advice: making a mold to withstand oven/autoclave temps?

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#1

Hey there gang,

I’m currently manufacturing cosmetic carbon fiber parts using the VARTM method, but am getting tired of several aspects of the process. (Highly skilled technician requirement for carbon layup, high potential for parts with air in them, slow layup times, etc…)

So, I’m looking to make the move to pre-preg.

I’ve asked several of my suppliers and none of them seem to know anything about anything other than selling me what they have in stock, so what I need to know is what do I need to do to manufacture a mold that will withstand the temperatures required for a headed cure cycle.

Let’s say the tools need to withstand 320* for 4 hours just to build in some overkill.

If anyone has any prepreg tooling and or layup advice, I’d be very appreciative to hear your thoughts.

Also, I’m thinking the best way to go may be a curing oven vs. the absurd expense of an autoclave.

Thanks for looking,
Alan

#2

Yes infusion is a pain at times, has some advantages but, pre preg is much easier in most respects. Especially in the control and ease of cutting. And there are lower curing resins available, like 275F range or even 180F.

So for tooling… You can use metal of course, more expensive but reliable and long lasting. There is composite, which have matching CTE and also good lobgevity. And you can do foams, which are much more fragile but relatively cheap. Another option is plastic faced plaster which are cheaper yet than foam, but also somewhat fragile. Surely there muxt be a few other options but these come to mind as my first choices.

#3

For autoclave use, I make carbon infused moulds, or prepregged ones. It depends by their size and shape (and budget…).

This is a mould for a Chevrolet Camaro EuroV8 Trophy I made recently by infusion with MTI hose and Axson 2035 resin.

https://www.dropbox.com/s/rojtcdrrouav3kh/2014-02-01%2020.13.48.jpg

https://www.dropbox.com/s/xz146dw634r8gt0/2014-02-01%2020.13.26.jpg

#4

The best way to go is an autoclave if the budget will stretch that far.

A good way to make composite tooling is as follows:

Machine an epoxy tooling board to the required shape (same as component). Seal the board with paint or board sealer. Release the board.

Lay up and cure a tooling prepreg (these cure from about 100°F upwards) a long cure time is likely required. Demould the tool from epoxy tooling board.

At this stage the tool is in a partial state of cure and will not have the required temperature performance therefore a freestanding postcure is required. Heat slowly (40°F/hour) to just below 400°F and hold for at least 4 hours allow to cool. Your tool should now have temperature capability approaching 400°F.

In terms of lay-up a standard tooling lay-up is the so called 1-8-1. 1 layer of a lightweight fabric, 8 layers of heavyweight fabric (rotated to be isotropic and produce a balanced laminate and cte) followed by a layer of lightweight fabric.

The companies selling tooling prepregs usually have processing guides to help with the step by step process.

If you are curing in an oven you will likely need a tooling gel-coat as surface pits will be an issue. Regular debaulking is also important as are gloves (tooling prepregs are usually sticky).

#5

I’m relatively uneducated on this end of the spectrum, but clearly recognize it as the next step in growth for my business.

That having been said, I can afford a DIY curing oven today, whereas I can’t afford an autoclave, even used. The molds I need to use are up to 80" L and 41"W and I’d like to cure 8 -10 molds per session, per day. (up to two sessions per day, once I can grow into the need.)

Right now I’m using polyester resin and orange tooling gelcoat to make molds for Vacuum Infusion.

I was hoping there was a chemical equivalent so I could continue making molds the way I do now. If I need giant blocks of metal and a CNC machine, it pretty much stops me in my tracks. We’re talking potentially hundreds of thousands in investment I can’t shoulder right now.

#6

I think we’ve done well, given the limitations of our ability to produce. (Vacuum infusion takes a specialized tech, as you all know too well.)

If I can make the move to prepreg, I can use a less skilled tech and turn it into a full production facility.

#7

Huntsman makes high-temp epoxy gel coat and tooling resin, for making glass molds (as normal), but used for prepreg. Hexcel and I think Cytec both have tooling prepregs, that you can machine a part in tooling board, coat, shine, and then use the tooling prepreg (Hex Mat? i forget) and lay up. You then seal and use that as the tool. I’ve seen some that are machined after layup (maybe so avoid a matching master mold…just lay up this stuff, cook it, and then machine later.)

#8

I currently work at a milling facility (mouldcam europe) where we produce moulds to be used with prepregs, Tg of about 80 degrees celcius. That is enough to cure the prepregs to about 65-70 degrees. After demoulding the prepreg laminate gets a free standing postcure, so the ultimate Tg of the prepreg material is reached.

I have made prepreg parts the same way, with a free standing postcure, because the moulds could’t have more heat. Bagged carbon moulds too btw. (a small percentage of air, which you will have with bagging, isn’t really an issue with out of autoclave prepreg curing.

The simplest way I produced production moulds for OOA use was with an epoxy tooling gelcoat, followed with an high temp. epoxy (sicomin SR1660)
Hand layup, vacuum bagged. Followed with a moderate post cure up to 60 degrees celcius, demould, and a (lengthy and slow) freestanding postcure to ultimate Tg. All with carbon fibre to match the product (also carbon) properties. For glass products I woud have used glass moulds.

I produced OOA prepreg moulds too, it’s much faster when the mould can take the heat anyway, because you don’t need the lengthy postcure.

If I can make the move to prepreg, I can use a less skilled tech and turn it into a full production facility.

Not necessarily true. Once you have the whole process up and running it’s quite easy to train personnel. But to get the whole process up and running takes some time and knowledge.

For instance: your personnel doesn’t need to know what the different fibre orientations do to part properties, or ease of layup. They only need to cut to the templates, and know where and how to put them in the mould.
If they don’t do it right (because the it isn’t clear or so) they will have troubles with laying up and part quality. But is is an easily simplicated process!

#9

Thanks everyone. This information has been very useful.

The prepreg we’re looking at cures in 1-2 hours at 310°F. (160°C)

Right this second, I can’t afford CNC’d molds, so getting it right chemically is something I need to get right the first time.

#10

You mean 310 for the tooling prepreg, or the part prepreg? In the last case, I would check if I could use something lower temp, to ease mold production.

Anyhow, whether being prepreg or infusion, you will need to train your personel, and provide them with clear instructions.

Once you cut the process in small parts, you can use relatively unskilled workers. Make a “cookbook” which shows all steps. Takes you a weekend or so, but worth the hassle.

#11

You may wish to look at this page from PTM&W. http://www.ptm-w.com/index.asp?pgid=5 There is a lot of good technical information there on mold making. They also have many resin systems which will work at the temperatures you referenced.
Have you considered a lower curing temp prepreg? Most of the advances in prepreg technology are in the 250f and lower range. There are many advantages to lower cure temps.
Also be prepared for a learning curve with prepreg. It is not as simple as dropping the material into the mold and getting a cosmetically perfect part.