Causes of Cast Film Thickness Uniformity and Precautions
Whether the film thickness is uniform is a key indicator for evaluating the quality of film products. Non-uniform film thickness not only affects the tensile strength and barrier properties in various areas of the film, but can also cause ridge bands (bulges) on the rolled film surface after winding. The film at these ridge bands undergoes permanent deformation, which in turn affects subsequent processing of the polymer, such as uneven film surface, slack and sagging film, and further impacts downstream processes including lamination, printing, metallizing, and bag-making. Therefore, the uniformity of cast film thickness is closely related to equipment, materials, and processing conditions.
1. Equipment-Related Causes
The casting process involves melting and plasticating resin in an extruder, extruding it through a flat die lip, and under the action of edge-fixing devices, air knives, and vacuum boxes, making the melt adhere closely to the casting roll (also known as the chill roll). The melt is then longitudinally stretched and cooled on the casting roll to form a film, which subsequently undergoes pre-trimming, thickness gauging, web guiding, corona treatment, edge trimming, and winding. The control of film thickness uniformity involves the following equipment-related factors:
Extrusion System: Improper screw and die design, worn extruder barrel and screw, or malfunctioning drive equipment can cause variations in film uniformity.
Die Lip Gap: The die lip gap is a primary factor affecting transverse thickness uniformity. Adjusting the die lip gap is the most precise and common operation for controlling transverse thickness uniformity. Modern cast film lines are equipped with automatic thermal expansion bolt adjustment systems for this purpose. The basic principle is to heat or cool the bolts that control the die lip gap, utilizing thermal expansion and contraction to adjust the gap. If the thickness gauge detects that a specific transverse zone of the film is thicker, the system sends a heating signal to the corresponding thermal expansion bolt in that zone, and vice versa for cooling. Although this automatic control is relatively responsive, the expansion/contraction range of the bolts is limited; typically, the automatic adjustment range is within ±1.5 µm. Deviations beyond this range require manual adjustment. During start-up or when changing product thickness, manual adjustment should be performed first, gradually and without rushing, until the deviation is within ±1.5 µm, after which the automatic control can be engaged. Improper die lip gap results in consistent transverse thickness non-uniformity, i.e., specific zones are always thicker or thinner.
Air Knife and Vacuum Box: Intimate contact between the molten polymer and the cooling roll surface is crucial for cast film formation. The quality of this contact indirectly affects the film’s appearance and physical properties. To prevent air bubbles between the film and the cooling roll, an air knife directs a uniform stream of air tangentially at the contact point between the film and the roll, ensuring close adherence. To further improve this effect, a vacuum box is used to evacuate air from between the film and the casting roll. The air knife width is similar to the casting roll length, while the vacuum box width is similar to the die width. During air knife operation, airflow must be carefully controlled: excessive airflow causes the molten web to flutter excessively, increasing thickness deviations; insufficient airflow reduces adhesion, causing transverse web fluctuations and significant thickness variations, leading to an uneven, unprocessable film surface (especially noticeable during start-up when moving the air knife closer to the casting roll). The air knife angle and position are also critical. An incorrect angle can cause bubbles and poor adhesion; the optimal angle directs airflow perpendicular to the tangent of the casting roll’s cross-section at the initial contact point of the melt. The air knife position refers to the stopping point of the air outlet (nozzle). Closer to the die lip results in a shorter contact line between the film and roll, stronger chilling, better quenching, greater web flutter amplitude, and generally better thickness uniformity of the final film. Therefore, positioning the air knife nozzle closer to the die lip is more favorable for controlling thickness uniformity. The airflow from the air knife must be uniformly distributed across the melt web. If foreign matter adheres to the air knife nozzle, it disrupts airflow uniformity, causing localized thinning of the film in the corresponding area due to differential cooling, resulting in vertically oriented transparent streaks with lower haze. Thus, the air knife must be kept free of any contaminants. For the vacuum box, the key is to match the extraction airflow rate to the specific production conditions. Excessive suction can cause holes in the melt web; insufficient suction fails to remove air between the film and roll. For the same product thickness, higher line speeds generally require higher vacuum box airflow than lower speeds; for the same line speed, thinner products require higher airflow than thicker ones.
