Low-temp metal for casting - any receptive to gold plating?

[AdamHunter]

I'm looking at doing a few larger scale casts (6"x6"x2") in metal. I want to use a metal with a low melting temp so I can use a heat resistant silicone for the molds (like Mold Max 60 or similar). At the end of the day, I'd like to have these pieces gold plated, hence the dilemma:

Originally I was considering something like a lead-free pewter, but from what I've read it's so reactive that the plating process involves copper plating, then nickel plating, and finally gold plating.

Are there any other alloys that are low-temp, that are more receptive to gold plating, potentially not requiring these intermediary steps?

Along that same vein, how much does a gold-plating service typically cost on something of this scale? I know it will vary vendor to vendor, but is this like a $100 job or a $1000 job?

 

[lmgill]

Ok, a couple of things here.
Just about any metal requires copper as a primer when electroplating. In fact, even copper items a often plated with copper first. So these "additional steps" have little or nothing to do with pewter being "reactive" per say.
Two reasons for the nickle or silver after the copper, because the gold goes on so thin, the copper color will effects the color of the gold, and over time the gold will be absorbed by the copper. So nickle or silver is used before gold plating. This is more often the case, so it does not make the cost any different, since "it's how its done".
Cost of gold plating is based on the current cost of gold, which currently is not cheap. They do make synthetic gold for casting (much higher temp than pewter), but there may be a similar thing for electro-plating that simulates the color, but is cheaper. Brass plating also does this.

Cost for something this large, may approach $50 to $100 each, depending on who does it, and how many you do.

Surface finish; This has the biggest single effect on the look of your plating. If the surface is not finely finished, you plating will exaggerate any imperfections. It also effects how the final gold looks. A satin finish or a polished finish require a different surface prep. You can get matte gold, and I believe this has to do with how the nickle is applied. Also, the nickle can go on bright and shiny, so this can improve your surface.
BTW- the copper and nickle will make your piece more resistant to surface damage, since they are harder than the pewter.

Pewter is a catch word for many types of low melt alloys, so there are many types to choose from, and I know of no real difference in plating ability. So this is a good choice. Conquest industries has a number of "pewters" for casting.
Let talk about the size: 6 inches by 6 inches by 2 inches is a massive amount of pewter to cast, unless hollow. The issue with a solid this big, is the amount of heat your silicone has to deal with, and metal shrinkage, which will distort you casting. Also, pewter is not cheap, so a casting this big will use $30-$40 worth of pewter, depending on which alloy you pick.

Can you post a picture of the thing you want to cast?

 

[lmgill]

Bueller......Bueller.......

 

[AdamHunter]

Bueller......Bueller.......

Sorry about the delay, I've been meaning to get back to you. It's been an absolutely insane week at work.

First off, all awesome info. It's good to know that those intermediary layers are a requirement regardless, even when plating other metals. I appreciate you taking the time to write all that out, and educate me a little more about the process. The more I know the better!

I've also gotten quotes all over the place, ranging from $120/piece to upwards of $300/piece. So it's good to know the 120-guy wasn't completely insane. This is the first time I've actually vetted something out to be gold plated, so I wasn't sure where those numbers would fall and what seemed fair.

Onto surface finish. Of course I want to shoot for as clean of pulls as possible, but I also take it that polishing would be a good idea based on your insight? I'm hoping for a pretty shiny and clean gold plate. Is pewter something you can finish sand if there are small imperfections that are a little too large for a polishing wheel? If so, any recommendations for sanding (i.e. avoid wet sanding, specific type of paper to use, etc.)

I'll take a look at Conquest. I'm still in the process of modeling the masters so this is all more "prep-reaserch" than "I'm stuck". I'll do some more digging on that site and see what catches my eye. I appreciate the lead here too!

Lastly the object:
6x6x2 was just a rough parameter of the overall dimension of the finished objects, but wasn't really a good indication of the volume of the actual cast, apologies for the confusion. I've attached a photo of the objects, everything that's printed in gold is intended to be pewter and plated. The dimension of the green stone is about 4.5" for reference. I'd prefer to try to cast these as single pieces rather than casting as two hollow halves. Should be interesting figuring out the venting requirements for the blue one, but the other two seem pretty straight forward. Given the significantly reduced amount of cast volume, do those topics you mentioned still post a concern? If so, is there something specific I can do to help mitigate some of those issues?

Thanks!

 

[lmgill]

It looks like you may be able to slush cast these pieces in pewter. I do not have much experience with silicone / pewter casting, most of what we cast is in vulcanized rubber molds. But with the correct silicone, it should be similar.
If you pour in the pewter, and pour it out again, you will have a shell of pewter, and therefor material use, and shrinkage should not be an issue.
If these are solid, you may still have some issues with shrinkage and surface blemishes without getting the mold temp and pewter temp perfect and I do not have enough experience with thick casting to offer much advice.
Here is an example of a pewter cast part from a vulcanized rubber mold, using a spin caster (you can see this equipment on the Conquest web site). This is an ACES spacesuit parachute harness release and it is functional. It has been "Black-nickle" and bright nickle plated. (the rectangular piece with the orange sticker is plastic) The thickest parts are the bright nickle "tongue" and main frame, which are both about 3/16" thick. The pin, may be a little thicker.

