Saturday, April 13, 2013

Yuck, sanding.

One of my least favorite tasks associated with building vehicles is sanding and bodywork.  To me its a dusty mess that magnifies any slight mistake or shortcoming in technique.  Getting these plastic parts from a rough RP surface to something with a decent finish requires naught but time, sandpaper and elbow grease.

I left the parts after a first round of sanding and filling primer.  They looked decent from 5 feet away but on closer inspection still had decent evidence of the stair-stepping artifacts of the layer-based FDM process.


Any surface imperfection will be faithfully reproduced in the mold, then the wax, and finally in the cast steel part so the idea is to make the surfaces as smooth as possible.

Back to the sanding room with a bit of glazing putty and some sandpaper results in what looks like even more of a mess:
The glazing putty is Bondo-like substance that does not require a hardener and can only be applied in very thin layers.  The lack of a hardener means not continually having to mix up a fresh batch and since I only have to fill in .005-.010 voids the thick layer restriction is not an issue.

Once that dries its time for a pair of gloves and a bunch of sanding boards/paper/sponges.
These parts are small and intricate so the usual auto bodywork tools, designed to do large smooth surfaces, can't be used.  Instead is some small boards, sponges and dowels wrapped in sandpaper to get in all the nooks and crannies.

A few hours later and the parts are starting to look pretty good.

Now its time for another coat of filling primer in Peter's paint booth and an overnight dry.
Below the parts is a hood to Peter's 1969 Ford pickup Cummings turbodiesel conversion.  Hint: making sure other parts are properly covered before spraying primer goes a long way to happy shopmates.  Putting things away helps too but that's been a problem of mine for a long time.



The parts are really starting to look nice.
I think one more round of glazing putty, filling primer, and then a final sand and normal primer coat and they will be ready for a sealing coat of clear gloss.  Then they will be ready to have silicone molds cast and wax injected then on to the foundry!

That's all for now.

3 comments:

  1. Brilliant stuff. I'm really enjoying watching this come together. I wondered if this lost PLA casting link might be useful?
    http://3dtopo.com/lostPLA/

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  2. Cool link, thanks.

    I was aware that the PLA can be burnt out but chose not to go that route. If we only needed one part then a PLA burnout could work but the approach I am taking with the extra step of making a mold for wax allows easy production of more than one part. Also, you can see in his pictures that the finish pattern of the RP machine is clearly reproduced in the final metal part. Some of my parts will be visible and I'd like to have a high quality surface so they look good. Primer and bondo do not burn out cleanly like the PLA does. Without a very clean burnout it is difficult to get an acceptable part.

    There are RP machines that print directly with a castable wax but you get one part per relatively expensive RP pattern. Since I was able to get these FDM prototypes made for nothing (but a nice bottle of rum for Dave) and only am spending my time to get them finished properly, the cost of getting several sets of steel parts is only the casting costs, which are minimal.

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