Looking for advice about casting fundamentals

HonestAbe

HonestAbe

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So I'm relatively new to casting (only a few years experience) and realizing that I've been getting by alright but when things don't go well (miscast, porosity, investment problems) I don't have a strong foundation in the fundementals to troubleshoot what's going on. I don't really have anyone that can look over my shoulder and point out what I'm doing wrong. I get the sense that I'm doing most things correctly but there's so many variables that sometimes a combination of things adds up to me messing something up. The areas I perceive I'm weakest at are:

Getting my torch set correctly for different alloys
Optimal sprue technique

I'm just wondering if anyone has some resource they can point me at to read up or watch where I will hopefully notice things I am doing sub optimally. Most of what I cast are full gold crowns, with the occasional pfm, custom abutment, or post.

EDIT: I suppose I could also start documenting stuff with photos so we can laugh the next time I mess something up and hopefully figure it out, but for now I'm mostly just looking to read and study something to see if anything jumps out at me.
 
Brett Hansen CDT

Brett Hansen CDT

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What fuel are you using? What type of casting machine?
 
HonestAbe

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I'm using the city gas, broken arm casting machine I'm not sure the brand but I can check when I can.
 
Brett Hansen CDT

Brett Hansen CDT

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I would google around for info on casting with different fuel sources. Many years ago, when I was still new to this world, we were using acetylene(!!!!) to cast with. It was dirty and we were having miscasts. I started googling and found a father son duo who had a great article about casting with different fuels and found out real quick that acetylene should not be used :). We changed over to propane and oxygen. Natural gas should be fine also.

Spruing is the other big concern. Making sure you have big enough reservoir is key. Also keep in mind how your molten metal is flowing. I try to limit the mount of directional changes the flow has to make before it gets to the margins.
 
CatamountRob

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I use propane and oxygen, you can cast with acetylene but it’s much dirtier. I’ve never had access to natural gas but it works fine, maybe requires a different torch?

I cast gold crowns with a direct ball reservoir sprue. You want the ball in the center of the ring, the top of the crown less than 1/4” from the top. No sharp corners, the ball needs to be bigger in cross section than the largest part of the object your casting. I burnout full gold at 1300 F but let it cool to below 1000 before I cast it.

I cast copings for PFM’s on runner bars in oval rings, again, no sharp corners and keep them angled back relative to the direction of spin. I burnout those rings at 1400 F, I preheat the alloy and crucible in the burnout furnace with the rings.

If you are casting high palladium alloys be careful with the 02, if the torch is really hissing it’s probably set too high. Palladium has an affinity for oxygen.

Post a few pictures of your sprued crowns or copings, it would definitely help.
 
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HonestAbe

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I use propane and oxygen, you can cast with acetylene but it’s much dirtier. I’ve never had access to natural gas but it works fine, maybe requires a different torch?

I cast gold crowns with a direct ball reservoir sprue. You want the ball in the center of the ring, the top of the crown less than 1/4” from the top. No sharp corners, the ball needs to be bigger in cross section than the largest part of the object your casting. I burnout full gold at 1300 F but let it cool to below 1000 before I cast it.

I cast copings for PFM’s on runner bars in oval rings, again, no sharp corners and keep them angled back relative to the direction of spin. I burnout those rings at 1400 F, I preheat the alloy and crucible in the burnout furnace with the rings.

If you are casting high palladium alloys be careful with the 02, if the torch is really hissing it’s probably set too high. Palladium has an affinity for oxygen.

Post a few pictures of your sprued crowns or copings, it would definitely help.
Thanks to you both for the advice, I will have some pictures tomorrow before I invest.

I am using prefab sprues with the ball built into them for the gold crowns. I need to learn more about sprue position and how to angle things relative to the direction of spin. Do you mark the ring in some way to indicate how to load it into the cradle? I'm currently using oval shaped ringless formers so I guess it can only go in one of two ways.
 
CatamountRob

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I put an arrow on the base former so it shows on the bottom of the ring. It’s more important with bars than with direct sprues. Your sprue always attaches to the thickest portion of the object you’re casting.
 
Brett Hansen CDT

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Prefab sprues are great, but as Rob stated, depending on the size of your crowns, you may need to increase the reservoir size. I used to just dip the prefab reservoir in my dipping wax to add a uniform layer to thicken if needed.
 
CatamountRob

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When you cast full gold crowns that reservoir ball needs to be full but you don’t want much more alloy than that, you want the button on the bottom small or non existent. The crown has to cool first and the reservoir last.
 
HonestAbe

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When you cast full gold crowns that reservoir ball needs to be full but you don’t want much more alloy than that, you want the button on the bottom small or non existent. The crown has to cool first and the reservoir last.

Hmm well my button is definitely larger sounding than it should be then. I know this is difficult without pictures so I will definitely post those tomorrow to make sure I am understanding correctly.

