The basics of stretch wrap
Ngày đăng : 18/07/2019

Stretch wrap is a highly engineered product that is meant to provide unitization, protection, and containment properties to your pallet loads. But without the right knowledge, you may be throwing your money away, not getting what you need for prices that are way to high.

The basics of stretch wrap

When it comes to stretch wrap, it’s critical to know some basic terminology. Here are some of the most common terms used, as well as a downloadable sheet with additional terms available here.

Banding: the act of applying multiple wraps of stretch film to a certain area of a load to reinforce and/or unitize a number of layers or products. Banding is used when loads need to be completely secure. Banding packaging makes it much easier to ensure the product will reach its destination in good condition.

Blown stretch wrap: a film extruded by the blown balloon type inflation system. Beads of resin are fed through a heated machine that has a circular die. The heated resin is forced through the die and then blown out vertically into a bubble. As this formed bubble finishes the process of being transformed into rolls of stretch film, it is cooled by the surrounding air. This type of blown stretch film generally costs more to make because the output per hour is less than with cast films.Typically blown film is a tougher but noisier film than cast films.

Cast stretch wrap: molten plastic 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 because of this process. Stretch film extruded by the cast method typically have greater tear resistance, greater aesthetic values and are quiet to unwind, which is not typically found in film manufactured by the blown method.

Cling (single side – two sided – differential): a bonding agent added to stretch film to increase the stickiness quality of the film. 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.

Film force (film tension): 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: 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 memory: the key 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 self. This ensures that the load integrity is maintained even as the load shifts or settles during transit. It is this film memory that 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 (strapping) where prestretched film memory takes up the slack and continues to secure the load.

Film recovery: describes the extent in which a material returns to its original shape and size after having been deformed or subject to stretching. This assures that the film will maintain a tight load during transportation. 
Force to load: varies by number of wraps, gauge being used, force to load settings on machine, pre-stretch level of machine. The most typical force to load seen is 15-26 lbs in the field.

Gauge: a unit of measurement that is used to measure stretch film thickness or caliper. An example value would read as: 40 gauge, 60 gauge or 100 gauge. Also note: Often used as a synonym for film thickness.
Load types (A-load, B-Load, C-Load): 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: This load is normally cubed with no protrusions.This type of load has clean edges and are the easiest to wrap.

• B-load: This load type may have a few protrusions creating irregular sides. These pallets will need a better performance film that offers more puncture resistance.

• C-load: The most difficult type of load. These pallets are very irregular with many corners and sharp edges that are particularly difficult to wrap. This type of load requires a high performance film.
Neckdown: as film is stretched there is a tendency for the film to narrow (similar to bubble gum). This narrowing of the film is called neckdown. Neckdown 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 neckdown.

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: 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.

Wrap parameters: the variable settings on a stretch wrapping system that can be adjusted to meet the load retention requirements of the product to be wrapped. These parameters typically include settings like: Top wraps, bottom wraps, film carriage vertical speed, turntable (or arm rotation) speed, and film force.