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Plastic Stretch Film Tech Terms
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| 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. This area typicall 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 wraped 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 wraped 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 a 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. 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 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. Not only can you use stretch wrap machines for creating tension but Stretch Wrap Dispensers. 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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