Foam Molds for Low temp Oven Cure

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#1

Hey everyone, this is my first post here. I have a question about using foam molds (40lb density) for low temp cure prepreg (~176 F). We’ve cleared it with our foam supplier that the foam will work up to 200 F without problem, but we are wondering about the materials we use to do the mold surface.

Traditionally, when we’ve done wet layup, and have used duratec and bondo. Will these two materials still work? We are worried about cracks forming due to the differences in CTE’s when they are heated for the 8h cure cycle. Any experience with this? The molds are about 4’x 8’ so metal is not an option. Any input is appreciated. Thanks!

#2

My experience with PU foam is the huge thermal expansion which creates havoc on your mould surface. Even EPS is for lower temps a better choice, as the forces are much less.

As a mould surface you could look into epoxy resins, but also things like plaster, or Master Works.

#3

How many parts are you planning on making? Any pressures greater than vacuum?

#4

This is the foam that we’ll be using: http://precisionboard.com/pdf/PBLT-8-DataSheet.pdf

We’ll be doing about 3 or less layups per mold, but we’d like to have them withstand more for the worst case scenario. Pressure will be maximum vacuum pressure for higher temperature cures.

#5

First post here too, but if you are using urethane tooling board the duratec will be fine.

However, do not use bondo under the duratec. They are not compatible at temp and the duratec will crack. Either improve your machining/prep before duratecing. We have successfully used laminating epoxy before to fill in small cracks before duratecing.

Hope this helps

#6

We are planning on making at the maximum 5 parts from each mold, and we hope that the molds don’t get beat up too badly in case we want to use them in the future.

We primarily use the bondo to fill in low spots like places that were oversanded. Our shop bot isn’t the most rigid and tends to leave places that need to fill in. I’ll take a look at using laminating epoxy. How does it sand down?

#7

We were having problems even after the first cycle. The laminating resin was more a quick fix that we did last year. Took sometime to sand, and was not the best option, just the most readily available. Duratec does offer a VE mold repair putty. I have some coming in myself next week. I can report back to you on how similar it is compared to bondo and whether we have any problems with it cracking the top coat.

#8

For the amount of time and hassle sometimes required for PU moulds, I really think it can be just as advantageous and cost effective to machine from aluminium: Particularly if the parts arent overly big.

#9

RITSAE - That’d be great, thanks!

findhan - I wish we could machine it out of Aluminum, though the parts are 4 feet by 8 feet (1.2x2.5 meters) so the logistics of machining something that big in metal and let alone buying a chunk that big (if it’s even possible) on a college student budget is daunting. I definitely agree that it’s a hassle, but I think it’s our only option

#10

Not sure what you are building… But would it be possible to make your foam mold , vacuum bagging plastic over foam mold , lay your part up and bag it again , put it in the oven. :slight_smile:
Very simple , effective way of creating parts with out surface coats. Plastic gives it a reasonable Finnish. There are bagging films out there that go up to 600’F so your low temp pre preg will suit. :slight_smile: and it the cheapest easiest way for a small budget :slight_smile:

Tim