Step 3: Method#2- lost foam molding

Picture of Method#2- lost foam molding
I used this method to create an air duct for the oil cooler on a friend's Porsche GT5R racecar. This is a very quick and easy way to mold a hollow component or a part that has severe undercuts that would not alllow it to be molded in a single piece female mold.

The first step is to cut a pattern from blue insulation foam and wrap it with clear packing tape.

Next, cover the taped pattern with a mold release wax.

Now cover the pattern with woven fiberglass cloth and wet out the resin. Make sure to leave a section uncovered- this is where the foam will be melted.

After the resin has cured, melt out the foam by pouring acetone on the uncovered section (note that in drawing #4 it says kerosene- this is incorrect). Make sure you have a decent sized tray or bucket under your part- this will get messy! As the foam melts you can reach into your hollow part and pull out the packing tape and you are left with a hollow molding.

manuel.roales5 months ago

thanks sir for being being generous in sharing your knowledge. such a big help to someone starting a project. THANKS again sir!

stevo675 years ago
Honus you are a huge inspiration, I love those sketches, and thank you for your most helpful and detailed instructional. I am going to hook in and make my own panels for my circuit car. Cheers from Down Under.
Honus (author)  stevo675 years ago
Thanks! Make sure to post pics when you're done!
Bergzee245 years ago
You mention using clear packing tape to seal. Is there a reason why you use clear over the brown packing tape?
Honus (author)  Bergzee245 years ago
I use clear because it's non absorbent. Brown packing tape would soak up resin and it's also not as flexible.
dk-info5 years ago
 Just came across this instructable, looks great, I am trying it this weekend. What did you do with the Acetone/foam sludge? Soap and water cleanup?.

ferrous5 years ago
I was taught to coat the foam with thinned down white-glue as a sealant, then heat-wax as a mold release... do you think the packing tape method is superior? if so, why?
Honus (author)  ferrous5 years ago
It doesn't really matter as long as it's sealed.
R4NS0M5 years ago
I agree with everyone! This is one of the clearest and most concise instructables I have come across so far. I'll be honest, I wasn't searching for fiberglass molding methods when your instructable surfaced, but now I am anxiously intrigued and looking forward to experimenting! thank you very much! :0)
latemtech6 years ago
I been thinking about doing fiberglass project for a long time and never got around to it. then I started getting the materials I needed to get started on a fiberglass project. I decided to make a hood scoop for my mazda millenia. It was to have a venturi tube inside (cone shaped) and I couldnt figure out how to cast one until I came across your instructable. Thank you so much for this! One question, do you use any syraform? and where do you get it? I be sure to post an instructables on making your own hood scoop, that for sure!
Honus (author)  latemtech6 years ago
Do you mean styrofoam? I use blue or pink styrofoam purchased from Home Depot.
pantonpride6 years ago
This "how-to" has probably been the most helpful one I have seen to date! Thank you SO MUCH for your in-depth illustrations and step-by-step instructions. I have wanted to play with fiberglass for so long but have had no success on finding someone to teach me. Once again, thanks man!
Honus (author)  pantonpride6 years ago
Thanks- glad you like it so much!
BluesRider6 years ago
Shortshift,I'd like to see a tutorial on making a bike tank from lost foam.That's a project I've been considering.Thanks!
shortshift7 years ago
Hey, cool that you posted this up, I like your illustrative sketches a lot, they're clear and simple to understand. I would encourage you to break these techniques into separate instructable sections just to make them more readily searchable. While they're all related they're also quite useful in and of them selves. Anyway, I was going to make a lost foam page for a motorcycle tank, but you beat me to it :) One challenge I've had is coming up with the correct patterns for cutting up glass when laying up around complex shapes. That air duct for instance; what were the glass shapes that you chose for the "crotch" section where it bifurcates into two? Seems like this could be a new instructable topic altogether... Thanks for posting.
Honus (author)  shortshift7 years ago
Thanks for the comments! As for figuring out how to cut cloth for complex shapes you can first cover the area with masking tape and then cut that into sections and peel it off your pattern. Then flatten it out to give you some patterns to cut cloth from. It's not perfect but it works reasonably well.
mf708 years ago
This is a wonderful idea, but... KEROSENE? I've never seen any foam product that would dissolve in kerosene. Mind you, I HAVE melted foam with solvents by accident. The solvents have generally been really "hot" paint additives. Mark
Honus (author)  mf708 years ago
You're right- I forgot I used acetone when I did this. Don't know what I was thinking.... I'll change it- Thanks!