Metal Casting Zone

So here is my issue, the surface on the pewter looks like a moon crater!

I made a mold of a vintage pipe tamper out of RTV and it is supposed to be good for taking the temperature of pewter but I am getting the typical pock marks on the surface.

The tamper is an old Victorian era tamper of a woman's leg. Picture if you will, a persons leg sitting with the thigh at a 90 degree angle to the calf with the toes pointed down.

Here are the knowns;

1. Lee melter that pours from the bottom with a thermometer
2. RTV 2 part mold that is supposed to handle the temperature of lead/pewter.
3. Mold has a smooth surface where I am getting pock marks.
4. I have two vents at the top of the leg, one at the knee cap and one near the hip. The pouring hole is in between the knee and the hip.
5. I have no other air escape vents.

What I have tried;

1. Pouring at different temps 500-600
2. Talc powder on the mold
3. Graphite on the mold

I have a mold of an Indian toy soldier and I tried pouring into it and the figure came out just fine.

So here are the differences between that mold and mine;

1. The material of the mold feels harder than my RTV mold
2. They had some vent "lines" cut in at different areas in the mold. Strangely enough no metal "flashing" went up them?????
3. The toy Indian figure was a bit thinner in most places, whereas the tamper was up to about 1" x 1/2" thick at it's thickest point. However the Indian had smaller extremities compared to the straight forward tamper.
4. The same pewter was used.
5. Graphite had been put in the Indian mold thus I tried it in my mold.

Geez, I didn't think it would be this damn difficult... I feel like a moron!

I am not sure what exactly the problem is. Two general suggestions with poor resolution are.1) More vents. Maybe small blind vents that you can cut off later if there is no obvious missing area. 2) more mass behind the casting. put a big resivour above the spruing to make the pewter come in with a little force. photos could help with more specific suggestions.

Another possibility is if you made the mold yourself, a RTV poured block mold that is, and you did not either pressure cast it or vacuum de-gas the rubber while it hardened; the mold will have bubbles inside under the surface making weak areas that the metal will force out against. You can not fix such a mold.

The thin vents you mentioned on the Indian mold were ideal, enough for air to travel through but too small for vicious pewter to flow through.

From the few experiments I have done with silicone and pewter, (and I mean a few only) graphite protects the rubber from heat damage and speeds the flow far better than talc. My talc did nothing for helping against heat. Brush the mold with graphite, rub it in with either your finger or a vinyl gloved hand and then clap the pieces together over a trash bin to shake out the excess.

If you are not using pure graphite, and something like lock lubricant, DO NOT breath the stuff use a canister respirator or do it outside or at least use a dust mask. That stuff was not meant to be breathed.

Keep at it, you will figure it out.

Hi all,

Thanks for the advice.

I will take pictures tomorrow and try to figure out how to post them on here, perhaps then you might see the error of my ways!

Hi Everyone,

Hopefully someone can help me... I am having zero luck.

I thought pewter casting was supposed to be relatively easy... guess I was fooled!

Here is a link showing the RTV mold that I made along with the many failed castings!

http://s34.photobucket.com/albums/d106/bjohns01/Casting/

Any ideas would be appreciated, as I feel like giving it up right now.

Very frustrated!

The images are helpful Bruce. The acorn nuts worked out well for you. I never got around to trying them, I'll have to do that some time soon.

NOTE: My experience is almost all in resin and silicon.

Firstly,
This is not air bubbles inside the silicone. In that case the the imperfections would of been convex instead of concave.

I am sure about,
Your spout is unnecessarily small. Larger volume in the spout would give you greater (weight) pressure to fill out the mold and force the air bubbles out. You could just as easily caste the spout on the cut upper surface of her thigh a thicker area that would of needed less final trimming.

The good news is, it seems can cut away more spout area in your block mold. (Image 7 or DSCN4839.jpg confirms you have a lot of rubber that could be removed easily.) A little cleaner where it enters the leg cavity and more importantly a bigger chamber of molten metal above to press out the air.

