Hi folks,

This is a general question regarding the strength and rigidity of a wet layup. Would there be any mechanical difference in the finished item using fewer laminations of a heavier weight glass than more of a lighter weight, i.e. 3 @ 200g as opposed to 2 @ 300g.




Cheers, Marty

as far as I know its better to use 3x200 than 2x300

What if you put the in the same direction both (3x200,2x300), would the 3 sheet ply be more vunerable to delamination?

Hi, really depends what you mean by glass, if your talking chopped strand matt then very little. Because the orientation of the fibres is random once the resin breaks down the binder then each layer effectively becomes one anyway.

If you mean woven glass then more lighter weights would give you more opportunity to orientate the direction of the weave on each layer of the laminate. This way you can change the characteristics of the finished laminate. ie build in a degree of flex or make it more rigid. By using something like biaxial glass you can do even more. This doesn't necessarily mean a stronger laminate (failure load) just one that behaves differently. Also it is usually more beneficial to keep a balanced laminate ie 2,4 or 6 layers for example.

Hope that make sense Warren

Carbon Copies Ltd

Hi Warren,

It's twin twill glass and epoxy that I'm using on my scale heli project.  I've just done a test lamination on the tail section using 1@ 100g and 3@ 200g.  The gauge of the layup seems to be about right 0.9mm but it's very flexible.  I've pigmented the resin (no more that 5%) could this be the cause or is it down to curing temperature? 

Also, I've got some air trapped under the first layer(see attached), despite using a bristle roller after the layup. In future, should lay down the first lamination, roller it, let it tack off a bit and then lay the remainder of layers consecutively?

Many thanks, Marty    

Hi Marty, temperature (or lack of) is probably the main issue for it remaining flexible, and it being a bit resin shy. Much depends on what hardener (fast/slow or a mixture) you are using and how quickly you get the layers down. It looks like the first layer (mould side) may have started to gel before you finished the remaining layers. If that happens it is very difficult to remove any trapped air even with a bristle roller. Most epoxy resins need curing at 60 degrees c to reach their best mechanical properties. Has it cured just at ambient temperature?
Not sure how big the part you have done is. You could try wrapping it in an electric blanket and post curing it that way. Just keep a close eye on it for safety reasons and see if that improves the stiffness (it should)
You could also try orientating the layers. Make a template from some paper, place that on the cloth and mark around it. Then flip the template over and mark the second layer and so on. That should give you 90 degree orientation of the weave once placed in the mould. If stiffness is still a concern you could look at using a biaxial cloth.
P.S Sorry should have said if your going to try post curing put the part back in the mould if that's practical to help prevent distortion of the part.

Warren

Carbon Copies Ltd

Hi Warren,

Thanks for that.  The tail pylon section is about 500mm long and the OAL of the model is 1840mm - see attached. 

I've done another pull of it yesterday but laid a 50g twill down first. It's so thin that I could check for air pockets even though the resin was tinted. I then let this tack off for an hour or so and then followed up with one 100g and three 200g layers alternating their weave direction. I had a gentle heat on (70*) it for 8 hours The finish is perfect but it still feels a bit flexi? Might do one in clear resin and compare.

Cheers, Marty

Just a few thoughts....

I think you'll notice the difference with a good post-cure.  It really does make a massive difference

A 1mm moulding of one side would be fairly flexible as the section is quite tall with minor detail.  Once joined the full round section should be far more rigid.  On a model of this size I'd imagine any flex would be bad (assuming the shell is structural, I beleive most heli's aren't). 

Have you considered using a core material?  The model is certainly large enough and shaped appropriately to use one.  To trial this you could laminate 1x100g, 1x200g, a piece of 3mm balsa sheet, 2x200g.  The only weight you'd add would be the balsa sheet and the rigidity would improve significantly.

You could also add some carbon tape to the critical points.  Add some bands and a lateral line to give it some further rigidity.

Just some ideas that I hope help

If it's not structural then use as light a layup as possible, I know some UAV covers just use a single layer of 90/100gsm carbon

Hi guys,

The shell will be semi-monocoque with some internal framework for the undercarriage, motor and transmission. I have considered a core material but not balsa.

I have a friend in Israel who has built an 8 foot long CH53 SkyStallion in carbon with Soric core and has kept the whole structure under 3.5Kg. To go that route I would have to invest in some vacuum kit - which I am prepared to do but for now I'll see what can be achieved with a good wet lay-up.

I've got the two test mouldings in a warm sunroom now to see if they firm up a bit.

Many thanks for the input, I'll report back.




Cheers, Marty