New to Casting and need Help... Please

Beaker

oh and yes, there are two choices for your above senerio, either vent the mold in the places that air is collecting or use a pressure pot... powdering the mold may help to some degree but if it is simply an issue where the air has no place to go then it is one of the two options.

Steamtech

When using the pressure pot do you pour your resin into the mold and then place the entire thing in the pot? Put the unmixed resin in? The mixed resin?

I'm interested in getting a pressure pot for some upcoming jobs but I'm really clueless as to how you actually use it in the process! Can anyone give me a quick explanation?

Beaker

If you are going to cast under pressure, first make your mold under pressure! Very important step... once the mold is made under pressure then yes, pour the uncured, mixed resin into the mold and place the whole thing into the pressure pot, clamp shut and cast at 60lbs of pressure

Makrel Johnson

Beaker, understood about Harbor freight. I've gotten adept at picking through the BS tools there. I think I'd only need slight pressure as my bubble count was 3-4 and they were really small. It's a shame I'm such a perfectionist... I'll try a few tests with a shot of primer prior to molding, using a powder and just plain poking the crevices with a pin. BTW my block does have an air vent and I make sure I fill that completely when I inject. I think it's just one of those things where you needs the planets aligned and the dog on the right side of the couch.

I'm going to try to attach some images...
Mold, Originals (right) and pulls (left)


Closeup of original and pull:


and 3rd that works well enough...

Makrel Johnson

...in the post above the swing arms (top images are ~2.5" long and the bottom image, the mounts are 1" wide. Just a note for scale. The arms are from this model: https://picasaweb.google.com/meffen/VR052F and are the first to break. The mounts are from a Revell Robotech Changers Axoid kit I inherited from a friend... naturally missing some parts but I have about 7 or so in various states of decomposition that I can mold parts from and rebuild.

I'll have to get my butt in gear and snap pics of my current project for that thread. (this molding issue/intermission is what I do while paint cures and putty dries)

Beaker

pressure pot will help this... next time you could try a squash mold also... I made a very successful motorcycle tail light lens doing this
http://www.smooth-on.com/images/gall..._castings1.jpg

Makrel Johnson

Beaker, I had better success squashing the mounts from above by pre-filling, closing then injecting. The swing arms will have to wait until I get pressure going.

I did cobble together a vacuum system consisting of an old 2qt nonstick pot, 1/4" acrylic, a PMC 121-30 gasket and a Harbro Freight 3cfm pump with ball valve. I'll post pics later tonight. It's small and suits my needs well. Next I'll look into pressure.

Thanks again for your help.

Steamtech

I never would have thought to make the mold under pressure too! That is a fantastic tip.

Thanks for the advice, can't wait to try it out soon

Makrel Johnson

Beaker, one last question... Smoothcast 300 and 321 and Task4, do these need to cure in room temperature or cooler? I have tried placing these in my hobby oven and I notice foaming... it's ~100 F. Maybe a sticky on resin tips would be a good thing.

Update - I had extremely good luck with Smoothcast 321 (which I forgot I had). The viscosity was super low (like water) and I pulled very well from all molds. I'm going to chalk trouble with the 300 to being old and the Task4 to needing pressure and being too viscous for the tiny molds.

Hopefully someone can use this info and save some time

Beaker

Makrel,
Glad you had luck with the SC321... foaming is simply moisture, about the only thing that causes foaming... in fact any urethane foam is simply urethane resin with a controlled amount of moisture built into the catalyst.