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Junior
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Would the following be suitable/compatible? I'm struggling to find anything cheaper.. |
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FLD
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I buy stuff there but I wouldn't use the reblend resin. It is just the dregs stirred together. Sometimes it's great, sometimes it's terrible. I gave up using it after a terrible lot.
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Junior
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So would the top two work together or is there a better alternative? Did someone say you can use a regular Gelcoat rather than a 'Tooling Gelcoat' like this:
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FLD
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The top two work great, I use them myself. The general gelcoat you've just added works fine too (I also use that) it's just a bit softer so you won't get as many pulls from the mould. I've a mould in that gel and with the GP resin that's up to around 50 pulls and is just fine if that helps. Not sure how many you're looking to do before retooling. Regarding the whole warping issue, I've used the materials you linked for a 1/2 car sized mould without loss of shape or dimension. In my head I consider the resin to exotherm as it cures so it turns solid in a hot state. Cooling that solid then makes it shrink. If you can do individual layers and allow them to cure you stop it getting hot so it doesn't shrink. I suspect warping comes about from hotter areas as they've been made thicker so any contraction is then uneven. A little care is all that's required. Regarding mould release I use easylease and wax (sometimes both) and I've had no problems with the materials you linked. I do tend to go OTT with it though so a good 12-15 coats of either at the very least. GRP mouldings then almost fall out, infusion made parts need a little pry but come out easily.
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Junior
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That's good to know it has worked for you, to be honest I'd only be looking to make 2-3 pulls, 5 at the very most over a period of years! Sounds as though the polyester Gelcoat could be the best option then.
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Hanaldo
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FLD (21/04/2017)
Regarding the whole warping issue, I've used the materials you linked for a 1/2 car sized mould without loss of shape or dimension. In my head I consider the resin to exotherm as it cures so it turns solid in a hot state. Cooling that solid then makes it shrink. If you can do individual layers and allow them to cure you stop it getting hot so it doesn't shrink. I suspect warping comes about from hotter areas as they've been made thicker so any contraction is then uneven. A little care is all that's required. Not quite as simple as that; effectively all you are doing is controlling the shrinkage to its minimum. Resins shrink due to the presence of uncross-linked free radicals after the polymerization stage (read: after the resin has turned to a solid). Regular unsaturated polyester resins typically have the highest percentage of uncross-linked free radicals, vinyl esters and epoxies have slightly less due to the presence of the more reactive epoxy groups. Polyesters can shrink as much as 7% by volume, vinyl esters are closer to 3%, and epoxies are >1%. The reason you get more shrinkage when the exotherm gets too high or you have too much catalyst is because the free radicals are actually produced before the resin gels. Cross-linking takes place between the gelled stage and the solid stage, and it requires the free radicals to be able to move around. If the exotherm gets too hot or there is too much catalyst, then the movement of the free radicals is slowed and the reaction stops before many of the free radicals can cross-link, leaving you with a higher percentage of uncross-linked monomers and thus a higher rate of shrinkage. In short, ALL resins shrink, it's just how polymerization works. Polyester tooling resins actually shrink just as much as regular polyester resins, but they are filled with a thermoplastic additive that expands at the same rate that the resin shrinks, thus giving almost truly zero shrinkage (0.00-0.1% volume). I think the important thing is how much shrinkage you can actually tolerate. Especially at hobby level, the tolerance for shrinkage is generally pretty high, you aren't working to thousandths of a mm and for the most part you probably aren't going to care if there's a bit of print-through. Apologies for the chemistry lecture, I hope others find it as interesting as I do!
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oekmont
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I totally agree that thermal shrinkage could cause even more problems than chemical shrinkage.
But I disagree that we are talking about inaccuracies in a range if thousands of a millimetre. The chemical shrinkage is about a few percent, and the fibre reinforcement reduces that further. But the linear shrinkage is not the main problem. The problem is that this small shrinkage could result in a warped surface, or - even worse - in a twist of your part. This could be very significant if you are producing an unclosed structure (like car hood). In this case the deflection could be several centimetres.
But I case of a canoe you could eliminate the twist during the bonding process, forming a closed structure, wich is much more resistant to torque.
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Hanaldo
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oekmont (22/04/2017)
I totally agree that thermal shrinkage could cause even more problems than chemical shrinkage.
But I disagree that we are talking about inaccuracies in a range if thousands of a millimetre. The chemical shrinkage is about a few percent, and the fibre reinforcement reduces that further. But the linear shrinkage is not the main problem. The problem is that this small shrinkage could result in a warped surface, or - even worse - in a twist of your part. This could be very significant if you are producing an unclosed structure (like car hood). In this case the deflection could be several centimetres.
But I case of a canoe you could eliminate the twist during the bonding process, forming a closed structure, wich is much more resistant to torque.Yes you're right, we aren't talking about only a few thousandths of a mm in terms of the polyester inaccuracy, I know first hand just how badly a part can distort. What I meant was that it can generally be handled, and if you can avoid major distortion or surface warping then most hobbiests will put up with their laminate being a bit out - their parts don't need to be accurate to within thousandths of a mm. In all honesty, I don't use GP resin for these exact reasons. It is far too inaccurate for me. In fact, I don't even use vinyl ester for final part production, everything I do is with epoxy. But that is my experience using my techniques with these materials, and I know that certainly locally there aren't many people who do things the way I do. 99% of people here, many of them at a professional level, still build moulds and parts with GP resin, some use vinyl, extremely few use epoxy. And by all accounts, they are getting on just fine. So it can definitely be done, but you have to be aware going forward that you are going to have some dimensional innaccuracy.
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FLD
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I thought the chemistry was more complex than that? The chain propagation is also radical based as the catalyst is just a radical initiator. If so chain entanglement is causing the gelled state and the cross linking is radical termination. The whole thing is a ballancing act with catalyst loading. Polymer chemistry is nuts, lots of maths involved which is why I went towards drug discovery. Thinking about the warping issue I suspect I've escaped it so far as I always add bracing panels / flanges to my moulds. This coupled with my slow lay up has probably been lucky for me. The shrinkage is an odd on too, I have a car bonnet / hood that fits back into the mould. Even after 3 years! Is this because I'm a hobby moulder and tend to demould a week or so after the final layer?
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