Crosslappers are of two types:
1. Horizontal crosslapper
2. Camel back crosslapper
The laying speeds are higher with the former and most of the modern cross lappers are therefore of horizontal type.
In crosslapping card web is laid by a laying/feed lattice on a delivery lattice, which runs at 900 of it. Speed of laying lattice is many times that of feed lattice and as a result, a number of layers of card web are laid one over the other in a zig zag manner.
Thus a batt of much higher thickness and weight is made from card web. The number of doublings depends upon the relative speed of laying and delivery lattice, web width and laying width.
Let
♠ Laying lattice speed = F m/min
♠ Delivery lattice speed = D m/min
♠ Laying width = L m
♠ Web width = W m
♠ Time taken to lay one layer = L/F
♠ Distance moved by delivery lattice during laying of one layer = (L/F)×D
♠ Number of doublings =W/[(L/F) × D].
Thus, the number of doublings can be increased by reducing delivery lattice speed or increasing laying lattice speed. To get the same batt weight and increased doublings card web has to be made finer and laying lattice speed increased.
Production rate of crosslapper therefore depends upon the number of doublings. Since the direction of web is changed by 900, the preferential orientation of fibres in the final batt is in cross direction. As a result, a higher strength is found in cross direction than in longitudinal direction. Laying speed depends upon the type of fibre, fineness and length, crimp, sticking tendency, web weight, spinfinish and humidity and temperature of department. With short and waste fibres, lower laying speeds are used.
Laying speed with modern cross lappers range between 150 - 270 m/min with card web input speed ranging from 120 to 200 m/min. Control systems are usually incorporated to control the card web as it enters the top carriage. This minimises web control problems encountered when top carriage approaches card and this facilitates higher laying speeds.
A major problem in crosslapping is the disturbance caused by removal of air entrapped in the fibres of card web leading to unevenness. To overcome this problem, NSC has developed a system to remove entrapped air. In addition, controls are used to prevent web accumulation at the reversal point. Dilo crosslapper prevents disturbance by air turbulence by guiding the web between two conveyor belts. Laying width in crosslappers can be varied from 1 m to 10 m with step-wise adjustment of 25 - 50 cm. For paper maker felts laying widths go beyond 10 m.
