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antonkov
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I am building moulds for a pickup truck camper, meaning my dimensions are rather big. I completed 2 out of 3 needed moulds in polyester (gelcoat + CSM + infused cored laminate) and there is a quite noticeable shrinkage. In my case, it is rather sagging than warping of the planes, but it still doesn't make it any better. For my third mould, I am contemplating going with epoxy, which I already have from another project and which cost me the same as locally sourced polyester. I understand the benefits of gelcoat in polyester system (hard and polishable surface layer that can be sprayed), but do I really need it in epoxy system? My thought process is as follows: - even if there was an epoxy-based gelcoat, I still would hesitate to spray it for safety reasons, - PE-based and epoxy compatible gelcoats are expensive, not easily available and with a short shelf life - if I use a textured PU coat on the final product, then the quality of the surface finish of the parts is mainly for easier de-moulding than for cosmetics. The plan would be to slightly thicken my infusion grade epoxy with fumed silica and apply it with a roller, then partially fillet the corners, then hand laminate with 316 g/m2 cloth, then infuse with heavy skins and 4mm core mat. 
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Hanaldo
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If you are planning to do a skin coat with epoxy and the 316g cloth, then why not use a vinyl ester tooling gelcoat and then a vinyl ester skin coat. Let that cure, then infuse your heavy reinforcement with the epoxy. Epoxies should bond well to vinyl ester (personally I would still key the skin coat first just to be safe), and you don't have the issue of having to hit your infusion within the bond window for your epoxy skin coat.
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antonkov
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+x...why not use a vinyl ester tooling gelcoat and then a vinyl ester skin coat.. that is because the gelcoat together with hand layed skincoat will make a good fraction of the total thickness, I am afraid it could make a fraction substantial enough to affect the shrinkage. The idea of going with epoxy all the way is to minimize the shrinkage. Another reason is to have a confirmed result, with epoxy I will know in a week how it turned out, with the *esters it will depend on time, temp, sun, future exotherms, etc.
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Hanaldo
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Not necessarily true, epoxies aren't that much different to the esters. They will still be vulnerable to warping and changing shape in the future due to temperature changes if you aren't post-curing them. They are all thermoset resins. Typically vinyl esters will actually handle temperature changes better than your run of the mill epoxies will. The vinyl ester gelcoat/skin coat with infused epoxy reinforcement is exactly how many in the high end boat building and renewable energy sectors make their moulds, so it isn't an impossible approach.
Regardless, I can understand your hesitation. You can certainly do what you are suggesting. Be aware that a silica thickened epoxy doesn't make a terrific surface coat. Especially if you are using an infusion resin as a base, you need quite a bit of silica to thicken it to the point where it won't sag off the verticals. It also won't resurface nicely if you do need to make any surface corrections. But it will work, I used to make my own surface coats in a similar way. Your biggest challenge will be getting everything done within the open time of the epoxy. This is absolutely critical, and when infusing it isn't nice to have a time limit on making sure you are leak free with no bridging etc.
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antonkov
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Unfortunately, none of the datasheets I checked specified the shrinkage percentage. Industry publications offer the shrinkages in the range of 2% for epoxy, 7-10% for VE and 5-13% for PE. Linear should be close to the cube root and would be affected by the laminate schedule. I don't feel very comfortable going by these ranges and betting on 2x4m mould. Currently, I am setting a number of experiments to measure volumetric and linear shrinkages of the resins I have on hand. Appreciate the suggestion about coats, will keep it as an option. Another advantage of a proper gealcoat would be UV protection. If I could trust it with dimensional stability, I would gladly avoid rolling or brushing infusion epoxy, totally agree here. Does VE have different handling when it comes to open window for post-infusion? I planned to either prep and infuse it the next day or, if missed the time, sand it before infusing.
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Hanaldo
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+xUnfortunately, none of the datasheets I checked specified the shrinkage percentage. Industry publications offer the shrinkages in the range of 2% for epoxy, 7-10% for VE and 5-13% for PE. Linear should be close to the cube root and would be affected by the laminate schedule. I don't feel very comfortable going by these ranges and betting on 2x4m mould. Currently, I am setting a number of experiments to measure volumetric and linear shrinkages of the resins I have on hand. Appreciate the suggestion about coats, will keep it as an option. Another advantage of a proper gealcoat would be UV protection. If I could trust it with dimensional stability, I would gladly avoid rolling or brushing infusion epoxy, totally agree here. Does VE have different handling when it comes to open window for post-infusion? I planned to either prep and infuse it the next day or, if missed the time, sand it before infusing. Yeh nobody will specify the shrinkage because it isn't a constant thing, it changes based on lots of variables. For what its worth, I've built a few moulds bigger than 4x2m without any shrinkage issues. These were all made using a rapid tooling system, very similar to Unimould. Vinyl ester gelcoat and skin coat with a filled polyester tooling resin for the bulk reinforcement. CSM construction. I have also made a few infused epoxy moulds, all smaller than 4x2m, and had shrinkage issues with some of those. So it is by no means a guaranteed approach. For me, the only time I make epoxy moulds is when I need a high temperature (over 100°) system. Everything else gets done with vinyl ester or rapid tooling systems.
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antonkov
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Group: Forum Members
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+xYeh nobody will specify the shrinkage because it isn't a constant thing, it changes based on lots of variables. For what its worth, I've built a few moulds bigger than 4x2m without any shrinkage issues. These were all made using a rapid tooling system, very similar to Unimould. Vinyl ester gelcoat and skin coat with a filled polyester tooling resin for the bulk reinforcement. CSM construction. I have also made a few infused epoxy moulds, all smaller than 4x2m, and had shrinkage issues with some of those. So it is by no means a guaranteed approach. For me, the only time I make epoxy moulds is when I need a high temperature (over 100°) system. Everything else gets done with vinyl ester or rapid tooling systems. Shrinkage is a physical property of resin, just like tensile strength, hardness, etc. Yes, it would depend on a number of factors, but it still can be measured under standard conditions and specified in a datasheet along with a reference to the test, just like all other datasheet values. I believe shrinkage is omitted rather for marketing than for technical reasons. In my initial post, I forgot to mention that the surface layer was laminated with a general-purpose PE resin. When preparing for the reinforcement infusion, I noticed my flanges peeled from the table, in some areas flange edges raised 2-4mm. May not be a big deal for the flanges, but it should have given the clue. Below is a pic of quite a prominent distortion of the part that came out of that mould (reusing in a mirroe plug). The infusion was with a "low-shrink" PE infusion resin and I hope it is not the one to blame but the GP PE layer. Still struggling to envision how shrinkage leads to the effect of implosion where the final length is actually longer than a straight line. Anyhow, getting ready for the next mould and talking to my supplier. VE gelcoat seems to be rather rare and isn't in stock, they suggest using regular tooling PE gelcoat, VE laminating resin and then epoxy infusion resin. In theory, they all should bound, but something makes me think twice. How does PE gelcoat / VE skin coat / epoxy infusion scenario look like, any words of caution?  P.S. Regarding Unimould, it relies on specific conditions of the exotherm, which I don't see how to achieve/control on a cored laminate. My skins are thin and are facing different ambiences, sounds like a recipe for an uneven shrinkage/LPA compensation.
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