Rapid Prototype Molds

General Composites Discussions Composites Talk
#1

Hi everyone…

I’m a naval architect by trade… composite engineering primarily. Feel free to hit me up with questions.

For a hobby project, I’m making some molds for room temperature, vacuum cured carbon-epoxy laminates. I’ve got the part and the tool designed in solidworks, and because it’s a complex part, I want to have the molds cut by NC means.

I’m looking into Rapid prototyping as an alternative to 3-axis milling. Has anyone got any experience with making molds this way? Specifically, what rapid prototype material is best? Will reinforced ABS bond to duratec for mold release surface finishing??

Anybody that can help, it would be most appreciated.

Cheers,

Redbeard.

#2

Many polyesters bond to ABS. I would ask for a sample piece of the ABS used, then do a small test with Duratec. You might need a bonding layer, but that is not sure.

How large are your parts? RP still is only suitable for smaller scale products, and milled parts can be a lot bigger.

#3

Thanks, Herman. Good answer. The parts are within the size range they can do. 24" x 10" x 5" outside dimensions.

I’m talking with Redeye about what would be entailed to RP the parts in their strengthened ABS material. I’ll ask them for a sample. If anybody has any first hand experience with the company or making tools this way, please let me know!

#4

What are you planning on using for your resin? Any plans to postcure?

#5

I’m not very strength limited in this case. I was leaning towards a readily available consumer-end resin… maybe pro-set? Matrix? If I can find an easy supplier for the low volumes I need, maybe Ampreg 22.

I hesitate to call what I am planning a post-cure, though technically I think it qualifies. I was planning to do a long slow ‘cook’ at about 100-120F just to ensure full cure and strength of the resin after assembly (two halves). But it won’t be in the mold at that point, so it won’t impact the decision on the mold making. Is that why you ask?

#6

Free standing postcure can be done, but be careful. make sure you have the available data on the datasheet, or at least a Tg development table vs time.

In general, a 5 degree C per hour temp rise for large parts (boats and the like) or 10 for small parts is doable. After that, keep the temp at elast 18-24h at the final temperature, which should be 10-15 degrees C below ultimate Tg. This way you will reach or almost reach ultimate Tg. Never ever cure over ultimate Tg, this will distort the part.

#7

Herman is right on- you need to know and take into account the final temp of your parts including heat soaking and abuse loads (aircraft parked on tarmac in the desert, manifold located above hot engine, kayak left in a trailer in the sun, sporting good left in a hot car trunk, etc.).

Also, plan on some sort of fixture for postcuring. You will be close to the Tg and the assembly will creep and distort if you don’t.

And one final thing- if the parts must be very accurate or nest tightly with other components, then make sure to take into account the thermal expansion differences of your tooling and your carbon parts or you might be unpleasantly surprised. That’s why for elevated cure precision components carbon tooling is typically used.

#8

Hi Redbeard, Re Ampreg 22, I use Ampreg 22, 21 and 26 on an almost daily bases. You might like to consider the Ampreg 26 which has has similar TG properties but would be much easier to work with especially with carbon as its mixed viscosity (cP) is almost half that of the 22 across a wide temperature range which makes the wet out of fabrics quicker and easier to control if doing hand lay up. TG wise there is very little between them both each having marginally stronger values for different post cure schedules. The Ampreg 22 hardeners are tinted while the Ampreg 26 gives a much better clear finish when cured if this is required.

The Ampreg 26 is only slightly more expensive but the manufacture sells here in the UK in 6.6kg and 20kg packs. Which are handy sizes to work with and transport.

I also use Ampreg 26 with the slow hardener for infusing its 251 cP at 30 degree C has given good results and a 5hr (same day) de-mould time.

You might find these datasheets of interest re Tg development table vs time temp cure/post cure etc.

http://www.gurit.com/files/documents/Ampreg_26_v11.pdf

http://www.gurit.com/files/documents/ampreg-22v17pdf.pdf

#9

Thanks for all the info guys. I’m making a musical instrument, so the highest temp they will hit is 120F if left in a car or something… Light and stiff are great, so Carbon is a good choice, but like I said, I’m not even close to marginal on properties.

Wight, thanks for the resin input. I’ll see if I can get A26. Sounds perfect for what I’m doing. For your artistic composites finished clear, do you use an in-mold clear coat of some type? Or do you just finish with poly-urethane or similar after?

#10

Hi red my best results have been with an automotive 2k clear applied after moulding. For your project I would ponder on the need for a finnished clear coat product. As a musical instrument would lead a cosseted life protected in a nice case when compared to automotive or marine parts which live outside and spend their lives exposed to harsh uvs. Try buffing the resin finish up after its cured fully.

#11

<big grin thinking of musical instruments used during acid rock concerts>

#12

Thanks, I hadn’t thought of buffing the epoxy… I’m experimenting with a prototype now that I pulled off a male tool, no bag. I had to fill the weave with epoxy in several places. So I’m now trying just a Minwax clear semi-gloss poly which I’m comfortable with from wood projects. The semi-gloss avoids fingerprints, but still gives some shine. I suppose you could buff to the desired amount of shine… an interesting thought.

#13

Harrisonaero, you got it one. This is a very very unconventional electric instrument.

#14

Well you better go with a carbon/aramid blend for fracture toughness… and if they light the thing on fire ala the infamous Jimi Sacrifice you will have to consider that too.

It’s funny how many structures end up getting designed around abuse loads. Think stiletto heels on a lightweight aircraft floor panel.

#15

Well, if they light it on fire, at least I’ll get a burn test validation of my process. They can tell me my fiber fraction. :slight_smile: (Geek joke!)

My prototype isn’t even vacuum bagged, and I can’t break it over my knee. Pretty sure I’m not going to have a problem!!!