kraft is produced from chemical pulp produced in the kraft process. kraft paper with high elasticity and high tear resistance, designed for packaging products with high demands for strength and durability.Wood pulp for kraft paper is made from softwood by the kraft process.The long fibers provide the paper its strength and wet strength chemicals are added to even further improve the strength. Both white and brown grades are made. Kraft paper is then produced on a paper machine from the wood pulp. The paper is microcrepped to give porosity and elasticity. Microcrepping is done by drying with loose draws allowing it to shrink. This causes the paper to elongate 4% in the machine direction and 10% in the cross direction without breaking. Machine direction elongation can be further improved by pressing between very elastic cylinders causing more microcrepping.
A paper can be made of several layers of paper depending on the toughness needed.Kraft paper is produced on paper machines with moderate machine speeds. The raw material is normally softwood pulp from the kraft process.Maintaining a high effective sulfur ratio or sulfidity is important for the highest possible strength using the kraft process.The kraft process can use a wider range of fiber sources than most other pulping processes.
- Normal kraft paper is strong & relatively coarse. It has high tensile strength. The grammage is normally 140-300 gsm.
- Kraft paper is high elasticity & high tear resistance, designed for packaging products with high demands for strength & durability.
There are usually two tests conducted to measure the strength of the box:
- Bursting strength test
- Edge crush test.
Bursting strength is measured by applying the maximum hydrostatic pressure required to puncture the paper used in manufacturing the corrugated box.The measurement of bursting strength is define the strength of the boxes. Bursting strength is determined by a test. The paper is held between two clamps and pulled tight. This helps to create a uniform pressure throughout the material. The pressure on paper can hold in this state is the bursting strength. The structure of the paper determines its strength. 
Bursting Factor
Different Bursting Factor (BF) of kraft paper are available in India
16 BF
18 BF
20 BF
22 BF
24 BF
28 BF
The Bursting Factor of the papers can be calculated by dividing the basic weight of the specimen and is expressed in grams/sq. meter. Bursting factor is calculated to determine the amount of pressure that a paper or cardboard can tolerate easily before it collapses. It varies with the weight of paper.
2. Edge Crush Test
The second test conducted to measure the strength of the box is the Edge Crush Test, a performance test, directly relating to the stacking strength of a carton. A minimum of 6 bursts must be made, 3 from each side of the board.
- In single wall corrugated box, the maximum weight of the box and its content is 20 lbs. The maximum outside dimensions is 40 inches, and the minimum edge crush test is 23 lbs per width.
- In a double wall corrugated box, the maximum weight of the box and its content is 80 lbs, the outside dimensions were 85 inches, and the minimum edge crush test is 42 lbs per width.
- In a triple wall corrugated box, the maximum weight of the box and its content is 240 lbs in a box with dimensions of 110 inches, and the minimum edge crush test 67 lbs per width. Corrugated boxes can come in many different shapes and sizes, so it is easy to find a box that fits your needs. Unless otherwise specified, box sizes are always measured using inner dimensions.
Period.
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