1.Polypropylene - PP
Extrusion temperature: Wide range, approximately 190-250℃. It has a distinct melting point but a high enthalpy value, requiring sufficient plasticizing and cooling capacity.
Melt strength and sagging: Low melt strength is one of the biggest challenges in the extrusion of PP sheets (especially homopolymer PP). At the die exit, the sheet is prone to sagging under its own weight, which impairs thickness uniformity. High melt strength PP (HMSPP) or copolymer PP is a better choice.
Crystallinity and cooling: As a crystalline polymer, the cooling rate and temperature (especially the calender roll temperature) exert a significant impact on crystallinity, transparency and post-shrinkage rate. A precisely controlled gradient cooling system is required.
Hygroscopy: Extremely low; drying is generally unnecessary (unless stored in an extremely humid environment).
Shrinkage rate: Relatively high (1.0-2.5%), requiring close attention to the dimensional stability of finished products.
Key characteristics / Challenges:Strengths: Light weight, excellent chemical resistance, microwaveable, and high cost-effectiveness.Challenges: Controlling sagging and edge warpage; achieving high transparency requires quenching processes and water tanks, making production more challenging than that of PS.
2. Polystyrene - PS
Extrusion temperature: Approximately 180-230℃. There are slight differences between GPPS and HIPS; the processing temperature of HIPS is usually slightly lower due to its rubber component.
Melt strength and sagging: Good melt strength, especially evident in GPPS. The sheet remains stiff when drawn out of the die with minimal sagging, enabling easy traction and shaping.
Crystallinity and cooling: An amorphous polymer with no crystalline phase transition. The cooling process is mainly a glass transition, with a very low shrinkage rate (approximately 0.4-0.7%) and excellent dimensional stability. The temperature control of calender rolls is relatively flexible.
Hygroscopy: Low, and pre-drying is usually unnecessary.
Key Characteristics / Challenges:Strengths: GPPS features excellent optical transparency, high rigidity and good printability; HIPS has good impact resistance. It has a wide processing window and is one of the easiest sheet materials to extrude.
Challenges: GPPS is highly brittle; the surface gloss of HIPS is generally inferior to that of GPPS; its heat and oil resistance are moderate.
3. Polyethylene Terephthalate - PET (APET)
Extrusion temperature: Relatively high, approximately 260-290℃. It is highly sensitive to temperature and requires precise control.
Melt strength and sagging: Melt strength is moderate, yet melt viscosity is extremely sensitive to temperature and moisture. Sagging control is easier than that of PP but more difficult than that of PS.
Crystallinity and cooling: The crystallization rate is relatively slow. High-transparency, amorphous APET sheets can be produced through rapid cooling (using a cold water tank). Insufficient cooling will lead to premature crystallization, causing an increase in haze and embrittlement.
Hygroscopy: Extremely high! PET is highly susceptible to hydrolytic degradation in the molten state, resulting in a decrease in molecular weight and loss of strength. A dehumidifying dryer must be used to strictly control the moisture content of raw materials below 50ppm (0.005%)-this is the lifeline of PET extrusion.
Key Characteristics / Challenges:Strengths: Excellent transparency, gloss, rigidity and barrier properties (especially to oxygen and carbon dioxide).
Challenges: Stringent requirements for raw material drying and equipment cleaning; equipment must be corrosion-resistant (due to acetic acid by-products); high difficulty in recycling and reprocessing waste materials.
4. Polyvinyl Chloride - PVC (Rigid)
Extrusion temperature: Relatively low, approximately 160-190℃. It has poor thermal stability, demanding precise control of residence time and temperature.
Melt strength and sagging: Features good melt strength and easy formability. However, the melt has obvious elasticity and significant die swell, which requires a suitable die lip design for compensation.
Thermal stability: The biggest challenge. PVC decomposes and releases HCl gas at processing temperatures, which has an autocatalytic effect. Heat stabilizers must be added, and the equipment's screw, barrel and die need to be made of corrosion-resistant alloys (e.g., bimetallic barrels).
Cooling: As an amorphous material, its cooling is relatively simple. Yet due to its low thermal conductivity, an effective cooling system is required.
Key Characteristics / Challenges:Strengths: High hardness, good flame retardancy, excellent weather resistance, and flexibly adjustable performance via formulation.
Challenges: Complex formulation (including stabilizers, lubricants, modifiers, etc.); corrosive to equipment; and increasingly stringent environmental and health regulations.
5. Acrylonitrile-Butadiene-Styrene Copolymer - ABS
Extrusion temperature: Approximately 200-240℃. Excessively high temperatures will cause degradation of the rubber phase and impair toughness.
Melt strength and sagging: Good melt strength with controllable sagging.
Hygroscopy: Relatively high. Though not as sensitive as PET, moisture can cause bubbles or crazes on the sheet surface. Drying at around 80℃ for 2-4 hours is generally required.
Cooling and shrinkage: An amorphous material with a moderate shrinkage rate (approximately 0.4-0.9%). However, due to its high glass transition temperature, the calender roll temperature needs to be set relatively high to avoid surface stress cracking of the sheet.
Key Characteristics / Challenges:Strengths: Excellent balance of toughness, surface hardness, gloss and processability.
Challenges: Relatively high raw material cost; moderate resistance to ultraviolet rays and long-term weathering (additives required); extremely high requirements for the finish of calender rolls when pursuing a high-gloss surface.