Friday, November 9, 2012

First Resin Casting

Following up on my two previous posts on making Silicone molds from OOGOO, I actually managed to cast a part today.  I ended up with mixed, but promising results as you will see. 

I started by cutting risers and sprues into the mold from my previous post.  Resin is poured into the sprues and air/resin escapes through the risers.  After the pour has finished, both the sprue and riser serve as reservoirs of excess resin to counteract shrinkage of the poured resin as it cures.

I used West System epoxy as a casting resin, which it is not specifically intended for.  It is however locally available and relatively reasonably priced.  Epoxy is also one of the least objectionable resins, much less stinky than polyester resin and apparently has substantially less shrinkage.  It's also easy to get the proportions right if you buy the pump kit.  But on the downside, it's meant for making boats, not casting.  Oh well. 

I don't have any pictures of pouring the resin, since I didn't want to gum up my phone, but in the photo above you can see the mold with mounting hole rods inserted and sprue/riser filled with epoxy.  I used vaseline as a mold release, especially thick on the metal parts since I was concerned that the epoxy would bond to the posts and scrap not only the part but the mold as well.  After pouring I plugged both holes and tumbled the mold by hand, hoping to get epoxy into all the nooks and crannies.  I failed, but more on this later.  I was concerned initially since my excess resin hardened quite quickly, but the resin in the mold appeared to harden much slower, judging by how gummy the resin at the top of the sprue and riser was.  So I left it for about four hours and luckily everything seemed solid when I checked it.

Here's the bottom half of the mold removed.  Unfortunately I ripped off the bottom half a bit too vigorously and tore the center plug.  Anyway, it was only a first try.

Here you can see the part almost completely separated from the mold, the only bit remaining is in the central hole which is the bit that I accidentally ripped off.  The vaseline also worked really well as a release agent; a quick easy twist of the posts with some pliers and they slid out cleanly.  This photo also shows the pegs left by the sprue and riser.

I then cut off the sprue and riser and filed the surfaces quickly to clean up the filament marks from the mold.  The bottom half of the part turned out pretty well, with only a few small bubbles.  However the piece looks pretty terrible due to the residual rust from the central pegs being oxidized by the Oogoo acetic acid as well as the yellowish color of the resin, although the photo makes it look worse than it actually does.  I was pleased to see that the accuracy is excellent; after a light sanding of the central hole, the linear bushing I'm using slides in perfectly with no play, although it might need a little dab of glue to keep it seated while in use.

The top portion of the part did not turn out so well due to trapped air bubbles.  The chunks missing in the photo above are all due to trapped bubbles, some of which are over 5mm across.  That's quite big for this part, which is only about 40mm on a side.  The white flecks are dust from the quick filing that is caught in the open surface bubbles.  This part is good enough to be usable, but I will try to produce a better quality version.

I realize now that the way I designed the mold was not the best.  Rather than have the Ooogoo fill the central hole, I could have put a patterned blank in as was done for the mounting holes, making it easier to separate.  Additonally, since this part only needs a single true surface, it could have been case in a single-piece mold which would have allowed me to directly see which areas were not reached by resin and also to intervene.  Having this ability would allow me to avoid the spoiled corners that can be seen in the last photo.  I would also spend more time on the cosmetic details of the pattern since every minor surface flaw was transferred to the mold and then to the finished part.  While this doesn't effect the function of the part, it does annoy me.

As an experiment, I consider this to be a success.  Ugly though it may be, this part MUCH stronger than the original printed pattern.  It is also dimensionally accurate with good surface reproduction, leading me to believe that Ooogoo molds with epoxy resin is a viable method for producing small-run parts, if one that requires some practice.  I expect the next attempt will be considerably improved. 

1 comment:

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