My team is struggling with controlling warp during production of single face pads using bleach kraft bag paper as the liner and medium.
The pads are curling down to the liner side of the single face after the knife section at the sheeter. I believe this is a paper/material issue since this problem only occurs with this paper combination. When using natural kraft as the liner and the same material as the medium we rarely have warp issues.
Is it the case that the slower drying side of the pad is the side that the pad will warp to? Example: liner dries slower than the medium causing the single face pad to warp in the liner direction or down.
Neither our roll labels, nor the specs provided with them list the moisture content. I probe the rolls with a Delmhorst P2000 moisture meter and find that they run within .5%. Our machine does not have machine temperature measurement systems implemented, but we do have IR thermometers on site which we use as needed.
I also I have noticed that our preheat roll does not rotate while running which may be part of our issue.
Do you have any recommendations to help alleviate this issue?
Typically with a rotating preheat roll the temperature is more constant and consistent, there is a more even distribution of heat, and better thermal transmission. So, you may want to look into that roll. Uneven moisture indeed does cause warp. The greater the moisture content in the paper, the more the paper shrinks and the board will warp toward that direction. Also, the difference in drying speed between the two sheets of paper can cause warp. Regular Kraft is stronger and less porous than bag, or sack, kraft. The bag kraft tends to be more elastic and this could potentially be increasing your warp. The fibers and the porous nature of the substrate don’t provide the stability to stand up to the forces applied during the curing process. Below are links to some document that may help you team get engaged. – Ralph