Foam shaping/finishing - help needed please

[TuskenRaider]

Merry Christmas everyone. I thought I'd use the Xmas holidays to get some projects fished and I'm a bit stuck on one of them. I'm trying to make a popular prop from a well-loved movie (high 5 to anyone who can guess what it is, but no prize since it's not too difficult! ). I'm struggling with shaping the foam. I've sandwiched a 6mm layer of MDF between two 10mm layers of closed-cell polyethylene foam and I need to shape the foam smoothly. I've used a sharp knife to cut as best I can and was then hoping to use sanding tools/paper to sand down the foam to the desired shape and a smooth finish but it seems to be tearing up a bit. I don't know how to explain it but it's sort of breaking down into fibers rather than smoothening out. Can someone please offer me some advice? Am I using the wrong foam, wrong sanding technique (I've tried sandpaper and emery boards etc). Will the fibers smooth out when I take a hot air gun to it etc? I've picked up some tips from Bill Doran/Punished Props but I can't recall seeing this on his videos.

Sorry for the poor pictures but My camera won't take good photos in the dark without getting my tripod out and I couldn't find it.
In the picture below on the left you can see where I've cut with a knife, you see the vertical 'stripes' from where I've see-sawed the knife back and forth. On the extreme right you can see where I've started sanding and it's tearing up...


And then a few more photos showing the sanded areas which are looking a bit crappy


Thanks,
Rob

 

[DizzyStrmtroopr]

haven't seen one of those in foam before..

what you are currently doing is too slow and rough for the surface you want to achieve. for example a pencil eraser slightly deforms before material is removed, this is what is happening. the best thing if you have it, is a belt sander with very fine grit, at high speed.

 

[brandomack]

What grit sandpaper are you using? Its possible it is just too aggressive for this particular brand of foam.

 

[TuskenRaider]

Thanks for both your suggestions. I have a rotary tool which I can try using to increase the speed and I'll use as fine a sanding/grinding drum as I can. I'm using around about 120 grit, I think - it's cheap stuff from my local store which just comes in a pack labelled 'Fine'!

I'm limited with the tools I currently own so I thought a foam version would be easier for me to get the bevelled shape. I sandwiched the mdf in the middle to keep it rigid. As it's a display piece I also wanted to keep it light. I think the original stunt version was made of foam too and judging by the picture below they didn't worry too much about the rough edges!

 

[cavx]

I found that rotary tool with sanding drum is perfect for the rough work, then switch out to a ceramic bit for finer work. Be careful as the sanding disc can "bite in" and ruin your work. Also, test on scrap first, but you can work out the best combo of running with the direction of the rotation of the bit to make it smooth better rather than have it bit in.

Nice Hover Board. BTW, if you want "screen accurate" then the marks left by the blade are perfect. Is this the "hero" board with the flatter tappers? You can see here the marks where they hacked off the edges to shape it and give it a bevel on the edges.

 

[penabled]

Interesting. I suspect the difficulty in getting a nice smooth edge is due to the foam type, it's that soft almost mushy stuff isn't it? If it is craft foam from the craft store then you need to go down to a finer grit...try 400 grit sandpaper, it will take longer but will give you the smooth look. You could soak the edges with CA glue to harden them up and then sand them down once it cures. It might use more than one bottle of glue though.