Molding Airframes Knowledge Exchange Insider Thread - Page 5 - RC Groups
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Nov 17, 2010, 12:49 AM
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MUTCAKE's Avatar
Man, that's funny I looked at the time posted above my avatar for my last post and boy is it off, must me east coast time. It's 9:49pm Tuesday evening right now.
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Nov 17, 2010, 02:57 AM
Like a Boss
Heliman420's Avatar
Originally Posted by rotagen
Thanks harpye, I'm thinking depron is a better way to go than using 2 part foams or that "great stuff" liquid spray foam that is used for insulation? I think I've seen some of your stuff, you use epoxy JBweld for making the mold?

Actually I don't think I will make too many of them, unless I keep crashing, I just want to learn how to mold.

I will check out your website today. I'd like to make a mold for my peregrine even though I think I could make one by hand-bending my depron. Mainly because I don't think I could make a bird with such nice smooth curves like that spanish website had without using a similar method to his. Perhaps he used fiberglass as the fuselage material? I have so much to learn.

Rotagen, its me who uses jb weld for the molds, theres lots of stuff out there that could be used instead, from looking at your link it seems as he is doing FG molding, as can be seen on the pics, fg cloth.

For such a falcon pu foam can be used for the body, and depron for the wings, pu foam's main draw back is the weight, but its an easy way to achieve max detail, and it can b e used where there are undercuts, depron cannot.

Mutcake, nice stuff! never would have thought of using blue foam to make the masters..... seems like a nice simple and cheap way to make prototype parts
Nov 17, 2010, 05:28 AM
Crashing with style
deadbird's Avatar
Yup, as Freddie B said Harpye, Plaster of Paris (PoP) is the same stuff in the UK too. Ive tried a number of different types. They all have different properties when dry. Flexural & Compressive strength are the stats I look to when choosing. The higher the flexural & compressive strength the less likely it is to go pop in the press - in theory. I think heating above 100 degs c changes things somewhat though.

Annoyingly the plaster with the highest flex & compressive rating :Cassinis Outdoor, gets it because its also incorporates a resin which vents nasties when heated - formaldehyde being one of them I'm led to believe. It was a shock when I first opened the oven on a Cassini mold. Fortunately there was a door to the outside world close to hand, so blinded and gasping for oxygen, I managed to get to fresh air pretty quick and the outside tap served as running eyebath.

However I decide to persist with the appropriate protective equipment: Formaldehyde mask and swimming goggles (its a good look), because, by then I'd made all the molds. (Lesson learned - a more scientific approach required)

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In fact the venting does subside with time but I will be trying Crystacast next (almost as good compressive strength) as I am only able to use the Cassini molds when none of the family are in and all of the doors and windows are open. Which, with winter here is infrequent to say the least.
Nov 17, 2010, 07:26 AM
Thanks for the Fish!
aerogel's Avatar
Originally Posted by Harpye
You use PU for forming ??? would be curious to see a detailed process description of the workflow which leads o a part !!!
Harpye will do start with pilot lots of request for them and they will work in all process bee it Depron,PU Durobatics, or just old school buiding stick and tissue..
Nov 17, 2010, 11:57 AM
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MUTCAKE's Avatar
I'll be reading this thread over again, to see if I can get the level of detail others are getting. But I'm glad to have learned this, as I am confident I can now cast lightweight parts for just about any design I can imagine.
Nov 17, 2010, 04:20 PM
Build it again, Sam!
Harpye's Avatar
Mutcake: It is also known as gypsum plaster here, but I did not know the expression plaster of Paris - I used this stuff for canopy positive molds in the past

Aerogel ... The process from mold to part should be the most interesting to have comparable alternatives to reach the desired target ...
Nov 17, 2010, 06:26 PM
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aerogel's Avatar

Molding and Casting urethane rigid foam Pilots

Here is a step by step method I use for casting full scale lightweight pilots. The pilot to be used is a plastic WWII 1/32 scale spitfire pilot to be installed in a ERC Micro Spitfire with a full cockpit repainted with RAF desert scheme and US Insignia decal.
This process can be used with any scale pilots.

