Korsnas produce paper sacks in large quantities. One of their customers, Sainsbury's wanted to introduce potato sacks with a window on the side, so that the consumer can see the size and quality of the product. A rotary knife was put in line with the bag making machine (while the bags were still a continuous web), which was to be synchronised with the speed of the machine. The controller looks at signal received from a sensor and gradually adjusts the velocity of the knife until its cutting position is at the correct point on the bag. However rotary knives are expensive, and Korsnas wanted to be able to use one rotary knife to cut a range of bags with different sizes.
Pan Controls designed a solution to this which involved synchronising the knife to the motion of the web during the cutting phase. Normally the pitch of the bags needs to correspond to the circumference of the knife cylinder. If the bag pitch is longer than the knife circumference, the controller accelerates the knife during the non-cutting phase by the exact amount to make the cut of the next bag in the correct place. The operator merely has to enter the bag pitch and the effective diameter of the knife cylinder, and the controller calculates the correction tables automatically.
This novel technique could be used in a wide range of processes where a rotating part of a machine applies some operation to a continuous web (e.g. printing machines).
The control system which Pan Controls supplied enabled Korsnas to use a single rotating knife mechanism to cut holes in a range of different bag sizes.