Where the
world gathers for metal finishing
Q&As since 1989




-----

Bubbles in black e-coat

we stamp a part and then stake a nut to the thru hole. The part is then sent out for E-coat. This part is used as a height-adjustment slider in a seat belt. For the last 12 months we have had a 5% fallout of bubbled e-coat between the staked-nut and the slider. We have been making this part for 6 years and up until a year ago this was not a major issue. Our customer has indicated that we are not alone with this and several coating facilities have indicated that their supplier( PPG or Dupont)has changed the formulation of the e-coat. Our coater has said that there is not much they can do, the cleaner used by them will get trapped and then boil out.
Since I am new to this field I would like your input and what can we do to help our processor. The part is stamped using a water soluble oil, it is then staked without oil. By the time the processor gets the parts 7 days have passed and the parts are pretty much dry to the touch.

Timothy Morris
Quality Director - Dolton, Illinois, USA
November 17, 2010

---

How are you cleaning the cutting fluid off of the part before shipping it? I am greatly bothered by your saying that the part is nearly dry to the touch after 7 days. Shipping wet parts is an invitation for a disaster.

If I read your post correctly, some cleaning fluid is being trapped between the stamping and the friction fit stake. At you end, I would reduce the hole as much as possible to decrease the space between contact spots and the pin.

Since you have cutting fluid trapped between the stake and the part, I would add a cleaning step to the flat and possibly the stake before the staking operation.

The painting vendor can help by putting the parts in a clean rinse tank for a couple of minutes before going into the cleaning tank. This traps water and not cleaning fluid, which should reduce the problem. A couple of trips between the hot rinse tank and a cold rinse tank prior to cleaning tank will help even more. This hot/cold cycle will also help to remove some of the reduced amount of cleaning fluid.

Finally, can the parts be sent thru a warm oven prior to the actual painting to remove as much liquid as possible.

James Watts
- Navarre, Florida
First of two simultaneous responses -- November 18, 2010

If you already know that the bubbles are coming from some cleaner that is stuck to the part and boiling out in the cure oven, I would look at your rinse stages after the cleaning stages. What is your washer setup? Do you have a dip clean and then a spray clean followed by a dip rinse and spray rinse or is it dip stages in the whole washer? If you have a dip rinse I would look at getting more agitation in your rinse tank after the cleaner or dumping the stage more often. If you have a spray rinse: look for plugged or blown off nozzles and adjust spray nozzle pressure to 12-15 psi. Hope that helps.

Adam Krey
- Whitewater, Wisconsin, USA
Second of two simultaneous responses -- November 19, 2010

---

November 24, 2010

I will give the coater the benefit of the doubt that he cleans the parts properly.

In this case I would look for the rupture voltage.
If your e-coat solution is contaminated, the rupture voltage is low. As your supplier to check your rupture voltage.

Is the solution's conductivity OK or too high?

In any case, I would start with a simple process - purging permeate.

Sara Michaeli
Tel-Aviv-Yafo, Israel