I’m about to buy another block of foam, to make sandwhich longboards and sailboards from I usually use SL grade which here in oz is 12.5 kg /cubic metre. Would I gain much more strength if I bought the next grade up (S grade) which is about 15kg/m or what would happen if I went the other way and used even lighter foam?
Also does anyone know the metric equivilant for EPS foam of 1# EPS foam?
Regards,
Bluejuice.
If your doing sandwich construction then 1#EPS (dont know the conversion but you can google it) is fine since most of your strength is in the skin.
12.5 kg/m3 is about the same as 1lb/cubic ft.
Just started looking at what is available in the uk and came up with the conversion 1lb/cuft = 16.02 x kg/cubic metre
Hi Bluejuice -
http://www.onlineconversion.com/density.htm
12.5 kilogram/cubic meter = 0.78 pound/cubic foot
15 kilogram/cubic meter = 0.94 pound/cubic foot
1 pound/cubic foot = 16.02 kilogram/cubic meter
All are marginally durable compared to the 2 lb/cu ft densities. With proper compsand skin, the 1 lb is OK.
I believe that some people are using approx. 1 lb/cu ft for vacuum bagging compsand skins and approx. 2+ lb/cu ft for conventional hand lay up with multiple cloth layers.
Here in US EPS blank distributors are mainly listing popular 2 lb/cu ft density with stringers.
I’ve obtained numerous foam samples from various sources and the higher densities seem to have smaller well fused beads - features worth specifying if you are able. The higher density foams are dramatically more rigid and harder to dent than the spongy 1 lb/cu ft and less densities. On the other hand, the light weight EPS seems more springy… dents pop back out. The thumb dents I work harder to put in the heavier EPS stays.
Possible non-venting, non-sealing capabilities and less water intrusion through leaks with heavier foams.
Bluejuice, I used S (1#) grade in one compsand and SL (.75#) in the other. To tell the truth I couldn’t feel any difference. I doubt you’d get better dent resistance using S. An SL core would probably flex more, so a board made with that would be more resistant to snapping - specially if you’re using Boatcote. I posted on this one a while ago and Greg Loehr and BB both said go as light as you can - but I didn’t think you could get anything lighter than SL?
Thanks for the info everyone. Looks like the Oz boards will be slightly lighter as the foam we get is slightly lighter. I was told the heaviest foam that can be supplied in west Oz is VH grade and that is 24kg/m. Pinhead your right that SL is the lightest but one company said they can make me a lighter block on request (but they won’t do one heavier than 24kg).
I was talking to an engineer who believes a lighter foam will mean a weaker board. He believes that if a force is applied to say the flat bottom of the board then the foam inside compresses and the load is then transfered to the rails of the board and the rails then break depending on how strong the rails are. He said a higher density foam would not compress as much initially and therefore the load would not be sent out to the rails ( probably explains why perimeter stringers are a good idea ). I just wonder if a couple of kgs in foam density would really make that much a difference, in theory = yes. In practicle use ???
Thanks for clearing that up John, All along I thought I was using 1lb foam!!
I got a special block made at 11kg/m3, so this would be about 3/4lb!
built about 8 boards from it so far (balsa compsand) with good results for weight and strength.
I believe this is what Bert uses in his balsa boards for greater shear movement?
Hi, bluejuice. Sounds like the engineer you talked to just explained the “morph bottom” bert is talking about. I think bert is actually taking advantage of the bottom being compressed.
I’ve used two different weights, 1pound(16kg/m3)and 1.5pound(25kg/m3)and for sure the 1pounders come out ligther and somewhat flexier,but for paddling and performance it’s hard to tell the difference in between the two. Well thats just how I feel.
And there’s more than just core weight that plays a big role i.e skinthickness,glassweight…
Jimmy yoshio shibata.
Yoshio I missed that post by Bert on the morph effect, I will have to do a search.
bluejuice,don’t know which thread but I’m pretty sure it’s been disscussed.
The engineer who said a lighter foam means a weaker board was no considering all factors concerning board strength. Engineers have a tendency to do that. It is a fact that a lighter foam is weaker and more compressable just considering foam. So if your board was just foam he would be right. Or with a very thin rail glas schedule sure it could blow out. But you can engineer those problems out. Once you add epoxy and fiberglas to the equation you change things considerably. With a lighter foam you can add additional glass layers to the deck botton and even the rails. Which will increase board strength considerably more than an equivalent foam weight increase and still maintain a lighter overall board weight.
That said the thing that will increase foam strength without not necessairily increasing foam weight is fusion. How long the foam is left under pressure and injected with steam affects the foam strength. The longer this process the better the fusion of bead to bead. The better the fusion bond the stronger the foam. Adding more pressure and material to the mold will increase density and weight. Using smaller beads will alter the formula. The ideal formula has yet to be discovered. I would look to Mctavish in OZ or Marko in CA for these advancements.
BTW the archives highlight your keywords, (awesome new feature) so now so if you key in morph bottom bert it will be real easy to find it in the posts. Have fun.
it all depends on the construction …
for sandwich , lighter is stronger …
for shape and handlam heavier is stronger …
the heavier the core , the less impact is spread over a greater area and the more it will be concentrated in one area …
heavier core boards will show signs of damage at the point of impact more , where as the lighter core absorbs and spreads load …
you can run a sliding scale and get good results with a range of different techniques …
as a general rule , light core thicker sandwich , as the core gets heavier , the skin needs to get thinner …
the 2 biggest reasons , 1 it keeps performance functional , 2 it keeps the weight realistic …
some of you guys maybe changing cores and changing boards at the same time , so may have missed the subtlties , but the 12 kg compared to the 16 is way different from a flex feel , if running the same finishing method …
so adjustments have to be made in the skin thicknesses and glass content to compensate for the different feel …
just like shaping , where you add one thing and take away another to compensate , keeping everything in balance …
construction follows the same rules , to get that just right feel means compensating in other areas if you change a certain material or aspect of the construction …
hey mike , anything interesting been happening back there ???
also mike where are you based , would you like any repairs sent your way ??
regards
BERT
I sent out some emails to EPS manufacturers a week or so ago to gather data on foam strength with goal of determining break strength for various foam weights and comparing to published data on PVC and wood for stringers. I’d like to end up with some guidance on how (if at all) stringers contribute to break stength and how foam density contributes.
I’ll ask for a peer review from Doc. That way my math gets corrected off line. ha.
Give me a week or so. I’m an engineer, but I’ve enlisted a PhD structures engineer to guide me. Cross your fingers.
(Hey, Bert, is Greg OK? Seems to have fallen off the map.)
the only falling greg has been doing ,is into snow covered slopes and snow pipes …
that wouldnt bother me if i was there with him …
so while hes not carving the slopes , hes doing a heap of work …
hes in cali at the moment , so i suspect its a case of not being close to the pc …
regards
BERT