John Mazurkiewicz
Baldor Electric Co.
Fort Smith,Ark.

Automatic shrink-wrapping equipment most commonly uses conveyors for product transport. Usually, an adjustable-speed motor and drive control conveyor speed for proper feed rate into the machine.

To size systems properly, realistic cycle times for each labeling step must be defined. Otherwise, time allotted for certain moves may be insufficient.

The typical labeling process begins with wrapping (printed with names, company logos, and other information) that's pulled or stretched over product — whether containers of several items, or individual units such as juice, soap, or bleach bottles. Then, with shrink-wrap labeling, heat makes labels tighten onto the products.

PVC is the most common type of shrink film. Polyolefin film is more elastic; it's commonly used with foods because it does not release harmful chemicals when heated and stretched. It's also used for products having sharp corners, because it is less brittle and resists breaking.

For high production capacities, automatic equipment most commonly conveys product into machines, with adjustable-speed motors and drives controlling conveyor speed. For example, for thicker, slower-shrinking film gauge, feed rate might be slowed.

Motion in labeling?
After a product unit is inside the machine, it is pushed or placed into a labeling position. This labeling position is the station at which labels are affixed to the product. Here, some legacy sleeve wrapper designs use continuousrun rotary motors and clutches to push product from the conveyor belt into the labeling position; their speed and feed rates are limiting factors.

Actuator arms then pull film labels onto the product. In newer equipment these arms are positioned with servos. Pneumatically operated grippers at the arm ends grab the material and stretch it open slightly, and then the arms pull the label onto the product and align it. Once in position, the grippers release the label. Then the product is placed back onto the conveyor, to ride through a label-shrinking heat tunnel. The heat must be sufficient, but not so high as to damage product.

Overcoming challenges
Labeling must be fast, but as accurate as possible; there are always new applications in which product must move faster than existing machine speeds. It's during these times that application engineers often raise issues that designers haven't considered. For example, target load rates are often based on existing technology, designed when the goal was not speed, but to get a machine operating.

The faster the move, the more force or torque required. Identifying load is as simple as placing it on a scale. The caveat here is that load includes more than just product weight; it includes actuator arms and grippers, mounting plates, bearing plates, and even friction. To measure the latter, connecting a simple scale to the load and pulling it across the surface in question gives a fairly accurate value.

Defining a machine's move is as easy as identifying task cycle time: This is the total time in which the designer wishes to accomplish a certain move. For example, say one axis pulls the product into position while another grabs the label and pulls it onto the product; then the grippers release and the product is pushed back onto the conveyor. If a designer allocates 1.6 sec to this operation, will the motor have sufficient time to complete its tasks?

Maybe not. Moving the product into the machine might take 0.2 sec; after moving the product into the labeling position, pulling the label onto the product might take another 0.2 sec or more. Then, the gripper needs time to release before product can be pushed onward. If the gripper requires, say, 0.4 sec to release, the motor is left with only about 0.4 sec to move product into position, and the same to push it back onto the conveyor. What appears to be an easy move is actually rather fast. If the distance is 10 in., an assumed speed of 10 x 2/1.6 sec = 12.5 in./sec is actually 10 x 2/0.4 sec = 50 in./sec

Make a difference
Servos and motion control improve labeling equipment by boosting speed and improving consistency for lower reject rates and improved part quality. Traditional inverter and vector technology reaches peak torques of 150% to 200%, but servos can attain 300% and even up to 400%. This allows for high acceleration torque, which mean faster positioning. By increasing machine speed and acceleration, servos increase the number of items wrapped and reduce average product cost.

In labeling, servomotors and controls improve the cycle time from 3 to 1.5 sec — an 87.5% improvement. Additionally, machines are better at accurately and correctly orientating and positioning labels, reducing the reject rate by 75% on average. Maintenance downtime on labeling equipment is normally one week per year; with servo motion it is reduced to one day per year. So, if a labeling machine boosts profits by $6,000 per day, the improvement from servo control in the cycle time alone increases that to $11,250.

Software has taken the place of oscilloscopes for easier tuning and easier servo motion implementation. Windows-based graphical tools display parameters so engineers can adjust systems and monitor key machine variables more easily. Also, newer fieldbuses allow parameters to be changed over a common bus. Ethernet Powerlink is one example; it is a software interface utilizing Ethernet hardware. With it, motion and drive parameter setups are controlled over the bus from a central location.

Hardware choices include rotary and linear servomotors. Rotary servomotors utilize either ballscrews or belts and pulleys to convert rotary to linear motion, whereas linear motors drive directly. Each approach has advantages: Rotary systemsare comprised of standard parts, many of which are usually in stock; linear motors are built to order. Additionally, each technology has speed, accuracy, and repeatability performance-to-cost advantages. A rotary motor with ballscrew can provide speeds up to 50 in./sec, compared to linear motor speeds in excess of 200 in./sec. Keep in mind, however, that overspecifying is not recommended.