Glossary of Stretch Wrapping Terms

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Banding: Applying multiple wraps of stretch film to a certain area of a load to reinforce and/or unitize a number of layers or products.

Between Frame Rails (BFR): The distance between the 2 outside frames on a conveyor section. Sometimes referred to as effective BFR, which represents the usable width of the conveying surface.

Blown Stretch Film: Film extruded by the blown balloon type inflation system. Typically blown film is a tougher, with greater puncture resistance. Blown stretch film is noisier, hazier, with less aesthetic value than cast stretch film.

Bottom Wraps: 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 lower area to increase the load stability of the unitized load.

Brake Roller: Mechanical roller that provides resistive force that reduces film feed rate being supplied to the load. The resulting drag increases tension between the load and the stretch film roll.

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Cast Film: Film produced from synthetic resins (such as polyethylene) by the cast extrusion process. In this process, the molten resin is extruded through a slot die onto an internally cooled chill roll. Alternatively, molten plastic is extruded over full width of a die, then cooled and crystallized over a drum. Cast film typically is quieter, more transparent and glossier than blown film.

Clarity: Freedom from haze; transparency.

Cling: Cling is a bonding agent added to stretch film to increase the stickiness quality of film so film sticks to itself, but not the product. Cling is desirable. Layers of applied stretch film bond to 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. The bonding agent is most commonly applied using co-extrusion where it is a layer that is co-extruded during the manufacturing process.

Co-Extrusion: Co-extrusion is the process of feeding, melting and / or pumping materials through multiple extruders, then merged to create a multi-layer film.

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Dancer Bar / Dancer Arm: Pivoting roller that measures the required film tension in a pre-stretch system. A dancer bar provides feedback to the control system in order to compensate for the varied feed rate demands caused by load corners, and thus maintains constant film tension during wrapping. As the pallet rotates, the stretch film needs to accelerate and decelerate around the corners to maintain a constant film force (or tension) on the load. The dancer bar pivots as the load demands more or less film fed.

Dart Drop: Technique used to measure the impact strength or sturdiness of a film. A test by dropping a crescent shaped weight (dart) onto film.

Degradation: Change or break-down in a material's chemical structure.

Dimensional Stability: Absence of dimensional change of a material when subjected to changes in temperature, humidity, heat or aging.

Directionality: Tendency for materials to have properties imparted by the flow direction through a machine.

Down Gauge: Use of a thinner film than had been previously used. Often, down gauging is desirable to lower material costs if load and / or package stability is not compromised.

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Elastic Recovery: Stretch film's ability to recover and return to its original form when stretched, strained and / or deformed.

Elmendorf Tear: Testing means for measuring a material's ability to resist tearing forces. This method creates a tear in a sample material, then measures the amount of force needed to tear the sample apart. Test result value is referred to as the tear value.

Erasable Programmable Read-Only Memory (EPROM): Electronic device chip that provides a means for storage and communication for a microprocessor program associated with the PLC.

Extruder: A piece of equipment that uses mechanical and thermal actions to change solid polymer into a molten polymer.

Extrusion: A manufacturing process of forms a thermoplastic film by forcing the polymer melt through a shaped orifice. Resulting extrudates are then configured to the fabrication of the end product.

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Film: Generally used to describe a thin plastic material usually not more than 75 micrometers (0.003 inch) thick.

Film Density: Ratio of weight of a body to weight of an equal volume of water at a specified temperature.

Film Feed: Speed or velocity at which film is applied to a pallet load. 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, film tension would increase as at the corners causing potential damage or film breakage issues.

Film Force / Film Tension: Measured in pounds, a retaining force applied by stretch film on the product being wrapped. During wrapping, film is fed out at a constant tension. By delaying the response speed of the carriage film feed, film tension is increased. A film force dial is typically located on the control panel or carriage.

Film Force Release: Feature 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: 1) Stretch film pulling out of the clamp at the beginning of the cycle and 2) detaching of the trailing tail at the end of the cycle are typically remedied by film force release.

Film Memory: Film memory is the structural foundation 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 original unstretched state. Prestretching's use of memory ensures that load integrity is maintained as a load may shift or settle during transit. The use of film memory differentiates prestretched film from non-prestretched film or other means of unitizing. For instance, settling that occurs during shipment can loosen other methods of unitizing such as strapping. Prestretched film memory takes up the slack and continues to secure the load.

Film Overwrap: Length of stretch film that folds over the top of a wrapped load, improving load retention. Overwrap amounts can be changed by adjustment of a load sensing photo eye.

Film Tail: Stretch wrap end piece created when cutting the film at the end of the wrapping cycle that is subsequently applied to the next pallet load to start a wrapping cycle.

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Gauge: In North America, film thickness is usually given in gauges. A 100 gauge shrink film is one mil, or 1/1000 of an inch thick. In Europe, film thickness metric is the micron. A quick equivalency equation is: 1 mil = 25.4 microns.

Gloss: Surface shine or sparkle. In LDPE stretch films, gloss refers to the amount of light that is reflected from stretch film surface. High-gloss attributes are typically found in cast stretch films.

