Help! Casting Defect Drove Me to Go Gray!

Hey! I recently joined the boards with the hopes in having a problem trouble-shot...I honestly had no help elsewhere and AlloyAvenue seems to be deserted.

My wife recently plucked a gray hair that wasn't there from my head after I took one of my castings from the mold. It was something that I've seen time and time again and that I still haven't been able to fix...

This damned shrinking:



Some quick background: I've been teaching myself to do this for the past 2-3 years, I have no background or training in any of this, and much of what I've learned so far is from trial and error and some input from a friend with just a bit more casting experience. The project is replicating the original stunt/special-effects hilts from the original STAR WARS movie, as it was originally made. They were sand cast aluminum and later machined to fit a small enough motor to make the blade effects work.



... So these have to be perfect. Every time. Any major defect like I've been having issues with, I can't machine or remedy externally and have to recast the piece.

I've had some successes but my chances are not good. I have a 25-35% success rate and the problems always the same: shrinking/collapsing on the surface, always by the pour gate. I have to get them to look like this before I can do anything with them:



So what exactly am I doing wrong? From my readings, I presume it's a heat problem; I'm getting the molten aluminum too hot and the piece is too thick and it cools unevenly...I think. The thick solid "body" section stays molten too long, from what I understand? But I have to get it hot enough to flow and fill in the details, because there's a lot of choke-points, otherwise I'm left with gaps in the cast and I have to remelt, and the rate at which I have to do that has become untenable.

If my problems are heat, I've bought a pyrometer and I'm hoping it'll help me fix my casts when it finally arrives. If not, I really hope some of the members here might shine some light on my dilemma. Is it my gate? Is it my pouring speed?

For reference, I've only fairly recently upgraded from a charcoal set-up to a propane gas forge, and for the longest time, my only tell for when I knew to pour was melt color. It's been all by eye and gut for the most part, so it's most likely my fault now with the newer forge.

This is my current set up: Devil Forge (nice little thing) with an A7 crucible, a small-home gas tank to feed it, I flux with the stuff pictured next to the forge after I "degas" with a bit of sodium carbonate (the Myfordboy method).



Any and all help would be greatly appreciate. Help me Home Foundry board, you're my only hope!

 

Hello Brian,

First thing is get rid of the sprue extensions. That piece should flow easily.
I would gate like this. Sprue on the left, risers on both ends.


I have a similar style furnace and judge pour temp by time.
After the last ingot has melted and disappeared in the melt, I start a timer.
For your piece I would turn the burner off at thirty seconds. Skim and pour.
For my large 8" x 15" plaques I use 2 minutes, (to give a reference).
I've developed this by trial and error and know my furnace/burner combo. Another aspect is the time taken to skim and pour, I try not to deviate from pour to pour because your metal is cooling.
If you have clean metal and your burner is tuned to neutral/oxidizing, there is no need to flux or de-gas.
The less time you keep your metal molten, the better the pour.

 

When I started out, I originally had the feeder go directly into the thickest section with a large gate and while that worked okay, I'd still have a pocket formed right by the gate, and most times, deeper than what I was getting here. I then moved it to its side, and on the counsel of a friend, advised be to add the extensions to help push the molten aluminum down, and where the sprue and gate are now, to remove that but now I'm just getting a shallower dip over a wider surface area.

Should I get rid of the extender on the fatter sprue and just pour in that, leaving the riser on the other end? Get rid of them together like the suggestion below?

I think I'll do that on my next cast. Would placing it at its thickest help, you think?

My aluminum comes from stock I get at the local hardware store. Stuff in the welding aisle. I assume it's as pure as I can get affordably, readily, and locally. It's certainly given me better results from the soda cans I was using when I started.

I unfortunately can't do that...Or rather, won't allow myself to do that. I'm kind of taking a "living history" approach and have become really obsessed in recreating the exact method as well. To this end, existing pieces from the original production have cleaned-up sections on the cast's sides and bottom, as I've frustratingly tried to replicate. I've also had some die-hard collectors contact me to do this for them as well, and a number of these would go towards making up the investment I put into this already just to tinker with what I've accomplished already.

I wish I had your confidence. I'm not experienced enough yet to know the finer points of my furnace and burner; it's still all very much a learning experience for me. I do try to not keep the melt from getting too hot (I've picked up on getting it too hot and it affecting my pieces), I think my problem is not striking that sweet spot between the material hot enough to not only met, but optimally pour. As you say, it seems to really be a matter of time. I've not got my pyrometer yet but I will be applying what you've said to my next cast.

 

If you're buying stock it's probably extrusions. It shrinks more than casting aluminum.
Find something that has been cast before, alloy rims, cylinder heads, pistons.

