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When stretch wrapping
you want to stretch the film as far as possible
because this reduces the amount of film that must be
taken from the roll to wrap a load, saving money.
If a 48"x48"x60" high
load is spiral wrapped with 2 layers of film top and
bottom, 4" of film over the top of the load, 4" film
overlap between vertical (spiral) layers, and 1" of
film neck-down (a typical wrap pattern), then the
following annual film costs prevail, if we wrap
10 loads per hour single shift (2,000 hours per
year), and pay $35 for a 20"x5,000' roll of film:
| 10% stretch (handwrap or full web non-prestretch
applications)..... |
$20,618 |
| 40% stretch..... |
$16,200 |
| 110% stretch..... |
$10,800 |
| 250% stretch..... |
$6,400 |
| 350% stretch..... |
$5,200 |
It is obvious that,
given any wrap volume at all, stretching the film as
far as possible will pay for the mechanical device
necessary to do so with a single year's film savings.
It may pay for the whole machine.
Why not just use a
thinner film rather than prestretch? The tensile
strength of a film is a result of its manufacturing,
and is higher for thicker films. Tensile strength does
not change as the film is stretched. If we used a
thinner film to begin with, we sacrifice the
load-holding abilities of the film.
Three major
problems prevent achieving high stretch levels with a
"conventional" brake system (a non-powered stretch
device in which the pallet pulls on the restrained
film to stretch it):
Most manufacturers use
electromechanical brakes, or clutches, to restrain the
roll of film for stretching. These brakes are not
precision parts and were not designed for the purpose
for which they are being used. They vary in
effectiveness, just as the brakes on your car are more
effective at slow speed, and vary dramatically when
they are cold vs. when they are hot. Film stretch
levels change as the brake heats up with use and as
the roll of film accelerates and slows (or stops) its
rotation during the wrap cycle. The end result is that
these brakes cannot stretch film to high stretch
levels because of their inherent inefficiencies.
 Also,
when the web width of film is not supported (usually
between rubber rollers) when it is being stretched,
much of its elongation is taken from the width of the
film. If the web width is supported by non-powered
rubber rollers (Durastretch I and II) which are driven
by the film being pulled through them by the product
being wrapped, the film will "neck-down" after leaving
the last rubber roller - though not nearly as much as
if it were not supported at all.
Narrower film forces us
to raise and lower the film elevator on a spiral
stretch wrapper slower, so we lose in additional
vertical layers and wrap time much of what we gained
in film length.
Also, unless we use a
motor to both stretch the film and relieve the
stretch force from the load, the force required to
stretch to high stretch levels will crush some loads
and/or their boxes, or pull them from the
stretch-wrapper.
"Powered prestretch"
has become so popular because it eliminates the
shortcomings of a "conventional" brake system. But,
powered prestretch as it generally exists today has
restrictions of its own.
Video: High Profile, Spiral, Semi-Auto, Prestretch,
Photo Eye*
It takes both force and
time to stretch film to its limits. In fact, ASTM
testing of stretchable materials designates both force
and time requirements to insure that all similar
materials are tested equally.
The stretch wrap
industry knows that stretching film quickly reduces
the amount of stretch that can be achieved and causes
films to be used at levels far below their ultimate
potential.
A quick look at the
technical data for any currently popular
stretch film will demonstrate that the film can be
stretched as much as 600% in laboratory testing.
With today's "average"
powered prestretch unit a stretch film is elongated
just under 150% in the span of approximately
one-sixteenth of a second.
If the distance between
the two rollers is increased to allow more time for
stretching the film, more of the elongation comes from
narrowing of the film. Again, narrower film forces us
to raise and lower the film elevator on a spiral
stretch wrapper slower, so we lose in vertical layers
and wrap time what we gained in film length.
 One
costing concept describes both the amount of
film "stretch" received, and the resulting width of
the stretched film. It is called "Film Gain," and is
expressed as a percentage - as is film "stretch".
If a 20" wide piece of
film which is 10" long is stretched until it is 20"
long, the film has been "stretched" 100%. If this film
sample is now only 10" wide, it has still been
"stretched" 100%, but its percentage of "Film
Gain" is 0%.
The film sample still
contains 200 square inches of film (20" x 10").
High "Film Gain"
efficiency is much more important to users of stretch
wrap equipment than "stretch" levels (which it
encompasses) because "Film Gain" efficiency translates
directly into lower wrap costs and quicker wrap times
for customers. Because the film is wider, fewer
vertical spirals of film are required to wrap the
load.
A Durastretch system
will give a "Film Gain" as high or higher than any
other comparable prestretch device because it is not
only adjustable in stretch level, its design keeps the
film as wide or wider than any competitive equipment.