Materials used
• Pilot , I sometimes call the plug
• RTV Silicone, hardness of 20 and above
• Mans Mold Release 200
• Gram Scale
• Cups and mixing sticks
• Legos for mold boxes
• Sheet of Plexiglass or Plastic something smooth and flat
• Enamel Paint Airbrush recommended but not needed spray can be used
• 3m 77 Spray, White glue a Non Permanent Adhesive

A vacuumed chamber can be used to degas the silicone I.E takes the air bubble out of it but is not necessary if done correctly.

More to come.
Last edited by aerogel; Nov 18, 2010 at 10:26 AM. Reason: added more materials
Nov 18, 2010, 10:15 AM
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aerogel's Avatar

Molding and Casting urethane rigid foam Pilots

Start laying out your parts so you can begin building your mold boxes
As you can see there will be 4 separate mold boxes for this build. I do this because it’s easier to pull pieces after being cast…and If I mess up in molding well I only lose that one mold and just make another.

There is a sheet of Plexiglass that we will glue the lego boxes to.
Note all parts but the head will be two part molds. The head is glued to the plexiglass I used CA.
For the mold boxes to the Plexiglass we will use 3m 77 spray.

Note allways calibrate your scale with a proper weight...just becuase your scale says 10 grams Doesn’t make it so.
Last edited by aerogel; Nov 18, 2010 at 10:28 AM.
Nov 18, 2010, 11:32 AM
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aerogel's Avatar

Molding and Casting urethane rigid foam Pilots

Now that my mold boxes are built. I use an old card board box and spray the 3m 77 on it as you can see this has been done many times. I simple press each mold box down on the sprayed box move them around a little to spread to spread the glue…I also use wood or white glue sometimes and just brush the bottom of the legos ..I like the 3m because it’s instant. Now I press the mold boxes onto the Plexiglas and we are ready to start mixing up some Silicone.

The method we are going to use is a simple drop method for a two part mold some prefer to use clay and build up one side of the part...I find this to be a utter pain in the ying yang and still use it now and then when needed..if you are going to do it that way you have to do it before building the boxes and then build around the clay.
Doing this with the drop method you have to watch out for air bubbles in the silicone I will go into further detail on the next post when I begin mixing and pouring and dropping.
Nov 18, 2010, 01:18 PM
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aerogel's Avatar

Molding and Casting urethane rigid foam Pilots

Ok, First I will have to say this is a utter pain to do while trying to hold a camera at the same time!

With that being said…On with the show!
First we start pouring our two part silicone the one I am using has a mix ratio of 100 to 10 I will use a little more of the pink stuff the 10 so my curing time is faster Product says 16 hours..It will be around 8 with the extra pink stuff. Also has a working time of 45 minutes around 15 to 20 with the extra pink stuff.

Mix and Mix well...Once it’s mixed let it sit and let gravity take effect you will see air bubbles will rise If you have a chamber stick it in it and start sucking the air out.

I use brush’s sticks ,pin’s etc on parts I see that air could get trapped in on the pours as you can see the pilots goggles I use a piece of carbon fiber to fill them in first and his face this will help prevent bubbles from forming and running the face. I also do this on the pilots back torso just because when I drop it in his straps I think will cause some bubbling.

Started pouring the head first then moved from box to box as you can see.
Some of you might be thinking well you wasting a lot of silicone form part to part. Not really Silicone can be reused as a filler you need a heavy duty meat grinder..Nothing you’re going to find at a Wal-Mart, Got mine off ebay it will grind bones! Remember pretty much anything will not stick to silicone but it to its self.

I let the Silcone set and level in the boxes as you can see more air keeps rising to the surface I help it along with a pin a poke at it like it’s a pimple.

I set my Iphone for 10 Minutes before dropping in the parts I’m also testing the silicone in the cup to see if it’s getting thicker once I’m satisfied I start dropping in the parts. If I’m too early the parts will sink to far sometimes all the way but we can allow for a little because we can trim before pouring the next part of the mold.

Notice I did the back first reason is if any air get trapped in it won’t show up with the pilot sitting in a seat.

