Haul-Off Unit in Sheet Extrusion: Tension Control and Speed Synchronization

Every haul off unit sheet extrusion line depends on a mechanical bridge between the calender stack and downstream equipment —pulling the cooled sheet away from the calender rolls at a controlled speed while maintaining consistent tension across the web width. haul off unit sheet extrusion involves specific considerations that differ from general extrusion processes. Despite its apparent simplicity (a pair of powered rolls gripping the sheet), the haul-off unit's tension and speed behavior directly determines whether the sheet arrives at the winding station flat and dimensionally stable or riddled with edge waves, center bagging, and residual stress that will cause problems in thermoforming.

The function of the haul-off fits within the broader sheet extrusion technology process chain, where each section's speed and tension must coordinate precisely to maintain product quality from die to final roll.

Rubber Roll Nip Design and Grip Mechanics

The hauling nip consists of a driven steel roll and a pneumatically loaded rubber-covered roll that together squeeze the sheet with controlled force. The rubber roll cover —typically nitrile (NBR), neoprene (CR), or silicone —provides the compliance needed to maintain uniform contact pressure across the full sheet width, compensating for minor roll deflection and sheet thickness variations.

Nip pressure selection involves a trade-off. Too little pressure, and the sheet slips against the rolls —destroying speed synchronization and creating tension instability. Too much pressure, and the rubber cover deforms excessively, generating heat at the nip that can mark temperature-sensitive polymers like PET and cause roll cover degradation over time.

Typical nip loading ranges:

Roll diameter also affects hauling performance. Larger diameter rolls (150-300 mm) provide a longer contact arc with the sheet, improving grip and reducing the line pressure needed to prevent slip. Smaller diameter rolls occupy less floor space but require higher nip loads to achieve equivalent traction.

Tension Control Methods

Sheet tension in the haul-off section must be maintained within a narrow band —typically +/- 5-10% of the target value. Below this range, the sheet sags and develops edge waves. Above it, the sheet stretches permanently or necks down, causing thickness reduction and width loss.

Three tension control strategies are commonly employed:

Draw control is the simplest approach. The haul-off runs at a fixed speed slightly higher than the calender output speed, creating a constant draw ratio that generates tension through the speed differential. This works adequately for thick, stiff sheets where small tension variations have minimal effect on dimensional quality. The limitation is that tension varies with sheet temperature, thickness, and polymer viscosity —none of which the draw control method actively measures or compensates for.

Load cell feedback control places a tension sensing roll in the sheet path between the calender and the haul-off nip. The load cell measures the total downward force on the roll, which the controller converts to tension (force per unit width) by dividing by the sheet contact length. The controller then modulates haul-off speed to maintain the tension setpoint. This closed-loop approach handles process variations that open-loop draw control cannot, including gradual temperature drift and minor thickness changes.

Dancer roll control uses a weighted or pneumatically loaded idler roll that moves vertically to absorb tension variations. As tension increases, the dancer roll lifts; as tension decreases, it drops. A position sensor on the dancer drives the haul-off speed through a PID loop. Dancer systems provide excellent tension stability during acceleration and deceleration, making them popular on lines that start and stop frequently.

Speed Synchronization with Calender and Winder

The haul-off unit operates within a three-section speed chain: calender, haul-off, and winder. Each section must run at a precisely coordinated speed to maintain uniform tension throughout the downstream path.

During steady-state operation, the haul-off typically runs 0.5-2.0% faster than the calender to establish sheet tension. The winder then runs at a speed calculated from the haul-off speed minus the small tension differential needed between haul-off and winder. These speed relationships must hold across the entire production speed range —from threading speed (2-5 m/min) to full production speed (50-150 m/min depending on line configuration).

Speed synchronization becomes critical during line speed changes. If the haul-off accelerates faster than the calender, sheet tension spikes and the web may break. If it accelerates slower, the sheet goes slack and develops wrinkles. Modern systems use electronic gear or electronic line shaft (ELS) control, where all downstream drives follow a single master speed reference with individually adjustable ratio setpoints. This ensures that speed transitions happen simultaneously across all sections, maintaining tension stability throughout acceleration and deceleration ramps.

