Combine Cure in Autoclave

General Composites Discussions Composites Talk
#1

Hye everyone ! i’m new here and need advise.

I planning to combine cure in the autoclave, but i’m not sure if it possible and appreciate if someone can provide me the answer for my question below.

  1. Is it possible we cure composite part with different tool material. For example: combine cure with tool fabricate from invar and steel. Is it possible, since both material has different thermal expansion.

  2. if it possible, how we determine the heat up rate for the cure cycle. since i need to meet the cure cycle requirement i.e heat up rate if i combine the cure with both tool inside the autoclave.

Thanks

#2

It is possible, but you have to take into account all the variables.
Using different cte materials, of course some stress well arise with the increasing of the temperature.
Most of problems come from using carbon fiber, since it’s cte it’s almost zero, and requires carbon tooling when curing at high temperature. At least on female tools.
Glass is more tolerant.
About the heat rate, you have to use the one required by the resin,keeping in mind the following:
The temperature must be read with a thermocouple on the part, not on the oven air.
In case of thick lay ups, you might risk exothermic problems, so you have to lower the heat up rate.
If you use metal moulds, keep in mind that the the layers in contact with it heat up more slowly than the ones on the bag side, this lead to two problems

  1. you have to increase the curing time so that you are sure that all the resin into the laminate has been cured properly.
  2. the outer layers will cure early than the other ones, so they will expand earlier. This leads to some kind of distortion of the parts that you have to take into account when you design the mould.
#3

Not quite sure what you mean: Do you mean is it possible to cure a part using a tool that is made from Invar and Steel? Yes is the answer. Depends on the shape of the part you’re making but, the vast majority of tools have an invar surface and a steel frame for example to save on cost. Yes, you can get slight variances and tool stresses at the joints for example, but ie, for an aircraft tool, making it all out of invar is rather expensive.
Or Do you mean curing say two completely different parts, one on an invar tool and one on a steel tool?
If this is the case, yes it’s still possible.

  1. If the latter is the case, best to just heat the part slowly, eg 1 degree C per minute to ensure a slow heating rate of both tools
#4

1: also confused on what you mean by combine cure. What material are you curing? Prepreg? Wet layup and then autoclave?

2: Your resin should come with a cure cycle. If not, call the manufacturer. Every resin is different!!! Use a thermocouple on the top, middle, and tool side of part to make sure the part sees the correct temp. If you heat too quickly, and short of a time, the middle of the part, if thick, will never get to temp.

#5

fidhan,

  1. yes, what i mean is to cure two different parts(prepreg) one with invar tool and the other is steel tool.
  2. You mean we need to control the cure with slow rate until the end of cure. is true? because in aerospace mfng industry, we need to meet the cure cycle req i.e heat up rate, cool down rate, dwell/hold time. Fyi, most of our part is carbon prepreg and cure at 350F 0r 180 deg C.
#6

180°C, carbon prepreg and steel moulds lead unavoidably to laminate stresses unless you use very thin laminates and superslow heat up and cool down rates. The use of UD tape worsen the situation.
You better switch to carbon moulds.

#7

roberto,

we not be able to switch the carbon tool because we’re using tool given by our customer.
I want to know any other drawbacks when carbon prepreg cure with different thickness and moulds?
also, i want to know how to control the ramp rate cure when using different mould tool i.e steel vs invar in the autoclave.

Have anyone try it? :slight_smile:

#8

Of course, an Invar mould is much easier to work with, thanks to its low CTE.

The only problem arises from its thermal inertia that could lead to eneven heat propagation through the part thickness.
A simple cure is to use insulating blankets over the bag to limit direct heat to the part.
PS: are you hiring?

#9

ok understood. Is there any formula or calculation we can estimate the heat propagation between the tools.

Also, is the any software which can simulate the whole activity in the autoclave.

PS: Sorry, no available vacancy open for non-citizen.