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Old 06-18-2014, 08:06 AM   #1
kyleblake

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Default Autoclave Questions

After doing quite a bit of research I have come to the conclusion that I am going to use an autoclave method for curing my prototypes.

I have found countless video demonstrations and articles detailing how to do RTM setups and curing methods, but I haven't found many providing great detail on Autoclave methods.

First I have a few questions on the autoclaves used for curing composites:

Do they pressurize or create a vacuum? Are the moulds bagged when put into the autoclave? Are pre-pregs only used? or can I put a wet layup or an RTM into an autoclave to help achieve a higher CF to resin ratio?

The product I am trying to prototype will require a male and female mould. Do these have to be made from metal when using an autoclave?

I am trying to prototype a music instrument neck; guitar, violin, bass guitar, etc. I have many ideas on different approaches, but I am fairly certain in order to achieve the quality I desire, an autoclave will be necessary.

Accessibility to the autoclave isn't a major issue as I work in an industry where access is available. I just need to get on the right track on getting my tooling created properly.

Any "I did this already and learned the hard way" type of tips would greatly be appreciated!

Thank you!
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Old 06-18-2014, 08:41 AM   #2
sammymatik

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Auto claves are pressure vesseles/ovens

the parts are bagged and vacuumed. Prepregs are used in autoclave. You woulnd't do a wet layup for autoclave. RTM molds are clam shell type made from metal and machines to the strict tolerance of the part dimensions, pressure is applied from a press, heat, and the injection of the resin. They are not intended to be used in autoclave and wouldn't gain anything from autoclave since RTM can be higher presure than autoclave.


A mold for the auto clave would not be male/female. It would be an open faced mold, and then bagged.

Musical instrument? Sounds interesting but, I think you might want to do out of auto clave. Autoclave is very expensive compared to just an oven. Parts that are cured with vacuum are very high quality as well. If you do need more pressure than you can do a silicone mandrel and two piece mold, or as you suggest RTM. But RTM is very expensive as well. The advantage to RTM and two part molds is that you get a mold surface finish on all the exteriors, meaning less post finishing.

Why do you think you need to use an autoclave? It is cool that you can use one for free, and that alone may be justification for it. Though you can get fine quality without it and other instrument manufacturers do their process out of autoclave.
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Old 06-18-2014, 08:54 AM   #3
kyleblake

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I have been told by someone who has been making instruments from CF for 35+ years that an autoclave is necessary to achieve a high enough CF/resin ratio for the instrument's neck to be stable enough for the application.

I am not opposed to other methods. I do need to have a precision part right out of the mould though.

Just so I understand, the part is bagged and vacuumed, then pressure is created inside the autoclave?

If I don't have a clamshell or male/female type mould for autoclave, how do I get a perfectly flat surface on the "open" part of the moulded material? just use a stiff and heavy "bag" for the top? could a waxed machined metal plate be placed on top of the part, then the bag go on top?
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Old 06-18-2014, 09:49 AM   #4
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To answer you last set: Yes, the part is bagged, vacuumed and then put into the clave. The clave is then pressurized (with N2) and heated.
The only way you can get a good secondary surface is to have a caul plate. Now, despite what Sammy said, you CAN use a 2 part mold. This will give you both surfaces perfect. The pressure in the part will NOT be hydrostatic, and will just compress the 2 parts, which can help, but for a complex shape, not all parts will see the same pressure!!! That is the beauty of the autoclave....hydrostatic pressure. All parts see the same pressure in all directions.
You can TRY and get away with a flexible top caul plate. One that will not allow print through, but can be flexible to be bagged, and then the autoclave pressure will put the pressure on the part evenly.
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Old 06-18-2014, 10:14 AM   #5
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Or if the part has an opening and is bagged... then you could see the equal pressure in the clave by allowing the pressure into the mold. Not sure if heating evenly would be an issue at this point, but air flow would be greatly reduced.

or just use a heated press if you have access
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Old 06-18-2014, 11:52 AM   #6
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I have thought about trying to put together a multipart mold from machined metal that would have its own heating elements/thermocouples and it would be sealed to allow vacuum to be established and held inside the mold during curing. I worked in injection molded plastics years ago so I kind of have a vision on how it could be put together, but my fear is it could take me 2-3 (or 25!) generations of the molds to have a perfect finished product! This could amount to a great expense.
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Old 06-18-2014, 11:57 AM   #7
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I agree... metal is awesome, but expensive. I guess that depends on how many you plan to make. for 1-10... not worth it. For 500 maybe

But you need the money up front. Plus if you make a mistake...

Though of course you can prototype it with something cheaper first, like foam. The tooling foam we use is pretty good stuff. It works fine for vacuum but I'm sure it wouldn't be useful for any sort of pressure.

as for a heated tool, yes it's nice. But you can also make a tool that is bolted together then put into a press or oven. We use both methods and they work great.
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Old 06-18-2014, 01:30 PM   #8
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Ok so I could make a metal clamshell (with insert because I want the part to be hollow) that is sealed completely and left in an oven to cure while the mold is internally depressurized? I am thinking I can make the mold so it just bolts together with heavy machine screws.

Is this a form of RTM? Is the resin injected while inside the oven? Is the resin heated? I think I want to pack the CF tight into the mold to get as high of a CF to resin ratio as possible, and then inject the resin into the mold. I have heard that prepreg is the way to go though...

We have a tool room here at work, and I have a couple of friends who own tool and die shops where injection molds are produced, I think I can get the molds done for materials only. At least for the 1st prototypes. I have a budget, but not a very good one at this point.
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Old 06-18-2014, 03:15 PM   #9
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An RTM tool is two part with seals and is bolted together. RTM can use high pressure so it needs to withstand the pressures of injection and heating. With RTM you would have carbon preforms that you put into the mold, then bolt the mold closed, and inject, heat and cure.

I have not used or made RTM parts. It's a bit more involved and you need the proper equipment in order to do this process. There are specialized resin systems for RTM as well as carbon that is stiched together as to not be deformed when you inject the resin in.

It sounds like you want to make a part with a two piece mold to get a good final finish. I think RTM isn't the process for you. I'm not sure what shapes you're working with but, something like a guitar, would probably be bonded together after you make the neck/back and then a front. It's possible to do it in one piece, but more complicated. You'd probably be making a silicone mandrel or air bladder. These mehtods offer good compaction but have their own issues.

If it was me, I'd get my molds to be vacuum bagged and then autoclaved (if I had access), these would be dependable and relatively easy compared to RTM. Or else just do vac bag in the oven, which would be fine for a guitar. If you wanted to be ambitious you could make a two part mold but, it sounds like you are on a budget. It's better to do a process you know would work and would work well. Laying up a prepreg guitar should be fairly straightforward and with a nice shiny tool, would look good. Of course if you do use aluminum be conscious of the CTE difference of the metal and design accordingly.

I woudln't worry too much about resin content and fiber ratios. If you use pre-preg you're gonna be around 60/40 unless your prepreg is high/lower in resin content. You can also mess around with bleeder and drain resin. Firstly make a good mold, and then just make some good parts. After you have a good metal mold you can mess around some with the layup schedule to optimize strengths and weights.
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Old 06-19-2014, 05:57 AM   #10
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You can do a composite mold and use it in autoclave, less expensive than aluminun, but less resistant. In autoclave we use both types.
RTM makes sense if you have thousand and thousand pieces. For a few pieces (but not only few..) you can try resin infusion, you need only a oven and a vacuum pump, mold can be less resistant, and you can try to do a male and female molds and infuse between to obtain 2 mold surface finish
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