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Hand Wrap / Hand Film: Stretch film designed to be post-stretched or tensioned stretched by staff manually wrapping film around a pallet load. Typically hand wrap does not have the stretching abilities of machine wrap and is wound on smaller and lighter rolls for easier manipulation.

Haze: Lack of clarity or inability to see through a film. Measured by the percentage of light not transmitted through a film sample. Stronger haze is a typical characteristic of blown stretch films.

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.

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Idler Roller: Idler rollers are used to bias stretch film direction as it travels through the prestretch carriage. In some cases an idler roller ensures that stretch film is held against prestretch rollers to prevent the film from slipping by the prestretch roller.

Impact Strength: Resistance of a material to shock, defying rapidly applied destructive forces such as from dropping from significant heights and / or hard blows.

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Load Types: Stretch film and machine manufacturers have divided the types of loads wrapped into three load types based on the degree of difficulty to wrap the load or product.

A-Load: Simplest load type with a standard pallet, no sharp edges, clean sides without obstructions that may interfere with application of stretch film.
B-Load: May have a rough pallet, uneven sides and / or an uneven top layer.
C-Load: Most difficult load type. May be unstable with severely uneven sides with sharp edges that may have an effect on wrapping speed.

Load Diagonal: Measurement of pallet or product load across the diagonally opposite corners. This dimension is critical in determining the overall size of a stretch wrapper. The dimension of the largest and smallest loads to be wrapped can effect which stretch wrapper is chosen for the application and / or how the wrapper needs to be designed.

Light Resistance: Ability of material to withstand exposure to light, usually sunlight or the ultraviolet part of the light spectrum, without change of color or loss of physical and / or chemical properties.

Low Density Polyethylene (LDPE): Flexible, odorless, transparent, 100% recyclable thermoplastic polymer film.

Linear Low Density Polyethylene (LLDPE): Blended more pliable and softer form of LDPE. LLDPE film has much more flexibility, tensile strength, and more conformability than LDPE.

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Machinability: Ability of film to run on packaging equipment.

Machine Direction (MD): Direction that film moves through the packaging equipment; perpendicular to film width.

Machine Film: Film designed specifically for film equipment.

Master Roll: Large roll of film wound during a film formation process, which is normally slit into smaller rolls for processing or shipment.

Metallocene: New polyethylene resins developed using Metallocene change the polyethylene chain structures resulting in a new breed of stretch films. Metallocene stretch films can achieve increased puncture resistance and clarity while blends offer balanced film properties and universal stretch percentage applications.

Micron: Metric measurement used for measuring film thickness. One micron is equal to one millionth of a meter.

MIL: Measurement used for measuring film thickness equalling one thousandth of an inch.

Mullen Test: Widely used on film packaging materials to determine the relative bursting strength.

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Neckdown: Narrowing of film width as the film is dispensed and stretched. Neckdown reduces the coverage a revolution of stretch film provides thus potentially increasing the number of revolutions required to wrap a pallet load. The larger the distance between the primary and secondary prestretch rollers stretching the film, the larger the neckdown.

National Electrical Manufacturers Association (NEMA): An association that rates electrical devices for uses in various environments. For example, NEMA 12 rated devices are intended for indoor use and provide a degree of protection from circulating dust and dripping noncorrosive liquids. NEMA 4 rated devices are intended for indoor/outdoor use and provide a degree of protection from blowing dust and splashing noncorrosive fluids.

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OD: Outside diameter. Often used to describe the diameter of a roll of film, or the max dimension of a machine's roll diameter capacity.

Opacity: Material's resistance to transmission of light through it.

Oxidation: Material's reaction, and likely degradation, to oxygen.

Overlap: As stretch film is typically applied to a pallet load, applied layers of stretch film are partially applied over previous layers. Overlapping film layers improves load retention. Slowing the vertical movement of the stretch film carriage increases the overlap. Quickening the vertical movement of the stretch film carriage decreases the overlap.

Overwrap: Amount of stretch film applied over the top of the load. As stretch film reaches the top of the load, vertical movement of the carriage continues so film angles over the top of the load. Stretch film on top of pallet loads creates a downward force on the load. Overwrapping 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.

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Pallet Covers: Poly film covers commonly used to protect pallets from dirt, dust and/or conceal pallet contents.

Post-stretch: Stretching film by using the load to pull the film out at the same time as film is applied to the load. Although post-stretching allows for some increase of film tension, tension levels are inconsistent, can damage loads, and possibly break the film. Also known as tension stretch.

Prestretch: Process of stretching the film in a prestretch carriage prior to applying it to the pallet load. As stretch film passes through the film carriage, it threads around the primary roller and secondary rollers. The secondary roller is usually larger with rotational speed geared to be faster than the primary roller. This difference of size and speed, where the secondary roller is pulling the film from the primary roller, causes the film to stretch between the two rollers, thus prestretching the film prior to wrapping the load.

The prestretch process can increase film strength, improve load integrity, reduce amount of stretch wrap film needed, and save on stretch film packaging costs.

