Web Lines | Why Bigger Cores?

Published: November 16, 2012, By Timothy J. Walker

Bigger cores avoid many winding defects. I like to joke that the ideal wound roll is a gigantic core with one wrap on it. Think about it.

Bigger cores have fewer layers for a given length of web on the core. Fewer layers of build up mean less magnifying of long-term thickness variations, less significant hardbands, and less winding-induced bagginess. Fewer layers mean less near core pressure buildup and high-pressure defects, such as coining, core impressions, blocking, starring/spoking, and core crushing.

A smaller change from core diameter to final roll diameter means a smaller torque range requirement for a given roll and a winder with a wider tension range able to handle a great variety of products.

The greatest benefit of bigger cores is in torque transmission and associated telescoping. Near core layers have two strikes against them in the challenge of transmitting center winding torque into outer layer winding tension. First, they are at a mechanical disadvantage to the tension at the outside of the roll. Second, they have less area per layer to develop the friction needed to transmit torque. Bigger cores eliminate the near core layers and their low torque transmission capacity.

Diameter will increase, but the diameter increase in moving from a 3-in. (75-mm) to 6-in. (150-mm) inner diameter core is not much. For a 20-in. (0.5-m) diameter roller, moving from a 4-in. outer diameter to a 7-in. outer diameter (assumes a 0.5-in. core wall thickness) only increases the final diameter by 0.85 in.

Yes, I know your customer wants everything on a 3-in. core. Yes, I know that 6-in. cores cost more than 3-in. cores. Yes, I know that 6-in. shafts and core chucks cost more than 3-in. shafts and chucks. This just means the change has to be justified.

Count up your waste from hardbands, core impressions, bagginess, blocking, spoking, and telescoping and see if the move to larger cores is justified. If you are winding rolls that simply move from your coater to your slitter, calculate the costs and savings of winding on reusable metal drum cores.

Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; tjwalker@tjwa.com; www.webhandling.com.