Surface finish:
I would say the biggest "mistake" I see here on the RPF is lack of surface finishing. especially lack of surface finishing before molding. You have to get a nice surface on your master, as the cast part will never be better than the master, always as good or worse.
Taking the time surface finishing makes a huge difference, but people are generally impatient or in a rush to get something done. (This is an unfortunate side effects with all these build shows trying to create drama by "Building (fill in the blank here) in a day".)
You can polish pewter, but it is more time consuming than surface finishing the plastic master, and you only have to do it once. Also with polishing pewter, is you have to be careful not to "burn-off" your corners. As the buffing wheel encounters an edge or corner, it has less surface area to dissipate the force you are putting against the buffing wheel (or sander) and you will remove material on edges and corners at a much faster rate, thus rounding them over, "burning them off".
Before you mold your part, I would suggest a semi-gloss / semi-flat (same thing) surface on your masters. (I use Krylon semi-flat black often) Take the time to make sure the entire surface is free of build lines and scratches. With 3d printed parts, we start with a high-fill primer, let it dry for a day before we start sanding.

Sanding:
There is a reason for sand paper comes in all these different grits, utilize them. 120-180-220-320-400-600, depending on the material, you can get away skipping 180 and 320, but in most cases, skipping grits just gives you a false sense of saving time. Also, Wet sand. Not just damp, but wet enough to be dripping off your paper and part. The water lubricates and cools the sandpaper, as well as carry away particles of sanded material, and this gives you a faster cut, and better quality surface.
Good light, another "mistake" I often see. Under estimating how much light you need. Good bright work lights will highlight any surface defects much better and general room lighting. Also, dry your part before you inspect the surface, as the water will fill scratches and reflect light in a way to make them invisible.

Polishing: Polishing is just sanding on a finer scale. Polishing compounds have "grit sizes" as well. For pewter, you will want a compound for soft metals. They sell all types of felt polishing tips for Dremel like tools. But, if your cast piece, has an even matte surface, free of scratches, the nickle plating can be applied smooth and shinny (mirror), so you starting part does not need a mirror polish.

Another process, one I have only done once, but was quite common for reproducing metal parts like yours, is electroforming. Which is the process of building up a part in a mold by electro-plating a heavy layer of copper in your mold. Electro-plating is not too difficult, and once you have a power supply (rectifier) and the plating solutions (liquids), you can do your own plating as well. The rectifier is basically a DC power supply. Gold likes 3-5 volts and less than an amp, but nickle like 5-7 volts and 3-5 amps. I'm not sure about copper.
I will say, that getting the nickle to go on super bright (mirror) is the trickiest step. So it could be worth having a professional shop, tank plate, these is you really want a mirror finish.
I'm currently decorating a hardened steel armour (16th century English) and in addition to etching, I am pen plating nickle and gold onto the steel. (In this case, as long as the steel is clean, I can dispense with the copper) Using plating solutions from "Gold Plating Services" on Amazon. I am also not necessarily wanting a super bright/ mirror finish to the gold, since the original armours where "mercury gilded" and this results in more of a matte finish.
An 8oz. bottle of their gold plating solution is $355 and this should be able to plate 6 or 7 of your pieces.
Here is one of the finished gauntlets with 24k gold.

 

[Duncanator]

Thanks for sharing so much insight into this process!

 

[AdamHunter]

It looks like you may be able to slush cast these pieces in pewter. I do not have much experience with silicone / pewter casting, most of what we cast is in vulcanized rubber molds. But with the correct silicone, it should be similar.
If you pour in the pewter, and pour it out again, you will have a shell of pewter, and therefor material use, and shrinkage should not be an issue.
If these are solid, you may still have some issues with shrinkage and surface blemishes without getting the mold temp and pewter temp perfect and I do not have enough experience with thick casting to offer much advice.
Here is an example of a pewter cast part from a vulcanized rubber mold, using a spin caster (you can see this equipment on the Conquest web site). This is an ACES spacesuit parachute harness release and it is functional. It has been "Black-nickle" and bright nickle plated. (the rectangular piece with the orange sticker is plastic) The thickest parts are the bright nickle "tongue" and main frame, which are both about 3/16" thick. The pin, may be a little thicker.
View attachment 1300974View attachment 1300975
Surface finish:
I would say the biggest "mistake" I see here on the RPF is lack of surface finishing. especially lack of surface finishing before molding. You have to get a nice surface on your master, as the cast part will never be better than the master, always as good or worse.
Taking the time surface finishing makes a huge difference, but people are generally impatient or in a rush to get something done. (This is an unfortunate side effects with all these build shows trying to create drama by "Building (fill in the blank here) in a day".)
You can polish pewter, but it is more time consuming than surface finishing the plastic master, and you only have to do it once. Also with polishing pewter, is you have to be careful not to "burn-off" your corners. As the buffing wheel encounters an edge or corner, it has less surface area to dissipate the force you are putting against the buffing wheel (or sander) and you will remove material on edges and corners at a much faster rate, thus rounding them over, "burning them off".
Before you mold your part, I would suggest a semi-gloss / semi-flat (same thing) surface on your masters. (I use Krylon semi-flat black often) Take the time to make sure the entire surface is free of build lines and scratches. With 3d printed parts, we start with a high-fill primer, let it dry for a day before we start sanding.