Many stretch films are designed to achieve optimal prestretching values that can be attained with leading-edge high-performance stretch wrap machines.

Prestretch Carriage: Assembly on a stretch wrapper that stretches the film prior to applying it to the pallet load. Located on a vertical traveling slide, the prestretch carriage applies a spiraling layer of stretch film to the pallet load, encapsulating the load in stretch film.

Prestretch Ratio: Measure of how much a film is elongated by the prestretch process. Prestretch ratio is defined as a percentage change in film length. For example, a prestretch ratio of 200 signifies a change in length of +200%. The film length in this case is three times the length it was prior to stretching.

Primary Roller: 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.

Programmable Logic Controller (PLC): Electronic microprocessor device that stores and automatically executes a series of programmed commands that produce a machine's sequence of operation.

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Resistance: Resistance of film to tear. This attribute is quantified by measuring the force needed to propagate an initiated tear.

Roll Formation: Term denoting qualitatively how evenly, smoothly, and regularly film is wound on a roll.

Roll Stock: Any flexible packaging material that is in roll form.

Roping: Gathering the full width of film using a bar or roller to create a rope. Stretch film that has been roped is nearly unbreakable and is often used to lock a pallet to the product. Roping can be relatively simple to accomplish with a turntable style stretch wrapping machine. With rotary arm style stretch wrapper, an air cylinder can be used to actuate the roping process. In some cases, roping eliminates the need for other packaging materials such as strapping.

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Secondary Roller: 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.

Silicon Controlled Rectifier (SCR): Motor control board that controls a DC motor's rotation speed.

Slip: Measure of coefficient of friction (COF). Ability of film to move more or less easily over hard plastic, metal, ceramic platforms, or against another piece of film. High slip means low COF. Low slip means high COF.

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Tensile Strength: Measurement of maximum amount of force a material can take without breaking. The greater the tensile stretch measurement, the stronger the material.

Tension Stretch: Stretching film by using the pallet load to pull the film out at the same time as film is applied to the load. Although post-stretching allows for some increase of film tension, tension levels are inconsistent, can damage loads, and possibly break the film. Also known as post-stretch.

Top Sheet Dispenser: Equipment that applies a poly sheet of plastic top sheet to the top of a pallet load to provide a water resistant layer. The top sheet film is unrolled, cut to size, and applied to the load automatically by the top sheet dispenser. Once applied, a top sheet is subsequently wrapped with stretch film to secure it in place.

Top Sheet Film: Poly plastic film that is applied to the top of the pallet load.

Top Wraps: Revolutions of stretch film applied to the top portion of the pallet load to be wrapped. As top layers of pallet loads are susceptible to shifting in transit, additional layers of stretch film are applied to top layers of pallet loads. These revolutions are typically set using the top wrap counter located on the stretch wrapper's control panel.

Tear Resistance / Tear Strength: Ability of a film to resist the propagation of a tear.

Tensile Strength: Amount of pull a film can withstand without tearing apart or stretching.

Threading: Placing of a web material through various rolls and stations of any web-fed press such as a printer, laminator, or wrapper in preparation for production.

Tracking: Film that follows a desired path on a packaging machine without constant adjustment is defined as tracking well.

Translucent: Permitting passage of light, but diffusing it to such a degree that objects cannot be seen clearly; not quite transparent.

Transparent: Allowing transmission of light so that objects can be clearly seen through the material.

Transverse Direction (TD): Direction across the width of film, perpendicular to the machine flow direction.

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Ultra High Molecular Weight (UHMW): Type of plastic commonly used in applications where a strong linear bearing between sliding assemblies is required. Low wear and reduced friction properties make this a commonly used product in stretch wrap equipment.

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Vapor Barrier: Layer of material through which water vapor will pass slowly or not at all.

Variable Frequency Drive (VFD): Device used to provide full control of an AC motor's shaft rotation speeds. VFDs offer a wider range of features as seen on Silicon Controlled Rectifier (SCR) DC motor control board. Features include, display diagnostics, multiple-step speed control features, and ability to reset controls resulting from voltage problems such as power surges.

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Web: Continuous length of paper, film, foil, or other flexible material as it is unwound from a roll and passed through a machine.

Wrap Parameters: Variable settings on a stretch wrapping system that can be adjusted to meet load retention requirements of the product being wrapped. Typical parameters are bottom wraps, top wraps, carriage vertical speed (effects film overlap), turntable or arm speed (effects wrapping speed), photo eye (effects film overwrap), and film force (effects load retention).

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Yield: Area per unit of weight, usually expressed as square inches per pound.

Yield Strength: Amount of stress a material can withstand without permanently having plastic deformation. Prior to reaching the yield point, material will elastically deform but will return to the original shape once stress is removed. The yield point is the precise stress degree that permanently deforms the material.

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Zippering: Lack of resistance to an initiated transverse direction (TD) tear or cut. Once initiated, the tear will rapidly spread.

Please contact Pro Pac or call 888-318-0083 for your stretch wrapping materials and stretch wrapping machinery.