Pressure indicating film with moderate temperature tolerance?

[JCain]

Hello,

I'm dealing with a thermoplastic composite hot-pressing operation, and I want to be able to quantify applied pressure as a function of location across the surface of my composite plate.

Sensor Products makes a nice film product called Prescale that does this very nicely with ink-filled micro-capsules at temperatures up to 95F/35C. But my press operation runs at 270F/132C, and at that temperature the Prescale measurement just clips out completely.

Does anyone know of a product that will operate at pressures up to at least 7ksi/48MPa (preferably 10ksi/69MPa) and temperatures to at least 270F/132C, on a surface at least 22"x22"/56x56 cm? I am open to use-once products like the Prescale, or some kind of paint; and I'm also open to a reusable product with or without real-time measurement capabilities.

Thanks.

 

[rstupplebeen]

Unfortunately, I am not aware of a product that fits the temperature needs. However, I wonder if you can use the film on a cold press under these scenarios:

1. Cold press uncured laminate with pressure film
2. Place cured part in press and cold press with pressure film
3. Measure the fiber volume of the part at various locations
4. Use FEA to predict contact pressures

I hope this helps.




Rob Stupplebeen

http://www.optimaldevice.com

https://sites.google.com/site/robertkstupplebeen/

 

[JCain]

Thanks for the response, Rob.

I've tried option 1 before, and it works okay, but I'm fairly sure that the pressure distribution at temperature is different than when pressed cold, due to increased compliance and migration of the matrix. However, it absolutely has been useful for getting a sense of what's going on.

I've tried something similar to 3, by measuring panel thickness. This gets me a good qualitative sense of pressure distribution, but I don't know how to reasonably quantify it with any accuracy.

Option 4 would be a good option, but I'm not a modeler, and haven't been able to interest/cajole/bribe any of the modelers in the organization to take this up.

However...option 2 sounds very promising. I think I'll give that a shot and see what happens.

Thanks!
-Jason

 

[Compositepro]

You really have to be careful in using and interpreting the pressure sensing film. It is true that various micro-balloons break at different crush pressure, resulting in more or less color. But are you really sensing pressure, or just variation in gap between two parts as a function of the compressibility of the film? If the pressure variability is due flatness variation a rubber sheet will compensate create a uniform pressure. If you truly want to know flatness variation, then a better tool is something like Verifilm, which is a modified adhesive film that cures in the gap but does not stick to the parts. It creates a map of the gap variation after you remove it from the gap.
The point is that in many cases, it is the gap variation that is the independent variable. The resulting variation in contact pressure depends on what is in the gap. Pressure sensing film may have a very different pressure map than the actual materials that you are processing, and therefore, may not be very meaningful.

 

[JCain]

Usually I'm using a 1/16" sheet of silicone rubber already, so gap is not an issue. But even when not using the rubber, I'm measuring pressure while pressing a polymer/polymer composite (read: compliant) so there are no gaps. So the film is indeed indicating pressure as desired. I just need to do it at temperaure.

 

[JCain]

CompositePro: I just reread your post and realized I had not made things clear regarding my process, including in my response. Sorry.

I'm using the pressure film in the press while the unconsolidated composite laminate is in there too. So I'm placing the film between my compliant part and the steel caul plate/press platen (or the rubber pad when I use that), and am NOT placing the pressure film alone between the cauls.

We have some of that gap measuring paste in the lab, but I'm not concerned with those sorts of issues; I only care about the applied pressure distribution.

Hopefully that makes more sense of what I wrote. Thanks for replying.

-Jason

 

[Compositepro]

Okay, but if you are processing a plastic material it should flow until pressure is uniform every where. The film is telling you what the peak pressure was at each point though the whole process, not at any particular time. I'm just wondering how does that help you. I'm really curious, because since I first saw the film 20 or so years ago, I can't think of how it would be more useful than other less expensive methods for my applications.

