Ask the Expert
What is Dragout?
How can I clean blind holes?
I have heard the phrase “dragout.” What does that mean? Is it
important?
“Dragout” is fluid trapped on the parts, and especially the
containers holding the parts, when they are removed from some cleaning or
rinsing tank.
Drainage of this trapped
liquid can reduce “dragout” if time for drainage is allowed before the parts
are moved on to the next process step. Vibration of the parts encourages this
drainage.Liquid carried on the parts, “dragout,” contaminates the next process
step with fluid from the last past process step. Usually, this means some of the
cleaning agents and soil are conveyed on the parts to the rinse baths. Then
parts aren’t rinsed with pure fluid – their cleanliness won’t be as high.
Sometimes, it is equally important that a tap water rinse is conveyed
into a final rinse with mineral-free (deionized) water. In that case, the final
rinse isn’t with mineral-free water and mineral stains may be left on the
parts.
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I am embarrassed! Three years ago I ordered a new cleaning system. To
date, it has been a total flop. We are removing a silicone oil and some
particulates from a narrow (3/8") blind hole, and not doing it very well. This
supplier told me that the major method for removing solids or liquid from blind
holes is to blast it out with pressurized jets. Well we tried that, and I have
seen with my own eyes that it isn’t so. The jets don’t do anything unless
they are aimed to hit a blind hole. When they do hit a blind hole and it quickly
fills with water, and not much debris comes out. Just how do you clean blind
holes, and what can I do with my spray cabinet cleaning machine?
Neither aqueous or solvent technology offers impossible limits to
cleaning blind holes. Both technologies can and have been effectively used to
clean debris from blind (one-sided) holes.
The most common method for removing debris from holes is to flush it out. It is
not generally necessary to blast debris from blind holes – any velocity will
do. The flushing process involves three steps:
- To start, the parts are inserted into a
container (basket) and fixtured so they are held fixed in ANY orientation. The
hole is filled with fluid, when the basket is immersed in a cleaning bath. No
aimed high-pressure jet is necessary. The hole can be filled with the fluid
being circulated by a pump within the cleaning bath.
- The hole is emptied of fluid, usually by being
turning the basket upside-down. The fluid contains some debris, which is removed
from the blind holes when the fluid is drained from them. This cycle is repeated
dozens to hundreds of times as the basket is rotated via mechanical action.
- The debris is swept away from the holes so it
doesn't reinfect other holes. A relatively high velocity (~10 fps) of fluid is
necessary to suspend even small particles with the moving fluid. The key point
is that there must be some effective way of shaping the cleaning bath so that
the flow of fluid and particles is directed to exit the cleaning bath and into
some external filtration system. If the moving stream of particles and fluid
hits a bath wall or other parts in the basket, the particles may be detached
from the moving fluid. In this case, they may be reintrained by some other
fluid eddy and redirected into a hole.
This method will work when the particles are just resting or loosely held within
the hole. Sometimes, the particles are trapped between surfaces or within
crevices in the holes. In this case, that hold must be loosened – usually via
high pressure jets. Then the flushing method is employed to recover the freed
particles from the blind holes.
The use of ultrasonic transducers is another proven method for removing debris
from blind holes. Note that it only replaces the use of pressurized jets to
loosen the particles within the blind holes. Ultrasonic transducers do not flush
particles from blind holes – they only liberate them from being stuck within
the hole. A flushing system must be simultaneously used.
The
most common mistake is to believe that debris is best removed from blind holes
via blasting it out with high pressure jets of fluid. There are two problems
with this approach, which makes a great deal of intuitive sense, and is very
commonly used –though without a great deal of success.The
first problem is that the debris must be removed from the blind holes, not just
dislocated or dislodged within the hole. The effect of high pressure jets is to
move the debris within the hole. Since the fluid jets are pushing inward, they
push the material within the blind hole.
Granted,
some particles can be removed by high velocity jets. If the hole size is much
larger than the jet size, particles can be carried out by the fluid which must
be displaced from the blind hole after the liquid spends it's momentum. This
action is seldom repeatable because it depends on the orientation of the debris
within the blind holes.The
second problem is that the high pressure jets must be aimed at the blind holes
to have even the above effect. If the jet misses the hole, the hole does not get
hit with fluid. So the issue is fixturing the parts so the fluid jets always hit
and penetrate all the blind holes. Obviously, this is very difficult to
implement.
Don’t
mistake the usefulness of spray cabinet washers. For multiple parts on a
conveyor line, for large surfaces which must be cleaned quickly, or for racks of
some small parts, spray cabinet washers are just the ticket! Maybe you can have
one of those applications for your spray cabinet washer.
And
don’t be embarrassed either. The person who sold you that spray cabinet washer
knew you would be dissatisfied and should be embarrassed.
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