Hey ahhh, why these parts getting so soft in the sun?

 

How can I fix this? I do resin infusion, is it time to get a parts oven and switch to a higher temp tooling gel?



If Brute Force Isn''''t working your not using enough...

One of the disadvantages of epoxy resin is that parts get soft in the sun. You have to use an epoxy with the highest possible tg and if possible post cure the parts.

whats post cure?



If Brute Force Isn''''t working your not using enough...

Hi Carbon Tuner,

I think NikCFC speaks about oven curing.

Bye,

Seb.

Hi,

First of all you have to check the tg (Glass Transition Temperature - The glass transition is the temperature where the polymer goes from a hard, glass like state to a rubber like state) of the epoxy you're using, the higher the better.

Post curing is actually curing in an oven / autoclave at higher temperatures raising the temperature little by little after initial curing has taken place while the part stays in the mold to avoid distortion.

You can post cure the parts using an infrared heat lamp the same type of lamps used for curing paint any thing from a single 30cm single bulb lamp to a large multi bulb array 

Hi Don,

All of the comments about post-curing are right, what's happening when you parts are getting 'soft' in the sun is not that the resin is at its Tg (glass transition) temperature but just that it was not fully cured when you demoulded it and then still not fully cured when it went out into the sun. For as long as the resin stays at the room temperature (or even quite a bit higher) than it first cured at, it will feel hard but when it's been exposed to the much higher temperatures of direct sunlight then it's effectly 're-curing' at the higher temperature. You'll find that these parts that seem to have gone soft in the sun then won't do it again because they have now effectively been 'post-cured' in the sun and so now have a higher HDT (heat distortion temperature).

Direct sunlight can take a black carbon fibre part to 80'C (176'F) quite easily which is enough of an increase to make the resin soften and then re-cure.

Post-curing is best done in the mould (which means moulds that can take elevated temperature without distorting themselves) but can also be done out of the mould. If you post cure out of the mould then you should go for a much steadier heat-ramp, maybe several hours at 40'C, several hours at 50'C, several hours at 60'C. If you're in the mould (particularly if you're still under vac) then you can be a lot more robust with the post cure, maybe even just dropping the mould straight into a 60'C or 70'C oven.

Most standard mould making materials (even those not designed for elevated temperature use) can take 60'C and you should find that this is all the post-cure you need to anneal the resin and prevent any HDT problems caused by high operating temperatures.

Heat lamps, as Bruce suggests can be used by you should excercise caution because you can accidentally get some very hot spots which would be beyond the temperatures you can safely expose the resin to. If you use heat lamps or other very localised heating be sure to diffuse the heat through breather cloth or some other insulating material which will dissipate the heat more or even flip the mould over (if it's still under vac) and shine the heat onto the reverse side of the mould, rather than directly onto the part.

--Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

THANKS everyone for your replies,

Well I guess I need to get into post curing.

 

I think I am going to get started by doing in in mould by heating the back of the mold.

whats the highest temp "in mould" should I go?



If Brute Force Isn''''t working your not using enough...

Hi,

You need to check which temperature your mold can survive to. If you'r using unimold, its about 90°C. If you're using mold making puty + green epoxy gelcoat, its about 60°C.

But, do not forget to post cure the MOLD itself before post curing your part in the mold....

Post curing your mold is easily made following post cure cycles given by the EC team on their pdf data sheets which you can find on the webpage of the product you want to learn about.

Hope it helps.

 



 


    A $1000 electronic device will always protect a 10 cents fuse

I guess you could leave it a few weeks if you cant get the elevated temperatures.

if you cant stretch to an oven, you can use electric blankets.  The one i have heats to 40 degrees celcius which although not as good as 60 degrees, can still be better than just room temp.

Another way of doing a cheap oven if you have space is to use breeze blocks to make a basic oven shape, line it with insulating foam (the sealed foil covered stuff). then use a 2kw fan heater to heat the box up.  Only problem is they need careful temperature monitoring really.  You could probably buy some thermostat controlled heating element but i wouldnt want to leave such a thing unattended really if its not a proper oven.