What is draw control?

Draw control is simple. Any time you drive the web with two or more rollers, you have draw. Draw control usually implies that there is no feedback used to monitor or control tension between the two driven rollers. Each driven roller can have its own motor or share a motor using a timing belt, chain, or line shaft. Driven rollers in draw control work as a team. They speed up together, slow down together, and they don’t necessarily care what the web is doing.

Draw is a machine property. Many times people imagine that if they know the machine draw, that they know the web strain. Not true! Machine draw can change the web’s strain, but you need to know the input web’s strain before you can calculate the strain of the web in a draw zone.

Sometime draw is called ratio control. People will talk about setting the ratio control to 1:1.05, which is the same as 5% draw.

Why use draw control (open-loop speed ratio control)?

Sometimes, the web strain or stretch is more important than the tension, especially if your web is inelastic (a web that doesn’t fully recover when tension is removed).

Draw is a great way to limit or control the stretch of your web. For nonwoven, crepe paper, and low-modulus films, applying too much tension will stretch the product beyond its elastic limit. Even high-modulus materials such as polyester or steel will be best handled in draw control when their temperature weakens their mechanical properties.

If you have a series of driven rollers, think about the draw of each roller relative to the first driven roller—what I would call the total draw. Having a small 0.5% stretch between driven rollers doesn’t seem like much until you do it ten times for a 5% stretch that yields or breaks your web.

Small draws are common for any multi-roller driven sections (presses, slitters, small-wrap over rollers, and unnipped pull rollers) and for low-modulus or easily yielding webs. Large draws are used for film orienting and for separation after sheeting.

  1. Fragile nonwovens – If you handle a nonwoven web with little binder holding the fibers together, applying tension may pull the fibers apart. Often, the product properties, such as pressure drop or density, are a direct function of how much stretch or slippage occurs in the fibers during handling. Since the force to pull the fibers apart may vary, running in tension control mode will create product variations. However, running in a percent draw mode independent of tension controls the critical variable, percent stretch.
  2. Low yield point webs – Similar to nonwovens, other webs, such as crepe paper, need to ensure that the desired stretch properties of the web are not pulled out by web tension. Setting a process in a known draw ration limits the amount of yielding or pull out in the process, leaving the desired mechanical properties in the web for the customer,
  3. Orientation – Film machine direction orientation (MDO) or other stretching processes create desired material properties or thickness by a set percent change in the web’s length via a speed ratio.
  4. Delamination – When peeling apart a laminate, the peel force may vary, making delaminating under constant tension a challenge. Delaminating in draw control ensure the laminate is pulled apart, letting the peel force determine the tension it needs.
  5. Closely spaced driven rollers – In many processes, two or more closely spaced, high wrap angle rollers are used to create high traction without nips. With no space for tension feedback, the ease of driving the rollers from a single motor (or closely coupled motors), and good relative diameter accuracy, draw control is a logical option. Closely spaced draw controlled rollers are common in pairs (such as an “S” wrap pull roller station) and in larger series (such as the driven section of a slitter-rewinder or the driven rollers before or after a machine direction orientation process).
  6. Registered processes – Registered printing and die-cutting process are commonly run in a synchronized speed mode, using a coordinated time-based motion to maintain pattern-to-pattern machine direction alignment.
  7. Roll transfers – At-speed splicing on both winders and unwinders requires speed matching of the new input roll or core prior to being attached to the process web. Immediately after the new roll or core are coupled, winders or unwinders will usually switch over to torque or tension control.