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Bonobo
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Following on from my previous thread in "Pattern and plugmaking" http://www.talkcomposites.com/623/Pattern-Making-and-CAD,-need-advice in which you can see the design and pattern making of this spoiler  My plug is now nearly ready  I'm now thinking about how to make the mould, i'm considering making a 2 part split mould as illustrated cross section below, i haven't done this before so please add your comments on how you think it would work out or how i can do it better. what would be the best material to use for the barriers? 
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Bonobo
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Ok, my reason for a two part mold is the undercut, i put an undercut where the spoiler meets the body of the car because i need this area to be a really good fit and didn't want this to be where i trim the final part, the undercut means i can trim at the hidden part.
hope this makes sense???
i'm totally open to suggestions as to how to make this work and would really appreciate the advice.
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fgayford
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Nice work you are doing.
There are a number of ways to make your flanges. I have used plasticine, wood and metal sheets at various times.
When I watched the Easy Composites video of the car hood molding, I decided to try that. It worked great and was far faster than my other methods.
The sign board is cheap, flexible and the epoxy did not stick to it. Where the sign board meets the plug surface I tried the yellow wax EC sells and it is fantastically accurate. I used the hot glue gun method they show to glue the pieces together and that worked well also.(before I tried I was sure that it wouldn't stick. but it does)
One thing I would recommend is that you continue the lips where you intend to cut, oversized. So you have a full thickness to cut through of uniform laminate thickness. If that makes sense. You know how the edges of your molded part tend to taper in thickness at the outer edges of your part.
Hope this helps.
Fred
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Bonobo
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Thanks for the feedback Fred, i'm at the stage now of flatting and polishing the plug. I've got myself some correx, a gluegun, and filleting wax so I'll be adding the flanges ready for mould making. I'll post up the pics soon.
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Bonobo
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 Plug polished up with release agent and wax
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brasco
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whoa! i have the EXACT same issue! i have a foha spoiler for my 87 bmw 325 that is long out of production. i am going to mold it on the car. also will make the bottom on the car too.then bond the bottom to the spoiler and possibly use windshield urethane to attach to the decklid. thinking of filling it with lightweight foam as well. good luck on your spoiler !!
CarbonFiberCreations
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Bonobo
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 started making the mould now, backing up with CSM and polyester resin. next step will be to make the second part of the split mould. using the threaded inserts i built into the first part i'll be able to align and clamp the second part.
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Bonobo
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brasco (11/01/2013)
whoa! i have the EXACT same issue! i have a foha spoiler for my 87 bmw 325 that is long out of production. i am going to mold it on the car.
also will make the bottom on the car too.then bond the bottom to the spoiler and possibly use windshield urethane to attach to the decklid.
thinking of filling it with lightweight foam as well.
good luck on your spoiler !!Filling with foam is a good idea, will be doing that too. regarding moulding the bottom part off the decklid, i want to be able to make several of these in the future so i'm guessing i'll have to first take a mould off the decklid, then use this to make a male mould / plug and use this to make my spoiler bottom parts?
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SRM
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What kind of 2k filler primer did you use? I've heard of using both polyester and urethane. Does one of them sand and polish better than the other?
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Hanaldo
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Not really, I've used both and they are quite much of a muchness. It's not the base resin that makes the difference, it's the way it is formulated. Some primer fillers (like Duratec surface primer or Scott Bader Primecoat) are intended for pattern surfacing work and so have been formulated to polish up well. Regular 2k polyester or urethane automotive coatings are not intended for use as a top coating, so they haven't been formulated to retain gloss. They will all sand easily though. The thing I like about the polyester based pattern coatings like Duratec and Scott Bader is they are quite flexible in their application. Not only can you adjust the catalyst percentage to adjust cure times, but you can also mix the coatings to adjust your gloss level. So you can either use straight primer and polish that up and that will give you a reasonably gloss surface to mould from, or you can mix in some of the high gloss coating (Duratec High Gloss Top Coat or Scott Bader Glosscoat) to give a higher level of gloss whilst still maintaining good primer filler characteristics and sandability. The more gloss coat you mix in, the better the gloss and the less primer filler functionality the coating will have. That degree of flexibility is just so useful when pattern making.
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