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Old Oct 16, 2010, 09:59 AM
Build it again, Sam!
Harpye's Avatar
Germany
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Molding Airframes Knowledge Exchange Insider Thread

Now - finding so many activities on molds and molding foams / films and other materials here in the channel I think it is time to organize a kind of little Knowledgebase so not everyone has to develope the wheel on his own and so could share and sameway participate from the ideas of others.

Now the
M.A.K.E. I.T.
is opened!!
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Old Oct 16, 2010, 10:13 AM
Build it again, Sam!
Harpye's Avatar
Germany
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Guess I will begin with my experiences giving you a short overview:
I use extruded styrene foam like Depron Bluecor or PB3 for molding

Material strength I experimented with is 0.8mm 1mm 3mm with negative molds
and 1mm, 3mm and 6mm with ducttape and positive formers

For the fine and detailed molds I use polyacetal / POM resine blocks ... giving a very smooth and fine surface when structured by CNC router. Polyacetal is about factors more expensive than Uriol.

For the bigger molds I go with 720gram / dm^3 Uriol which is finally brought to limit of its rating during the "baking process" but it is also easy to route and fulfills its duty..

For positive formers which are not to be heated in oven - bigger Planes up to 1,6m WS I use a combination of Plywood and Depron or for smaller models I go for styrene foam positives.. Using foam cores in combination with foam Skins is possible because the heat is applied from the outside so the inner core in not that hot as the outher skin gets.

Workflow...

The the skinmodel is created in 3D Cad .. all details are created on the plane skin... afterwards those skinns are cut in parts - for the single molds and placed in virtual stock material ... the Material is routes and the mold is ready... now the press core is created mostly from MDF or POM or Uriol ( in the case both sides are to be structured on surface... the press core sould leave a gap of about 80 to 90% of material strength between the mold and teh core and have a smooth and slippry surface..

The material is carefully placed in the mold and then the plug / core is set loosely the setup is heated up to 110 to 115 C and afterwards teh plug is pressed in end position.... the assembly is left for some more minutes in oven afterwards removed and let cool down .... teh rims of the foam are trimmed and afterwards the plug and the part are to be removed carefully

Thats all - in principle!

Hansjoerg
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Old Oct 16, 2010, 10:41 AM
still learning
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Amazing and impressive.
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Old Oct 16, 2010, 10:49 AM
Build it again, Sam!
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Hopefully others will follow....
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Old Oct 16, 2010, 01:41 PM
Like a Boss
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Canada, BC, Whistler
Joined Jun 2006
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Ill join, i use epoxy for molds, a plastic kit for the master, and 025 sliced depron for the material, heat the molds to 110-115 put a piece of depron on top the female and press in the male, put into a press and wait for it to cool down, and there you have it a perfectly molded part.... i challenge anyone to look at a painted plastic part and a molded foam part and spot the difference without touching them.

These old pics are from my thread.
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Old Oct 16, 2010, 02:16 PM
Build it again, Sam!
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Perfect!!
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Old Oct 16, 2010, 02:24 PM
Like a Boss
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I probably should mention that i use jb weld epoxy.. but any high temp metal filled epoxy should work, also forgot to mention the male is cast 1to1 then sanded to size... mold some parts if you get tearing in areas and the foam is super thin then sand that area and repeat till it stops.

Some areas need more sanding then the others... theres really no set way to go about it... experiment and then modify. But BE CAREFUL do not sand it too much or you will ruin the male plug.

Since i use 025 foam.. and others use thicker foams, plug size will vary.

If one doesnt get a smooth surface on the first try, the part can be repressed numerous times untill it does, with proper mold temps and a good press there shouldnt be too many of those occasions.
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Old Oct 16, 2010, 02:38 PM
Lawn dart pilot/builder
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wow very cool .
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Old Oct 16, 2010, 03:25 PM
Build it again, Sam!
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Heliman - thats what I forgot to say - the first benefit of this thread so to say....
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Old Oct 16, 2010, 04:01 PM
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jarel design's Avatar
Southern Oregon
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Quote:
Originally Posted by Heliman420 View Post
I probably should mention that i use jb weld epoxy.. but any high temp metal filled epoxy should work, also forgot to mention the male is cast 1to1 then sanded to size... mold some parts if you get tearing in areas and the foam is super thin then sand that area and repeat till it stops.

Some areas need more sanding then the others... theres really no set way to go about it... experiment and then modify. But BE CAREFUL do not sand it too much or you will ruin the male plug.

Since i use 025 foam.. and others use thicker foams, plug size will vary.

If one doesnt get a smooth surface on the first try, the part can be repressed numerous times untill it does, with proper mold temps and a good press there shouldnt be too many of those occasions.

Curious if vacuum forming a thick sheet of plastic into the female mold, trimming it and then pouring in your male mold would do the work of sanding the male mold smaller? (I'm assuming that you are trying to achieve a uniform thickness across the entire surface betwen the male and female mold? There would be "some" thinning around the parting line, but only by .010" or so...)

Just a thought.

Thanks for sharing all your hard work!

Best,
Richard
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Old Oct 16, 2010, 05:03 PM
Like a Boss
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Canada, BC, Whistler
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Hi Richard, no im not going for a uniform thickness, some areas need more some less, ive tried it before with jewellers wax sheets, and also using a already molded part with mold release on it... results were disastrous... as can be seen in my original thread, i had to discard both molds, these were my first attempts from last year.... at this scale (1/72) and the thin 025 foam... sanding is the most reliable way i found...

Im sure there are ways of doing it without sanding i just havent found it... hopefully the more people get into this.. the more these techiques can be refined.


Now on a bigger model it will be the way to go as one will be using thicker foams.. and sanding a large plug would be very time consuming.

Theres just so much that can be done here, the possibilities seem almost endless!

Thanks for starting this thread Harpye!
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Old Oct 16, 2010, 07:30 PM
still learning
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west Georgia, USA
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Also here's a link regarding molding in one of Heliman420's posts from his 1/72 scale plane thread...
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Old Oct 17, 2010, 02:44 AM
Build it again, Sam!
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Another thing:I made the experience when pressing the plug with the depron in the mold at the location at the rim, where the depron is pressed ( plain surfaces transition to the mold / plug) there is the risk of cracks when the material is overpressed what easily is to be done with 0.8mm foam ...
As both the plain areas keep the plate in position ( huge normal forces cause friction) and the plugg part is further pulling the depron into the mold ... so I decided to modify my plugs in the way that I engraved a little 2mm wide rim around the plug positive... shown below ( Plug o.t. wheelpants. Thats a ratio of about 2mm width to about 0.2 mm overpressing ...= 10 what normally would do the job I also try to break the edges a bit with a fine sand paper

For thicker materials I eliminated the plain area from the plug and us a free swimming one which centers itself.

Good thing that the foam expands some percent due to the appliance of the heat and so rebuilts the surface details thich otherwise would be gone due to the deepdrawing process.



Hansjoerg
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Old Oct 17, 2010, 06:35 AM
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Great info, Keep it coming!
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Old Oct 19, 2010, 01:49 AM
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USA, WA, Bremerton
Joined Mar 2009
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Question for Heliman: Do you have to use any kind of release agent for your molds? If so what? How long does it take to set up (I guess you could read the package). Temp seems to critical also. What do you heat in and what do you use to measure temperature.
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