Wolf HR
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| posted on 6/2/08 at 06:31 PM |
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GRP tips and tricks
Would it be OK if I started a therad where less expirienced (or inexpirienced, such as myself) members could tun to those 'in the know'
for help regarding questions about making composite parts for their projects?
For examole I'm planning to build a rather large tub (GRP sandwich)- and seeing it's wider than widest fabric roll (60" and I
can't take an easy way out, I've decided to do it properly (±45° fabric orientation). The first question that pops to mind is- how much
overlap should I have between pieces of fabric in the same ply, would something like 10mm suffice? Another question is whether I should orient fabric
on different layers perpendicular to each other (the fabric in question, 7781 has, oddly enough, higher strengh in fill than in warp direction)?
TIA
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r1_pete
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| posted on 6/2/08 at 07:53 PM |
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Dont know anything about GRP, but to answer your 1st point - thats what the site is all about, exchanging ideas and views to achive everyones aims.
Rgds.
Pete.
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Delinquent
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| posted on 6/2/08 at 08:00 PM |
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A very good idea, one tip I'd give is to buy a book called "competition car composites" by Simon Mcbeath. It's not a massively
thick book that's going to take you longer to read than it would to build your car, but for the price of 1/2 kg of resin it'll save you
one hell of a lot of headaches and wasted materials!!
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Mr Whippy
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| posted on 6/2/08 at 08:22 PM |
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According to my book 'how to fiberglass boats' the overlap for fabric is about an inch and each layer of fabric should at 45 deg to the
last.
Fame is when your old car is plastered all over the internet
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twybrow
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| posted on 6/2/08 at 08:46 PM |
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Wolf, there are a few of us composite people on here. I will add my 2p worth, and I'm sure others will have plenty more to add on top.
What is the part? Can you give us a drawing/photo/dimensions/loading?
GRP is a very forgiving material to work with once you have the basics sorted. You are using a +/-45 fabric, which wont be as cheap as playing around
with CSM (chop strand matt). If you have not done any laminating in the past, then I would get some practice on a non-essential part before committing
to the final job. Learn how much resin you need, how to apply the fabric and how to get the air/voids out. Depending upon the shape, you would also
need to think about which order to work (corners etc).
Once you have that sorted, you can think about layups. If you are making a sandwich structure (grp/core/grp) then you can get away with a bit more
than a straight laminate. Even so, you should try to make your laminate 'balanced' - for each layer at 0degrees, you have one at
90degrees, for each at +45degrees, you have one at -45degrees. Symmetrical is exactly as it sounds, symmetrical about the centeline.
eg [0,90,90,0] = balanced and symmetrical
[0,+45,-45,+45,-45,90] balanced, but not symmetrical etc etc.
As said already, you should if possible avoid putting layers down with more than 45 degrees between them, but its not the end of the world.
Overlaps should be around 1" as already mentioned. If working with CSM then you can 'feather' the edges to avoid buildup. If not,
then just make sure you stagger your joins when adding layers to keep the laminate flat.
Lastly, don't be tempted to put down to much thickness in one go. Exotherming resin can be rather dodgy (fires, fumes, burns etc) so take it
slow, with slow/low levels of catalyst to maximise working time.
I hope this helps!
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Wolf HR
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| posted on 6/2/08 at 09:30 PM |
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Thanks guys. 
Twybrow- here's the monocoque/tub with glued reinforcements (1220 mm wide and 240 mm deep). Sandwich would be 4 layers (and gel-coat) of 7781
cloth*, then a 3/8" H-80 core and 5 layers of the same cloth on inner facesheet.
There will be 4 Al alloy bulkheads (box section) glued to it- fore and aft carrying suspension (and engine), and two in the middle section for
stifness/impact protection. (Sorry, haven't done any sketches of the layout, but will do soon).
* 8 harness satin of E-glass, seems to be quite low price and high performance cloth (used in aviation). Here are properties of a epoxy based
composite with it:
http://rapidshare.de/files/38509415/7781_Material_Properties.pdf.html (68kb)
[Edited on 6/2/08 by Wolf HR]
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Fred W B
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| posted on 7/2/08 at 06:16 AM |
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I'm no expert, but have you considered doing that as just 2 parts - a complete inner and outer moulding, bonded together. That way you can get
a "good" surface on both sides.
Cheers
Fred W B
You can do it quickly. You can do it cheap. You can do it right. – Pick any two.
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Delinquent
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| posted on 7/2/08 at 10:11 AM |
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quote:
Originally posted by Fred W B
I'm no expert, but have you considered doing that as just 2 parts - a complete inner and outer moulding, bonded together. That way you can get
a "good" surface on both sides.
Cheers
Fred W B
Exactly what I was thinking Lends itself to it rather nicely.
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