Key Terms Every Multi-Wall Valve Bag Buyer Should Know

Valve BagThe following is a list of key terms and phrases commonly used when referencing multi-wall paper valve bags (PVSE bags):

Anti-Skid - Coating applied to the outer surface of a bag or bag that prevents it from sliding.

Basis Weight - The weight, measured in pounds, of 500 sheets of paper in that paper's basic sheet size. 

Bulk Density - The weight of a unit volume of a substance expressed as pounds per cubic foot or kilograms per cubic meter or equivalent units.

Burst Strength - The resistance of paper to rupture under pressure expressed in pounds per square inch.

Cubic Capacity - The volume of material that a bag will hold.  Cubic capacity cannot accurately be determined from measurements of an empty bag, but must be determined by filling trials, taking into account entrained air in the product. 

Ear - The pointed extensions which appear at the corners of bags after they are filled and closed.

Face - The front of a bag or the side opposite the side of the bag on which the longitudinal seam is located.

Flat Tube - A tube that has no gussets.

Freeboard - The distance between the top of the product and the top of the bag when the bag is placed upright.

High Performance Extensible Paper - A stretchable paper with a smooth appearance and high energy absorption properties.

Kraft Paper - A paper made entirely from wood pulp produced by a modified sulphate pulping process.  The paper is coarse and noted for its strength.  The name is obtained from the German word "kraft" meaning strength. Its natural color is brown, but by using semi-bleached or bleached sulphate pulp it can produce lighter shades including white.  Kraft paper used for shipping bags and bags is most commonly made in basis weights from 25 to 80 lbs.

Liner - A poly film inserted in the paper plies of a bag to provide barrier protection for the product packed.

Patch - A paper patch pasted over the top and/or bottom of a pasted valve bag to prevent sifting. The patch adds strength to the bag. 

Perforations - Small vent holes punched through the walls of a bag to allow air to escape from the bag rapidly during the filling operation.

Ply - A wall of a bag. A single thickness or fold of paper.  Plies are described from the inside ply of the package to the outside ply.

Polyethylene - A thermoplastic material composed solely of polymers of ethylene abbreviated PE.

Porosity - The characteristic of a sheet of paper that permits the passage of air through the sheet. 

Protected Glue Seam – Interior fold style on the longitudinal seam of the bag designed to prevent abrasion between the product being filled and glue seam.

PVSE – Pasted Valve Stepped End

Reduced Valve – A valve tube opening that is smaller than the top and bottom width of the bag.

Sleeve, Double Trap - Separately applied offset pieces of paper and plastic film in the valve opening to prevent sifting.

Sleeve, Poly-lock - A separately applied piece of plastic film in the valve opening to prevent sifting.

Sleeve, Tube - A separately applied piece of paper and/or plastic film in the valve opening to prevent sifting.

Sleeve, Tuck-in - An outer sleeve extension on a valve bag that is folded and tucked in by hand after the bag has been filled.

Slide Angle - The angle at which paper will start sliding when in contact with another piece of the same paper and under standard test conditions. 

Stepped-End Tube - A tube in which each ply is cut individually in a stepped pattern rather than all plies cut together and straight across.  Successive plies extend, one beyond the other, so that when the bag is pasted shut, each ply is pasted to itself.

Trapping - A term originally used as a measure of how well one ink printed on top of another. Trapping involves creating overlaps (spreads) or underlaps (chokes) of objects during the print production process to eliminate misregistration on the press.

Tubing – The forming of continuous webs of paper into a cylinder on a bag-making machine and then cutting to length to produce a tube for a given bag size.

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