Corrugating Medium (CM) / Fluting - normally comes in various grade measure by basis weight. The range of basis weight normally ranged from 100 gsm to 200 gsm.
In the past, the lowest basis weight was 125 gsm and today new paper machines are able to produce up to 60 gsm; presently this grade is still at introduction stage . The lower the basis weight the weaker the paper in the construction of boxes. However, due to cost competitiveness and better box design technology most of the CM used today ranged from 90 gsm to 110 gsm.
Nevertheless, the application of lighter weight (90-110 gsm) CM is much dependant on the required strength of boxes and the ability of a corrugating machine. A corrugating machine corrugate the CM and joint them together with 2 pieces of Linerboard to form a board. This board is eventually converted into a printed box.
In view of the development in paper technology, we expect more lighter weight CM to use in the construction of boxes. The advancement in corrugating technology also shortened the learning curve in applying the light weight CM paper. Lastly, the new advancement in chemistry to strengthen the properties light weight paper after corrugating had further encourage the used of light weight CM for the construction of boxes.
Kraftliner (KLB), Kraft Top Liner (KTL) and Testliners (TL) (100% made of recovered paper) - are commonly produce between 125 gsm to 440 gsm.
The most common type used in the Northern America is Kraftliner board which has the highest strength and can withstand high humidity. Most of the kraftliners are produced using virgin pulp which are normally categorised as Unbleached Softwood Kraft Pulp (USKP). USKP has longer fiber length compared to any hardwood species. The lengthy fiber properties gave this paper grade a superior strength compared to Kraft Top and Testliners. The high content of lignin in the fiber also allowed this type of grade to withstand cold weather. Lignin provide a coat to reduce the absorption of moisture. Normally, KLB are used to construct boxes to store tobacco leaves, seafood products and other products which are stored at cold room. This grade are normally produced in North America, Scandinavian Countries, Australia and New Zealand due to sustainable forest to provide a long term supply of USKP. Normally the basis weight comes from 125gsm, 150gsm, 175gsm, 250gsm, 275gsm, 337gsm and 440gsm.
The development of Kraft top (KTL) and Testliner (TL) are mainly by the Asian mills due to non availability of long fiber pulp (USKP). In the recent years, the replacement of KLB by KTL and TL had become more apparent. Many believed that such substitution occured as a result cost competition and some believed that existing KLB was above the box specification. Most of these grades are produced in China, Japan, Korea, Taiwan and other South East Asian countries. At present, sources mentioned approximately 85% of Asian boxes uses KTL and TL as outer layer/liner and remaining 15% uses of KLB due to high quality requirement. The common basis weight for KTL and TL are 110gsm (latest), 125 gsm, 140gsm, 150gsm, 170gsm, 180gsm, 220gsm, 250gsm and 280gsm.
B. Quality measurement
Test methods
There are many quality measurement used to detect the strength of CM. The most common ones are Concora Test, Ring Crush Test and Plybond. A good reference of test methods and parameters can be sourced from TAPPI (a body that develop standards for paper industry).
The concora test (CMT) is to apply pressure to fluting to simulate the strength of the board when passing through the printing pressure. Ideally, the fluting will collapsed after a certain force and that is the concora strength. It is measure in Newton or kgf.
The ring crush test (RCT) is to used a piece of paper in fold it to ring form with application of force from top. Once the ring collapsed the strength of paper will be determined. This test is also applied to KLB, KTL and TL and mesured in Newton or kgf.
Bursting strength (BFT) which is commonly measured to determine the strength of a piece of paper before it punctures. This measurement was critical in the past when railway was used as the main transportation. The high movement of goods between locations required the boxes to withstand side forces . However, it is no longer essential as today's transportation mechanism are much developed. Nevertheless, there are still many customers (box users) who requested for such test. They are measured in kpam2/g or kgf/cm2
There are also other tests for Containerboard paper namely SCT (short compression test), plybond (measured delamination), surface roughness and cobb (water absorption test). The selection of test is essential to ensure the said paper fit it purposes.
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