 

[JCain]

So it's a polymer composite, but the reinforcement fibers are crazy strong and stiff (for a polymer). Yarn moduli are well in excess of 1000 cN/dtex. So while the matrix undoubtedly flows some, and the fibers are known to shift around some, it isn't a monolithic block of polymer that can flow around as it pleases. If it were, it would squirt out the sides of the press, because this is just a hot press, not a closed mold. So the pressure doesn't equalize across the face as you might expect. In reality, we see lots of small scale (~1cm) variations in pressure on the order of 2-3x due to local inhomogeneities of the material itself when we don't use the rubber sheet, and we see pressure gradients of 2-3X from center to edge when using the rubber sheets, which was surprising but is backed up by evidence from other measurements, including modeling; it isn't a function of alignment or gap issues but the nature of how the silicone rubber sheet flows at 5ksi and basically goes into hydrostatic compression near the center of the plate, and a mix of hydrostatic and deviatoric near the edges. Or so the modeler tells me as I smile and nod knowingly.

Anyway, I'm rambling. You're right that the film records the peak pressure over the press cycle, and that's another problem that I have addressed by the simple expedient of assuming the stress redistribution is small enough that the results I'm getting are still meaningful. This is probably a reasonable approximation, though not exactly true of course. So basically I'm running cold presses with the film and assuming that the pressure distribution is grossly similar to what is seen during hot press operations. This is obviously not totally accurate, but it's the best I can do at the moment, and in reality I think it's actually a surprisingly good estimate given the nature of this specific material (it wouldn't be for many other materials like homogenous polymers.) Essentially, even though the matrix softens or melts and wants to flow, the fibers stack up in contact with one another vertically and carry load through the plate, and don't flow like the matrix.

-Jason

 

[Compositepro]

All of what you say is true and can be deduced from logic alone. If you are press curing a laminate the goal is generally to get a flat part of uniform thickness and smooth surface. In that case one would not use a rubber pad because you want the high spots to be squeezed down and the low spots get more resin. You will see vastly different results between cold and hot prepreg.
So how does this information help?

 

[JCain]

Nope. First, I'm not curing anything whatsoever; as I said, these are thermoplastics. It's simply a melt/freeze cycle. And I could not possibly care less about a smooth surface.

Second, if all I said could be deduced from logic, we wouldn't be here now, inasmuch as you have produced nothing that is germain to the subject I raised. With respect, I don't think you understand the situation.

Third, I'm guessing "generally" applies to production situations. I work in R&D for a national lab. My output isn't widgets per hour, but refereed journal papers per year. My mandate is to understand the micro mechanics involved in processing these materials on a scale of micron.

We use a rubber pad, precisely because using one does NOT cause the "high spots to be squeezed down and the low spots get more resin." Not least because there is no "resin", because these are thermoplastic systems.

I encourage you to google recent papers from O'Masta, Deshpande, Wadley, Fleck, et al (especially O'Masta's paper from late '14, which is absurdly good). Some are DTIC, but most are public. Those guys are ridiculously proficient and crazy smart, and will explain, better than I can, apparently, the difference between pressing some PC or other monolithic polymer, and trying to optimize pressing of a highly engineered composite material.

Cheers.

 

[adfergusson]

Sounds like a really interesting experimental challenge.

A bit off topic (bear with me) but there is quite a big a metal forming group at Imperial College in Mech Eng. They do a lot of work on creep age forming of Aluminium alloys, for which they use temperatures around about what you're using (thats not a mistake, they do use some really low temperatures for this forming method). If there was a pressure film solution for this sort of thing I'm fairly confident that they would use it and that I would have seen or hear about it.

However, what they do frequently use is an optical metrology system called Argus by GOM. I've not used Argus but am familiar with GOM's Aramis product which has a built in feature to allow for the comparison of observed strain fields with prediction ones from Abaqus. The program will produce a 3D surface map showing deviation between of the strain fields Aramis observes and what Abaqus predicts. I'd presume that Argus does similar and provides more of a metrology type platform aimed at the comparison of results with CAM data rather than FEA results (although they do a lot of this too). If your project budget can stretch for it then it might be worth seeing if there's a company or research group around that can do a couple of days of experimental work with you and using such a system? Such a system won't directly provide you with pressure distribution and evolution data but it should help in inferring whats going on.