Step 1: Raw Material Pretreatment and Conveying
Drying: For highly hygroscopic plastics (such as PET, PC, and nylon), the pellets first enter a dryer to remove moisture. Moisture will generate bubbles during high-temperature extrusion, leading to defects in the sheet or a decrease in mechanical properties.
Ingredient Batching and Mixing: As required, the main material pellets, masterbatch, and functional additives (such as antioxidants and antistatic agents) are put into a mixer in proportion for thorough mixing to obtain the desired color and properties.
Vacuum Feeding: The mixed raw materials are sucked into the hopper of the extruder through an automatic vacuum feeding system, enabling closed and dust-free continuous material supply.
Step 2: Melting, Plasticizing and Extrusion
Screw Extruder: Raw materials fall from the hopper into a screw with a large length-diameter ratio. The screw rotates inside the heated barrel, and through external electric heating and its own shear friction heat generation, the plastic pellets are gradually melted, compressed, and homogenized into a viscous and uniform melt.
Melt Filtration: The melt passes through a device equipped with a porous filter plate and a screen changer. The filter screen traps impurities or unmelted pellets that may be present in the raw material, ensuring the purity of the melt. The screen changer allows the replacement of dirty filter screens without shutting down the machine, thus guaranteeing continuous production.
Melt Metering: The homogenized melt is delivered to the next stage stably and at a constant pressure. Precise control of the screw speed and temperature zones (feeding zone, compression zone, homogenization zone) is the key to quality.
Step 3: Molding and Initial Shaping
Flat Extrusion Die: The melt enters a wide and flat T-shaped (coat hanger-type) die from the circular flow channel. The die has a precision-designed internal flow channel, ensuring that the flow rate, pressure, and temperature of the melt are completely consistent from the center to the two ends.
Lip Exit Sheet Formation: The melt flows uniformly out of a thin, elongated adjustable lip at the end of the die (usually with fine adjustment of the opening via bolts), forming a "molten curtain" with a specific width and initial thickness. The flatness and adjustment precision of the lip directly determine the uniformity of the sheet's transverse thickness.
Step 4: Precision Cooling and Setting
First Cooling Roller (Cast Roller)：The molten sheet hanging down from the die first adheres to a large-diameter cast roller with a highly polished surface and cooling water (often ice water) circulated inside. It is tightly "attached" to the roller surface via electrostatic adsorption or an air knife for rapid cooling (quenching). This process determines the sheet's surface finish (mirror or matte) and crystallinity (which affects transparency and hardness).
Cooling Roller Group：The sheet then passes through multiple cooling rollers in an "S" shape, undergoing gradual and uniform cooling until it is fully solidified and set. Tension control between the rollers is crucial.
Step 5: Thickness Monitoring and Feedback Control
On-line Thickness Gauge: After the sheet is cooled and set, a beta-ray or infrared thickness gauge is used for non-contact continuous scanning, which generates a real-time thickness profile curve across the entire width of the sheet.
Automatic Feedback: The thickness measurement data is transmitted back to the central control system in real time. The system fine-tunes the local opening of the die lip through an automatic lip adjustment system (thermal expansion bolts or push rods), realizing closed-loop automatic control (APC) of thickness and ensuring the tolerance reaches the ±0.01mm level.
Step 6: Trimming and Winding
Trimming Device: The two sides of the sheet are usually uneven due to the "neck-in" effect, so an on-line trimming process is performed on both sides using circular knives. The trimmed scrap is crushed for recycling and re-fed into the extruder (the proportion must be strictly controlled).
Traction and Calendering Rolls: One or more pairs of traction rolls provide stable pulling force for the entire production line. They match the extrusion speed to control the longitudinal stretching and tension of the sheet.
Winder: The fully cooled, set, and neatly trimmed sheet is finally wound into large rolls by a large-diameter automatic winder. Modern winders adopt constant tension or taper tension control, which can produce mother rolls with flat surfaces and uniform tightness, facilitating subsequent transportation and slitting.