2. Process-Related Causes
The thermal properties of the raw resin and improper temperature settings across the die heating zones can lead to non-uniform, non-isokinetic melt flow. Changes in melt velocity (e.g., screw speed, take-off speed) also affect melt distribution and consequently the transverse thickness.
Temperature Settings: Different resins require different processing temperatures. Barrel temperature settings typically decrease along the material flow direction from feed to discharge. A typical temperature range (with significant differences between PE and PP) is: feed zone 150–215°C, middle zone 190–230°C, rear zone 210–240°C, adapter and die body 200–230°C, and die lip 210–240°C. Improper extrusion temperature settings cause unstable output and longitudinal thickness fluctuations. Improper die temperature settings cause non-uniform melt flow across the die width, leading to transverse thickness non-uniformity. Die temperature distribution is typically set higher in the middle and slightly lower at the ends, resembling a saddle-shaped profile across the width.
Take-off Speed: Primarily the rotation speed of the casting roll. With constant extruder output, faster casting roll speed produces thinner film, and slower speed produces thicker film. Thus, unstable casting roll speed results in unstable longitudinal thickness. Roll speed refers to linear velocity, which depends on both the roll’s circumferential uniformity and its angular velocity. Typically, casting roll speed is quite stable. If periodic longitudinal thickness fluctuations occur and the period length corresponds approximately to the roll’s circumference, the roll’s linear speed should be measured with a tachometer.
Contaminants in Die Cavity or Die Lip Degradation Products: Contaminants in the die cavity significantly affect transverse thickness uniformity by obstructing melt flow. When melt passes through a contaminated area, flow is restricted, resulting in a thinner zone in the final film, creating longitudinal transparent streaks and, on the wound roll, a corresponding ridge band, severely affecting film usability. Contaminants in the die cavity must be removed immediately, typically by increasing die gap opening, purging, and manually scraping the cavity with a copper blade from the center outward until the contaminant is expelled. Degraded material (burned polymer) adhering to the die lip also affects transverse uniformity, though usually less severely; this is often referred to as “die lines” or “draw lines.” The solution is to clean the die lip.
Die Lip-to-Casting Roll Distance: This distance should be as short as possible. Since the polymer exits the die lip in a molten state, a longer distance makes it more susceptible to external disturbances (e.g., ambient air currents), causing web flutter and gradual thickness changes, thereby degrading thickness uniformity.
3. Material-Related Causes
Resin density, melt flow index (MFI), and the formulation ratio also influence cast film thickness uniformity.
Neck-in: During cast film production, the polymer width becomes narrower than the die width, a phenomenon known as neck-in. The molten web narrows under tension during air stretching, causing edge thickening. The difference between film width and die width is defined as neck-in. Greater neck-in leads to thicker film edges, thus increasing edge trim waste and reducing product yield. Neck-in is related to the surface tension and elastic modulus of the molten film and is caused by film shrinkage. Its extent depends on resin characteristics (e.g., density, MFI) and casting conditions (e.g., melt temperature, air gap, die lip width). Under constant casting conditions, lower density or MFI reduces neck-in. Regarding conditions, a smaller air gap, wider die lip, lower take-off speed, and lower melt temperature all reduce neck-in. The equipment component for controlling neck-in is the edge-fixing device, commonly using high-pressure air or high-voltage discharge (electrostatic pinning). High-pressure air is suitable for higher speeds or thinner films; electrostatic pinning is critical for stable thickness uniformity control, especially for wide products. Unstable edge-fixing significantly affects thickness uniformity at both edges. The root cause of edge thickness uniformity control is controlling neck-in stability and magnitude. While choosing resin characteristics and optimizing casting conditions can influence neck-in, the primary control method is the edge-fixing device. The key to electrostatic pinning operation is positioning the pinning wire/needle correctly, with fine adjustments based on resin type and casting conditions. The correct position ensures stable film edge width under the given process conditions, without lateral oscillation or with controlled, minimal oscillation.
Formulation Ratio: Non-uniform raw material blending or unstable feeding leads to variations in formulation composition, which in turn affect melt temperature and viscosity. These variations alter melt flow within the die, causing thickness variations in the final film.
