Greensand Mold Lifespan

[David Durman]

I have a question concerning the longevity of a greensand mold, specifically what is the maximum time period I can make them in advance of casting before they begin to crumble? I am taking a metal working class at my local university and, due to time constraints, I only have 1 class left and I am worried that I am not going to be able to cast my bronze projects. It occurred to me that I could make my molds at home and then cast them at my last class, but I am concerned about the molds drying out beforehand; could I coat the outside in parting dust and plug the risers to prevent dry out? Thanks for any help.

[ThomasPowers]

Where I currently live: very short times!  Where I used to live: quite some time. Where you live: who knows.  I would have opted for an oil based sand like petrobond in your situation.  I would also worry about transportation vibration to a rammed greensand mold.

[David Durman]

Fair points. Looking at the weather, the humidity is projected to hover around 40%, so not particularly good. Do you think that it would have a 'sweating' problem if I sealed it up? I thought about the vibration from the transportation, but I don't know how to counter it if it is even practically possible. Do you think a different method that greensand would work?

[Smoggy]

I would have thought, you would have have plenty of time to ram up a green sand mold while the crucible gets up to heat, before you add the bronze and wait for it to reach pouring temp. What may be of benefit, is to practice ramming up molds beforehand, so you can make all those little errors befor casting day.

[ThomasPowers]

The instructor won't let you come in early or stay late?

[David Durman]

Smoggy - That is my hope, but the class is only 3 hours; I have 5 molds to make, one of which has to use the same pattern so presumably I would make that one first so I can get the pattern out us it again within the timeframe.

Thomas - I discussed it with him briefly last night (the class is on Thursdays); he said we can talk about it next class (the last one for me) and maybe get me in there on a Saturday or something. It was vague enough to cause concern so I'm trying to figure out a workaround.

[Smoggy]

Is there any chance you can combine a couple of molds in one flask? that would cut it down to 4 or even less....may even have time to mold a few spares flasks up "in case" something goes all to pot!

 

ps penny just dropped, it's yourself that's casting the jewelry box isn't it, I've been watching that thread with interest.....

 

[Frosty]

Green sand molds have a limited lifespan and taking them on a road trip is a deal killer. We'd team a couple kids to move a large flask or if the kid wasn't strong enough to move his mold from the table to the pouring area. Even in the double period occupational class we waited till the melter was heating to ram up molds, even an hour was risky. Green sand casting MEANS go fast ramming it up. However, I went to school in S. California and high desert conditions applied.

You really need to be using resin or petro bonded sand and then not move them more than necessary.

And oh YEAH, "gang mold" as many in one flask as you can. The instructor should be able to tell you if there's room and more importantly . There may even be a large flask so you can ram and cast quite a few.

 Frosty The Lucky.

[David Durman]

I stupidly didn't think of that; I was just counting individual objects. I'm not sure if I can combine patterns into the same flask, though; two of them are using the same pattern so that is two flasks, one is using a 12.5% tin bronze I mixed up separate, and the remaining two are boxes that will need, as I understand it, complex spruing to avoid failure.

 

Yes, I'm the one planning on making the bronze Jewelry Box (2 actually, one for each of my daughters). The dimensions have changed because I ran out of time to carve them from pine and just bought a couple of generic boxes from a craft store for my pattern. The walls are about 1/8" thick, which is good, but I'm worried about the bottom since it looks to be 1/32", although I can probably just fix that with some plasticine. My long term goal with them is inlay silver wire into the grooves and to etch them at home with flowers, then use gold and silver loose leaf to gold the etching. It'll be pretty depressing if I don't get to cast them.

[JNewman]

Something that can save you a lot of time if these are patterns you have to cut a parting on is to make up a follow board for the pattern.  Good pattern equipment can make a huge difference in moulding time. None of my foundry customers will take a loose pattern with an offset parting without a follow board cutting in a parting just takes too long. 

I would avoid plastercine on any draw faces bondo is much better even on one offs.  But make sure you don't leave any backdraft.  Using a pre made box with no draft is going to make your moulding much more difficult.  Try to add some draft to your box.  Use a sanding block and some sandpaper to add a little draft.  Fillets in inside corners and rads on outside corners of the pattern also help with mouldability.  A rule of thumb is not to try to draw anything that is deeper than it is across

To thicken up the bottom of a box I would be more inclined to use thin plywood but even several layers of bristol board would be better than plastercine if you want it flat. 

[David Durman]

Thanks! Good to know.

 

i also just noticed that the word 'gild' was autocorrected to 'gold'.

[JNewman]

1/8' sounds a little thin for something as big as  a jewellery  box in terms of getting the bronze to run.  An easy way to add to your walls and bottom and to add draft to the inside would be to make a wooden plug that is slightly smaller than the inside of the box with draft on the plug.  Sand a radius on all the outside corners of the plug then using paste wax wax up your plug.  Do at least 2 coats polishing between coats after it dries for 15 minutes or so.   Then place a couple of small shims in the bottom of your box the thickness you want to increase the bottom by.  Mix up a big batch of bondo and apply it to the outside of your plug which you then squish into the box.  There is a bit of a knack to avoiding bubbles.  When the bondo gets to the rubbery stage (NOT BEFORE)  trim all the extra that squeezed out and spilled on the outside of the box. 

The sanded corners on the plug apply all your fillets all at once

[David Durman]

That's a good idea, too; Thanks! I'm not sure if I'll have time for that, though. If we agree next class on a time for me to come back I will give that a try.

[M_Gard]

I know this is a little late to answer your question, but if other people are wondering about this I always would put my whole mold into a trash bag to help keep in moisture. I would do this if I couldn't pour before the shop closed for the night after I made the mold. The longest I did this for was probably 20 hours. After that, I am not sure.

[Foundryman]

I make green sand moulds professionally and think nothing of leaving moulds over the weekend. I have 3 boxes on the floor at the moment containing 15 components each that I moulded yesterday (Friday) and won't cast until Monday.