Vacuum Drop

General Composites Discussions Composites Talk
#1

A flat tool of 3meter x 2.5meter long made of cast iron, i used to cure a flat carbon laminate under 0.8bar (min) curing till 177degree C,
For last 10 curing i have encounter the problem of vacuum drop from 0.88 to 0.6bar during heating and sustained 0.6bar during dwell for 2hrsl and during cooling again attain 0.8bar vacuum.
i cannot find actual cause for this problem.
Earlier same tool, same material i didn’t get drop

I have tried by changing new connectors, hose line, excess loop during bagging, two adjacent layer of sealant tape, extra layer of breather / bleeder and increasing no of suction line, but still i can’t.

#2

Probably the vacuum bag has dried so much that has lost its properties.

#3

Could it be the breather is struggling to maintain the air paths at high temperatures but then recovers when cooled and consequently the vacuum increases.

Is there a higher grade of breather you could try?

If the vacuum is only dropping to -0.6 from -0.8 is it a huge issue.

You might be better off targeting a stronger starting vacuum e.g.below -0.9

#4

How much laminate & what’s the stack you are using?

Also, I am presuming it passes a static drop test okay.
A part that big would drop ideally around 1 mbar 15-30mins if you did not want to have problems at temp.

I would also try to cycle the table without fibre on it to see if you have the same result, perhaps the cast iron has slight porosity issues.

I have seen this similar behaviour in autoclaves where the breather becomes so compressed at pressure that it is no longer operating, but if your pulling 0.88 then this shouldn’t be a problem with the correct vac stack.

Good luck,
Jim.

#5

Minus hidden leaks as others have said. Outgassing?

#6

Friend,
same tool cured in different autoclave from curing start to end it maintained 0.91bar to 0.9bar vacuum.
but in different autoclave during heating vacuum from 0.9 will decrease to 0.7/0.75 bar and sustained 0.70/0.75 bar in dwell and during cooling phase it will increase from 0.70/0.75 to 0.86bar. I didn’t understand why its happing.
I tried so my trail, what you suggested still i cat resolve this problem.
I thought autoclave valve or connecting ports opens at high temperature so i did small envelop bagging and connected one with suction and one with measuring and cured for 180 deg C for 5hrs i didn’t have any drop in the autoclave. when if any tool is used vacuum drop is occurs.

#7

Looks like your quick disconnects are at fault. Check and service them all.

#8

A little off topic here:
I simply wonder why you keep full vacuum during autocleve cure.

#9

Then the vac table should be sound, and your looking for a leak with the Autoclave vac system.

Which style of connections are you using?
Don’t mix and match brands/manufacturers as this can give trouble.
Check very carefully that they are in perfect condition. Eg: Do not let the male ends drop on the floor. They dent and then they leak!

Which breech units are you using… The flat, round screw in type also give problems and we try to use perforated tubes (Flutes) inserted through a pleat instead.

Have you tried swapping the hoses for other ones? ( they maybe damaged and collapsing during the cycle!)

There are ways to help the system too, make sure the vac reader has a good vac path to it within the bag by inserting more layers of breather around the perimeter of the panel. (2x 300gsm)
Using more vacuum connections as well - This usually will help stop the false pressure readings.

If you get time to answer the previous questions about the stack, drop test and tell us what pressure your going too in the autoclave, it would be useful.

As I have said before, it’s not uncommon to see this happen, and most of the fixes have been suggested.

You will just have to keep methodically checking off the list one by one…or use the good Autoclave! :wink:

Jim.

#10

By what he writes, it seems that there are two problems: a) diferent vacuum on diferent autoclaves… and b) vacuum decrease when temp rises.

For a) it seems the vac system itself (bag + connectors, hoses, etc) must be checked ( he´s talking about VACUUM pressure, not autoclave’s pressure) it seems a question of leaks … (if I understood correct)

For b), vac drop with temp and increasing agaim when cooling… The origin seems to be the temp. It causes a dilatation on most materials. I would test one by one. MAYBE the table is openig microcracks by dilatation = leaks, desapearing when contracting = no leaks and vac increase… porosity increase… maybe the leaks appear on the bag by permeability change ? Metal parts as connectors, Ts, gauges … ? Test, test, tes. One by one… try to separate causes…

#11

Oh no this argument. There a number of reasons:

  1. 3bar - atm (0bar) = 3bar pressure differential, 3bar - vacuum (-1bar) = 4bar pressure differential.

