Thermoplastic Film Variables

Polyethylene is sealed by controlling the level of three variables - TEMPERATURE, TIME and PRESSURE.   Any one of these applied at an extreme level will make the polyethylene seal to itself.

For example, if two pieces of poly film are touching one another, they will weld together if the time is long enough regardless of the intensity of the other two variables. The amount of time required for this to occur will vary depending on the composition of the poly film. We recommend that polyethylene flexible packaging be stored in a location that is no warmer than room temperature since temperature is also one of the variables for sealing.

The Polypropylene Custom Poly Packaging uses to manufacture bags is a monolayer random copolymer that contains ethylene. This polypropylene allows our equipment to make a good heat seal and it provides a better moisture and air barrier than HDPE (High Density Polyethylene) or LDPE (Low Density Polyethylene).  Although it has good optics, the polypropylene we use is NOT the high barrier, stiff, easily torn and crystal clear polypropylene that is similar in feel and appearance to cellophane. That material is OPP or BOPP and is not something we use to manufacture flexible packaging.

Freezer Film is LDPE that typically contains EVA (Ethyl Vinyl Acetate) to maintain pliability at low temperatures.

WVTR (Water Vapor Transmission Rate) is a measurement of how quickly air and / or moisture will pass through a membrane, in this case, low or high density polyethylene and polypropylene films. All of these plastic films used by Custom Poly Packaging will allow air and moisture to migrate through the walls of the film. The key to an effective choice when selecting plastic films for flexible packaging is the rate that this occurs. Without noting the technical formulas and procedures to measure this event, the general progression of WVTR when applied to the plastic films used by Custom Poly Packaging to manufacture plastic bags, from highest WVTR to lowest is LDPE, HDPE and Polypropylene. Low Density Polyethylene is generally more permeable than HDPE or PP (Polypropylene).

Co-extruded LDPE - Co-extruded plastic films can be used to manufacture flexible packaging or bags with special characteristics. Coextruded film is manufactured to have two or more layers of poly film that emerge from the extrusion process as one. Coextrusion of multiple layers of polyethylene resin can provide one or a combination of higher strength, extra WVTR barrier, different performance between one side of film to the other and/or make the plastic film visually different from one side to the other. For example, Custom Poly Packaging routinely uses co-extruded poly that has a white layer on one side and a black layer on the other to manufacture polyethylene yard and lawn bag signs. The black poly on the inside of the bag sign blocks sunlight to insure the viewer only sees one side of the poly yard sign.

Colored FIlms - Color pigments can be added to polyethylene films in varying degrees to make the film appear "opaque", tinted, or clear with color tint that can be seen through. The limiting factors to the opacity that can be obtained are the thickness and the strength requirements of the poly. Actual "opacity" is relative to the thickness of the polyethylene film. If too much color resin is added to the recipe, the plastic film will become too weak to be useful. The most opaque films at the thinnest gauges are the co-extruded films.

Tinted Films - There is a difference between tinted polyethylene film and clear polyethylene film that is tinted. Film that is merely tinted may be translucent (cloudy or milky) and make it difficult to see objects on the other side of the film. Clear film that is tinted has a hue that is transparent. Although clear, tinted film has color, it is easier to view objects on the other side of the film than film that is tinted only.

Metalized LDPE - Adding a layer of aluminum metal to Low Density Polyethylene reflects light and heat and provides a good barrier against the transfer of heat through the plastic film. Metallized poly bags can be used to obtain the thermal barrier properties, the chrome / shiny appearance or both.

Recycled Film - Low density polyethylene and high density polyethylene that has been recycled and reprocessed can be used in some flexible packaging applications where FDA/USDA approved virgin poly films are not required, such as the manufacture of trash bags. Custom Poly Packaging makes Colorful Trash Bags® in about 10 different colors. Colorful Trash Bags® are typically purchased by organizations like Boy Scouts, school groups and clubs that want to raise money and offer their customers a useful consumable that is a little different from the usual candy or cookies. Some virgin resin is added to improve the repro film and facilitate the extrusion process.

Reprocessed polyethylene films are currently prohibited from use in food and medical packaging. It is difficult to obtain consistent results when manufacturing flexible packaging from recycled or reprocessed plastic films. The way in which reprocessed polyethylene will accept ink and the ultimate strength of the reprocessed film packaging is unpredictable.

UVI (Ultra Violet Inhibitor) can be added to poly resins to slow the degradation of the film when the flexible packaging is exposed to ultraviolet light. This is a concern when the bagged product is set outside in sunlight as it is sometimes for landscape products.

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