Pre shaping core material?

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Pre shaping core material?

sijandy
Does anyone here pre shape they're core before doing the lay up and if so how??? Without preshaping i get problems with the layers of glass/peel ply/breather etc get sucked under and fold around the rails while the core is being vac'd down by the bag. I use Paulawnia for the core and managed before to shape it by brushing car laquer both sides and then vaccing it in the rocker table and once cured it held its shape nicely. For some reason i just cannot replicate this, as in everytime since using the same methods/materials it just springs back immediately even having left it under vac for over 24hrs!!! Has anyone tried steam treating or see any reason why it would not work? The only other option i can think of its to leave the core flat and simply do the lay up one side at a time but i'd really prefer to do it all in one go as i've pretty much got my method nailed except for the core issue. How does everyone else get around this issue? cheers, Simon.
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Re: Pre shaping core material?

downunder
I do not do all in one go, coz of the reasons you described. Maybe vac and all in one go is a no go
Pneumatic press might be (think "big boys", ski industry etc).

I think you need to do what Matt did. Cut the core in strips to make it more flexible and to keep the rocker.

Also, you mention folding around the rails, it was discussed here to keep some excess wood all around. I've had the same issue when I didn't leave the excess. Now if it folds there, I don't care...
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Re: Pre shaping core material?

mattma
Administrator
In reply to this post by sijandy
Hey mate,

I've always used flat cores and just let the vac bag pull it down. I did have the problem of material getting sucked under once when i was playing with different breather material. However, I am pretty sure the issue was that i had folds in the vac film because I thought this was necessary to get the vac film to conform to the 3D shape of the deck. I now don't think that is the case but this depends, i guess on the vac film your using. I've used the proper nylon vac film and also tried very cheap builders plastic ( the kind that is used under concrete to create the water barrier). With cores as thin as what we typically work with I think that there is sufficient stretch in these plastics so that you won't have problems with bridging. I don't pull the vac film tight but I now never have any visible wrinkles in it prior to applying the vacuum. I am pretty sure that by not having wrinkles in the vac film the chance of the material being drawn under the flat core are very remote. Also, sometimes as the vacuum starts to pull I put my hand in the middle of the board to at least assist with the rocker. This, inadvertently, has the affect of reducing the space available under the rails for the material to get sucked under. I also, leave a good 10cm of extra fibreglass around the edges of the board. This helps suck up excess resin but may also help avoid the fold under issue.

I started out doing one side at a time but found it wasn't much more effort to do both at once. I try to do the resin work in the cool of the evening to help slow down the curing and although the pot life of the resin I use is about 20 mins I can still get a good result taking up to 1 hour to do the lay up both sides. Once the resin is applied and thinned out it looses heat easily unlike in the pot when the heat has the affect of speeding up rate it cures at.

Hope this helps

Cheers
Matt


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Re: Pre shaping core material?

sijandy
Ok, yeah i leave an excess of about 10mm of core around the outside of the rail but i have my rocker set pretty high which means when i lay the core in the mould theres around 50-60mm of gap to the bottom of the table so plenty of space for the materials drapped accros the top of the board to get pulled under. I think for me pre-shaping the core will produce the best results if i can figure out a good way of doing it. Still miffed as to how come the laquer technique worked so well with my first core but had zero affect on my second??? Another question for you both, once out the mould how do you cut out the board? I use a jigsaw but it's a bit aggresive and tends to delam the glass a bout 1-2mm either side of the cut. Even sanding with a strong grit 40/80 delams and causes the edge of the glass to go fluffy. Got any tips or techniques to get a nice clean edge when taking back to the rails?
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Re: Pre shaping core material?

Chris
Try cutting 2-3mm from the finished edge then file/sand back to the finished profile. If you are getting too much grab from the blade then there are a number of factors to look at:

Worn out blade
Blade too hot (slow it down)
Blade too course (try something inbetween metal and wood)
not enough pressure on the jigsaw so it jumps
Too fast/slow cutting speed
Trying to cut too fast i.e. don't push it forwards too quickly

If you are getting fluffy edges then the glass may not be fully impregnated, try consolidating more before vaccing down. If you are trying amarid then you'll get this anyway so always finish it a little before the finished edge so you are just cutting the glass/carbon

I've always found cut with jigsaw/fretsaw, then file back with diamond edge file or 80-120 grit glasspaper (not sandpaper) then go up to 180 then 500 then wet and dry 1500 for a really smooth finish. When you are sanding you need to sand along the edge rather than across it (unless you are sanding a thick laminate then you can sand gently down NOT up otherwise you'll catch it again)

If you're just after a quick edge finish for handling then jigsaw, quick rub with 120 then quick rub with wet&dry and wash.

Although, if you are getting delamination while sanding normally then the resin isn't bonding to the deck so the laquer is effectivly acting as a release agent to a point.

Chris
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Re: Pre shaping core material?

downunder
In reply to this post by sijandy
I am using T101D Bosch blade with a jigsaw. Every 5-10cm I change a
blade (of 2).
Going near to the rail as possible so minimum sanding needed.  The
best was to work with a big (shop) belt saw tho. Very, very, precise.
A dream to work with. Both tools give good results, no issues you're
describing here (bamboo is hard, your wood maybe is not?).

Some ppl are using a router with a bearing and the same template to
flush the excess. Might try on a next project just to avoid sanding.

About your rocker table, I did have a go on high setup like 10-15cm (
my current board, photos on a blog), no problem folding or anything.
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Re: Pre shaping core material?

Chris
In reply to this post by sijandy
Check the video out in this link... about 13mins and onwards if you want to skip the H&S stuff

http://www.easycomposites.co.uk/Composites-Tutorials/How-to-Cut-and-Shape-Carbon-Fibre.aspx

They also do vacuum adjustable regulators or look on RS or Farnell for SMC regulators

Chris
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Re: Pre shaping core material?

sijandy
Cheers lads, very helpful as usual.

Chris -

Worn out blade
Blade too hot (slow it down)
Blade too course (try something inbetween metal and wood)
not enough pressure on the jigsaw so it jumps
Too fast/slow cutting speed

...combo of all the above really ha. The rails came out fine after all as you can see in my Board No2 thread, wet n dry starting 80grit up to 1200 done the job nicely.
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Re: Pre shaping core material?

mattma
Administrator
Hey Guys

I found that the best tool for the job is a multi-tool.



with a hardened steel cutting blade like the circular one shown in the picture.

The blades are very thin and it makes a very fine cut which doesn't rip and tear like a jigsaw can. The action it fine enough that you can can cut without having to leave more than a millimeter or two of excess.

I bought a really good brand blade which was around $50 and I've cut 4 four boards with it with no sign of going blunt. Despite that the oscillating action, instead of rotating, it means that it if it was going to go blunt it would on an isolated place on the blade so you can just change the orientation of the blade and cut with a fresh section.

Cheers