Core Components of a PE/PP Sheet Production Line
1. Feeding and Batching System
Function: Automatically conveys, dries (if needed), meters, and mixes PP or PE raw materials (granules, powder). For PE, especially HDPE, processing aids are sometimes required.
2. Extruder System - The "Heart" of the System
Function: Melts and plasticizes solid plastic granules into a homogeneous melt.
Main Equipment:
Extrusion Machine: For PP/PE, a single-screw extruder is typically used. The key is screw design:
PP: Higher processing temperatures require a screw with strong shearing and mixing capabilities.
PE (especially HDPE): Higher melt viscosity requires more energy to melt; the screw's compression ratio and length-to-diameter ratio are particularly critical.
Screen Changer: Essential. Used to filter impurities in the melt, ensuring sheet purity and quality. Due to the processing characteristics of PP/PE, an automatic continuous screen changer better ensures production stability.
3. Die and Calendering System - The Core of Molding
Function: Shapes molten plastic into sheets, giving them a smooth surface and precise thickness.
Main Equipment:
Coat Hanger Die: The mainstream choice. Its internal flow channel design needs to be optimized for the rheological properties of PP/PE to ensure uniform material distribution.
Three-Roll Calender: Requires higher temperature control: PP/PE has high crystallinity, and the phase transition from molten to solid releases a large amount of heat, with a narrow crystallization temperature range. Therefore, the temperature setting and cooling rate control of the three rolls are crucial, directly affecting the flatness, crystallinity, and internal stress of the sheet.
4. Cooling and Traction System
Function: Further cools and solidifies the sheet, and provides smooth traction.
Main Equipment:
Cooling Support: Composed of multiple cooling rollers, ensuring uniform and slow cooling of both sides of the sheet to prevent warping due to excessively rapid cooling.
Traction Machine: Similar to an ABS production line, typically one or more pairs of rubber rollers, with speeds strictly synchronized with the extrusion and calendering speeds.
5. Cutting and winding system
Function: Processing according to the final product form.
1. Extrusion System Optimized for Polyolefins
Features: Employs a single-screw extruder specifically designed for PP/PE. The screw structure (e.g., barrier screw) and L/D ratio are optimized to ensure efficient delivery, compression, plasticization, and homogenization of this high-viscosity material.
2. Precision Temperature Control System
Features: PP/PE is a crystalline plastic, and its melting and crystallization processes are highly sensitive to temperature. The equipment is equipped with a multi-segment, independently controlled precision temperature control system in key components such as the extruder, screen changer, die head, and three-roll calender.
3. Coat-Hanger Die Head and Automatic Thickness Adjustment
Features: Employs a coat-hanger die head to ensure uniform melt distribution across the entire die lip width. High-end equipment features an automatic die lip adjustment system, allowing for micron-level online adjustment of the die lip opening via heated bolts or micromotors.
4. Three-Roll Calender System
Features: Consists of three large-diameter, high-precision, mirror-polished rollers, with internal oil or water circulation for precise temperature control. The middle roller is usually fixed, while the upper and lower rollers can be precisely adjusted to control the roller gap and the thickness of the sheet.
Characteristics of PP sheets:
High hardness and high melting point.
Low heat distortion temperature (100℃), low transparency, low gloss, and low rigidity.
High impact strength; the strength of PP increases with increasing ethylene content.
Good surface rigidity and scratch resistance. PP sheets are modified by adding glass fibers and metal additives, resulting in a flow rate (MFR) ranging from 1 to 40. Low MFR PP materials have better impact resistance but lower tensile strength. PP sheets typically do not suffer from environmental stress cracking.
PP sheets have a relatively high shrinkage rate. As a semi-crystalline material, the shrinkage rate of PP sheets is generally 1.8% to 2.5%. Furthermore, the directional uniformity of the shrinkage rate is much better than that of materials such as HDPE.
Excellent moisture resistance, acid and alkali corrosion resistance, and solvent resistance.
