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TURK
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It's always a good idea to lay a fibreglass skin over a foam pattern/plug just so you can then sand fair a thick primer or filler until it touches the firm skin here and there. Then add further primers etc.
If you just have the soft foam underneath then it becomes difficult to sand any paint or filler as if you sand to touch the foam then it digs a hole in it!
Won't a few brushed on coats of Pattern-Coat Primer harden the PU foam ?
That's a good tip there Fasta  my intention was to build it up with body filler, then Pattern-Coat Primer.
The polyurethane I'll be pouring into the makeshift moulds, is to get the original contours of the OEM headlight. I had to do it that way as I don't have a CNC machine, to get both headlights identacle shapes in reverse was impossible ........ I know, I tried !
As the OEM headlights are a really poor fit, I want my custom headlight covers to be a tighter fit, so they blend in better with the van panels. So in some places ( top and right hand edges ) I'll be using 5mm EasyCell75 to help the build-up. Then I intend using the procedure above, body filler for final form, then many coats of Patter-Coat Primer.
Does anybody foresee any issues glueing two different foams together using a hot glue gun ?
Here's the OEM headlight, with added DAS modelling clay to give me a vacuum formable shape >>
I'll be using PETG for vacuum forming those, Then I'll add the apertures for the quad headlights, then pour PU foam in them ...... that's the general idea anyway. |
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Hanaldo
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Actual PU foam (not the aerosol stuff) is closed cell, so the pattern coat primer won't harden it at all as it won't get absorbed. It will give you a hard shell, but once you sand through it then the foam underneath is still soft and will dig out very quickly. And you will definitely sand through, it can't be avoided. In a way I agree with Fasta, adding a layer of fibreglass will give you that solid base. However having done both methods, I prefer not to use fibreglass, simply because the nature of wet laying fibreglass naturally leads to high and low spots where you get resin rich areas. On parts where the dimensions are critical, this can be annoying because you sand through your coating to the fibreglass, and you aren't truly flat. So you have to build up the rest of the structure to make it flat, and before you know it you're a few mm off in places. I prefer to use profiles to give you the parts dimensions, then skim the foam with body filler before sanding down to the profiles. Yes you will sand through and chew into the foam in places, but this isn't such a big deal as you just fill them again. It's a bit of a tedious rinse and repeat procedure, but it allows you to follow your profiles very accurately and know that you are true to your shape.
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TURK
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Thanks Hanaldo for your time and expertise, very much appreciated. It makes great sense what you're saying, but because of the shape that's needed, contoured face, angled sides and two cone like apertures in the centre, I really have no option than to pour some sort of solid ( or near as solid as possible ) material into my mould that I'm creating. I've bought some Vac-Cast Epoxy casting resin, but can't find any how-to's on the web, so not really sure how to use it. I was hoping somehow that I could simply pour it into the mould to get a very durable male mould for vacuum forming. As it contains Aluminium, it's ideal for vacuum forming as the 'plug' will be subjected to high temperatures. I had considered using the profiles approach as in Paul's video on how to 'Pattern Making', when he demonstrates the Carbon Airbox. But with my project having two cone like apertures, I could never sand down the two 'holes' to the degree of accuracy needed unfortunately. We need a How-to video on how to cast resin I think !  |
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Hanaldo
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Apologies, you had mentioned that's what you were doing, I had forgotten. You might just want to be careful with using the casting resin, what sort of high temperatures are you talking about?? Unfortunately aluminium doesn't increase the Tg of the resin, it only increases the stability, so rather than getting hot spots the whole thing will stay at a similar temperature. So if your casting resin doesn't have a high Tg then it still won't be suitable.
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TURK
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Hi Hanaldo, I assume the 'Tg' is the 'Heat distortion temperature' of the resin. No 'Tg' information on the EC Vac-Cast Data Sheet as such, but there is a 'Distortion' figure shown as 80 °C ABS becomes pliable between 88°C & 120°C which is it's 'Glass Transition Temperature' (Tg), varies according to thickness. Forming temps I'll be using should be in the 90°C range for about 90 seconds. So if I manage to make my plug solid enough, I may get away with it 
I hope the Vac-Cast Epoxy Tooling Resin is up to the task, as it's sold as such 'for vacuum forming tools', as I can hardly send it back now !
Well spotted there Hanaldo, if you hadn't mentioned it I would of been none the wiser ! ....... learn a new thing everyday  |
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Hanaldo
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In essence, the HDT rating of a resin is a pretty useless value for most users, as it is the temperature at which the resin deflects under load. This is normally a temperature that is somewhat higher than the actual Tg of the resin (hence why manufacturers like to quote it in their data, as it makes their resin sound like it takes more heat), which means your resin is likely to be softening up before it gets to 80 degrees. If that's the case, I'd be cautious with it if you need to go as high as 120 or even 100. You may find that it will lose its form at those temperatures, ruining all the work you put into getting it dimensionally accurate.
That said, I don't have any experience with that resin or with vacuum forming. If they say specifically that it can be used for that purpose then I'm sure it will be fine.
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TURK
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Hi Hanaldo, I hear you, and totally agree ........ but I haven't got to where I am with my project by 'playing it safe' ! I constantly take chances on my projects, I think we all do when confronted by new procedures, as we're not sure if they'll work or not. I think this is how we learn what works and what doesn't, and from that, we tend to develop our own 'style'. I'm still at the research stage at the moment, hence all the questions. In fact, the mould which will culminate to a working plug, may even turn out to be a female mould. It would most certainly suite my final ABS part, as the shrinkage of the material would ease the release of the part, as a male mould would have the effect of clamping down onto the plug making demoulding that much harder. Many thanks for all the info guy's, armed with my new found knowledge I now have a clearer picture of what to expect. I'll keep you all posted. Alternatively, you can see the entire project on the 'vwt4forum'. TURK
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Hanaldo
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Absolutely agree mate, trial and error is all part of the fun! Honestly your project isn't really a field I have very much experience in, I more like to contribute my thoughts and experiences on a matter and hope it stimulates some good ideas. I look forward to seeing how you get on!
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TURK
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Thanks Hanaldo, I fully understand mate, I did read some of your posts, sounds like interesting stuff ...... shame all the pictures have gone though, as most of it I don't understand without seeing what's happening ! 
When I get to my Carbon Fibre parts I'll be back to pick your brains.
I may be going about this in a totally unconventional way, but it seems to be working out for me at the moment. The biggest learning curve at the moment is knowing the capabilities of the materials, and what can be achieved ...... and obviously how to use them correctly. And which material to use when.
I think once you have mastered that, got your technique sussed, then you should be able to design and fabricate just about anything.
........ next project, extended wheel arches !  |
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Zedok
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Hanaldo, Sorry for the late reply. Just wanted to say that I have used expanding foam to make a canoe plug. Initially, I had made the error of building a female mould without a plug... I know, just ridiculous. Anyhow, I used three cases of 750ml containers to fill-in the would be female mould. It worked perfectly except that if you spray too much in one go, the interior part of the foam mass will not expand. Otherwise, by spraying layers of maximum 10cm, waiting for expansion and dry, you can add as many layers as you want. Sanding is a breeze and you can then cover the plug with body filler before finishing with getcoat. Z
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