Shafts

General Composites Discussions Composites Talk
#1

We have a interesting scenario:confused:.
We have a kayak shaft paddle shaft light weight, high stresses made by a company who wraps either glass, carbon all pre pregs or a combination of all around a mandrel, tapes it then cures it in a oven.
We found out if shaft comes out cured of the oven and has blemishes or faults like not consolidated properly etc , firstly that is a indication the shrink tape did not do its job so its already starting the second process with a questionable product , instead of binning it they hand sand it down which has issues of how much have you sanded in places and not others and then add a few more layers, wrap it, oven cure it.
My opinion is when the resin in pre-preg “melts” during the cure process it links from the mandrel side to the outer side as sort of one molecular unit.
To me if you then add a second layer which equates to re-joining it in a second stage after the first has had a full cure process it can never have the same properties as if it were done in one stage as your molecular link is not the same, they say it is as it re-melts the first layer again, I don’t see (know) this is not the case as its cured resin not hot melt glue.
What is the opinion of those out there, is doing it twice above as strong or not, will it have more a chance of burst delaminating under bending pressures as we have found a few shafts that “burst” for no real reason and suspect this may be the reason?

Thanking in advance for replies.

#2

You do have secondary bonding, but I do not feel that is the cause of your problems. Do you have pictures of your shaft? :wink:

Without pictures it is about impossible to determine cause of failure, with pictures it is very hard to say at the least.

And my crystal ball just broke…

#3

Hi all,

Let me try explain the reason for the posting so you can advise me.

I have been doing composites for 40 years and have kayak, aircraft and race car structural part school fees behind us, most stuff vac bagged or resin infused, all oven cured etc, we don’t know it all but we also not new to the game. E.g. our best is 6.5 kg for a ICF race KI kayak.

We not trying to determine if and what the problem is, which is why we have not posted photos plus the factory who makes the part won’t let us take a photo of their faults being covered up, we know why they have the fault being the shrink tape was incorrectly applied and product was not consolidated as witnessed by my wife who saw them sanding shafts down for a second stage application, she asked why and they said that’s how it’s done, she said its weaker, they told her she does not know how it’s done. Ok they consider her a fool being female, little do they know she builds full vac bagged parts for various applications and knows how to use lathes, milling machines, so she is far from a average house wife.

We know what’s wrong and why it happens and not trying to find a solution, solution is apply shrink wrap tape correctly, just need opinions on:

Will a shaft that has a secondary bond of pre preg applied after first one has cured be structurally as sound as one that is made in one go,does applying a second layer in a separate stage on a shaft at 120cm comes in at 240 grams reason for concern as there is not really any safety factor as its as light as what it can be.

Will:
SHAFT 1 made by sanding half off original composite to remove blemishes and faults and then re applying in a second stage what you just sanded off

be as strong as

SHAFT 2 the one made correctly in one proccess?

#4

Do they not use some form of stress/failure mode testing as a manufacturer? The answer or truth could be best answered there.
Herman had the only crystal ball that I knew of so we are screwed there,lol. You’ll have to do a test process to prove your point to them, but they still may not listen .

In theory, as long as the second post cure doesn’t exceed the resin tg of the first laminate , it could come out stronger. However…did it meet the specified final weight? Most likely not.

Always best to fight the good fight and figure it out.

#5

so to answer your question no the shaft will not be as strong. If you do a secondary bond, it’s only a mechanical bond, meaning it’s just a surface bond of the epoxy/resin gripping to the other surface. Rather than the chemical bond of having all of the layers of carbon bound together by the crosslinking process. Having all of the resin cure in one step makes for a stronger part.

