Hey guys,

I've been playing this game professionally for a few years now, and it's been awhile since I've had a problem that I couldn't diagnose. But I've recently come across a new one, so thought I'd see if there's anyone that has experienced this sort of thing before.

So I recently made a mould of a bonnet that I bought in pretty bad shape and have been slowly restoring over the last few years. It is for my own private car, so it has been a bit of a side project that I work on when I've got a bit of spare time. The original bonnet is fibreglass, and I could tell from the get go that it's nothing special, not very well made and done using low quality materials. It had been painted matte black by the previous owner, and had obviously got hot in the sun and sections of the outer skin had sunk a bit. It took a whole 4kg tin of body filler to get it flat again, so there is a lot of bog in it too. Fast forward a bit, and I had finished reshaping it and repainting it to get a finish that I was happy with - all of this was done with materials that I have used a thousand times, nothing new here.

Then about a month ago, I got on with making the mould. Mould system is a rapid tooling system equivalent to Unimould; vinylester tooling gelcoat and skin coat, polyester tooling resin. Typical layup method of 1 layer tooling gelcoat, 1 layer 225g csm skin coat, 5 layers 450g csm. Being in Australia with our intense summer sun, I've always been a bit concerned about making sure the future carbon bonnet doesn't distort, but I haven't got an oven big enough to fit this bonnet into. So as an attempt to harness a bit of the sun's power, I added a black pigment to the tooling resin - not an ideal way to do things, but better than nothing.

Anyway, I finished both the outer and inner skin moulds no problem, everything was sweet. So I left the mould out in the sun for a month to post-cure nicely. We had a few 38 degree days in that time, and I measured the mould surface temperature to be a nice 70 degrees at its peak. Then when it came time to demould, I found some issues... See attached pictures. The pictures are of the original bonnet, I haven't taken pictures of the mould. Unfortunately the damage is actually even worse than the photo's show, it is literally the whole bonnet that is covered in these 'dents'.

So far, the only thing I'm really certain of is that the problem was caused by heat. My first reaction was that the heat of having the mould out in the sun must have at some point been so intense that it caused the body filler on the bonnet to shrink excessively. But having thought about it, I used the same body filler that I have used for all my pattern making, some of which have consisted of FAR more bog than this bonnet and never suffered any ill effects, even when post-cured in excess of 90 degrees. Also the fact that a large majority of the distortion you can see in the bonnet is replicated in the mould, which means that it almost certainly occurred prior to the elevated temperature cure in the sun. Which led me to the conclusion that it must have occurred from the heat generated during the curing of the tooling resin - which I didn't measure, but I know from experience would have been around 65-70 degrees.

Then on further reflection, I realised there is major distortion in areas of the bonnet where I didn't use any body filler (and I know that there was any there previously as I removed everything back to bare fibreglass prior to starting my repairs). So I'm now certain that the issue isn't the body filler, and is more likely the actual fibreglass that the bonnet is made of. The things that stump me though, are why did the distortion occur in smallish dents like it did, why not just sink again like it had originally? Why did the distortions over the middle of the bonnet replicate in the mould, but the distortions on the edge of the original bonnet (which are admittedly a bit hard to make out in the pictures) did not?

My problem now is I'm not to certain how to fix the issue... The mould is beyond repair, there's no way I can carry out that degree of resurfacing and achieve a result I would be happy with, so I know I will need to remake the upper mould. But if I fixed up the original bonnet and tried again, I'm quite certain the issue would just occur again... So I feel like my only option is to pull a copy from the current mould and use that to fix up and remake a new mould. My issue with that is accuracy - without making an entire new bonnet including an inner skin, how do I make a skin with enough rigidity to ensure I maintain the dimensional accuracy? Even if I made the thing excessively thick, which I'm reluctant to do for cost and disposal reasons, the shape is too flat to have much rigidity on its own. Is bonding on a backing structure my only option? 

Ideas?

*EDIT* Getting an error message about having exceeded my available space when I try to attach the images... Will try another method. 

**2nd EDIT* Right, I'll try that again:

Could it be blisters of air within the moulding or lay up which with the high heat have expanded to create those type of raised areas.I have seen this before with moulds postcured at that sort of temperature, the trapped air in the laminate caused the blister type effect as it expanded with heat during the postcure when the resin softened.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Yeh I've come across that before too, but this isn't that. These areas are sunken not raised. If it were a metal bonnet, you might think someone had taken to it with a hammer.

Speaking to one of my colleagues, it might also be shrinkage during the high temperature part of the cure or potentially overheating in some spots if the gel had got a bit thick in those areas.

It's going to be virtually impossible to prove either way sadly. 

As you've suggested. your best bet is to try and work out the best way to recover the mould or part.  I would potentially take a "splash" part off the existing mould.  do some measurements against the car so its straight, then  modify it before doing a new mould., but more carefully on the "post cure" this time.  It could well of got quite hot quite quickly in the sun when left outside, so maybe properly done in an oven or temperature controlled environment would be best as you can control the temperature steps better.


Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

Thanks Warren, pretty much the conclusion I had come to.

So do you think it is more likely this distortion happened due to the heat of the sun? It seems somewhat hard to imagine that it could have been, given this part of the mould had been well and truly cured at an ambient of somewhere around 30 degrees for 3 weeks before it ever saw any direct sun. I think shrinkage (potentially in the adhesive between the two skins?) due to the heat from the tooling resin curing is the most likely, so I will do what I can to avoid it happening again.

I also think that as annoying as it is, the best solution for making sure the splash moulding I take is accurate would be to produce a completely new bonnet including the inner skin. 

That must have been some dramatically high reaction heat. I mean it was a car bonnet. Painted black. So it might have seen sunny days, cruising around, and than get parked in the sun, with an still hot engine under it. If your mold got hotter than this, you might consider laminating with fire ambulance predialed next time.

I know, that is exactly my thinking, which is why I am completely baffled. I have never come across any fibreglass materials that are SO adversely affected by heat, even the low quality stuff that I've used wouldn't have suffered like this. I have also made dozens of moulds with this tooling system through all times of the year, never once has the exotherm generated more than 75 degrees and usually on a mould this size it is a bit less, closer to 65. In fact, the NACA duct doesn't have any distortion at all and due to overlaps and its shape,  that ended up being 8 or 9 layers of quite resin rich 450g.

I know it didn't get THAT hot in the sun, I monitored its temperature on the hottest day we had, and it only got up to 75. Enough to affect a poor quality polyester system I know, but I don't think that is enough to distort both the mould and the original bonnet to this degree (again considering that this part of the mould was 3 weeks old by this time). 

It did have distortion originally though, it had obviously been affected by heat. But the original distortion was more what I would have expected - large sunken areas where the top skin was unsupported by the inner skin. It was certainly nothing like this where there is even distortion on some of the body lines that would naturally be quite stiff, and such small little areas that would suggest spots of intense heat).