MD and CD

Machine and Cross Direction on paper machine

 

In paper machine approach flow system, when stock passes through pressure screen, the fibers are oriented lengthwise. If the stock velocity from headbox slice is equal or less than wire speed, fibers which are already oriented lengthwise, will align in the direction of wire run. Fiber alignment can be altered to some extent if stock velocity is less than wire speed. So all papers have a definite grain direction due to greater orientation of fibers in the direction of paper machine run. This grain direction is known as machine direction. The cross direction is the direction of paper at right angles to the machine direction. Some of the properties vary with the MD and CD and hence the values are reported in both the directions. The sheet which have all relevant properties same or almost same in both direction are known as 'square sheet'.

 

While sheeting the paper, machine and cross direction are to be kept in mind and the sheet cutting to be done to suit the end use requirements. E.g. 1. All printing papers are to be cut in long grain (The biggest dimension in the grain direction). 2. Book papers fold better and the book stays open better if the sheets are out so that the machine direction runs up and down the pages. 3. Wrap around labels for metal cans and bottles are to be cut with the machine direction vertical to obtain greater flexibility about the can. Long grain and Short grain : The sheet is in long grain if the larger dimension is parallel to grain (MD) direction. The sheet is said to be in short grain if the larger dimension is parallel to cross direction (CD).

 

There is no sure way to determine the MD or CD of a sheet but one crude method which work is; cut a strip of about 1" wide and 2" long paper and moist it. Put this moist sheet on a smooth surface or hand. As sheet will dry it will curl. The direction of curl is CD as paper contract in CD more than MD while drying.