weight-saving techniques

[aggie79-HILO]

Dibond might be another material to look at. The 3mm thickness is about 0.75 pounds per square foot. It may be rigid enough to use with just a perimeter aluminum frame.

 

[afreegreek]

here's something that has a lot of good qualities.. Nida-core

it's a plastic honeycomb core with thin ply on each side.. at the shop I'm at, we use it for floors over an aluminum stringers.. 3/4 spans 24 inches and feels solid underfoot. it's not that pricey either.. and comes in several thicknesses.

I'm planning to use it to make my next trailer.. a 16-18 ft TT


http://www.nida-core.com/spanish/nidapr ... eneers.htm

http://www.nida-core.com/pdfs/pds/nidac ... _Lauan.pdf

 

[glassice]

afreegreek how are you going to finish the out side Lauan ?

 

[glassice]

ken that we found for a lightest trailer with a 20 year life span is 15 mm gel-coat 1 layer.010" Thick veil 1 layer of1.5 mat 1 layer of Style 7781 E-Glass Satin Weave 9 oz 1 layer of 3/4 mat 1" of foam 1 layer Weave 9 oz 1 layer of 3/4 mat through most of the field
Is's the airy around the doors that need more from people pull them shelf in and out' the back of the fenders rocks bush THEN this surprise me it the in side wall space where there buts hit when turning over then where the hatch tie in we build are for a 75 year it cost about $250 more in the mold

 

[afreegreek]

afreegreek how are you going to finish the out side Lauan ?

I'm either going to use a combination of polished and painted aluminum or wood veneer and polished aluminum.. either way it will only be decorative panels attached to aluminum "H" the Nidacore will essentially be an interior wall with 1 inch foam between the sheet and the skin.. the metal 'H' will be PL'd to the Nidacore, foam will be slipped in the groves of the 'H' and the skin will be either PL'd (veneer panels) and or 3M tape for the metal.

because the 'H' is attached to the Nidacore with PL there is no wood to metal contact and because the foam sits in the 'C' section of the 'H' it doesn't touch the wood either. I will leave a little space at the top and bottom of the foam so air can travel between the foam and other surfaces..

 

[kennyrayandersen]

here's something that has a lot of good qualities.. Nida-core

it's a plastic honeycomb core with thin ply on each side.. at the shop I'm at, we use it for floors over an aluminum stringers.. 3/4 spans 24 inches and feels solid underfoot. it's not that pricey either.. and comes in several thicknesses.

I'm planning to use it to make my next trailer.. a 16-18 ft TT


http://www.nida-core.com/spanish/nidapr ... eneers.htm

http://www.nida-core.com/pdfs/pds/nidac ... _Lauan.pdf

Nidacore looks to me to have a few issues:
The plastic core appears to be kind of flexible (thus they are suggesting an alternate test for the strength). It’s too darn bad NOMEX core is so expensive! (It’s really that good). Notice, all Nidacore is, is Luan and plastic honeycomb. I’d like to see some properties on the plastic core they are using in the Nida core, but I suspect that it is polypropylene (which is kind of flexible).

Also the cost for a 1â€

 

[kennyrayandersen]

Dibond might be another material to look at. The 3mm thickness is about 0.75 pounds per square foot. It may be rigid enough to use with just a perimeter aluminum frame.

I may be a bit confused, but after afreegreek’s post I’m a little clearer about what he’s trying to do. The problem with any of these very thin materials are that it just doesn’t take much load to make them go to poop. 3 mm is quite thin (Dibond), unless there is something to back it up. What I was trying to do is come up with a one-shot structural panel that would react the loads and be the finished product (another advantage to the fiberglass exterior). If the outside is Luan, then you still either need to glass it and paint, or use aluminum. That Dibond stuff is pretty expensive too (ouch!).

For the Nidacore to be used as just an inner, non-structural skin doesn’t make much [light-weight] sense; plus, it’s pretty expensive. Actually what would be hot is to make an aluminum sandwich, but I think with the coefficient of expansion so high for aluminum that you’d have to have a honeycomb core (it’s very free to expand in the plane of the facesheets) which would likely be cost prohibitive. Alternately, you could make a structural panel that does everything except the outside ‘finish’ (which could be aluminum sheet, which isn’t so expensive). You’ve already got the foam, and the inner could be made from the stuff they use on the interiors (kind of a plastic – non structural, but it’s pretty cheap, or you could just paint the luan, for that matter. On the exterior, if you could find a truck trailer repair place you could eliminate the seam which would be easier to seal.

