Polyethylene Terephthalate (PET) Film Line Settings: MD/TD Stretch Ratio and Tenter Temperature

In continuous cast-film orientation of polyethylene terephthalate (PET), the transformation from amorphous cast sheet to biaxially oriented film is controlled largely by PET Film Line Settings: MD/TD Stretch Ratio and Tenter Temperature. The cast sheet is generated from resin with an intrinsic viscosity of 0.58–0.68 dL/g, measured according to ASTM D4603, and is quenched on a chill roll at 15–28°C to suppress spherulitic crystallization before orientation. Desiccant drying at 160–180°C lowers residual moisture below 50 ppm; melt temperature at the die is held between 275°C and 285°C on a single-screw extruder with an L/D ratio of 30:1. These conditions preserve the metastable amorphous state required for later strain-induced orientation. In sequential biaxial film production, machine-direction drawing occurs first, followed by transverse drawing in a tenter. The MD draw ratio is defined by the circumferential speed ratio of slow and fast rolls, while the TD draw ratio is defined by the ratio of final tenter clip width to initial clipped sheet width. Tenter temperature refers not to a single setpoint but to a gradient across preheat, stretching, and annealing zones.

Because PET has a glass transition temperature near 78°C and a cold-crystallization peak commonly reported between 140°C and 170°C by differential scanning calorimetry per ISO 11357-3, the orientation window is narrow. If the sheet is heated too far above 95°C in the machine-direction draw unit, thermal crystallization begins before orientation and the film loses clarity and drawability. If the sheet is drawn below 80°C, tensile stress exceeds the cohesive strength of the cast sheet, causing film breaks. This thermal boundary is one of the main process conflicts on production-scale lines.

What Determines the Machine-Direction Draw Ratio Before the Tenter?

On sequential lines the cast sheet enters a set of driven rolls with independently controlled temperatures. The first rolls are typically maintained at 80–90°C to bring the sheet just above its T_g; final rolls before the fast draw roll may operate at 90–95°C. The fast draw roll speed is set to produce an MD draw ratio of 2.8–3.5:1 for balanced packaging films. Films requiring higher tensile strength may use ratios up to 3.8:1. The ratio must not be selected in isolation; it interacts with cast-sheet thickness, line speed, and intrinsic viscosity. A cast sheet with IV near 0.58 dL/g cannot sustain the same draw ratio as one with IV near 0.68 dL/g because lower molecular weight reduces melt cohesion and draw tension.

On a production line, the MD draw ratio is calculated from the ratio of fast roll linear speed to slow roll linear speed. For example, a slow-roll speed of 40 m/min and a fast-roll speed of 132 m/min yields an MD draw ratio of 3.3:1. Because the web width contracts during MD drawing, the actual tenter entrance width is smaller than the cast width. Width contraction during MD drawing is typically 10–25%, depending on draw temperature and roll gap. The cast sheet width is therefore set so that the final tenter exit width produces the target TD draw ratio.

The onset of strain-induced crystallization increases the load on the draw rolls. On high-speed lines operating above 300 m/min, this load can exceed the friction capacity of the roll drive, producing local slippage and gauge bands in the finished film. The defect is observed on production lines as periodic transverse thickness variation with a wavelength corresponding to the circumference of the fast draw roll. To avoid this failure, the MD draw ratio is reduced by 0.2–0.4 units when the line speed increases, unless additional idler rolls with high-friction ceramic coating are installed. Published data for exact high-speed thresholds is limited, but the interaction is well documented in roll-stack orientation equipment.

The transverse draw ratio is applied after MD drawing. The MD-drawn web is gripped by clips on a chain tenter and diverging rails stretch the film to a final width. For balanced films the TD draw ratio is commonly 3.2–4.0:1. The ratio between TD and MD draw defines isotropy: a TD/MD ratio of 1.0–1.15 produces relatively balanced tensile properties, while a TD/MD ratio above 1.2 produces anisotropic shrinkage and tensile behavior. Tensile properties are measured according to ASTM D882-18; density is measured according to ASTM D1505; thickness uniformity is evaluated by profilometry or beta-transmission gauges. No absolute ratio produces optimal film for all applications; the industrial setpoint is determined by downstream converting requirements such as flexographic printing, metallization, or lamination.

When Tenter Zone Temperatures Exceed the Stretch-Crystalline Window

Tenter temperature management directly controls transverse stretching force, crystallization rate, and final shrinkage. The tenter oven is divided into preheat, stretching, and annealing zones. In the preheat zone the MD-drawn sheet is reheated to 90–110°C before the rails diverge. In the stretching zone, zone temperatures are typically 105–140°C depending on film grade; the film is stretched transversely while crystallization is constrained by the draw. In the annealing zone, temperatures between 225°C and 245°C are used for heat-set films to increase crystallinity and reduce thermal shrinkage to below 2% at 150°C per ASTM D1204. For shrink-grade films, the annealing zone is operated at much lower temperatures or bypassed to retain orientation, with TD shrink levels above 40% at 100°C in some grades.

Temperature deviations in the tenter stretching zone have a sharper effect than in the preheat zone. If the stretching zone exceeds 135°C for a standard packaging grade, thermal crystallization competes with orientation. The resulting stress-whitening appears as haze above 3–5% measured by ASTM D1003-21 and reduces tensile elongation at break below 80% in the transverse direction. If the stretching zone is too cool, below about 100°C, the yield stress of the semi-drawn web rises sharply; clip pull-out or film tear occurs at the rail entrance. Production lines therefore maintain the stretch zone within a narrow band, sometimes ±5°C, for high-speed sequential orientation. The band shifts with MD draw ratio, intrinsic viscosity, and casting roll temperature.

Clip temperature also influences edge stability. Clips traveling through the oven carry heat into the sheet edges. Edge temperature can be 10–20°C higher than centerline temperature, producing lower viscosity at the edge and premature crystallization. This thermal gradient is managed by reducing edge heater output or using IR pyrometers to map cross-web sheet temperature. On production-scale lines, the edge bead is trimmed after wind-up; however, edge instability in the tenter can propagate inward and cause clip release.

Rail divergence angle in the stretch zone determines the transverse strain rate. Typical divergence angles are 7°–15°; narrower angles reduce stress but increase oven length, while wider angles increase stress and can initiate ductile fracture. The tenter clip pitch is often 150 mm or 200 mm, and the chain speed is matched to the MD-oriented web speed. Transverse strain rate varies by line design; published data for exact strain-rate windows is limited.

When the MD draw ratio and tenter stretch-zone temperature are not matched, gauge bands, haze, tear defects, and thermal shrinkage become immediately visible at the winder. A line producing 23 µm balanced film at 350 m/min cannot use the same settings as a line producing 100 µm roll-to-roll optical film at 80 m/min. For thicker films the thermal equilibration time in the tenter increases; the stretching-zone setpoints are therefore lowered by 5–10°C to avoid surface overheating while the core remains below draw temperature. Conversely, thin films may tolerate higher stretch-zone setpoints because their thermal mass allows faster equilibration. These process boundaries are controlled by closed-loop thickness gauges, IR thermography, and beta-transmission scanners. Film quality is verified against ASTM D882-18, ASTM D1003-21, ASTM D1204, and ASTM D1922. Operational limits include the incompatibility of PET with strong amine-based additives, which reduce molecular weight through aminolysis, and the need to avoid water contact above 60°C during processing because hydrolytic chain scission accelerates rapidly. Published data for all combinations of high-speed sequential MD/TD stretch ratio and tenter temperature is not complete; line-specific validation remains necessary.