What seems debatable,
You need to skim the slag from your pot and perhaps scoop some powdered waste that is residing in the bottom of your molten vessel. By the way it seems unnecessary stirring of the metal just increases the speed of building up slag.

Also keep using the graphite every few pours. And expect to do a couple pours that are only to heat up the mold and by the third or so you will be getting less porosity as the cold mold isn't cooling off the metal.

Do to my lack of experience with this medium wait to get some more feedback from others.

Good luck Bruce I will check back in over the next couple days.

Stephen

"Your spout is unnecessarily small. Larger volume in the spout would give you greater (weight) pressure to fill out the mold and force the air bubbles out. You could just as easily caste the spout on the cut upper surface of her thigh a thicker area that would of needed less final trimming."

agree.

Graphite i know nothing of.

Hi Stephen & MCTC,

Thanks for your comments.

This is the first 2 part mold that I have created and I know there is a lot more to learn.

I like the thought behind making the spout larger. I will try cutting it out tomorrow and pouring again and see if it helps.

Part of the reason why I put the spout at the top of the leg was to try and minimize areas the air could get trapped. I suppose in hindsight I could have just vented those areas.

In carefully examining the mold I didn't think it had pockets underneath the mold surface... I checked it out after you suggested it yesterday.

I am pouring the pewter at about 575F does that seem correct? I am also using a Lee 4-20 melting pot with the lift valve that allows you to pour from the bottom. I am thinking that perhaps I should set it to flow at a rather fast speed.

Thanks again everyone for your input!

Bruce

Hi spad 13

Looked at your pictures

ok The mould you have made looks fine ,the sprue openning needs to by bigger and wider,gets the metal in quicker. There never is a reaction with the pewter and the rubber to give this surface on a casting. You say you have a bottom pour melt pot ,great that is important ,do you constantly keep the molten metal stired while casting???. Skim of the dross on the surface before you do stir. talc powder your mould correct.
I'd experiment with the temp of the metal ,and keeping constant temp of the mould.with the shots you have posted the metal looks to be giving this surface becauce of the too high temp, when you next cast ,bring the temp up on the melt pot very slowly allow the pot to sit for several minuites at each setting .just check eactly when the metal starts to become liquid and note the temp. any alloy should be cast around 50oc above the melt point but you have to establish exactly the melt point of your alloy first.Raise the temp just a little and start casting. and keep casting drop after drop. you may need to drop several time 10 to 15 drops but as the mould temp rises the metal should fill the mould completly and start giving you better and better casts. if after 15 drops or more raise the temp a little more and do the same.
I explain this because the photos of your castings indicate to me (metal too
hot) . find the exact temp can be difficult. Once the temp of a mould item is found I write these in a book, as I am spin casting other perameters are also noted ,
Spin speed ,,mould pressure, metal temp. Even after 20 years of spin casting I still can have problems,one of the first areas I look at is mould temp. 10 oc too high can be the difference between pitted surface or a nice silver frosted surface, the latter is what I am after.
Have you also tryed pouring the mould with a bit of distant between the mould and melt pot ,,when gravity casting this extra speed of the falling metal can give better results when the metal has greater speed when entering the mould. this can be made easier if you pour the metal down a metal funnel placed on the top of the mould .just watch for splashing metal and wear some safety items doing this.

PS problems arise wth casting ,casting is not easy to perfect it take a lot of trial and error, I went throught a lot of hell in the early days.
so spad just stick with it you are no moron just because you fail .to quote a good line ,,,each dismal failer makes a point ..if nothing goes wrong you don't learn anything. Best of luck spad keep trying. JOHNCAST.

Johncast, thanks for the info!!!!! I really appreciate it.

I will try what you have outlined I will take note at the melting point of the metal and will pour giving the maximum distance I can given my setup.

I have a thermometer that sticks in the middle of the pot and will monitor that.

What do you use for flux in your pewter? I have heard everything from paraffin wax, sawdust to damn near everything else...!

I'm not giving up but find myself very discouraged but I do appreciate your kind words and the sharing of your years of experience.

Funny... I actually thought pewter casting would be "easy". Well I'm learning it is an art like many other things in life!

Thanks,

Bruce