What is going to take the shear load from trailer bending? It seems that the walls should be reacting that load, but I’m not sure how they are. What does the cross-section through the trailer at the axle look like? It seems you are making a metal frame, no? And how about the attachment of the panels to that metal frame? I think unless your frame is very robust, the walls still want to carry the shear load so they need to be tied to the frame somehow – how are you planning to do that?

Obviously, the lightest way to build is with some kind of monocoque, where the box (it’s so big, why not use it as structure!) is reacting most (if not all) of the loads. An airplane carries all of the loads on the outside surfaces because that’s the most efficient thing to do. If we can make the box (be it square or with the traditional curve) do the work, it’s the lightest way to go.

 

[afreegreek]

here's something that has a lot of good qualities.. Nida-core

it's a plastic honeycomb core with thin ply on each side.. at the shop I'm at, we use it for floors over an aluminum stringers.. 3/4 spans 24 inches and feels solid underfoot. it's not that pricey either.. and comes in several thicknesses.

I'm planning to use it to make my next trailer.. a 16-18 ft TT


http://www.nida-core.com/spanish/nidapr ... eneers.htm

http://www.nida-core.com/pdfs/pds/nidac ... _Lauan.pdf

Nidacore looks to me to have a few issues:
The plastic core appears to be kind of flexible (thus they are suggesting an alternate test for the strength). It’s too darn bad NOMEX core is so expensive! (It’s really that good). Notice, all Nidacore is, is Luan and plastic honeycomb. I’d like to see some properties on the plastic core they are using in the Nida core, but I suspect that it is polypropylene (which is kind of flexible).

Also the cost for a 1â€

 

[Basinranger]

I've been following this site & the discussion of using cheap insulation foam like you'd get @ home depot as a structural core.(I assume we are talking about blue dow foam insulation board - although there are MANY dow foams that are blue... only three are listed as structural foam - panel core 20 30 & 40 - and you don't find it at home depot...)

My first response is are you kidding? I'm looking forwards to seeing the results of using cheap foam structurally because if it works it would be great! It's a big leap from using thick ply skins with extensive solid wood infills and cheap insulation board glued in to an actual "composite sandwich structural panel" - meaning a panel where the skins take the load, giving you the weight savings & stiffness which make the trouble & expense worth it.

Never having made anything out of it I can't say it wouldn't work - but I have made many panels & structures out of structural cores - mainly balsa with both ply & glass skins bonded with epoxy.

Here's the argument for why it's a terrible idea - Without getting into too much detail the core material needs to have two main properties beside holding the two skins apart. It has to have sufficient compressive strength to keep the "top" skin from buckling - so it transfers the load efficiently to the other skin. Also it has to have enough shear strength to keep the skins from de-bonding & sliding off the core. It's also useful to have enough compressive strength to resist spot loadings from impacts.

Dow doesn't list the structural properties of it's cheap insulation foam - because it's not structural...

But here's how it's structural foam compares to the structural core materials I'm familiar with. Their properties get stronger as the density goes up in weight.

Dow Panel Core 20 - 20psi in compression 25 psi in sheer
Dow Panel Core 30 - 30psi in compression 35 psi in sheer

Baltek BalsaCore
917psi to 3813psi in compression
262psi to 712psi in sheer

Nida-core's H8PP 10 to 25 - the material in it's pre-made panels
188psi to 348 psi in compression
72psi to 87psi in sheer

Core Cell
80psi to 336 psi in compression
98psi to 287 psi in sheer

It's a good idea to know the basic properties of the materials you are using - often that explains their attractive price... I consider using 13.00 a sheet foam structurally to be a big risk - if anybody has experience with it or better yet Dows Panel Core product I'd love to hear about it, building lighter is better! Peter

 

[kennyrayandersen]

I've been following this site & the discussion of using cheap insulation foam like you'd get @ home depot as a structural core.(I assume we are talking about blue dow foam insulation board - although there are MANY dow foams that are blue... only three are listed as structural foam - panel core 20 30 & 40 - and you don't find it at home depot...)