"Effective Film Gain"
is more important than "Film Gain" because it affects
the survivability of the wrapped load as well as the
cost to wrap the load.
"Effective Film Gain"
is the "Film Gain" of stretching the film, subtracting
the film stretch lost as the film recovers (or
shrinks) after it is stretched.
Video: Film Threading and Gears*
There are two ways to
hold a load together in transit with film:
Wrap the load with
enough stretched film that the force of the film
after it has recovered will successfully hold the
load together in transit.
Wrap the load with
film under tension. This method uses much less film.
Most powered
prestretches today attempt to wrap a load with film
under tension. The problem is that most mechanical
devices which control the "load-holding", "load
tension", or "compression" force of the film going to
the load are not precise, and you must use more film
to get a shippable load.
A Durastretch powered
prestretch system, because of its design, will use as
much as 50% less film to deliver a shippable load when
compared to almost all other stretch-wrappers with
powered prestretches stretching the film the same
amount (The mechanical details of how this is done are
described below.)
No stretchwrapper
delivers a higher "Effective Film Gain".
All Durastretch powered
prestretch systems provide for independently adjusting
the load-holding force of the film around the load -
regardless of the amount of film stretch, load size,
shape, position on turntable, or turntable speed
selected.
The operator adjusts
Durapak's Durastretch system, by turning a knob, to
the proper load-holding ("Load Tension" or
"Compression") force of the film around your product -
adjustable from only two (2) pounds of force to as
much as the film being used will allow - strong enough
to hold the load together, but not so strong as to
crush light or soft products or boxes or to topple
unstable loads during the wrap process.
If you desire, the
control for load-holding force can be placed inside
the control cabinet, making it "non-operator
adjustable."
The importance of
consistent load-holding force cannot be overstated.
Besides not allowing your products to ship undamaged,
an inconsistent force-to-load, or too low a force can
easily double the cost of film used by allowing the
stretch in the film to be lost, decreasing the
"Effective Film Gain".
Durastretch power
prestretches incorporate a Nautilus-shaped cam to
provide automatic progressive resistance from
the prestretch motor's speed-control dancer-bar's
return spring. This system is vastly superior to
so-called "Full Authority" film dancer bars.
A powered prestretch
requires either a Nautilus-shaped cam or a complex and
expensive electronic system with a
load-force-measuring beam (as in an electronic scale)
to actually select a load-holding force for the film,
automatically maintained on both the corners and sides
of the load, without manually changing the
dancer-bar's return spring - even with a "Film Force"
knob (which actually only sets the motor's speed range
so that the speed-control dancer-bar can work without
either constantly stopping the prestretch motor or
locking-out at fastest speed in most machines.)
All electronic
units lower the load-holding force at the beginning of
the wrap cycle to allow their prestretch system to
begin operation without breaking the film. Doing this
applies film which will not help hold the load
together in-transit, wasting money.
A Durastretch system
automatically and constantly self-adjusts its motor's
speed to place an equal holding force on the entire
load, even during wrap-cycle start-up.
Durapak chooses to use
a Nautilus-shaped cam because it is not electronic,
and is simple in concept but superior in results.
1.
They work.
2.
The machine to use prestretched film costs less than
the machine to prestretch films. ($1,625.00 to $2,375
less for a machine to use 20" film.)
3.
These new films cost more than prestretching the film
yourself. (The prestretched roll of film costs
about 20% less per foot, but our machine will stretch
a foot of film up to 4.5 feet long. You don't
stretch "prestretched" film, you "tension" it around
the load, like handwrap.)
4.
The "prestretched" films do not allow for the control
of load-holding forces that a powered prestretch
system does.
Durapak's Durastretch
III system is one of the very few systems which
stretch film by progressively increasing stretch force
as the film the supply roll and travels between three
rubber rollers in the pre-stretch unit (multi-staged
stretch.) This increases the time during which the
film is stretched. The progressive forces which
stretch the films are adjustable at three points in
this stretch process. This is why the Durastretch III
system stretches film further than virtually all other
units.
 Durapak's
Durastretch II system stretched the film as it leaves
the supply roll and travels between two rubber
rollers in the pre-stretch unit (similar to
competitive units.)
The close proximity of
Durapak's rollers in both of these systems, plus the
fact that Durapak uses its Durastretch system's motor
to totally stretch the film before it
leaves these rollers, minimizes neck-down during
elongation. Thus, the width of the film is not
significantly reduced as the length is increased,
giving a higher "Film Gain."
In many competitors
systems a pre-stretched motor only helps, or "Power
assisted", the lead in stretching the film. This
results in increased neck-down.
