Making moulds of thin trailing edges... - RC Groups
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Jun 25, 2017, 03:09 PM
plane destroyer and builder
skybattle's Avatar
Question

Making moulds of thin trailing edges...


Hi,

I am making moulds of a very thin trailing edge (1 mm.) and I wanted to ask you if you have any 'trick' for making them.

For me it's quite difficult to build two halves of a 0.5 mm. of trailing edge being very precise...

I have watched some videos where there is what I suppose is a resin channel next to the trailing edge. Does it help in any way to get a better trailing edge in the positives?

Thank you very much for your help
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Jun 26, 2017, 05:50 AM
Registered User
1 mm is not "very thin" for a TE. Two factors play a big role: the material used for the moulds and the process used to make them.

It raises as well other questions: what is the moulding technic, what is the lay-up and what are the materials/fabrics used?

BTW it is better to have a slightly "too thick" TE but very square than a rounded one...
Jun 26, 2017, 11:26 AM
plane destroyer and builder
skybattle's Avatar
Hi, fnev:

I am using vinylester gelcoat with 1 layer of veil MAT and 4 layers of 450 gr. MAT, and polyester resin of low contraction as adhesive.

I set the positive in a wood silhouette to make the two-halves mold, trying to set it exactly in the middle.

So in this way is so complicated to set exactly the trailing edge at both sides...

After that I level the laminate in both parts of the mold. I imagine if one half-mould is less that 0.5 mm., it would be very difficult to keep it level, because the paint+glass fibre+1 mm balsa wood would be too thick...

Thanks!
Jun 26, 2017, 06:52 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by skybattle
Hi, fnev:

I am using vinylester gelcoat with 1 layer of veil MAT and 4 layers of 450 gr. MAT, and polyester resin of low contraction as adhesive..........
By this do you mean that you are using the above for both mould only?

Quote:
I set the positive in a wood silhouette to make the two-halves mold, trying to set it exactly in the middle........
This is commonly referred to as the parting board. The best way I know to set wing plugs at half depth in the parting board is to use templates.

Quote:
So in this way is so complicated to set exactly the trailing edge at both sides... .....
Adam of WyoWindWorks has posted good pictures somewhere and written about this. Do a search. If you don't find anything, post back and I will see what I can find.
Part of the explanation is to taper off the fabric layers at the trailing edge. I normally only have the two layers of 25 gsm veil at the final edge. The heavier outside layer finished short. 1 mm is a thick TE!
Quote:
After that I level the laminate in both parts of the mold. I imagine if one half-mould is less that 0.5 mm., it would be very difficult to keep it level, because the paint+glass fibre+1 mm balsa wood would be too thick...
I'm not sure what you mean here?
Last edited by Jim.Thompson; Jun 26, 2017 at 09:11 PM.
Jun 27, 2017, 07:52 AM
plane destroyer and builder
skybattle's Avatar
Thank you, Jim. Very kind.

Yes, I use templates to draw and cut the parting board.

I didn't know the Adam's videos, so I will take a look at them.

The most complicated thing of this process is that some 'problems' appear just when you are making the mould or laminating... things you have not imagined... Practical knowledge...
Jun 27, 2017, 06:16 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by skybattle
..........................
Yes, I use templates to draw and cut the parting board.
.................................................. .......................
The templates I am referring to are external 1/2 airfoil shaped templates that function as a gauge to set a wing mould into the parting board.

Quote:
The most complicated thing of this process is that some 'problems' appear just when you are making the mould or laminating... things you have not imagined... Practical knowledge...
Yes, it does take practice that is for sure.
Jun 27, 2017, 06:17 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by skybattle
..........................
Yes, I use templates to draw and cut the parting board.
.................................................. .......................
The templates I am referring to are external 1/2 airfoil shaped templates (typically the top) that function as a gauge to set a wing mould into the parting board.

Quote:
The most complicated thing of this process is that some 'problems' appear just when you are making the mould or laminating... things you have not imagined... Practical knowledge...
Yes, it does take practice that is for sure.
Jun 28, 2017, 04:55 AM
plane destroyer and builder
skybattle's Avatar
Hi, Jim:

Do you have any picture of these templates? I can't imagine how are they.

Yesterday, my positives (fuse and horizonal stabilizer) kept glued to the mould... bad news. I gave 8 coats of wax... but it didn't go well...

Thanks.
Jun 28, 2017, 07:01 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by skybattle
Hi, Jim:

Do you have any picture of these templates? I can't imagine how are they............
I will draw a diagram later tonight.

Quote:
Yesterday, my positives (fuse and horizonal stabilizer) kept glued to the mould... bad news. I gave 8 coats of wax... but it didn't go well...
Thanks.
That's bad news, sorry to read it. If you provide a full description of your process starting with the plug finishing, paint type etc., including the full release treatment method, one of us readers can assist with suggesting reasons for this happening. Pictures are also helpful.
Do you know how to post pictures here?
Jun 28, 2017, 07:10 PM
Everything's A Compromise
Larrikin's Avatar
Quote:
Originally Posted by skybattle

I gave 8 coats of wax... but it didn't go well...

Thanks.
In my experience, leaving time between each wax coat has made the difference.
While the instructions in the document at THIS LINK are brand specific, I do believe the difference is not applying the next coat too soon. Minimum 45 minutes.

David
Jun 28, 2017, 11:44 PM
Scott
Pylonracr's Avatar
Since he states polyester products and wax only my preliminary guess would be styrene migration. Without more information we can not tell for sure.
Jun 29, 2017, 12:40 AM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by Pylonracr
Since he states polyester products and wax only my preliminary guess would be styrene migration. Without more information we can not tell for sure........................
Yes!
Jun 29, 2017, 03:02 AM
plane destroyer and builder
skybattle's Avatar
Hi,

I waited few hours between coats, even a night sometimes...

I am using 34D automotive wax.

Yesterday I was talking with the shop's expert and he told me that there are problems when you try to make a mould from a positive painted. And he recommend me to sand it!! He has also recommend me a semi-permanent demoulding system.

I suppose my positive is painted with 2k paint. How do you do to avoid this, please?

Here you have some pictures of the fuse mould, and after cutting some glued parts (now is more clean). The fuse is now broken after demoulding.

Thanks!
Jun 29, 2017, 03:39 AM
Registered User
Quote:
Originally Posted by skybattle
... picture of these templates? I can't imagine how are they...
I believe he means something like this:
Jun 29, 2017, 03:58 AM
plane destroyer and builder
skybattle's Avatar
Very interesting...


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