Casting with zinc

Any tips casting this piece. Original toy was zinc, about the size of a tennis ball. I have made a 2 piece plaster mold but am concerned about the flow of metal. I was playing around with lead and it dries fast.

 

Well, welcome aboard. First off, lead pours considerably cooler than zinc. I think zinc is poured around 1000. I'm guessing you are trying to pour this thing in plaster??
That toy was probably die cast (which you will never do) or it was sand cast (which you can do) I am NOT a sand guy, but many here are. I suggest you visit ebay and get some
petrobond, make a box and learn how to sand cast this with zinc. Plaster can work, but it's a lousy mold material for hot metal. If you do not want to pursue metal casting further than this piece, try posting this in our request casting service. We have some guys here that like picking up the odd job. Getting setup to cast just one part is a lot of work. This help?

 

It was made in the 30's by a low income company, doubt it was die cast, probably sand but a 2 part sand mold is difficult, at least for me. The part is tapered so both pieces come out of the mold ok. Are there hi temp silicone products?

 

not for zinc that I know of. Even lead is pushing it for most platinum silicones. 500-600 is really the limit for silicone.

Here is a thought. How about make a wax of your part and run solid flask investment? Dewax, burnout and pour your zinc in under vacuum... See what I mean? This is a can of worms in terms of equipment, materials and learning curve.

 

Here is the plaster mold I made.

 

GOOD! Now soak BOTH pieces of plaster for 20mins. Shake and pat dry with a paper towel, and pour wax into the gap. Then you will have your wax positive. Invest that into a flask filled with ultravest after you add sprues. Dewax, burnout, and pour metal. Break it apart and out comes your part.

So as to not confuse you. Please understand you must get a wax version EXACTLY what you want as of your finished part. Not a negative. The positive is always what gets invested, melted out and filled with metal. Savvy??

 

What advantage do I have with wax vs just pouring into the mold? That ultravest is pricey and usually comes in 50lbs boxes for $50. Too much for this project both in qty and price. What does the soaking do?

 

Straight plaster makes a crappy investment. It contains bound water and is not the same as investment. Seeing you are pouring zinc and the temp is much lower... you might get away with it. Try it, what have you got to lose?

 

FWIW, there are plasters specifically designed for metal casting such as Hydroperm by USG, I don't actually know anyone that's used the stuff but it might be worth looking into if the plaster you're using now causes problems. And if you don't mind buying at least 50lbs, the price is comparable to plaster of paris.

 

Geez, ultravest is 50bucks. I bet if you walked into our jewelry store and asked nicely, we'd bag you up a few pounds. But here we are again, that old cheapness mentality comes up. People all too often associate castings with cheap. I assure you casting doesnt necessarily mean mass produced cheap shit. You could ask this guy if he will take 50bucks for his light, but ya might not like the answer. https://www.etsy.com/listing/709203...f=sr_gallery-1-1&organic_search_click=1&frs=1

 

I'm all for using the right tools for the job, in this case, something like a suitable metal casting plaster might be it. He's got a pattern from which he can make a mold, using that mold to make a wax impression to invest then burn out the wax to end up with a second mold to pour metal into seems like might be an unnecessary step if he can just make one mold from his pattern and pour metal into that instead

 

Let's see him make a wax of the part with that mold. If he cant make a wax, he will never make one out of metal.

 

So this is my first attempt. Where did I go wrong. Some of the zinc is shiny other sections are rough. It was one pour. Any tips?

 

I'm guessing steam from the mold, but my experience with casting into plaster was only couple of times about 40 years ago. My success rate then was zero, unless you count steam explosions in which case it was significantly higher. (I had almost no idea what I was doing)


What was your dryout process?

 

I let it air dry for 2 weeks. I didnt see steam when I poured the metal.

 

IMO, you won't see steam, at that point it'll be a steam explosion and it will spit molten metal back at you...at least that was my experience.

Some time in the oven to drive off extra moisture is my guess. Sorry, but no help on details, I abandoned the process before getting it to work well.

 

Try adding some vents to the mold and I think it will help immensely. Anywhere you see a shiny spot on these castings is where you trapped gas so try and put them there.

 

My guess is that's not good enough and a small amount of steam was created which may have caused the defects. Check the Wikipedia article on plaster to gain an understanding of the role water play in the chemistry of plaster.
The following comes from Georgia Pacific's metal casting plaster page, GP Metal Casting Plaster

 

doubt shiny is from steam. The slug in my ladle is shiny because the surface is smooth.

 

Looks like it was cold.