The close proximity of
Durapak's rollers in both of theses systems, plus the
fact that Durapak user Durastretch systems motor to
totally stretch the film before it leaves there
rollers, minimizes neck-down during elongation. This,
the width of the film is not significantly reduced as
the length is increased, giving a higher "film gain".
In many competitive
systems a prestretch motor only helps, or "power
assists", the load in stretching the film. This
results in increased neck-down.
Some competitive
equipment manufacturers interconnect their rollers
with chain instead of using more expensive gears.
Roller-chain stretches, requiring re-tensioning and/or
link removal to keep the rollers that stretch the film
from "snapping" in relation to each other; hard on the
film.
 Some
competitors use different sized rollers. It is
important to the long lifetime of the prestretch
system that as much film as possible be in contact
with the rubber roller to insure that the roller gets
a good grip on the film. The film must be stretched
rather than slide (unstretched) around the roller(s),
particularly as the rubber on the rollers hardens and
becomes contaminated over time. Decreasing the
diameter, and therefore the circumference, of a roller
is counter-productive to long-term goals.
Virtually all other
prestretch designs, except Durapak's, incorporate
rubber belts which change the machine's stretch
performance rather obviously as they wear, swell, or
break.
A one degree
misalignment in pulleys can reduce a belt's life
expectancy to one-fourth of normal. Adjustable pulleys
are naturally misaligned.
Because of necessary
design limitations, it is not a simple task to replace
these belts.
 Durapak
does not use rubber belts (unless "Operator Selected
Stretch" is ordered as an option) except for our
"Automatic Film Threading" POWER-THREAD option, which
has no effect on stretch levels.
Some competitors turn
their prestretch rollers in the same direction to
simplify film-threading, at the cost of film
neck-down, and possible loss of stretch.
To stretch film between
two rollers turning the same direction requires
additional space between the rollers that the film
must span, which increases neck-down, decreasing
Effective Film Gain.
The amount of contact
between the film and stretch rollers is also reduced,
increasing the probability that the film will slip on
the roller, eliminating stretch.
While elongation levels
are usually never changed in day-to-day operation, one
must have the ability to adjust the stretch
performance of the equipment to receive the highest
obtainable performance and value from stretch film, or
to test new and improved films as they become
available. Stretch films are constantly being
improved.
Gear sets for changing
stretch levels on any Durastretch system are available
FREE from Durapak on an exchange basis.
By using gears, a
Durastretch system lets its owner "lock up,"
preventing unauthorized change, the highest obtainable
stretch level for any brand, type or gauge of stretch
film insuring the lowest possible cost per wrapped
load.
Because the stretch
level is mechanically fixed (gears), it cannot vary -
giving Durapak customers a "Guaranteed Stretch Level."
If one wishes to
frequently make major random adjustments to
a Durastretch system's stretch level, "Operator
Selected Stretch" for Durastretch III, which
incorporates rubber belts, gives the machine operator
this control.
Any Durastretch system
self-adjusts to changing roll diameters. No operator
adjustment is ever required.
Semi-automatic film
threading, a standard feature of a powered Durastretch
system, is important in protecting your employee
because it does not require him to place his hands
near moving rollers.
A "Jog" button
activates the threading operation or clears the
rollers on Durastretch III, pulling the pivot roller
activates the semi-automatic threading operation on
Durastretch II.
Durapak's optional
POWER-THREAD automatically threads stretch film around
a Durastretch powered prestretch's rollers.
The film is placed over
a threading band, and the prestretch is activated by
pushing the prestretch's "POWER-THREAD" button.
 Within
seconds the Durastretch powered prestretch is threaded
and ready to use.
While Durapak's
standard semi-automatic film threading is not
difficult nor time-consuming, POWER-THREAD is safe,
instantaneous, and requires absolutely no dexterity
nor training.
POWER-THREAD is the
inexpensive solution to prevent threading problems
where untrained individuals will run the machine.
Unlike some of our
competitors, Durapak's semi-automatic and automatic
film threading do not require the amount of film
elongation to be restricted. Some competitor's
film-threading designs require that the film be
stretched 150% or less.
Metal anti-personnel
guard(s) between the rollers prevents an employee from
being caught or hurt.
Some competitor's
anti-personnel guards are plastic; not advisable in an
industrial environment. In some instances the plastic
guards are also a mechanical part of the prestretch,
which will not function if they are broken.
This popular option
saves labor on semi-automatic machines with powered
prestretch where the fork-truck operator also runs the
stretchwrapper.
At the end of the programmed wrap
cycle (Picture #1) the film is automatically cut near
a corner of the load being wrapped (Pictures #2 & 3.)