My first response is are you kidding? I'm looking forwards to seeing the results of using cheap foam structurally because if it works it would be great! It's a big leap from using thick ply skins with extensive solid wood infills and cheap insulation board glued in to an actual "composite sandwich structural panel" - meaning a panel where the skins take the load, giving you the weight savings & stiffness which make the trouble & expense worth it.

Never having made anything out of it I can't say it wouldn't work - but I have made many panels & structures out of structural cores - mainly balsa with both ply & glass skins bonded with epoxy.

Here's the argument for why it's a terrible idea - Without getting into too much detail the core material needs to have two main properties beside holding the two skins apart. It has to have sufficient compressive strength to keep the "top" skin from buckling - so it transfers the load efficiently to the other skin. Also it has to have enough shear strength to keep the skins from de-bonding & sliding off the core. It's also useful to have enough compressive strength to resist spot loadings from impacts.

Dow doesn't list the structural properties of it's cheap insulation foam - because it's not structural...

But here's how it's structural foam compares to the structural core materials I'm familiar with. Their properties get stronger as the density goes up in weight.

Dow Panel Core 20 - 20psi in compression 25 psi in sheer
Dow Panel Core 30 - 30psi in compression 35 psi in sheer
Baltek BalsaCore - 917psi to 3813psi in compression, 262psi to 712psi in sheer
Nida-core's H8PP 10 to 25 - 188psi to 348 psi in compression, 72psi to 87psi in sheer
Core Cell - 80psi to 336 psi in compression, 98psi to 287 psi in sheer

It's a good idea to know the basic properties of the materials you are using - often that explains their attractive price... I consider using 13.00 a sheet foam structurally to be a big risk - if anybody has experience with it or better yet Dows Panel Core product I'd love to hear about it, building lighter is better! Peter

You tell ‘em Glassice! (BTW, Glassice ain’t afraid to give you his opinion!)

Well, there’s structural foam and then there’s ‘structural’ foam HEhe

Dow doesn’t want to accept liability for the lighter-weight mass-produced foam, but the properties aren’t much different than the lower density foam that you listed (If I’m not mistaken, the densities are the same). If you scrounge around the internet, you’ll find that there are a lot of people using it to build all kinds of stuff.

My buddy is building and airplane out of it right now (from scratch, and not his first aircraft build!). He’s used it for the ribs, parts of the empennage, and parts of the fuselage. Interestingly enough, it wasn’t lighter to use it for the whole fuselage (he could have) as a wood (fir) stick frame (truss) turned out to be lighter. The whole plane including 3-axis control system and engine will be under the 254 pounds required to be categorized as an ultra light aircraft. He has already completed a successful static test of the wing (that’s pretty much the biggest and most important test as it’s generally considered bad form to lose the wings while flying!).

The process for the ‘structural’ foam might be a little more tightly controlled so that Dow can ‘guarantee’ the mechanical properties (or probably not), but pretty much the strength is dictated by the core density. If the core is denser, it can carry more loads (both tension and compression). Generally, and especially in the case of the Home Depot foam (also available at other bog box stores like Lowes) I’m guessing that it’s pretty much identical – it’s just mass produced. My airplane buddy is using reduced allowables (I think he is using 15 psi for the shear allowable), but when another airplane buddy of his did some testing, the strength was actually nearly 3 times that (around 70 psi if I remember correctly).

The problem with actual structural foams is that they are VERY expensive [as you may have noticed]; so, the key is getting a non-structural foam to do the task if possible (it isn’t always possible). For local load introduction we can use a puck (round insert which is generally potted into a composite sandwich and is either a metallic part, or made from potting compound, but we can use plywood, and it will do the same thing – it’s just not as fancy (we’re not about fancy, we’re about cost-effective (OK cheap, but don’t tell anyone)). I’d point out that there is a difference between the extruded foam (blue, pink, and sometimes white) and the white ‘beaded’ foam. The later is crap (and even it can be used if done carefully, but I personally wouldn’t use it).

