Thursday, March 10, 2016

Under pressure, part 1

As was mentioned in my previous post, incorrectly labeled "What's new for March 2017" (whoops on the date), I am going to attempt to cast car parts via pressure casting. And as such, I will need both a vacuum chamber and a pressure pot.

There are a few options available for both vacuum chambers and pressure pots. Most of the vacuum chambers can be sourced via Ebay and consist of a stock pot, a large acrylic lid, fittings and a vacuum pump. The pressure pots fall into two varieties: pots designed especially for casting (expensive) and home brew paint pressure pots turned into casting chambers (cheaper but some hacking needed).

After quite a bit of research, I decided to attempt to kill two birds with one stone and make a combination vacuum / pressure pot. My one major consideration was that the pressure pot would be big enough to hold an O scale car side. 

This requirement immediately ruled out the very popular Harbor Freight pressure pot conversion as the size isn't large enough. That left one option (without resorting to a professional model): the Grizzly Tools 20 Liter (5 gallon) Paint Pressure Pot. I was able to obtain this for the very reasonable price of $190.

As shown in the prior post:

One big advantage of the model (aside from its larger size), is the much more substantial clamping system that this pot has versus the Harbor Freight model. The pot has larger wing nuts that clamp down on metal protrusions from the pot, making a much more secure (and safer) hold on the lid.

One problem with this pot, however, is all the attachments that are installed on the lid, none of which I plan on using. All were easily stripped off with a wrench.

The now stripped top, after all attachments and fittings have been removed:

One large problem is the large hole now left in the center ... which of course is not threaded. The other holes are standard size and are threaded (1/4" and 1/2" in size).

After some internet research, it was shown that a 1" black pipe coupling with two caps and a liberal application of sealant filled the hole.

The view after the holes have been filled:

All but two of the holes have been plugged with caps. One hole holds a 60 psi safety / pressure release valve and the other holds the quick connect, a combination pressure / vacuum gauge and another 60 psi safety / pressure release valve (extra safety here!). I have a wall mounted regulator / moisture trap so that wasn't needed on the lid. Mounted on the inside of the lid is fitting that forces the air out at 90 degree angle. That way, when filling the pot, air won't blow directly on the molds splashing the resin. 

Why the combination vacuum / pressure gauge? With the still in transit vacuum pump I ordered, I'll be able to use the pot as a vacuum chamber to degas my silicone rubber by connecting a hose to the quick disconnect fitting and then to the vacuum pump. Vacuum can be monitored on the gauge. I can then use the air compressor to fill the chamber with air to act as a pressure pot. Double duty!

Right now as I'm still waiting on the vacuum pump I am running a pressure test to see how long the pot will hold air. There are no obvious leaks, so that is good. I don't plan on pressurizing the pot over 50 psi, but started conservatively at 40 psi.

Once I get the vacuum gauge I will test the vacuum potential of the pot. But, I figure that if it can hold air, vacuum shouldn't be an issue.

Also mentioned in my prior post was that I couldn't locate the "in progress" photos of my news stand. Well, I found them so here they are:

I did make the roof removable for easy painting.

And as shown last post, now on the layout painted and with magazines for purchase:

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