Key Steps Of Plastic Sheet Extrusion

Key steps of plastic sheet extrusion
1. Raw material preparation and ingredients

This is the starting point of the entire process and crucial.

·Drying: For hygroscopic plastics such as PET, PA, ABS, they must be thoroughly dried in advance. The residual water will vaporize during heating, causing bubbles, silver lines or surface defects inside the sheet, and degrading the polymer, affecting the final strength and transparency.
·Mixing and ingredients: Mix the main resin with the required color masterbatch, additives (such as antioxidants, lubricants), or recycled materials in precise proportions. Uniform mixing is the foundation for ensuring consistency in product color and performance.

2. Plastic plasticization and melting

The mixed raw materials enter the extruder.

·Feeding: Raw materials enter the barrel through a hopper.
·Heating and shearing: In the barrel, the raw materials are heated by a heater and strongly sheared and mixed by a rotating screw. Under the combined action of heat and shear force, plastic gradually melts and becomes a uniform, viscous melt.
·Temperature control: The barrel is divided into multiple temperature zones, and precise temperature control is required for each zone to ensure complete plasticization of the plastic without degradation due to overheating.

3. Melt filtration and homogenization

Molten plastic needs to be refined before reaching the mold head.

·Filtration: The melt passes through a "breaker plate" with a filter screen. The filter screen is used to filter impurities, unmelted particles or gel to ensure the purity of the melt. The mesh size of the filter is selected according to product requirements.
·Homogenization: This process can also increase the back pressure of the melt, making plasticization more uniform and extrusion more stable.

4. Forming through a flat die head

This is the key step in giving the sheet shape and initial size.

·Mold head channel: Pure melt enters the flat mold head from a circular outlet. There is a precise flow channel system inside the mold head (usually designed in a hanger style), which is used to evenly unfold the cylindrical melt into a rectangular strip of melt with consistent width and thickness.
·Mold lip adjustment: High quality mold heads are equipped with adjustable mold lips. By fine-tuning the bolts at different positions of the mold lip, the thickness of each point in the transverse direction of the sheet can be precisely controlled, thereby eliminating uneven phenomena such as thickness in the middle and thinness on both sides.

5. Cooling and calendering in a three roll calender

This is the core factor that determines the final surface quality (glossiness, flatness) and thickness accuracy of the sheet material.

·Introduction: The molten sheet billet extruded from the die is introduced into a three roll calender (usually composed of upper, middle, and lower rollers).
·Cooling and shaping: There is circulating cooling water inside the roller to preliminarily cool and shape the sheet. The sheet will sequentially pass around three rollers, forming an "S" shape or climbing path.
·Polishing: The precise gap and pressure between the rollers press the sheet to a smooth and even surface. By adjusting the roll gap, the final thickness of the sheet can be precisely controlled.
·Temperature control: The temperature of each roller needs to be independently and accurately controlled, which is crucial for the crystallinity of crystalline plastics (such as PP) and the surface properties of the sheet.

6. Auxiliary cooling and traction

The sheet has not completely cooled to room temperature after leaving the three rollers.

·Cooling rack: The sheet enters a cooling rack with multiple cooling fans and is further cooled in the air until it is completely cured.
·Traction: A traction machine typically consists of one or more pairs of rubber or silicone rollers that grip the sheet and provide stable, continuous tension. The traction speed must be synchronized with the extrusion speed, as it directly affects the final thickness and orientation (molecular arrangement direction) of the sheet.

7. Rolling or cutting

According to the subsequent process requirements, process the final product.

·Roll up: For thin and flexible sheets (such as PP, PET), use a large roll up machine to roll them into rolls for easy transportation and subsequent processing (such as thermoforming).
·Cutting: For thick and hard sheets (such as PC, PS boards) or products that require fixed length, an online cutting device (such as a circular saw) will be used to cut them into fixed length boards.
·Edge material recycling: During the traction process, an online edge cutting machine is usually used to cut off the uneven edges on both sides of the sheet. These edges are immediately crushed and recycled, and sent to the raw material system to reduce production costs.

Summary of Key Points for Core Process Control

·Temperature: The temperature control of the barrel, die head, and calendering roller is the lifeline to ensure plasticization quality, extrusion stability, and surface appearance.
·Speed: The matching degree between the extruder screw speed and the traction speed directly determines the thickness and intrinsic performance of the sheet.
·Cooling: The cooling rate of the calender and cooling frame affects the crystallinity, internal stress, and dimensional stability of the sheet.

Mastering these key steps and their interrelationships is the foundation for producing high-quality plastic sheets.