Also, Balsa is generally pretty heavy core (relatively speaking), so that’s why the strength numbers look so good. Nidacore, is polypropylene, which is reportedly tough as nails, but it’s a bit expensive and flexible for my liking. Structural foams other advantage is that they come in many graduated strengths so that you can tailor the strength requirements very closely by splicing different densities in the same panel, only using heavier core where it is required (using this type of material is all about weight-saving in the end).

I could discuss composite failure modes with you until your eyes rolled back into your sockets and you slipped into a coma from which you likely would not return, but I hate to do that to anybody. Suffice it to say that generally light-weight thin-skinned sandwiches fail in either core shear, or facesheet wrinkling. Core shear is core shear, so that one is easy to figure, but facesheet wrinkling is a function of not only facesheet thickness, but core stiffness (denser foam is stiffer). If you provide more stability (stiffness) to support the facesheet, the wrinkling allowable goes up; when you make the facesheet thicker, the wrinkling allowable goes up. So, you can basically use thicker skins, because we don’t want to use denser foam, since it is more expensive (it may not be perfectly optimized, but it’s a wee bit of a compromise to save a lot of additional cost). Also, you can reinforce the core with plywood locally if you need to (Like where the axles attach, for instance). Usually this reinforcement won’t have to be very large as the load, and thus stress, spreads out pretty quickly.

One of the biggest areas of concern (pointed out by Glassice) is that core can peel away from the foam (I think this isn’t necessarily how the failure starts, but it can easily be mistaken for this failure mode). Regardless of how the initial sheet-to-skin failure occurs, the delamination can spread. This isn’t unique to the Home Depot foam, but all foams. I read a recommendation by one foam manufacturer that said you should put some holes 2 mm dia. X 3 mm deep (5/64 X 7/64 in, or thereabouts) spaced from 50-100 mm (2-4 inches). It’s also probably a good idea to rough up the surface with 60-80 grit and of course remove any films or coating that might be on the sheet. I think it likely that the initial failure is a facesheet wrinkling, followed up by facesheet peel. The hole should help arrest that (time will tell).

One other source that I m checking out is there are apparently people working in the insulation business that can custom slice whatever thickness you want out of a big block (the standard block dimensions are 5’X5’X10’) of the stuff and can vary the density as well; so, theoretically, you could get a bit denser (stronger) foam if you need it and get uniform and specific thickness sheets as required. I’m still checking that one out – there are evidently a couple of those places near Ft. Worth (where my permanent home is located).

I will be doing some verification test of mechanical properties before my build begins. All of the tests are pretty standardized, so that part isn’t particularly hard to do.

BTW, I’m CERTAINLY not the first to do this – there have been several in the past. Check the Uber ultra thread – I made a finite element model of a tear body and put in all of the properties for the sandwich when I ran the model (I hear I wasn’t the first to do that either, but I did document it so others could take a look at it).

The only problem with pre-made sandwiches is that you can’t do a curved surface, which limits the profile a bit.

 

[afreegreek]

*puck

 

[afreegreek]

you know,, foam is basically a spacer to keep the two skins separated and keep them from moving in relation to one another. it isn't much of a structural component on the whole and unless your building something like a wing, pretty much any rigid foam will work.. you just have to make sure the glue or resin won't eat it away.. I'm not saying there is no need for true structural foam, it's just that for something like a TD it just won't matter..

 

[eamarquardt]

I think one important consideration in the GRP/foam construction technique is making the GRP thick and strong enough to resist denting to begin with. If you dent it and damage the GRP/foam bond then the delamination has a "foot in the door" and can progress more easily. More is more expensive and heavy but "everything in life is a compromise". My friend's fishing boats are pretty robust construction (but as they are displacement hulls, weight wasn't a big considerstion) and they, after 30 years of commercial fishing in the Santa Barbara Channel (pretty rough by some standards) are doing well.

I agree with AFREEGREEK that the strenght of the foam isn't that big a deal but you want to get the best bond between the GFP and bond you can and protect the integrety of the bond by making sure the GFP/foam isn't easily dented.

