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Surfboard fins by injection

This topic contains 18 replies, has 9 voices, and was last updated by  NowProject 1 day ago.

Yann lagrenouille

Surfboard fins by injection

21/12/2018 at 18:53

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 !

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21/12/2018 at 20:31

How does the fin came out feeling wise?… Is there any flexibility in the fins?

thank you

23/12/2018 at 16:25

I started fine tuning of the injection to find the right paramteres, some parts of the fins are very thin and then tricky to inject.

First trial on top of the picture with HDPE :
– 200°C for the rose one : absolutly not fully injected + cold cut on the jonction point.
– 270°C for the two yellows : fully injected but still this cold cut

Then I went for PP which I know easier :
– 240°C, 260°C, 270°C for the four blue and then the 2 first dark green : no cold cut at all, but not really fully injected (few miss on the bottom left)
– 280°C for the two other dark green : still no cold cut and perfectly injected

First conclusion :
– HDPE doesn’t have the same fluidity and displacement inside the mold, it look more like a front of cold plastic move inside, then doesn’t melt when it meet the second cold front
– PP gives the impression it rolls on the mold, so both front are still fused when meeting
– 280°C seems to give the right fluidity on PP to be able to go everywhere in the mold (I don’t want to heat the mold for the lost of energy it would imply).

@bbreve I think it is actually too flexible with only PP, so I then went to try fibre glass addition (not recyclable anymore though) the only trial done was to pinch a piece of fibre glass between the two mold part and inject around. It seems to work quiet well but is quit difficult to place properly. Also the fibre is in the worst position to be usuful (fins work under flexion, where the middle wil not receive any effort). So next research will be : PP + fiber glass, PS, ABS and PC.

26/12/2018 at 19:50

Hello!! Thanks for sharing!

I have try similar mould with Pp plastic! I found the result too soft..
We need to find more hard plastic!

looking forward for your fiber glass try!

07/01/2019 at 13:00

Some new testing during christmas vacation !

PS :
– 270, 280 & 300°C : seems to inject quiet well even at 270°C but need more speed than PP tho.
– The bright sound of PS make the fins feel cheaper will in hand
– Doesn’t really change the flexibility, still far too flex

– 280°C : not fully injected, not brittle
– 290 & 300°C : not fully injected, super brittle

PP + fiber glass :
– This time shredded fiber glass mixed with PP inside the injection barrel
– Very well injected at 280°C
– No big difference observed yet

Conclusion :
– Abandon of PS : no big difference observed on the flex, the plastic is less pleasant to work with, and the final product kind of feel more crappy
– ABS seams very promissing, but Precious Plastic machine doesn’t seems to be able to inject it.
– There is probably something to do with PP + FG, but I need a better mixing, and also a better connection between the fiber and the PP matrice.

To follow :
1 – Pre-mix PP and FG with an extrusion machine
2- If it doesn’t work, buit a better injection machine with a crank
3- If still not OK, increase the thickness of the fin

22/01/2019 at 19:02

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

23/01/2019 at 13:50

When shredding the PP/fibreglass mix did you notice any wear on the blades?

23/01/2019 at 18:20

WOWWWW!! so cool what you doing.. love the photos too

24/01/2019 at 12:44


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.

25/01/2019 at 00:01

If you mix it with fiberglass then it won’t be able to be recycled anymore, dont do that?

I am just interested, did you read any book about injection molding or you are getting all your info from this website?

25/01/2019 at 13:54

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.

30/01/2019 at 15:43


There are some book more interesting but expensive about composite material :
Daniel Gay
Composite Materials: Design and Applications, Third Edition

Plastics extrusion :
Harold F. Giles Jr et 2 de plus
Extrusion: The Definitive Processing Guide and Handbook

The first book is considering like a Bible for composite engineer and the other, it could be a complete and a good book about plastics extrusion.

You can find new development of composite and extrusion on thesis and scientific paper.

Google is your friend 😉

01/02/2019 at 19:07

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
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

04/02/2019 at 17:43

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.

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.

13/02/2019 at 18:22

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).

14/02/2019 at 20:04

@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.

14/02/2019 at 20:48

It may be worth making (or buying) a fin mount that you can use to test fins in your warm shop to get quantitative strength comparisons.

15/02/2019 at 09:06

@s2019, thanks for your inputs !

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 !

16/02/2019 at 00:34

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.

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