To continue on my path to surfboard products, I will try to produce some surfboard fins. I will go for a thruster set, as they are the most comon ones and also the smaller (more or less) so they take less strain.
I built a first mould of the central fin with V4 workspace CNC. There is two injection points as my first was that as this mould in thin, then I can injected the plastic faster, but it appears that it is creating a cold lap. This test was done with HDPE, but I will try others with PP intead that I find more fluid.
I want first to try with only plastic and see how they are reacting while surfing, but I might go to look for a mix with recycled fiberglass at one point. I keep you in touch !
For a composite of wood and plastic, if you are using wood fibers with plastic, it should work a bit like fiberglass, I know Futures developp something like that (real info on the plastic is hard to find though !!!). But if you inject plastic around wood for example, then your product will be absolutly non-recyclable, as non separable. While fiber of wood/glass/carbon mixed inside a specific plastic, can still be recycle by itself, even mixed again with this plastic without fiber, as they are no degrading the properties of thje plastic.
I am working on a new mold with FCS II system.
I also tried to incease the preasure with a air jack modified injection machine, compacity is better, but still have to be better.
For the mixing of plastic and fiber, I am sure I have some bond, but maybe not 100%. I try to document myself on it, but accesible data on how to bond fiberglass and thermoplastic are hard to find also.
Next test will be this week-end, I found some fishing net, that should be polyamide, theorically very tough. Let’s see !
I too am extremely interested in making surfboard fins! I am about to embark on building a precious plastic operation myself. I have funding sources who want to know what I’m going to start making first! Surfboard fins are a great choice in a lot of ways (I live in Hawaii), but a real challenge to make a quality fin.
How have you progressed with your design since your last post? You’d mentioned bubbles, which likely make the fins brittle where they go into the fin box. Maybe a FCS II or Futures design would be better?
I read about a guy who was making blocks of plastic and fiberglass and machining it into a fin. What are your thoughts on this? Or maybe even a composite of wood and plastic?
Oh, and just to let everyone know, I went to engineering school, but no degree. I still have lots to learn!
Super nice project !!
And congratulation for your work ! the fins look super well done !
By testing different fins form the market you would be able to define the specifications that you need to reach to have a fins which work. Decathlon is doing one with Polycarbonate, fiber glass and Styrene Ethylene Butadiene Styrene (SEBS) :
Do you have software to do theorical analysis ? I think that you could find solution faster by modelising the fins and test it on a software with different parameters. You will be able to define a “window” for the bending stifness and test different material / combination. And when you find something pretty interesting test it !!
i agree with @s2019, a mount (aluminum?), accepting 3 M6 bolts coming from the fin should work, and causes possibly less hassle with the used plastic. i notice this quite often that there isn’t really much useful you can do with recycled plastic but adding here or there some enforcement seems to do the trick.
I have a 1mm fillet at the base, so no sharp cut, but maybe not flared enough. I will have a big review on the drawings.
I made some in workshop load test before going into water, but my main mistake was not breaking any commercial fins to get a reference value… I am still very reluctant to this idea, but I guess I have to go through it !
I also wanted to test the flexibility of the fins, and the feeling of it, even if very doubtful about the overall resistance. This part is validated still, they are nice to ride.
Generally comercial fins are made with thermoset resin, I found only a few out of thermoplastic. Maybe the main point is that I want to built an impossible product, but I am very stubborn !
@lagrenouille Sorry to hear the fins failed, though I think your test method is a lot more fun than most mechanical test procedures.
Do you have a photo of a commercially available fin? One way to help the load capability could be to flare the base of the fin somewhat so that some of the bending moment is taken up by the flared base instead of entirely by the tabs. Also it is hard to tell whether you have a sharp corner at the base of the tabs.
Do you know what reinforcement the commercial fins use?
While PP strength falls off with temperature, I’m guessing that the water being too warm was not the issue.
Great work. It is important to post fails along with success.
I managed to make the first testing, and… it didn’t end well !
If during the whole session the fins were super nice to surf, no speed lost at all on turns, on the last wave, they sudently desapear ! The wave was slightly bigger, but still : not strong enough at all !
I tested then on shore other fins, injected with only PP, and they are even weaker. Who else already tested fins into water ?
Of course these two small legs are a big weak point, but it comes from the most comonly used plugs (FCS).
So two orientation now :
– Working on inserts inside legs
– Working on a better injection machine, with more pressure (bubbles in the middles of these fins).
@stjepanb
Thanks for compliment about quality, that’s what I try to do, but it is quiet hard to get something which is not only nice looking but on what you can also rely on. Hopefully, it should happend soon
I agree with you on books, you need to document first. And you need to try also, as most of the documentation comes for industrial machine, which doesn’t necessary apply for far weaker precious plastic ones ! I will take time on my next post to explain what on my mould I took from industrial mould making design, and what I change to make it applicable to PP.
