but I find you have to mix it within about 15 seconds and get it into the cast slushing quick, otherwise it will start setting up before you can cover everything. After the pour, slush around to coat everything inside within the next 20-30 seconds, then the moment everything is covered and you still have some liquid time left start flipping it in 180's to drip any pooling resin to the opposite side and even the coating out, and then finally just as it starts thickening keep it in constant rotation until it sets up and is ready for the next layer (you can touch the visor cutout to test it - should be soft but not wet, sticky but not so sticky it pulls away with your finger). It's speed is why I use a slower resin like 65d or 300 series for the first layer or two so there's more time to guide it into crevices, then onyx makes the nice super quick, super thick, and super smooth reinforcement layers after. The black on white colors also makes identifying weak areas easier, if you still see the white 300 anywhere, you need more onyx there.

You'll get a good sense of these timings and develop better slushing strategy the more you do it, and also the better you hone in on the exact amount of resin you need for each layer. Definitely don't feel bad for poor first casts, my early ones were all sorts of too thin with ridgy inconsistencies. Can be a good idea to keep a sorta mini-journal of each cast too with resin amounts, slushing strategy, number of layers, etc.

« Last Edit: Dec 05, 2019, 03:33 AM by Grimstuff » Logged

Grimstuff


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Re: casting advice?
« Reply #10 on: Dec 05, 2019, 03:29 AM »
Some great info above so I'll only add this bit:

Don't waste money on expensive mold release. A really cheap, effective and safe thing to use is liquid dish soap. A buddy of mine who used to do this stuff professionally (mass producing BBQ parts, amongst other things) used Sunlight dish soap. I've used it too and it makes life so much easier in that you just need to run the pull under some water to clean it.

Just brush thin layers of the soap all over the mold until it dries up, pour your stuff and when you need to de-mold, you're good to go. Also really useful when doing 2-part molds as a barrier between the 2 parts.

I've also used cornstarch as a mold release and it's super cheap and effective - just make sure you don't let it clump in the mold.

That's actually pretty interesting, I'll have to try it. Having to scrub off the release is definitely one of the steps I'd be more than happy to eliminate. Thanks.

Also reminds me that I read somewhere that you can actually spray primer into the mold too, and it'll bond to the urethane and give you an easy self-primed cast, but I've never tried it.

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Ori Dart


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Re: casting advice?
« Reply #11 on: Dec 05, 2019, 12:26 PM »
That's actually pretty interesting, I'll have to try it. Having to scrub off the release is definitely one of the steps I'd be more than happy to eliminate. Thanks.

No problemo. I was skeptical when he first showed me but it's great. Most liquid dish soaps will work. The plant-based one (I use Seventh Generation dish soap at home) just barely works, so go with one of the big ones.

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Oya!

Andry         ; )
 


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