Are you sure they gated in the side and what you are seeing is not just flashing they ground off?
I can't imagine a real foundry would gate into the side because there will always be shrinkage around the gate.
Got a picture of the original?

Using a pyrometer is not a cure-all. I pour each of my patterns at a temperature that produces the best finish.

 

the best way to cast a cylindrical object is upright , so the item acts as it's own riser, simply add one third to the length of the cylinder with intent to cut it off,
you dont need any special flasks, just cut a hole in one end of the flasks, this is your sprue opening ( I attached a picture ignore the vaccum) since your flasks will be on
their sides, cut two pieces of plywood and clamp the mold halves together using the plywood , flip them on end and pour, BTW this technique will save you alot of metal

if you want to save your current setup, you have one fatal flaw, the riser size is determined by its smallest cross sectional area including the ingate . so when you step the riser up as you have done you will get no appreciable additional feeding ,
there is sort of an exception to this, the top of an open riser should be tappered out a bit (1/3) to give some additional heat so the riser does not freeze over at the top and stop feeding early , topping off the risers with hot metal is a good substitute for this, though if you are using a big ladle and small risers, you might enlarge the top of the risers just to give a better target.

But in short on your current setup, the risers need to be bigger all the way to the casting .
if you are just pouring down the riser , I would only include the riser at the end, enlarge it to be as big as the rear of the handle, tilt the riser end of the flask up 15 degrees and pour , this will create a directional solidification , the thinner pieces getting the hottest metal, you can keep feeding hot metal into the riser , this is an advancecd pouring techniques, as you slow the pour as soon as the casting is full, and slowly add metal to the riser to keep it hot , works really well on very large castings


V/r HT1

 

Original:


A "perfect" casting of my replica (also currently my avatar):

I feel embarrassed posting these earlier monstrosities. This was back when I was experimenting casting the components all in one piece, unlike the originals were. I also tried adding a partial blind riser to some in the later ones, which worked from time to time.

I will most definitely look into this! I also realized I don't have any photos of my flask, if it'll help, I'll be sure to post them.

 

The end view of your "original" shown above shows porosity that is reminiscent of what I see at my sprue/runner transitions so it seems possible that it was sprued on the end with a heavily gated riser on the side. I could be wrong but it makes sense to me.

Pete

 

If you're unable to find a suitable source of virgin 356 ingots locally and you have to use scrap, a good rule of thumb to follow is, always make castings from castings, that way you know it's an appropriate casting alloy. Any type of cast aluminum scrap will be better than extrusions or cans. I've always used aluminum rims, they're easy to find, rims are often damaged in collisions, so check with local body shops, mechanics and auto wreckers and you shouldn't have too much trouble finding a few scrap rims.

 

Ooh, yeah. I dunno about that. I'm a really small operation. Anything I can't do by hand is almost out of reach for me. I just haven't the space for these heavier machinery.

 

A circular saw with an ordinary wood working type carbide tipped blade makes pretty quick work of cast aluminum, the only caveat is the chips produced are rather nasty so appropriate ppe is a must, a bench saw is better but a handheld circular saw will do. Here's a rim off of one of my old Sciroccos ready for the furnace.



A less desirable but viable alternative, chuck your scrap into a burn barrel full of wood and catch what comes out. Also, if you pull it out of the fire just before it starts to melt, it will break up very easily with a hammer, go easy, not a lot of force is required, again, you're going to want appropriate ppe if you do that.

 

I have to admit, I was a little nervous the first time I took a skill saw to a big chunk of metal. But it really is a pretty safe practice. You MUST wear jeans, long sleeve shirt, gloves, Ear protection, Safety glasses AND A FACE SHIELD! The flying chips will hurt like hell if you aren't completely covered. CLAMP YOUR WORK down and two hands on the saw. I use a 5 dollar cheapy diablo blade from HD and it goes through stuff like hot butter.

 

I was pretty nervous too the first time I cut aluminum with a circular saw and I was shocked at how well it worked. It's worth repeating again, ppe, PPE, those chips are small, sharp, hot and they get absolutely everywhere so protect yourself.

 

I used one of those and it folded over on me. Now I buy a thicker name brand.
Still takes a firm grip on the saw, no matter what blade you use.

 

Just reading those experiences make me nervous.

...Although I'm sure I'll get to that stage at some point.


I'm working on a new flask that will accommodate the changes you've all suggested to make to my molding. I'll have pics of that up soon and when I have the time, see how the next casting goes thanks to all of your input.

 

That was not our intent. It's not hard and it doesn't need dangerous as long as you take a few simple precautions like securing the work, ppe etc. and use some common sense like two hands on the saw.