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Stretch film & stretch wrap packaging terms defined.
Blown
Stretch Film
Blown-film extrusion is a process for forming plastic stretch films. Blown
stretch film extrusion involves a molten resin extruded through a circular die
and filling the tube with air. The air within the tube stretches the film to
obtain the desired thickness. Typically blown film is a tougher but noisier film
than cast films.
Bottom Wraps
The revolutions of film applied by a stretch wrapping system to the lower
layers of a pallet or load. Typically more than one revolution is applied to
this area to increase the load stability of the unitized load. These area
typical where machine stretch films will break. Out automatic stretch wrappers
deploy the dual hydro stretch system to help prevent this problem
Brake Roller
A mechanical roller in some stretch wrap machines that provides a resistive
force that reduces the film feed rate being supplied to the load. The resulting
drag will create an increase in tension between the load and the stretch film
roll.
Cast film
Plastic sheeting or stretch film produced from synthetic resins. A molten resin
is extruded through a slot or flat die to form a thin film or molten sheet.
Cast stretch film typically is quieter, more transparent, and glossier than
blown film because of this process.
Cling (single side - two sided - differential)
Cling is a bonding agent added to stretch film to increase the stickiness
quality of the stretch wrap. This is desirable to allow the layers of stretch
film applied to bond to the previous layers effectively creating a single wall
of stretch film. Depending on the desired effect the bonding agent is applied
to one or both sides. It is most commonly applied using co-extrusion where the
bonding agent is a layer that is co-extruded during the manufacturing process.
Dual Hydro Stretch System
Dual Power Hydro Stretch provides 2 independent levels of pre-stretch: one for
the bottom of the load the other for the stress of the pallet wrapped load.
Over 70 percent of film breaks occur at the bottom of the load where the pallet
meet the load. The Dual Power systems allows you to set the pre stretch level
at the bottom of the load independent of the pre-stretch level on the rest of
the load. This allows loads to be wrapped tighter at higher stretch levels and
lower cost.
Film Feed
As a film is applied to a load the speed or velocity at which it is applied is
called film feed. The feeding of the film is rarely constant as it must be
accelerated and decelerated to compensate for the corners of the load to keep
the film tension constant. Without this corner compensation feedback the film
tension would increase as at the corners causing potential damage or film
breakage issues.
Film Force (Film Tension)
Film Force is the retaining force applied by the stretch film on the product
being wrapped. This force is typically measured in pounds. Film force is
created two ways. The second method is by delaying the film feeding out of the
carriage. During wrapping the film is fed out at a constant tension. By
delaying the response (speed) of the carriage film feed, you electronically
increase the film tension. A film force dial is typically located on the
control panel or carriage.
Film Force
Release
A feature typically used on automatic stretch wrapping equipment whereby the
film force feature is disabled for a time at the beginning and end of the
cycle. This feature prevents unwanted tension at the two points during the
cycle where added tension would create problems. The stretch film pulling out
of the clamp at the beginning of the cycle and the detaching of the trailing
tail at the end of the cycle are typically remedied by film force release.
Film gauge
Is the measurement used for measuring stretch film thickness.
Film Memory
Film memory is the most important reason for prestretching stretch film.
Prestretching stretch film creates a memory in the film, which causes a
continuous elastic effect as the film tries to return to its unstretched state.
This ensures that the load rigidity is maintained even as the load shifts or
settles during transit. This stretch film memory that differentiates prestretched
film from non-prestretched film or other means of unitizing.
Hand Wrap
Stretch film designed to be post stretched or tensioned stretched by personnel
without equipment is called hand wrap. Typically hand stretch film does not
have the stretching abilities of machine wrap and is wound on smaller and
lighter rolls for easier manipulation.
Home Position
Home position is the position of all the moving sub assemblies on a
stretch-wrapping machine when they are at rest and ready to begin a new cycle.
Load Diagonal
Is the measurement of a pallet or product load across the diagonally opposite
corners. This dimension is critical in determining the overall size of stretch
wrapper equipment. The difference in this dimension in the largest load to be
wrapped and the smallest can also effect how the machine needs to be designed.
The equation for determining a load diagonal of any pallet is /(L2 + W2).
Microns
Is the metric unit of measurement used for measuring stretch film thickness.
Neck down
As film is stretched there is a tendency for the film to narrow (similar to
bubble gum). This narrowing of the film is called neck down. Neck down reduces
the coverage a revolution of stretch film provides thus potentially increasing
the number of revolutions required to wrap a pallet or load. The larger the
distance between the two rollers stretching the film (Primary & Secondary
prestretch rollers) the larger the neck down.
Overlap
As stretch film is applied to a load the stretch film is typically applied so
that the next layer of stretch film is applied over the previous layer. This
overlapping of film layers increases load retention. By slowing the vertical
movement of the stretch film carriage the larger the overlap created.
Overwrap
Is the amount of stretch film applied over the top of the load? As the stretch
film reaches the top of the load, the vertical movement of the carriage can
continue so that the film angles over the top of the load. The stretch film on
top creates a downward force on the pallet load. This is also used when a
plastic top sheet or corrugated top cap is applied on the top of the load to
lock them in place.
Prestretch Film
Prestretch is the process of stretching the film in a
prestretch carriage PRIOR to (Pre-) applying it to the load to be wrapped. As
the stretch film passes through the carriage it threads past two rubber rollers
(Primary roller & Secondary roller). The secondary roller is usually larger
than the primary roller and its rotational speed is geared to be faster than
the primary roller. This differential of speed where the secondary roller is
pulling the film from the primary roller causes the film to stretch between the
two, thus prestretching the film between one and the other.
Prestretch Carriage
Is the assembly in a stretch wrapper that stretches the film prior to applying
it to the load. Located on a vertical traveling slide, the prestretch carriage
applies a spiraling layer of stretch film to the load thus encapsulating the
load in stretch film.
Primary Roller
Is one of two rubberized rollers located in the prestretch carriage used to
create prestretched film. The primary roller is the first rubberized roller the
film passes by and is usually the smaller of the two. Its speed is also geared
slower than the secondary roller.
Secondary Roller
Is one of two rubberized rollers located in the prestretch carriage used to
create prestretched film. The secondary roller is the second rubberized roller
the film passes by and is usually the larger of the two. Its speed is also
geared faster than the primary roller.
Tension
stretch
Tension stretch (or post stretch) is the method of stretching the film by
pulling (or tensioning) while the film is applied to the pallet. Although this
increases the film force, the load being wrapped limits this tension. Empty PET
bottles or light boxes will be crushed or damaged by high-tension stretch.
Layers of product can be swept of a pallet and film breaks also increase as the
tension stretch is increased.
Top Sheet Dispenser
Top sheet dispenser is a piece of equipment that applies a poly sheet of
plastic top sheet to the top of a load to provide a water resistant layer.
Typically, once applied, a top sheet is subsequently wrapped with stretch film
to secure it in place. The top sheet film is unrolled, cut to size, and applied
to the load automatically by the top sheet dispenser.
Top Sheet Film or Plastic Sheeting
Is the poly plastic sheeting film that is applied to the top of the load.
Top wraps
Top wraps are the extra revolutions of stretch film applied to the top portion
of the load to be wrapped. As the top layers are susceptible to shifting in
transit extra layers of stretch film (top wraps) is applied. These extra
revolutions are typically set using the top wrap counter located on the control
panel.
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