48050

Controlling hard chrome electroplating  

March 5, 2008

Dear Sir

We are manufacturing cylinder sleeves and do internal hard chrome electroplating.

what are the parameters that can be controlled, to reduce reworking during the process. we are controlling current, bath temperature and proper cleaning of workpiece.

please help, what could be the other parameters that can be controlled?

Mohsin Siddiqui
product engineer - Karachi, Pakistan

The best way is what you are currently doing and then use a conforming anode for each part. The anode of my choice was lead with tin and a small amount of silver. We used tire weights as required to harden the mat. We cast the mat in a web or waffle pattern , bent it to shape (within 24 hours) and lead burned it together. We tweaked the anode as required by removing lead from the top and bottom or by adding lead as required. The top and bottom are high current density areas, so the anode needs to be further away (less) for the ends.

The anodes get "tired" after some use and slow down. We just melted them down and recast the mat.

ID plating generates trivalent chrome. Porous pot technology really helps control it. High current dummy--big anode, small cathode and very high current will convert trivalent to hexavalent, but it is wasteful.

James Watts
- FL

Dear Sir/Madam

We undergo OD Hardchrome electroplating for piston rods.

We are facing problem in surface finish of rod, According to the process we go for three stage grinding and maintain its surface finish to 0.4 Rz, than it goes to plating, but after plating the surface finish becomes 1.2 ~ 3 Rz.

Please suggest what could be the reasons for that.

Mohsin Siddiqui
- Karachi, Pakistan

On this side of the big pond, we use Ra. It appears that Rz actually has 3 different definitions which makes conversion difficult.
Chrome plating will always have a a rougher surface than what you started with, but yours seems to be extreme.

1st, I will guess that the grinding operation is slightly smearing the metal on the surface that is being unsmeared or opened up in the cleaning/etch cycle. Since Rz does not exclude large outlying measurements, it will not take much to increase the measured valley and also add some raised vertical
measurements. These will become high current density areas and plate several times faster than the surrounding area, so a tiny "hill" can become a mountain.

2nd, you may have grinding "dust" embedded in your parent metal which will become a high current density spot and generate a mountain.

3rd, you may have some very fine dust floating around in your tank. Normally, this will settle but since the specific gravity of chrome plating solution is rather high, some will float. Normally this leads to shelf roughness or roughness on the top of the part as it is racked--BUT-- it can also plate out on the sides.

I would take 3-4 parts off of the rack after the cleaning and etch cycle (premeasure the roughness) and measure in nearly the same area. This will give you some clue if you have a smeared part. Any increase in roughness will be increased by the plating. You may need to change the grit or the resin in your wheel and possibly the feeds and speeds.

Consider filtering the cold solution and see if this has any effect on the roughness of the plated part.

James Watts
- FL

Dear sir

Can you please suggest grinding wheels (grit sizes) based on three stage grinding.

This will definetlu help us improving surface finish, Also should we go for polishing after electroplating?

Please help

Mohsin
- Karachi

Hard chrome plating of bores is slightly harder than exterior plating , it seems you are doing every thing correctly , if you have the correct current and temperature , and conforming anode , it should e ok - what exactly is the problem you have ? depending on the size , I would plate at 2 amps sq" (should plate @.001 per hour ) . and have an airline below the part to allow the solution to be constantly changing through the part .

could you let us know the exact problem you have and give more details how you are processing the parts . thanks,

J Smith
- Essex, England

Look on web sites like Norton and similar companies for grinding wheel suggestions. They are the professionals.

James Watts
- FL

Dear J Essex

The actual problem we are facing is the surface finish, we are not obtaining the required surface finish.

We undergo three stage grinding of wheels grit size 80, 120/150, 700 respectively on three stages on outer surface of rod, we receive 0.5Rz of surface roughness, than we go to electroplating and receive surface roughness of 1 - 1.2 Rz, which is very high.

Is there any standard of surface roughness after hardchrome electroplating? So that it will help us to improve our electroplating process parameters to control surface roughness.

please help

Mohsin siddiqui
- Karachi, Pakistan