Views: 200 Author: Site Editor Publish Time: 2025-04-18 Origin: Site
The quality indicators of cast films involve several aspects, with thickness uniformity being the primary issue. Controlling the thickness of cast films is the most critical problem in the production process. There are many factors affecting film thickness, mainly as follows:
The selection of extrusion molding temperature is determined by the raw material. The temperature gradient in the extruder barrel shares similarities with, but also differs from, that in blow molding processes. As shown in the cast film process parameters for the three plastics mentioned above, the temperature of the extruder barrel and die head is 20–30°C higher than that for blow molding the same type of film, and 30–40°C higher than that for extruding the same type of pipe. The die head temperature is controlled to be approximately 5–10°C lower than the barrel temperature.
In terms of temperature distribution across the width of the die lip, a "saddle-shaped" profile is adopted, with slightly higher temperatures at both ends and lower in the middle. This is because the distance molten material flows from the extruder barrel to the two ends of the 衣架式机头 (coat-hanger die) is longer than to the center. To ensure uniform flow across the width, the temperature at the ends is slightly higher, reducing melt viscosity and improving fluidity compared to the center. This compensates for the longer flow path. Another control method is to maintain uniform heating across the die width while adjusting the die lip opening (narrower at the center, wider at the ends) using a restrictor bar to balance melt flow. Longitudinal thickness uniformity of the film is also important; it can be ensured by stabilizing process conditions.
Based on production experience (taking PET as an example), recommended die lip gaps vary for different film thicknesses. While the die lip gap is the primary factor affecting film thickness, other non-negligible factors include the linear speed of the cooling roller, stability of the cooling roller rotation, screw speed, temperature control of the extrusion system, and draw ratio. When the extrusion output is constant, increasing the cooling roller speed reduces film thickness, and vice versa. Fluctuations in cooling roller speed directly cause unstable film thickness. Increasing extrusion output increases film thickness. The cooling roller’s role is to uniformly cool the molten resin extruded from the die and draw it at a constant speed; adjusting the draw speed (with fixed screw speed) can change film thickness.
This distance should be minimized. When molten material exits the die lip, a large gap between the die lip and cooling roller makes the material susceptible to external disturbances (e.g., air flow), causing thickness variations.
Given the high-speed, large-scale nature of cast film production and the wide film width, online thickness measurement is a critical step. Automatic thickness gauges are used during production, with real-time display on a screen and computer-aided automatic adjustment. Due to the many factors affecting thickness, continuous monitoring is essential—any fluctuations should prompt immediate checks of process conditions.
The resin extruded from the die is at a high temperature (viscous flow state). When it contacts the cooling roller surface, it rapidly cools to form a film with low crystallinity, and the roller also serves as a draw mechanism. Close contact between the molten material and the cooling roller is key to film formation, directly impacting the film’s appearance and physical properties. To prevent air bubbles between the film and roller, an air knife blows air uniformly at the tangential contact point, pressing the film tightly against the roller. Smaller air knives at the edges are used to prevent curling and ensure edge adhesion.
The air knife’s airflow must be optimized: excessive airflow causes excessive vibration of the molten film, increasing thickness 偏差 (deviations); insufficient airflow results in poor adhesion and transverse waves. The angle of the air knife relative to the cooling roller is also critical (typically 30°)—an incorrect angle can cause surface bubbles.
Winding must ensure a smooth roll appearance. Unlike blow-molded films, cast films have higher elongation, so tension control during winding is crucial. Additionally, flattening the film before winding is essential to prevent defects.
