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.

What determines draw in a machine?

In many machines, the draw is a fixed value. The machine designers determine the draw by selecting gear ratios and roller diameters. Many slitter-rewinders and line-shafted presses drive two or more rollers with one motor. Each driven roller’s rpm is determined by motor speed and the gear ratio of the motor to the rollers. The rpm turns in to a surface speed depending on the roller’s circumference.

Roller speed is the roller’s circumference times the roller’s rotational rate (in RPMs or radians/s). Either of these two variables can be used to create the draw. In some machines, two or more rollers are driven at the same rotational rate but have an engineered variation in diameter and circumference to create the desired speed differential and draw. Other machines will use uniform roller diameters, but vary the rotational rates by motor speed or gearing to create the draw. Many machines use a combination of diameter and gearing to create the draw. In any case, understand that your web, especially high modulus and thick webs, are quite sensitive to even small draw ratios, so carefully calculate and ensure draws are calibrated properly in all applications.

Some machines have a programmable draw, allowing you to dial in the ratio or percent draw between driven sections. Closed-loop tension control using pacer and follower driven rollers also will use draw, but the draw will be moving up and down to satisfy the tension trim control loop.

What is the relationship between machine draw and web strain?

Draw is a machine characteristic. Often it is confused with strain, which is a web property. Draw and strain both can be described in percent, but where strain is always relative to zero strain; draw can be relative to any initial speed. An untensioned 250 mm (10-in.) web sample stretched 2.5 mm (0.1 in.) has a strain of 1%. Given two driven rollers, if the first is driven at 100 m/min (300 fpm) and the second at 101 m/min (303 fpm), this is a draw of 1%. For larger draws, it is more common to talk about a draw ratio. If two sections are driven at 30 and 90 m/min (100 and 300 fpm) such as in length orienting film, the draw ratio is 3:1.

How does draw control create strain?

Here’s the crazy part about draw control. As simple as it is to design, how it creates strain and the corresponding tension is confounding. Let’s see if I can demystify it.

Imagine a machine section with two driven rollers, the first at 100 fpm and the second at 101 fpm. The draw is 1%. What will be the steady-state web strain in this draw zone?

If you run an elastic web through these two rollers, you would be correct to assume the web will be stretched 1%. Yes, but stretching 1% doesn’t mean the web has 1% strain.

The initial condition is critical to knowing the final strain. If I stretch a relaxed rubber band 10%, the strain is 10%. But if I stretch it another 10%, the strain will be 20%. Draw control is a similarly additive process. The draw or stretching will modify the entering condition.

Asking you to predict strain from draw is a trick question. I can’t estimate the draw zone strain unless I give you three values: the speeds of both rollers and the strain of the entering web. So let me ask the fair question. Let’s say the entering web is strained 0.5%. Now can you tell me the tension? Hmmm. It starts at 0.5%, we stretch it 1% more for total of 1.5% strain. Correct? Yes, 1.5% strain is the anticipated steady strain in the draw zone.

That was a qualified “yes.” I chose my words carefully. To truly know draw zone strain, we need even more information. Why? Because draw zones have a time constant that determines how quickly the draw conditions will get to steady state or respond to changes in upstream strain or roller speeds.

How are draw zones time dependent?

In the above scenario, I asked you to predict the web’s strain in a draw zone, given three data points: the web speed at roller #1, the web speed at roller #2, and the strain of the web at roller #1. This info is enough to determine the system’s eventual steady-state condition, but without knowing the initial strain in the draw zone and the time the system has been running, this still isn’t enough information to know the draw zone web strain.

The draw zone’s time constant is equal to the web length between the first and last draw-controlled rollers divided by the web speed. When any condition in the draw zone changes, it takes 3x the time constant to move 95% of the way to the new steady-state condition.

For example, a printing press with 50 ft of web from the first to last print station and running at 200 fpm has a the time constant of 15 sec (50 ft/200 fpm). If the input web tension (and strain) is changed, it will take 45 sec for the change to feed through the system.

I’ve seen operators chasing their tails after changing the infeed tension. They try to keep the multi-station press in registration before the draw zone reaches a steady state. At 45 sec, this isn’t a long wait, but if the press is running at low speed, say 50 fpm, then you have an agonizing 3-min wait before you see the registration return.

This time delay is the biggest negative of draw zones. Where closed loop tension control and open loop torque control will get to their steady state quickly, draw zones take time. This is like waiting for the hot water to get to the shower head in the morning. Your shower would continue to act like a draw zone if the hot and cold knobs adjusted the flow 30 ft. away. Each adjustment would require purging the entire pipeline before you felt the new temperature.

Draw zones also are poor in handling slack webs. Since web strains and draw zone percentages often are less than 1%, if the slackness in a draw zone is 5% of the zone length, it will be some time before the excess material is purged.

What creates tension variations in a draw zone?

Asking this question may be missing the point, since draw control usually is intended to control strain, not tension. But since draw control is used in tension control, it’s worth talking about how strain becomes tension.

All the variables reviewed so far (speeds, initial strains, time) will lead you to the draw zone’s web strain. To find tension, multiply the strain by the web’s modulus. Is the modulus constant? It would be nice to say “yes,” but material properties change from moisture, temperature, cross-linking, strain rates, and the big wildcard—viscoelasticity. In many processes, the web’s response to strain will be predictably elastic, but be on the lookout for unusual mechanical properties and the resulting drawing confusion.