Common Haul-Off Configurations

Different sheet extrusion applications require different hauling arrangements. The most common configurations include:

Single nip haul-off —One pair of powered rolls. The simplest and most cost-effective arrangement, used on commodity sheet lines where tension precision requirements are moderate. Works well for thick sheets (above 1 mm) where the sheet's own stiffness helps maintain web stability.

Dual nip haul-off —Two sequential nip pairs separated by a short free span. The first nip provides the primary hauling force, while the second nip acts as a tension isolation point and ensures consistent grip at higher line speeds. Dual nip configurations handle thin-gauge sheet (below 0.5 mm) more reliably than single nip units because the second nip prevents any slip that might occur at the first nip from propagating downstream.

Horizontal versus vertical orientation —Vertical haul-offs (sheet travels upward from the calender) save floor space and keep the sheet path short, reducing heat loss for temperature-sensitive materials. Horizontal haul-offs (sheet travels horizontally) are easier to thread during start-up and provide better access for maintenance.

Understanding how the haul-off feeds into downstream equipment is essential. Center-wind and surface winders and surface winders both depend on receiving sheet at consistent tension and speed —defects introduced at the haul-off stage propagate directly into the wound roll as telescoping, wrinkling, or crush damage.

Troubleshooting Haul-Off Issues

Most haul-off problems trace back to either mechanical wear or control system drift. Rubber roll covers harden with age and UV exposure, losing their grip characteristics. When nip slippage begins occurring on a previously stable line, roll cover condition should be the first suspect. Most rubber covers require replacement every 12-24 months depending on operating temperature and polymer type.

winder tension problems sheet often originate upstream in the haul-off section, even though the symptoms appear at the winder. Tension spikes, oscillations, or gradual drift from the haul-off translate directly into winding defects that may not become apparent until the roll is unwound at the customer's thermoforming facility.

The haul-off unit serves as the critical speed and tension bridge between the calender stack and the winding station —a role that directly impacts sheet flatness, dimensional stability, and final roll quality. JWELL's haul-off units feature servo-driven rubber roll nips with closed-loop tension feedback, maintaining sheet tension within ±0.5 N across line speeds from 5 to 100 m/min —a consistency level that prevents the edge wave and center bagging defects common in poorly synchronized systems.

FAQ

What causes edge waves in extruded sheet at the haul-off?

Edge waves typically result from insufficient or uneven tension across the sheet width. If the haul-off nip pressure is too low, the sheet may slip at the edges while gripping at the center —creating differential tension that pulls the center tighter than the edges. Roll cover wear, nip pressure imbalance, or a crowned roll that no longer matches the sheet width can all contribute. Increasing nip pressure uniformly and verifying roll cover condition usually resolves the issue.

How is haul-off speed synchronized with the calender?

Modern systems use electronic line shaft (ELS) control where all drives follow a single master speed reference. The haul-off drive receives the master speed signal multiplied by a ratio setpoint (typically 1.005-1.020x calender speed). During acceleration and deceleration, all drives ramp simultaneously at coordinated rates to maintain constant tension. Older lines may use analog speed references with manual ratio adjustment.

What rubber material is best for haul-off roll covers?

Nitrile (NBR) covers handle most general-purpose sheet extrusion and offer good abrasion resistance. For higher temperature applications, neoprene (CR) or EPDM covers provide better heat resistance. Silicone covers are used where non-marking contact is critical, such as optical-grade sheet. Durometer selection (60-90 Shore A) should match the sheet stiffness —softer covers for thin, flexible sheet and harder covers for thick, rigid sheet.

Can a single haul-off handle multiple sheet widths?

Yes, provided the rubber roll width exceeds the maximum sheet width. Some haul-off units feature adjustable lateral positioning to optimize nip engagement for different sheet widths. On lines that run a wide range of widths, segmented nip loading —where the rubber roll is divided into independently pressurized sections —prevents excessive edge loading on narrow sheets while maintaining adequate total grip force.