What are the primary differences between terminal and intermediate guides?

Location and Function

• Terminal Guides: These are located at the very entry (unwind) and exit (rewind) of a roll-to-roll machine. Their purpose is to manage the master roll—either ensuring the web feeds correctly into the machine (unwind) or ensuring the roll winds up straight out of the machine (rewind).
• Intermediate Guides: These are located within the interior of the machine, typically just before a critical process such as printing, coating, laminating, or slitting. Their job is to shift the running web to align it for that specific downstream process.

Mechanism of Movement

• Terminal (Moving the Roll): Terminal guides function by moving the entire roll of material laterally. They typically utilize a "shifting stand" or "shifting base" on linear bearings to position the heavy unwind or rewind rolls.
• Intermediate (Moving the Web): Intermediate guides function by twisting or bending the web itself while the machine frame stays stationary. Mechanisms like Displacement Guides or Steering Guides displace the web by rotating or translating rollers over which the web passes.

Dynamics and Application

• Unwind vs. Rewind Dynamics:
  • Unwind Guides: Function as true guides. They shift the unwind stand to ensure the web enters the machine at a predetermined position.
  • Rewind Guides: Function as chasing systems. They do not control the web's lateral position; instead, the rewind stand moves to align the winding roll with the incoming web edge to ensure a straight roll.
• Intermediate Dynamics: Most intermediate guides (specifically Displacement and Steering guides) rely on the Normal Entry Rule, which states that a web will align itself perpendicular to the axis of the roller it is approaching.

Sensor Placement

• Terminal Guides:
  • Unwind: The sensor must be fixed to the machine frame (it does not move with the stand) and positioned immediately downstream of the shifting idler,.
  • Rewind: The sensor must be attached to the moving rewind stand so it moves with the carriage to "chase" the web.
• Intermediate Guides: The sensor is always located in the exit span (the span immediately following the guide roller) and moves with the web, not the guide mechanism. It should be placed as close to the exit roller as possible.

Actuator Requirements

• Terminal Guides: Because they often move heavy master rolls (potentially thousands of pounds), they typically require high-thrust actuators and robust mechanical rigidity to avoid resonance.
• Intermediate Guides: These move lighter guide frames and rollers, focusing more on dynamic response and low friction than raw thrust.