Sanding:
There is a reason for sand paper comes in all these different grits, utilize them. 120-180-220-320-400-600, depending on the material, you can get away skipping 180 and 320, but in most cases, skipping grits just gives you a false sense of saving time. Also, Wet sand. Not just damp, but wet enough to be dripping off your paper and part. The water lubricates and cools the sandpaper, as well as carry away particles of sanded material, and this gives you a faster cut, and better quality surface.
Good light, another "mistake" I often see. Under estimating how much light you need. Good bright work lights will highlight any surface defects much better and general room lighting. Also, dry your part before you inspect the surface, as the water will fill scratches and reflect light in a way to make them invisible.

Polishing: Polishing is just sanding on a finer scale. Polishing compounds have "grit sizes" as well. For pewter, you will want a compound for soft metals. They sell all types of felt polishing tips for Dremel like tools. But, if your cast piece, has an even matte surface, free of scratches, the nickle plating can be applied smooth and shinny (mirror), so you starting part does not need a mirror polish.

Another process, one I have only done once, but was quite common for reproducing metal parts like yours, is electroforming. Which is the process of building up a part in a mold by electro-plating a heavy layer of copper in your mold. Electro-plating is not too difficult, and once you have a power supply (rectifier) and the plating solutions (liquids), you can do your own plating as well. The rectifier is basically a DC power supply. Gold likes 3-5 volts and less than an amp, but nickle like 5-7 volts and 3-5 amps. I'm not sure about copper.
I will say, that getting the nickle to go on super bright (mirror) is the trickiest step. So it could be worth having a professional shop, tank plate, these is you really want a mirror finish.
I'm currently decorating a hardened steel armour (16th century English) and in addition to etching, I am pen plating nickle and gold onto the steel. (In this case, as long as the steel is clean, I can dispense with the copper) Using plating solutions from "Gold Plating Services" on Amazon. I am also not necessarily wanting a super bright/ mirror finish to the gold, since the original armours where "mercury gilded" and this results in more of a matte finish.
An 8oz. bottle of their gold plating solution is $355 and this should be able to plate 6 or 7 of your pieces.
Here is one of the finished gauntlets with 24k gold.
View attachment 1300966

Absolutely gorgeous work. The implementation of gold on that armor is stunning!

I never thought of slush casting a pewter piece. I'll definitely give it a try though! This is seriously a huge help, I can't tell you how much I appreciate you taking the time to write out all this information. The pieces I'm looking at casting will be SLA printed, rather than FDM so there should be much less clean-up to do, however, I'll make sure I take my time in the mold-prep process. It seems like a little more time up-front, can pay off dividends on the back end.

I just got my stones in, so I'm beginning the rough prototyping stage to make sure the frames fit properly. After that, I'll SLA my masters and get them ready to go. The rest of your info is going to be invaluable to this attempt. If you don't mind, can I give you a shout if I hit any hiccups along the way?

 

[sandbagger]

Why not cast/machine in aluminium, then anodise it in gold? 15 micron gold anodise should come up a treat.

Baz.

 

[AdamHunter]

Why not cast/machine in aluminium, then anodise it in gold? 15 micron gold anodise should come up a treat.

Baz.

Definitely open to it - although not sure what the cost to have something like this machined would be, and if there would be a savings doing it that way. There's also the thought that pewter would have a weight closer to gold than Aluminium would, although again I'm not opposed to it at all

 

[lmgill]

Why not cast/machine in aluminium, then anodise it in gold? 15 micron gold anodise should come up a treat.
Baz.

I suspect casting in aluminum will be pretty expensive, since you require a very clean surface. Also, unless Baz is referring to something other than anodizing specifically, anodized aluminum will not result in a shinny (mirror) gold finish, but rather a matte/ lustrous yellow / gold finish.
This is a gold anodized Apollo spacesuit O2 elbow:

If you are happy with gold anodize aluminum, then there is no reason for metal casting, since you can achieve this finish with paint.
Another option is to electo-plate or vacuum metalize a plastic casting.
A conductive primer on a plastic part allows it to be electro-plated with copper / nickle / gold.
Or you can vacuum metalize the plastic.
This helmet and chest plate have been vacuum metalized in aluminum, and given a transparent yellow finish, so they look gold.
(The chest is vacuum formed, the helmet is cast polyurethane rubber)

This spacesuit sun visor is vacuum metalized in 24k gold, but this is extremely expensive (This visor cost $850 to plate, and the guy will not do cast plastic pieces, since the off gassing of the cast plastic will contaminate his vacuum chamber)

The disadvantage to vacuum metalizing is the surface needs to be perfect if you want a blemish free end result. Also, any filling of imperfections in the casting need to be void free, because when the vacuum is applied, any trapped air can escape and blister the surface.