@ropmain
Thanks for the links, it seams to be proper books ! Did you read them though ? Because by the title, I am note sure they really apply here, it seams to get more detail on extrusion than injection and the composite seams to be more resin based than thermoplastic based ? I said above I don’t think book explaining industrial process fully apply, they might be a starting point, but you need to move from that and I think stjepanb need more feedbacks on which technics you can take from these books to apply on PP.
Hi @lagrenouille thanks for the explanation, I was wondering because i can see that your thing is higher quality then the most of the stuff around here, good job!
This next is maybe a bit off topic @ropmain
Yes i have heard about that guy google, before thanks! On the more serious note, maybe I missed those talks but I have a feeling people around here are not talking about actual engineering books enough but more just “lets try it and see what happens” approach. I am more like lets first read as much as we can about it, and I have a feeling you are something like that too. So what do you say could we make a small library of books that every person trying to work with plastics should read first, and also maybe collect the pdfs of the books? What do you think
As it is a comment I regularly have, here is my point of view on the subject :
Fins are not a single use product or assimilate, you barely broke them, and if you do, it is generally in water, which means that regarless of the way you built it, it is a waste, so it is best to build them as strong as possible.
Currently, they are made that way, strong, but out of new petroleum based material. As the best waste is the one that doesn’t exist, the best solution would be to stop surfing, but you can ask any surfer : I’d rather stop everything else in my life than surfing !
So I am in an in between with a fin that would still be 100% recycled at least, wtih PP wherever you can find it, and fiberglass out of lost cut from surf making industry.
Also the fact that mixing PP with fiberglass makes it non-recyclable anymore is quiet false. I can still shred it again and inject them back in another fin.
But you are right that it creates another plastic category, and it is hard to know how many fiberglass is inside if you want to make another product where you plan your resistance on this fiberglass. Howerver it never degrade the plastic as it would be if you mix different plastic or wood chips for example. So I could also just shred it and inject it in another PP product, mixed with full PP chips, it would still work. I will lost quickly the count of fiberglass inside, but as long as I don’t reckon on them, it will never make a weaker product than a full PP one.
But of course, it is only my point of view, open to discussion.
And to answer your question @stjepanb, I learn plastic injection during my engineering degree (material sciences), even if I focused more on metal work since, I still remenber some stuff or have acces to my basic courses, but unfortunatly, I don’t really have good book to share.
I didn’t properly checked, but it looks like there is no immediate damage visible. It might cause a premature wear of the blade I guess, which imply more maintenance or differente steel type.
There was some great improvements these two weeks. I figure out a better method to mix PP and fiber glass together :
1- Shred pp
2- Shred fiber glass (with scisor before finding a better solution) – average lengh around 10mm
3- Mix it together
4- Extrude this mix
5- Shred it again
6- Inject this new mix !
It doesn’t cause any trouble during extrusion or injection and is injectable as standard PP. The resuklt is clearly more rigid (25 % fiber ratio in mass).
While it is still not to my taste in term of rigidity, I feel that it is going somewhere.
Then I went for second test with carbon fiber pre-treated for injection with thermoplastic that I bought on internet. Average lenght is 0,1mm.
I tried both mixing it directly in the injection barrel and with an extrusion pre-mixing as I did for fiberglass. Both time the ratio was 25 % fiber in mass.
Conclusion :
– The Precious Plastic injection machine doesn’t allow a proper miw of the plastic and the fiber, an extrusion pre-mix seams to ne mandatory.
– With fiber 100 times small than fiber glass, I get more or less the same properties with carbon fiber. This can be explained by the change of material, but more likely also by a specific treatment that was applied on these fiber to make them compatible with thermoplastic.
– Using carbon fiber makes the final product black whatever the shreded PP is initially. Even if the black is quiet nice while pre-mixed (mat apsect), I feel that we loose some cool properties of Precious Plastic machines.
I will then concentrate my research in finding the right fiberglass ratio and a proper treatment to allow a proper bond between the fibers and the thermoplastic matrix.
1st picture : some graphic of the pre-mix process
2nd picture : last fins injected (PP + GF direct injection / PP + GF pre-mix / PP + CF direct injection / PP + CF pre-mix)
3rd picture : flex comparison with a approximativly similar load
!!
On the more serious note, maybe I missed those talks but I have a feeling people around here are not talking about actual engineering books enough but more just “lets try it and see what happens” approach. I am more like lets first read as much as we can about it, and I have a feeling you are something like that too. So what do you say could we make a small library of books that every person trying to work with plastics should read first, and also maybe collect the pdfs of the books? What do you think