Now we go do something else like EAT!
Nov 18, 2010, 01:33 PM
Build it again, Sam!
Harpye's Avatar
That is great! - now I know why lego is that famous to kids
Nov 18, 2010, 06:11 PM
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aerogel's Avatar

Molding and Casting urethane rigid foam Pilots

Little over four hours went back and check with the figure test…not sticky so we can begin pouring the second half.
I remove the first block of Lego’s and check if there is sever undercuts the torso had a little more drop then I wish but nothing that can’t be fixed..I simple trim out where I see there would be a severe undercut this will now be on the next mold. I also go around and start making my mold keys simple cut out the corners and notice I put a key by itself this helps with putting them back together in order for the casting also helps in the direction I will pull apart the molds when casting.

“Sray the corners pins you cut with the Mans mold release 200 then do a overall spray of the mold!”
“Sray the corners pins you cut with the Mans mold release 200 then do a overall spray of the mold!”
“Sray the corners pins you cut with the Mans mold release 200 then do a overall spray of the mold!”
“Sray the corners pins you cut with the Mans mold release 200 then do a overall spray of the mold!”

No I’m not stuck in a space time continuum loop you would not believe how many times I forget this step and end up with what! A block of silicone and most of the time destroy the original plug trying to cut it out!

Mix up another batch of silicone as before let a sit a little and then pour I did add a little more pink stuff to speed the curing . What is left over in the cup pour it out they make great CA, Glue, Paint, pads and can be used many times.

I will leave the pilots head in it will be fully cured on the pull when the others are finished.

Next will be pulling and Casting the fun part might do it tonight or tomorrow morning.

And Yes Lego’s Rule! A long time ago me and another person figured this out we argued on who was first to use them this was in the late 80’s This reminds me of the race to flight two different individuals different parts of the world figuring out something about the same time.
Last edited by aerogel; Nov 19, 2010 at 08:12 AM. Reason: typo city where that english major I married when i need her
Nov 19, 2010, 07:35 AM
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aerogel's Avatar

Molding and Casting urethane rigid foam Pilots

Now we are ready for the pull. Remove Lego’s from around the molds and begin pulling apart and removing the plugs from the mold as you can see the body has a few undercuts but the silicone easily allows me to pull the part with a simple bend.
Once pulled I mix up a batch of Kaki Enamel paint this is the overall color of the pilot and helps with the skinning of the PU foam. Will let it sit for a few hours to dry then we will be ready to cast our pilot.

I also cast without painting but have found it helps the skin of the foam in the strange world of PU sometimes it works and sometimes it don’t. but with painting you pretty much know right away if you have a good part..if not try ,try ,try again.
Last edited by aerogel; Nov 19, 2010 at 08:14 AM.
Nov 19, 2010, 09:53 AM
Thanks for the Fish!
aerogel's Avatar

Molding and Casting urethane rigid foam Pilots

Now we are ready to cast our pilots. Itís kind of cold this AM in my shop this could affect the cast I find that a temp around 76f seems to be the best but then again I have pulled great looking parts at 69f

Iím using a 3 pound density foam that will expand 20 times its original volume itís a 1 to 1 mix by weight this has to be very accurate. Too much of either will make different results. I use old Batteries, Steel, etc as weights to hold down the molds after pouring.

There was no way to take a pictures during the mixing and pouring so I did a short vid. You can hear me stirring product says 45 seconds I do it until I see the foam about to froth.
I start with the Torso to get the foam in the undercut and move along you can see in the vid the foam begins to rise as I pour it in each mold, you have to do this very quickly pop on the top mold and put weights on it.


The slightest vibration will cause cells in the foam to collapse for at least 2 hours!
In a few hours we will pull the parts and see what we get. The thrill of victory or the agony of defeat!

MOV09335 (1 min 46 sec)
Nov 19, 2010, 02:41 PM
Build it again, Sam!
Harpye's Avatar
Great ... I probably will try to take a short vid about the pilots head I did in a simlar way!! as soon as I have the possibility to get a smaller amount of long term epoxy resine

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