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Frequently Used Flexible Packaging Terms


Bags on a Roll: Polyethylene bags can put up on 3" cores (e.g. cardboard tubes) and perforated between bags. The bags can be oriented either to tear off side-by-side with the opening to one side and perpendicular to the perforation or to tear off top-from-bottom with the perforation causing the opening for each bag as it is torn off the role. Examples of the is product include rolls of trash bags and rolls of produce bags found in most grocery stores.

Bag Signs: A bag sign is a plastic bag that is stretched over a wire frame. Typically, the wire frame is rectangular with an open end. The two wire rods on the open end are pressed into the ground and the bag sign is stretched over the wire frame to display the printing on either side of the bag. The bag signs made by Custom Poly are printed on LDPE film with a special backing to prevent sunlight conditions from interfereing with the printed message on either side of the sign. Bag signs are most familiar during election years when candidates use them as part of their campaign efforts.

Coroplast Signs: An alternative to bag signs are signs printed on coroplast, a material that resembles corrogated cardboard but is made from plastic. Coroplast signs are cut to size from larger sheets of material and then screen printed. They are displayed in yards by inserting wire stakes into the corrogation in the edge of the sign board and then pressing the other end of the stakes into the ground. Because coroplast signs are printed using a screen print process, it is possible to do four color process printing to obtain amazing full-color results.

Flat bag: A flat plastic bag is the most basic poly bag product. Flat bags have no gussets (folds or pleats). Flat poly bags can be either sidewelded or bottomsealed. Flat bags have a width and a length dimension and one open end measured across the width. Sleeves are poly bags with two open ends on opposite ends.

Zipper Bags: Plastic bags can be made with narrow interlocking tracks, made from the same plastic material, that are sealed into the poly bag near the open end across the width. These are called zipper bags, zip top bags or reclosables. Typically, the user places an item in the bag and then pinches one end of the track near the seal at the side and slides their fingers across the width of the bag, forcing the poly zipper tracks to merge and lock together. Most zipper  bags can be opened and resealed a number of times.

Bottom-Fill Zipper  Bags: For many production processes, it is much easier to slide a product into a zipper  bag from the bottom with the zipper already sealed. The open end of the bag is then sealed with equipment that heats and presses the plastic at the bottom of the bag and fuses the the two sides together. The bags used in this type of production process are known as bottom-fill zipper bags.

Tamper-Evident Zipper Bags: Many zipper bag users (e.g. retail goods, food products) want to ensure that only their consumer utilizes the resealable zipper in the packaging. One way to accomplish this is to use a tamper-resistent zipper  bag, which is basically a bottom-fill zipper bag that is pre-sealed above the zipper. A perforation in the plastic above the zipper allows the consumer to tear away the seal and access the zipper and the bag contents. Another type of tamper-evident zipper bag has a zipper that will not open once it has been closed except by destroying the bag itself.


HDPE: High Density Poly Ethylene. This plastic film is most familiar for its use in standard retail store check-out bags (e.g. Wal-Mart).

LDPE: Low Density PolyEthylene. This plastic has a softer texture than HDPE and is often used for custom printed bags used for retail product packaging or for higher quality check-out bags (e.g. department stores, mall stores, Target). Most of the custom flexible packaging items we manufacture are made from this plastic film.

Mil or Thickness: The term "mil" is commonly used in reference to the thickness of plastic film. It expresses thousandths of an inch as a whole number. For example, "1.5 mil" is one-and-one-half thousandths of an inch (.0015 inches). When this measurement is included in a bag dimension, it refers to the thickness of the film for one side of a bag. Bag thickness is an important consideration when making packaging decisions if you have concerns about moisture transmission, ultra-violet light exposure or punctures/tears in the packaging.

MVTR: Moisture Vapor Transmission Rate. This is a measurement of how quickly moisture and air will pass through the plastic film. It varies depending on the type of resin used to extrude the film, the thickness, and the film's ambient environment.

PP: PolyPropylene. The Polypropylene we use to manufacture flexible packaging is a mono layer, random co-polymer containing ethylene. It is a better barrier to moisture and air transmission than either LDPE or HDPE (see "MVTR"), but not as complete as the stiff, crystal clear PP that is similar to cellophane.


Bottom Gusset: A bottom gusset is formed by pushing the bottom, closed end of a flat poly bag up, into the bag interior making an accordion type fold or pleat at only the bottom of the bag. Bottom gusset bags are always sidewelded. The size of the bottom gusset is measured across the open or unfolded dimension. The bottoms of plastic bags constructed in this manner sit more or less flat on a surface when filled.

Bottomseal: Poly bags that have a heat seal across the width at the bottom of the bag are manufactured by making a seal about 1/4" up from the bottom.

Header Seal:
A header seal is heat sealed across the width of the bag, usually near the top. Many custom poly bags used at point-of-purchase displays have been loaded from the bottom and sealed by the company that packages the product. A header seal keeps the contents of the bag from rising to the top of the bag and interfering with the hang hole or being obscured by printing in the header area. For small quantities or more flexibility to make changes on printed headers, a cardboard header with a hang hole can be used, but making plastic bags with a header seal as an integral part of the bag is more cost effective at larger quantities.

Side gusset: A side gusset is formed by pushing the side edges of a flat poly bag into the bag interior, making accordion type folds or pleats along both sides of the bag. Side gusset bags are always bottomsealed. The size of the side gusset is measured across the open or unfolded dimension. Plastic bags constructed in this manner conform well to box-like shapes.

Sideweld: Bags that have a heat sealed edge along the length of each side are manufactured by sealing the sides.

Zipper Profile: On zipper bags, the zipper profile is the narrow band across the width of the bag where the zipper (the two narrow, three dimensional inter-locking ribbons of plastic) is sealed to the bag film near the opening. When expressing measurements for a zipper bag, the height measurement (from opening to bottom) is measured from the zipper profile to the bottom of the bag.

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