If I want to do multiple crowns in the same ring (I often do 2),what I've been doing is forming a cone shaped sprue at the bottom of the former, then I take my two crowns and wax the end of the sprues onto the top of the cone in a V shape. In order to make sure they're not too far spread out to the side I've been melting the balls together so it kinda forms one larger reservoir. Like a venn diagram. I've experimented with different ways to have the crowns face but I'm not sure what's ideal and what's not.

Should I automatically be using a bar if I'm doing more than one unit?

With a bar, does the same principle apply about the button? Like the bar should be completely filled out with gold but barely have a button after that?
 
CatamountRob

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No, you can cast a couple gold crowns on the same ball. If I’m casting a couple on the same ball I remove the short sprue on top of the ball. I use small sections of wax rod and put the crowns a bit lower and angled out.
Just to be clear I’m adding a photo of a direct sprue. I almost always shorten the long portion to position the ball in center of the ring.
3DDFF4CE-FABA-48A3-94B0-659A0BCB789A.jpeg
 
HonestAbe

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Okay got it, I've been using 8 Gauge for gold, and I don't have 10 gauge so I've just been using wax wire with no reservoir for PFMs, but I'm getting those ordered.

I have been shortening the long part of the sprue to get my crowns in the correct area, but I can see how it would flow better if the sprue came out straight vertical from the top of the cone instead of at an angle.
 
CatamountRob

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Like this. You want everything above the red line full of alloy and as little as possible below it. I usually use round rings for full gold but mostly as a matter of habit.
61E76BB2-2F62-4545-9748-4066A03016EA.jpeg
 
HonestAbe

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Thanks that explains it perfectly. It also shows me that I've been using too much alloy. Do I want to aim for the same thing with all alloys or just gold?
 
Contraluz

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Getting my torch set correctly for different alloys
Optimal sprue technique

I'm just wondering if anyone has some resource they can point me at to read up or watch where I will hopefully notice things I am doing sub optimally. Most of what I cast are full gold crowns, with the occasional pfm, custom abutment, or post.
Check this brochure from Ivoclar:
 

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CatamountRob

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Thanks that explains it perfectly. It also shows me that I've been using too much alloy. Do I want to aim for the same thing with all alloys or just gold?
The principals are the same with all alloys but full cast is the least forgiving. If you were casting on a bar you’d want the entire bar filled as that is your reservoir and not much more.
 
kimba

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I would suggest weighing your patterns and sprues, This way you will know exactly how much alloy you will need to not end up with the huge button which as has been stated here , leads to porosity. It can also cause distortion of bridges as well as COCCr frame works if the multiple sprues are joined up..
just multply the weight of your pattern and sprue by the density of your alloy , and that gives the exact weight of alloy needed to cast.

also for full gold crowns , I lower the number of winds of my casting arm. For mine I use 3 for bonding alloyx , but 2.5 for casting alloys. This reduces turbulance in the melt. ( I think I learnt hat from this site) You will have to play around with your particular cating arm.

And Be Scientific . Dont change everything at once. Keep constants and variablesandtry and keeps notes , which I know is hard when busy in the lab.
search this site as there has been some good discussions on casting .

Good luck!!!!! the fact that you are on this site and reaching out makes you half way there. Even if you are a pinko commi Vaxcine Cuck ;)
 
doug

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When you're casting Hi-gold alloys you can get away with less tension on the casting spring. The ring is filled before the arm moves a 1/4 of the first spin. If you have it wound too tight the metal will splash around in the mould and cause problems with the density of the casting. Also, I tell you this from experience, if you over wind the casting arm you can shoot that molten metal right out of the top of the ring. That's a party.
 
HonestAbe

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I would suggest weighing your patterns and sprues, This way you will know exactly how much alloy you will need to not end up with the huge button which as has been stated here , leads to porosity. It can also cause distortion of bridges as well as COCCr frame works if the multiple sprues are joined up..
just multply the weight of your pattern and sprue by the density of your alloy , and that gives the exact weight of alloy needed to cast.

also for full gold crowns , I lower the number of winds of my casting arm. For mine I use 3 for bonding alloyx , but 2.5 for casting alloys. This reduces turbulance in the melt. ( I think I learnt hat from this site) You will have to play around with your particular cating arm.

And Be Scientific . Dont change everything at once. Keep constants and variablesandtry and keeps notes , which I know is hard when busy in the lab.
search this site as there has been some good discussions on casting .

Good luck!!!!! the fact that you are on this site and reaching out makes you half way there. Even if you are a pinko commi Vaxcine Cuck ;)
Appreciate it. And I have no problem working with, learning from, and being friends with people I disagree vehemently with so long as they go clockwise around Costo and they're not super late to everything.
 
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