Electroforming mandrel question

A. Bond .002-.005 nickel to your thief then immediately prior to use immerse in potassium dichromate solution at 12 oz gallon. Make sure the thief is waterbreak-free before the dichromate or it will bond.

A. Hi Jason. Electroformers face this same problem. Dichromate is the best release agent, but I have heard that egg albumin may be good enough, and it would relieve you of an environmental hazard.

I have never heard of using silver mandrels, Anandan. What are you making from silver mandrels and why are the mandrels made of silver?

A. It is most unusual to use silver as a mandrel and to electroform copper onto it. However, that is your decision. I would personally suggest making the mandrel from stainless steel, aluminium or plastic, as they are cheaper. I am surprised the chromate treatment did not work, but I do not know enough about your process. To get best results from this web page, give as much detail of the problem and process as you can. Perhaps you are not giving the chromate sufficiently long, so increase the passivating time to (say) 30 seconds with a 10% Cr2O7 solution. You can also try an electrolytic chromating. As an alternative, but I cannot be sure if this will work, but try putting a dip coating of permanganate onto the mandrel; this is a substitute for chromate. Alternatively try a very thin sulfide passive film. Obviously, once the passive film has been put down you MUST not clean it, just rinse it thoroughly and put it in the copper solution. There is one major factor about the sulfide coating- you can't put it into acid copper or it will dissolve off! Use an alkaline copper solution (e.g., cyanide); you may be able to get away with live entry as another way of protecting the passive films, but that is something you will need to try. Just one more thought; - is the mandrel designed to have good draught angles so the electroform can come off it?

A. I'm new to electroforming. The exact type of mandrels I want to mold are mostly made from nickel and/or copper alloys. Can I electroform directly on these alloys? How do I get the mold to remove? Ideally I want to get a electroform either from Cr Ni-Co due to its hardness (copper or nickel would be too soft).

Let me know if I can electroform directly on the metal, or if I have to transfer the shape to some other material. The fine details and hardness in resulting form is important.

Thanks a lot.

-John

A. Yes, you can electroform onto nickel or copper alloys (actually it's very common). Two widely used release agents are egg albumin and sodium dichromate. You seem to be approaching the project a bit confidently, and I hope it turns out as easily as your letter makes it sound :-)

First of three simultaneous responses --

A. The whole principle of electroforming is that the depositing surface mimics that of the mandrel. The trick to good electroforming is to get the electrodeposit to loosely adhere to the mandrel but to mirror the surface, so whilst there is a complete and positive adhesion, it must not be too well adhered. The nickel electroform should be stress neutral or very slightly compressive. The smoother the mandrel, the better the chance of release, but the mandrel needs to be passivated. Electropolishing is an excellent way of increasing corrosion protection, but I have never come across it being used a passivate. I would therefore suggest after passivating, the stainless steel is given an electrolytic chromate passivate - this should be strong enough to break through any phosphate layer on the steel that has ben produced by the electropolishing.

Second of three simultaneous responses --

A. Something that doesn't sound right is that you mention the plating side comes out brilliant. An electroformed nickel plate from a properly balanced bath must be mid-bright to dull. Brightness may be an indication of stress. This will lead to an early separation or even cracks. Check with your contractometer.

Third of three simultaneous responses --

You'll have to strike with Wood's nickel as part of your mandrel prep.

A. I must disagree with both Guillermo and Jeff above. Firstly, whilst brightness can indicate stress, it doesn't mean it is stressed. I have used nickel sulphamate electroforming baths with small amounts of sodium saccharin (0.1-1.0g/l) to give me a zero or slightly compressive stress. Not only have I had excellent electroforms, but the plating surface has been very bright as well as being hard, both of which I wanted. Indeed, many commercial electroforms are specified bright. Secondly, Woods nickel is usually used to promote adhesion - using it as an initial electroforming stage could result in the final electroform being too adherent to the mandrel to be successfully pulled. The trick to successful pulling of electroforms is to have the correct geometry (i.e., a taper) and a semiconductive passive layer.

Second of two simultaneous responses --

A. ASTM B842-93 "Standard Guide for Electroforming with Nickel and Copper" is very informative. Geometric factors are discussed. A stainless steel mandrel should be lapped or electropolished to a 0.05 micron finish and briefly passivated in a 2% sodium dichromate for 30-60 seconds at room temperature before use. Both the ~zero stress, chloride-free nickel sulphate and the Watts nickel (gives a matte finish) solutions are described. Stress reducers, levelers, roughness and mechanical properties are discussed.

So, Trevor's details seem entirely correct, and the brightness/stress relation described by Guillermo is also possible. Sorry, Jeff, the electroformed product must be removed from the mandrel.

A. Trevor,
As we have always dealt with electroforms subject to temperatures above 200 &C and our only concern is the mandrel side finish, I accept that my previous comment was wrong and partial. Indeed, sulfur bearing organics such as saccharin that promote brightness and compressive stresses are frequently used to maintain other desired properties. About the Woods strike suggested by Jeff, I also disagree. The contractometer check I still think is in order due to the separation problem reported by Don. Thanks for your continued teaching.

A. Yes, there are exceptions to every rule, and some electroforms have to exhibit a good finish on both sides. For example, small pneumatic bellows are manufactured by electroforming, and the bellows must be a functional part with a good finish inside and out and good mechanical properties as it expands and contracts.

A. Sorry, just saw that you are plating on #2 SS mandrels. I plated on #6 polished SS, and without a bit of Wood's Ni the plated part would separate partially or fully in the plating bath. It was a very delicate balance of just enough Wood's to provide enough adhesion to keep the plated part on the panel, but too much Wood's and the parts were difficult or impossible to separate.