Manufacturing
|
BUMP
TUBING FROM CONCEPT TO REALITY
-Charles
Sparacino, Davis-Standard Corporation, USA
|
Puller/Cutter and Control
The puller in a tubing line must grip the product tight
enough to ensure that there is no slippage between the product and the belts,
and convey it at a constant rate of speed through the sizing tank. Any deviation
in the speed either from an inaccurate drive or from slippage between the belts
will show up as an inaccuracy in the final product. To ensure that there is no
slippage belt material must be chosen that establishes a high coefficient of
friction between the belt and the product. Pullers that use rollers in place of
belts introduce distortion to the product. Therefore a puller with a set of
belts having a contact length from 12 to 18" is most desirable. Puller
drives should have a speed regulation of .5% or better.
The process of making bump, flare or tapered tubing, requires the puller to
change speed also at a constant rate in order to form the required shape.
Originally this was done with timers or switches to signal the drive when to
change speed, and the internal air
control when to change pressure. This cumbersome method limited the range of
products and the rate of production.
The microprocessor of today has changed the method by
simplifying the user interface and having the puller/cutter control communicate
directly to the drive of the puller, the servo of the air control, and the
cutter. As an example using a part configuration where the minor diameter runs
for a distance then transitions to the larger diameter than runs for a distance
then transitions back to the small diameter. The part is then cut at the sort of
the cycle, again in the center of the bump or larger diameter, and at the
overall length.
To set up the control we would set the speed of the puller in
FPM or MPM, and the internal air pressure in percentage, for the smaller
diameter, then the distance traveled before beginning the transition. Next set
the speed of the puller, and the percentage of air pressure for the transition
to the larger diameter. Then the speed, air percentage and distance for the
larger diameter. Then set the transition and return to the small diameter, and
last set the length of the small diameter. The downward slope normally requires
more points of adjustment to establish a smooth transition, due to the melt
strength of the product. Most controls offer twenty-five or more points for each
axis. To complete one cycle the cut is then positioned where required. This is
done by first positioning the cutter relative to the puller, using an offert
dimension, then inputting the desired cut length dimensions.
Conclusion
The entire process we have described sounds complicated and
difficult to operate, however there are process controls available that can
integrate a system to the point that the all of the process parameters once set
can be downloaded from one recipe command. All of the component parts of the
system can be monitored and controlled from one operator control station. A
process control of this type will also include alarm and event logging,
networking capability, process and equipment troubleshooting via real time and
historical trending, as well as Statistical Quality Control.
The main thought I would like to leave you with is that for
many years the extrusion of tubing in general and specifically bump tubing has
been considered more of a black box art than an engineering science. With the
introduction of the microprocessor, reliable drive control, and the vacuum sizer
tube extrusion has evolved into a determinable, valid process.
(Because of space limitation, we could not publish
detailed information available with us in the form of an article, which can be
supplied to MPDS readers on request.)
|
|
|
|