  2. Any out gassing or moisture is removed more effectively.

  3. If you have a leak any air will be more compelled to head to the way out.

  4. Why not, you are already running an autoclave so a few cents more to leave the pump on won’t be noticed.

#12

Well, I was simply wondering because this is not a standard procedure for a quality laminate.
1)With full vacuum on, you get a porous laminate, because entrapped air will expand to the maximum. (using the usual non perforated release film it’s very unlikely that any air/moisture will pass through, exept on the edges of the film)
2)In case of a leak, you could face problems with the other bags connected to the same vaccum source, and lessen the chance to minimize damages to the part because more airflow is let into the bag.
The worldwide standard procedure is to vent the inside of the bag to the atmosphere after reaching around 2 atm or leaving 0.2 atm vacuum if your autoclave is equipped with automatic leak detection system.

If you don’t believe it, make youself a test making the same part with the two technics and try to paint them…:smiley:

#13

I guess like on most maps New Zealand isn’t part of the world. The physics just doesn’t make sense that under vacuum a bubble will get bigger than when open to atm. Otherwise by that logic when curing without an autoclave you are better not vac bagging.

I guess we shall agree to disagree. And since tomorrow is my last day at this job I’m not going to think about it any more .

#14

Simply not true!

I have processed thousands of non porous parts and have conclusive laboratory analysis results that state the void content was undetectable and quantified it to be less than 0.1%… All using Vac and Autoclave.

You are much more likely to need to improve your debulking schedule, review the vac stack or increase your ramp rate/ change the cure cycle if your getting porous parts!

Any air (voids) are usually trapped between multiple plys of fibre. The Vacuum cannot act upon it directly, instead when you apply Vacuum the weight of the atmosphere compresses the bag/stack and the laminate in turn. The void is subject to this compressive force which makes the void smaller as air is a compressible media.
When you place the part into an autoclave, you are able to then increase the pressure above atmospheric, increasing the compressive force on the void. Now as you add heat and the resin becomes liquid ( liquids are incompressible) the void now much, much smaller can pass out easier as the resin flows into the stack ( if your using a perf film) or it stays in its compressed state cured in. Gases flow from high pressure areas to low so increasing the low pressure side by adding vacuum is only a good thing.
Ultimately- The best way to get a good quality laminate is to not trap air in the plys during lamination.
My 2 cents!..

#15

You are leaving a nice facility. Are you staying in field?

I am wondering if autoclaved parts are being post sprayed with clear ? Because those of us post spraying Infused parts will certainly find micro pores in the tow if any exist.

If a claved part were post sprayed, would it also ?

#16

I’m sort of staying in the field only much smaller fibres, I’m starting at Revolution Fibre who manufacture nano fibre.

We often clear coat and/or paint and we have no problems with pin holes. We have a customer that we just cook his parts, who now only uses autoclaved prepreg for that very reason.

Astute:
I agree, the ramp and dwells are so important for quality. I always have a dwell at a point where the resin is liquid but not gelling (80deg for a 120deg cure system) to allow for what I call a “hot debulk”. If you ramp too fast and the outside cures before the centre and you will get voids.

#17

Absolutely!

Generally, for monolithic parts I have found that people ramp much too slowly, 0.2 - 0.5C/min. This has a detrimental effect on the resin as it begins to gel too early.
Ramp as quick as your part/tool will allow to your target temps ( say 80C @ 1-2C/min and this will drop the viscosity of the resin more than a gentle warming of 0.2C/min ever will.

Laminate thickness, surface area and mould materials all play a part… This is where skill and experience are needed to tweak your cycle to avoid the common pitfalls.

Good luck in the new job BTW!

Jim.