Modern Plastics Worldwide - March 2010 - dae uptlax

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EXTRUSION
Q: You mentioned a black mark on the back of a screw. What
is it there for?
AG: A black mark is put there to make it easier to count rpm,
to make sure the rpm display is working OK, and to get rpm
in case it isn’t.
In Figure 2 (p. 36, slide 13), there is such a black area around
one of the four holes in the back of the screw. This could serve
as a counting mark, but it should really be sharper and darker
to be more distinct, especially if the screw is turning fast.
I didn’t mention the four holes, as they didn’t relate to my topic
of numbers, but they are important, too—they are the place where
the screw pusher is fixed. Not all screws have this feature, but it is
quite useful to help push out the screw with minimal damage.
Another image showing the rear of the extruder is Figure 1,
with the screw-cooling fitting. There may be a mark on that screw
as well, but it can’t be seen. It wasn’t necessary on this machine, as
the fitting wobbled back and forth with each revolution and was
easy to see. (That’s not a good thing, as it stresses the connections;
a proper installation would have the fittings immobilized in a way
that allows rotation but no lateral movement.)
Q: Most of this information can be used for extrusion
blowmolding, can’t it?
AG: That’s quite true. Extrusion blowmolding (EBM) is fed
by extruders, and the same needs apply—especially to wheel
systems, which are basically pipe extruders with a specialized
takeoff. There are more numbers to be gotten, of course, in
addition to the ones I mentioned, such as blowing air and
details of parison programming and mold cooling, but this
is like any other extrusion (film, sheet, etc.) with specialized
needs for the cooling phase of the operation.
There is an interesting “marriage” of these processes—
twin-sheet thermoforming of hollow objects. This was done
more than 50 years ago with collapsible cubical containers
used for battery fluids, then used for hollow cases for tools
with a smooth outside and formed “nests” inside, and even
automotive gas tanks, using multilayer sheets.
Q: Who are the best providers of computerized data acquisition
systems for extrusion?
AG: Most extruder and controller makers will help their customers,
and sometimes will help others that don’t yet have
their equipment but are good prospects for sales. The missing
link is the person on the plant floor to inventory what there
is and how well it is working, and eventually do the actual
wiring from the instruments to the processor. This should
preferably be people from your own factory, as they need
someone to understand what is being done. Plus, if an outside
source does all the wiring and goes away, who do you call
when things go wrong?
I do know some companies and individuals who might
do such a service and remain on call for problems, but can’t
express such preferences in public, to maintain my position
of independence. With private clients, we have a relation of
mutual confidence and I can tell it more like it is. MPW
38 MARCH 2010 • MODERN PLASTICS WORLDWIDE plasticstoday.com/mpw