The fork-truck operator can now
remove the load from the turntable without getting off
of his fork-truck to sever the film (although he may
want to wipe-down or tuck-in the film-end "tail"
later.) (Picture #4)
This
option can be ordered with or without the increased
control over the size and placement of the film "tail"
(Picture #4) that a variable speed turntable (with
controllable deceleration) offers.
Durastretch I functions identically to the Durastretch
1.5 system, except that I is not powered. The load
being wrapped powers the unit by pulling the stretch
film through the system.
This
tensioning system self-adjusts for the changing roll
diameter, however there is no load tension adjustment
and no automatic threading nor automatic cut-off
options. Load holding (load tension or compression)
force is varied by changing film thickness or stretch
levels.
If you
want to adjust the load holding force of the film on
the load mechanically, order our optional "Load
Tension" adjustment knob for Durastretch I.
However, it adjusts "Effective Film Gain" but not
"Film Gain" to adjust the "Load Tension" force, which
is not as efficient as Durastretch II or III.
Non-powered Durastretch is a component in a powered
Durastretch system and acts as a back-up tensioning
system in the unlikely event that the powered system
fails and requires lengthy repairs.
The
stretch you obtain is dependent on the film you use,
and the care you give it. Good film stretches farther,
so long as it has not been damaged.
Every
Durastretch's performance is adjustable - by changing
gears. Replacement gears for different stretch levels
are available from us FREE so long as the old gears
are returned within 30 days.
You can
"order" ANY stretch level when you order a machine.
All you need to know is that the film you will use
will stretch that far and still hold your load
together. However, we recommend that the stretch level
you order be approximately 2/3 of the maximum stretch
you can achieve with your film, to allow for
occasional minor film defects.
That said,
the "usual" stretch levels actually ordered for
"production" performance are:
Durastretch III
(powered).....200-265%
Durastretch II (powered).....170-230%
Durastretch I (non-powered).....80-120%
"Load
Tension" adjustment.
Prestretch
"On-Off" switch, with power indicator lamp for safety
in threading film.
Durastretch JOG Switch(III).
Durastretch JOG Switch if optional POWER-THREAD is
ordered (II).
Fuse for
prestretch system.
A
Durastretch system is fabricated from structural
tubing as well as 1/2" and 3/8" steel plate.
All parts
of a Durastretch system are mechanically fixed in
place, some competitors components are secured by
hinges and/or latches, which wear and allow
misalignment over time.
A
Durastretch system's rollers are manufactured of a
solid rubber compound tubing with a special textured
surface which will not allow film slippage. They also
cannot delaminate or allow "tackifier build-up" as
many competitive rollers can. Tackifier build-up
allows film slippage, reducing or eliminating film
stretch.
Because of
DURAPAK's design, film on Low Profile and RA-80
Overhead machines can be applied to within 1" of the
turntable, conveyor, or floor over which the machine
has been installed. (Actual results depend on the type
and gauge of film used - some films neck-down more
than others, and some have longer core extensions past
the end of the film. Both effect how close to the
turntable, conveyor top, or floor the film will
actually come.)
The
standard spiral Durastretch system is designed to
perform with all films to 12" maximum diameter and up
to 20" width, full-web to 60" width. Other film widths
are an available option.
Because
Durastretch units are self-contained, powered units
only requiring a 115V power feed, they are easily
installed on older machines; ours and others.
Simple,
reliable 115 V, 10 amp. AC electrics are used.
A powered
Durastretch system is additionally protected by
current limiting.
An
"On-Off" switch for a powered Durastretch prestretch
system is prominently placed for use in case of an
emergency.
The
prestretch control panel also contains a "Function
Indicator Lamp" to verify that every part of the
equipment is active.
Durapak
uses no PLC, custom "microprocessor" or "discrete
logic" (solid state) control circuits. These can be
difficult or impossible to troubleshoot, and expensive
or impossible to repair. Replacement boards or modules
can be very expensive.
NO special
parts are required. All components of this equipment
are of U.S. commercial manufacture and are readily
available at most supply houses world wide.
Component
fusing is utilized to provide maximum equipment
protection and safety.
Wiring
conforms to the National Fire Protection Association's
National Electrical Code (N.E.C.) standards, not just
the section Industrial Machinery (Article 670 - which
covers supply conductors and nameplate data only) some
manufacturers claim as compliance.
Only
Underwriter's Laboratory (U.L.) listed/approved
components are used.
The
powered prestretch motor is Totally Enclosed, Fan
Cooled (TEFC).
For
personnel safety all machine components are metal and
bonded. No plastic machine components are used.
The
"anti-backlash" circuitry of our prestretch system
stops the rollers immediately, preventing film from
wrapping the rollers, a common and frustrating
occurrence on some competitive equipment.
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