I also think keeping the completed tear out of the weather to the extent practical would go a long way towards maximizing it's lifespan. The heat/cool cycle and resulting expansion/contraction can't be good for the sandwich. Not that you can't take your trailer out in the sun, but even if it was used 2 days a week that still gives you 250 days a year you can keep it out of the weather (mostly the hot sun). Doing that might extend the usefull life by a factor of 3 (well beyond our practical lifetime).

Cheers,

Gus

 

[kennyrayandersen]

The heat/cool cycle and resulting expansion/contraction can't be good for the sandwich. Not that you can't take your trailer out in the sun, but even if it was used 2 days a week that still gives you 250 days a year you can keep it out of the weather (mostly the hot sun). Doing that might extend the useful life by a factor of 3 (well beyond our practical lifetime).
Cheers,
Gus

I agree with most of what you said to a point. The skin needs to be thick enough that it can withstand abuse loads and some impact damage. However, not every place on the tear is exposed to the same hazards. As Glassice suggested – the door and the hatch are areas that get ‘abused’ more than others. The galley area would probably be more prone to things being dropped or ice chests being slid around etc. I’d also add the lower front (where things get kicked up by the car), and the underside aft of the fenders (though the fenders should catch most of it). It would probably pay to put an extra ply or two in those locations. Small delaminations (under a nickel size or so) shouldn’t make too much difference unless you get a bunch of them). You would want to protect so that you could drop a butter knife hilt first, but perhaps not a hatchet.

 

[afreegreek]

little delams can be fixed if caught early too.. you just need to drill a small hole and inject some resin..


if you're smart, you can do it with your vacuum cleaner..

 

[afreegreek]

you can also drill holes in the foam and thread strands of glass through them.. (pull bundles from roving and use a crochet hook to thread them through- trim them an inch long on each side) this creates little piers between the layups.. the glass give some stiffness and helps wick the resin through.. this gives a lot of strength and stiffness for the work involved. it's really worth it around corners and openings.

I just use a piece of peg board and go to town with the drill

 

[glassice]

When I was poorer I put 10ea 20 d nails in a 1x4 to pock holds in the foam. It work great under leaves at the door to stop thieves

 

[eamarquardt]

I agree with most of what you said to a point. I really don't see than you are not agreeing with anything I said.. The skin needs to be thick enough that it can withstand abuse loads and some impact damage. Agreed. However, not every place on the tear is exposed to the same hazards. Agreed. As Glassice suggested – the door and the hatch are areas that get ‘abused’ more than others. The galley area would probably be more prone to things being dropped or ice chests being slid around etc. I’d also add the lower front (where things get kicked up by the car), and the underside aft of the fenders (though the fenders should catch most of it). It would probably pay to put an extra ply or two in those locations. All seems like common sense to me. Small delaminations (under a nickel size or so) shouldn’t make too much difference unless you get a bunch of them). You would want to protect so that you could drop a butter knife hilt first, but perhaps not a hatchet. Everything in life is a compromise. No such thing as a "perfect compromise" as all compromises trade one thing for another.

Cheers,

Gus

 

[afreegreek]

When I was poorer I put 10ea 20 d nails in a 1x4 to pock holds in the foam. It work great under leaves at the door to stop thieves

good idea on both..

 

[kennyrayandersen]

I agree with most of what you said to a point. I really don't see than you are not agreeing with anything I said.. The skin needs to be thick enough that it can withstand abuse loads and some impact damage. Agreed. However, not every place on the tear is exposed to the same hazards. Agreed. As Glassice suggested – the door and the hatch are areas that get ‘abused’ more than others. The galley area would probably be more prone to things being dropped or ice chests being slid around etc. I’d also add the lower front (where things get kicked up by the car), and the underside aft of the fenders (though the fenders should catch most of it). It would probably pay to put an extra ply or two in those locations. All seems like common sense to me. Small delaminations (under a nickel size or so) shouldn’t make too much difference unless you get a bunch of them). You would want to protect so that you could drop a butter knife hilt first, but perhaps not a hatchet. Everything in life is a compromise. No such thing as a "perfect compromise" as all compromises trade one thing for another.

Cheers,

Gus

Gus -- I was slightly disagreeing about the thermal damage you were suggesting -- I don't think it is there. Of course -- how would you know what I was thinking -- since I forgot to write it down!!