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Common Questions
Does a furnace have to be "SKIDDED"
prior to Shipment?
What are the optimum Air or Nitrogen flow
rates for thick film firing furnaces?
What factors adversely effect overall yield?
How do I clean the the furnace muffle?
Does a furnace have to be "SKIDDED"
prior to Shipment?
Actually, skidding a furnace isn't required and the vast majority of the
time it is not recommended (in my opinion)!
There are a few exceptions, but almost always in our business, furnaces
are loaded on and off air ride moving vans without skids. In my experience,
I have seen numerous problems, damage to the furnace and injuries to those
doing the work, occur as a result of trying to get the furnace off of
a skid. But, with the exception of most RTC designs, I have honestly never
seen an unskidded furnace damaged, which was on an air ride moving van.
Skids can be manufactured so that they place a dense layer of foam between
the furnace and the trailer truck. This will allow for a furnace to be
moved via common carrier. But the price of this special shock absorbing
skid plus the freight charges will always exceed the air ride costs. The
most that you can pay, for exclusive use of an air ride trailer, between
any 2 points in the country is under $3000.00**.
Of course, the customer can choose any shipping method that meets their
needs. We're happy to provide skidding and/or transportation services
at our cost. Or, you're welcome to talk to our providers yourself. Gary
Gardner at MAYFLOWER can be reached at 800/777-1205. (** be sure to ask
Gary for the LORAC rate, when you talk to him). Expert skidding services
can be provided by BEAU’S CRATES in Tempe, AZ. Our men there are
Mark or Matt, 480/966-3630.
With all due respect to them, the money you'll spend on a skid will be
more than you'll pay for the services of professional machinery movers (riggers)
at the receiving end. These folks can take the furnace from the truck
to exactly where you want it located in your facility, without the slightest
bit of shock or vibration reaching the furnace. Riggers will also eliminate
concerns of any possible damage occurring to your building (tile floors,
etc.) which can sometimes happen when customers handle things themselves.
Of course, this advice is intended strictly for customers in North America.
International container shipments should always be not only skidded, but
also crated with the furnace inside protected by a corrosion resistant
barrier bag!
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What are the optimum Air or Nitrogen flow rates
for thick film firing furnaces?
We believe that the following guidelines should apply to correctly
set atmosphere controls on continuous belt furnaces.
Entrance curtain and exit curtain: The curtain flows pressurize the inside
the furnace, P1 being > P2 (outside the furnace), the curtain flow
will be outward, preventing (along with the physical barrier of the stainless
baffle curtains) ambient from entering. This can ideally be verified while
observing the effect of curtain flow on a piece of dry ice, cigarette
or other method of producing a visible gas.
Burnout/Firing Sections: These sections of the furnace have a defined
length, width and height. This can be measured and expressed in terms
of volume. Furnace operators are encouraged to determine the recommended
atmospheric changes per hour (or minute) for their particular application.
Firing thick film materials, the recommended flow should consider the
amount of material being fired, both in terms of thickness and printed
paste area. Once the volume of the furnace and the frequency of completely
replacing the atmosphere within the furnace are known, it's fairly straightforward
to determine a flowmeter setting which will achieve the desired result.
For example, a furnace with a volume of approx. 1 cubic foot, could be
expected to receive a complete change of atmosphere once per minute (60
times per hour) if the total burnout and firing section flows were 60
SCFH. Two changes per minute would be achieved with a total flow of 120
SCFH. It's then useful to know the approximate TGA properties of the materials
being fired to know how to set the ratio of flow between the burnout and
firing sections.
Modern mythology, where the firing section flows are set higher than the
burnout section, "to push the flow counter to belt travel" is
incorrect. The majority of the flow should be in the burnout section where
it is required based on how the materials are reacting to an increase
in temperature. The direction of the flow within the furnace is determined
by the venturi (exhaust) settings, not by other means.
The exhaust flows are ideally set by monitoring the temperature of the
exhaust gas as it exits the furnace. With a thermometer in the exhaust
and a piece of dry ice at the entrance curtain, gradually increase the
exhaust flow, until the temperature becomes constant or until the curtain
flow reverses and becomes drawn into the furnace.
With some experimentation, a combination of flow rates can generally be
achieved that will optimize both the exhaust and curtain flow performance!
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What factors adversely effect overall yield?
It definitely is not only possible, but very practical,
to eliminate your furnaces as a contributing factor to ANY reduction in
yield that you may be experiencing.
As you know, there can be many causes for an apparent firing reject. Cleaning
might be the way to eliminate the problem in one furnace for MONTHS, while
an adjacent furnace might be producing rejects again in a few days. This
could be due to a furnace belt which has far outlived its usefulness and
must be replaced.
With regular, scheduled visits I can maintain the furnaces in an impeccably
clean state and I can also monitor your profile accuracy, airflow quality
and quantity and any other factors which could adversely effect overall
yield.
Service contract preventative maintenance/cleaning visits can be scheduled
to occur at off hours or on weekends at no additional charge. This can
result in a significant savings of manufacturing time, because the furnaces
must be allowed to cool prior to cleaning.
You can be guaranteed exact dates when your furnaces will be maintained
as far into the future as you deem practical. Furnaces will often maintain
cleanliness differently based on the effectiveness of their atmosphere
control system and the amount of material being processed. It may take
some experimenting to determine the schedule that works best for your
particular equipment. But, you are never obligated to any long term commitments.
If we begin on a monthly basis and after a few months, you decide to extend
the period between visits, that is entirely your prerogative.
Please give us a call to let us know if we can get you a proposal.
We'll need a list of the furnaces which are to be included in the maintenance
program and your estimate as to their current condition.
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How do I clean the the furnace muffle?
Muffle cleaning is always performed with the furnace turned OFF. The internal
temperature must be below 50C. Please allow ample time for natural cooling
to occur, furnaces should NEVER be force-cooled with fans
or extremely high atmosphere flow.
The first cleaning step is removal of the furnace belting. The belt is
placed into an ultrasonic cleaning tank, in D.I. water. The water is changed
as often as necessary, until it remains clean, indicating that all loosely
adhering particles have been removed.
Unlike abrasive cleaning methods, when using ultrasonics the tightly adhering
protective oxides are not disturbed. Therefore, the ultrasonic
cleaning process has absolutely no detrimental effect on belt life.
Next, all of the existing dust and contamination is vacuumed from inside
the furnace using progressively longer lengths of 1" diameter pipe, which
can be threaded together to reach the entire length of the furnace. The
vacuum which is connected to the last pipe in the chain, should be a Clean
Room Vacuum with at least a .25 micron HEPA filter.
The muffle is then thoroughly scrubbed using wire brushes and specialized
tools which fit into the crevices and corners of the furnace interior.
Once all particulate is dislodged, the vacuuming is repeated. The vacuum-scrub-vacuum
procedure greatly reduces the introduction of particles into the room
surrounding the furnace.
Special attention is paid to cleaning the furnace exhausts, the drive
system drums and the entire belt return system. The belt is then reinstalled
and the furnace is ready to be restarted.
You can expect the furnace cleaning procedure to completely eliminate
dust and contamination as the cause of any reject parts or other problems
you have experienced.
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