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RE: drain times & carry over


Hold times will very depending on the specific operation and are often based
on operator experience. Most shops are reluctant to decrease hold times
because it increases the potential for rejects. I'm sure many hold times
could be shortened by 5 or 10 percent without harm but there are always
exceptions. The minimum required hold time can also be a function of bath
condition. In a degreasing operation, longer holding times may be required
as the bath ages. Since the holding time is seldom varied as a function of
bath condition, the net effect is a potential increase in rejects over time.

Pumping the parts in and out of the bath can effectively increase agitation
and allow for shortened holding times. This practice is most effective in
cleaning and etching. In tank agitation can also help speed the process.

As for drainage time, I believe it's importance is over-rated (let me
explain this before I get flamed).  The total amount of solution removed by
drag-out is a function of withdrawal speed and drainage time.  For ideal
parts with flat surfaces, it is much better to remove the parts slowly from
the bath and allow a short drainage time than it is to remove the parts
quickly and allow longer drainage.  For a fixed amount of time between
baths, the least amount of dragout occurs when you allow two-thirds of the
time for withdrawal and one-third of the time for drainage.

For complex parts, this relationship may not hold.  Recesses can trap and
hold solution. Rather than withdrawal speed and drainage time, the important
parameter would be rotation and movement to allow drainage. The use of fog
sprays to quickly cool, dilute, and rinse off the part is also important.  A
fog spray can allow increased drainage time over the tank with a reduced
worry about solution drying.

Hope this helps.


-----Original Message-----
From: Karl DeWahl [mailto:dewah001@tc.umn.edu]
Sent: Friday, March 30, 2001 11:19 AM
To: P2tech(response) (E-mail)
Subject: drain times & carry over

A common reservation (barrier) to increasing drain time on parts going 
through an immersion process line (e.g. a plating line), is that increased 
drain time increases production time and cuts production capacity (if the 
line operates near full capacity).  The cost of reduced production capacity 
can dwarf  the savings in chemicals and water.

This could be addressed by cutting hold times in the bath to compensate for 
additional drain time.
I have thought about the theoretical trade off's (see below),  but does 
anyone know of:
1.  studies looking at compensating increased drain time with decreased 
hold time?
2.  an expert who might have experience with this?
3.  work to identify optimal hold times for different immersion 

Theoretical analysis
On the positive side,
- Contact time with the chemistry or rinse remains constant.
- Drainage flow will induce some turbulence and in-film mixing that should 
enhance the movement of contaminants away from the  surface or active 
chemistry to the surface.
- the longer the tank hold in standard practice, the less the effect of 
reducing hold time.

On the negative side
- access to fresh solution is reduced
- increased drain time can enhance channeling on the surface, and 
differences between the top and bottom of a part that could create uneven 
surface effects (should be minimal in rinses)
- the stronger the tank agitation is, the greater the possible effect 
lengthened drain time

Karl DeWahl
MnTAP / University of Minnesota