The two stage process these guys are doing could be ok if done properly and depending on what sort of strength is required. There is a chance however, that in the life of the part through use it could start to delaminate and thus fail. If you are paying for these they should inform you about this so you can agree that these meet your quality standards since these parts are technically being repaired. Though it sounds like they’re really doing more of a cosmetic repair than a structural repair. It sounds like they’re having wrinkles because the tape isn’t overlapped well. I would think they’d use a jig with tension to get them to come out evenly. But even still, really well made tube still has discrepancies and is often sanded down after curing to remove the lines from the shrink tape though, not to sand in order to apply a new layer of carbon.

not sure if that’s the answer you wanted?

#6

One of the questions is: Is a secondary bond on the location in the part loaded above failure? When in the laminate, probably not.

However, my concern is them sanding off about half of the laminate, then reapply that half. If that is the case (instead of sanding half off, then reapply the full laminate) then I doubt they can get it as strong as a good shaft. Sanding never has been precise engineering. In some spots you might end up with just 55% of the expected laminate.

If the whole shaft is 240 grams, I doubt sanding and reworking is cheaper than tossing and doing over, and spend the labour saved to make sure the tape is applied correctly. It is Always best to fight problems at the source.

My neighbour let me borrow her crystal ball:

#7

Well we knew this, but put this to you as at the end of the day we don’t know it all and you never too old to learn.

Answering a few questions posed:

Firstly the shafts are on the edge of their performance / weight ratio to start off with, some are used at World Championship level kayaking.

Secondly we leave the tape spiral on the finish as most don’t use a grip on shaft and this provides pleasant grip plus its difficult to hide blemishes by sanding them away.

But manufacturer says we don’t know what we talking about, their experience in composites about 4 years max, my experience about 40 years, my wife’s about 15 years.
Problem for now is they the only local manufacturer so we forced to eat their way of doing things as we don’t have time to make shafts plus logistics of pre preg. When you the only bloke making things such as shafts you can have a dismissal attitude to quality.
But then we have experimented in making shafts in aluminium moulds using a inflatable bladder to get a shaft that’s slightly oval around the hand area.
Maybe time to go down that road.
What annoys me more is the dismissive sexist attitude towards my wife who outclasses them all combined with technical capabilities for the mere fact she is a woman.
But thanks to you for your replies.

#8

Sounds to me like you should start looking for a new manufacturer… Not only are their attitudes disrespectful, it sounds like they don’t have the level of experience or expectations of quality that your product requires. If you’re making pro competition level equipment, you can’t just pass off second rate and defect equipment to your customer. I bet those things aren’t cheap and so the purchasers expect only the best.

Aluminum molds with bladders are great: http://youtu.be/cWom7oav6x4?t=1m23s

I’ve also used biaxial carbon sleeve from A&P technologies: http://www.braider.com/Products/Braided-Carbon-Biaxial-Sleevings.aspx

stuff is awesome. Not prepreg but if you do a closed mold and silicone mandrel the compression is more comparable to autoclave.

I guess this all just depends on your volume, how much you’re willing to spend on tooling and manufacturing, or finding some folks who can take care of the work. I’ve done similar types of projects for test purposes and know it’s very feasible and you’d end up with a far superior part than what you have now. And you could add improvements like the more ergonomic handle as well as possibly making a lighter, stronger, more durable product.

#9

Sammymatik, you summed it up, the problem is some accept the quality, to me it is either correct or not, if we remove the stuff you attitude from the equation most of their stuff is world class, but they also expect you to accepts that which is not up to standard by disguising it.
We build some models of wing paddle at below 700 grams complete, at these weights everything is on the edge of as light / strong ratio as you get.
Cost of what we prepared to pay, well within reason, money saver is use after hours of a CNC mill for a few beers.
Autoclave, well I have a 4/5ths built one, 1.500 x650.
Yes viewed that You tube video many times along with all the others.
Our experiments have been starting from end to end of shaft: first section round to go into blade, next section slight oval for controlling hand and the next round till the end where blade goes on.
But we going in the direction of doing it ourselves, just needed opinion of whether the above was acceptable strength and quality wise.
WHICH IT IS NOT.