Optimal Screw Parameter Selection: Tailoring Plastic Processing for PC, PMMA, PA, PET, PVC

When delving into the intricate world of plastic processing, the selection of screw parameters plays a pivotal role in determining the success of the operation. Let's explore the nuances of screw parameter optimization for 5 distinct raw materials: PC (Polycarbonate), PMMA (Organic Glass), PA (Nylon), PET, PVC.

1. Polycarbonate (PC)

Features:

Non-crystalline nature with a glass transition temperature of 140°C to 150°C and a melting temperature ranging from 215°C to 225°C.

High viscosity, sensitivity to temperature, and notable water absorption.

Screw Parameter Selection:

a. In light of its remarkable thermal stability and high viscosity, opting for a larger L/D ratio enhances plasticization. This recommendation is substantiated by data indicating improved plasticization efficiency with a higher aspect ratio.

b. The challenge in calculating the melting rate calls for adapting the compression ratio ε. Leveraging empirical evidence related to PC's machinability, a higher gradient A value is suggested, falling within the 2-3 range for larger L2.

c. The incorporation of a mixing structure in the screw design addresses both high viscosity and water absorption. Data-driven insights support the claim that this addition reinforces solid bed disintegration and aids in the transformation of water into gas, mitigating potential processing issues.

d. While maintaining consistency with other ordinary screws in terms of parameters such as e, s, φ, and barrel clearance, the emphasis on data-driven decision-making remains paramount.

2. Organic Glass (PMMA)

Features:

A glass transition temperature of 105°C, melting temperature exceeding 160°C, and a wide molding temperature range.

High viscosity, limited fluidity, and pronounced water absorption.

Screw Parameter Selection:

a. A gradient screw with an L/D ratio of 20-22 is recommended, considering the accuracy requirements of the final product. This preference is substantiated by empirical data illustrating the correlation between screw design and product accuracy.

b. The compression ratio ε, falling within the 2.3-2.6 range, is determined by the material's characteristics. This data-driven approach ensures optimal processing conditions for PMMA.

c. Addressing the hydrophilic nature of PMMA, the addition of a mixing ring structure at the front end of the screw is supported by data indicating improved processing outcomes, particularly in terms of water absorption.

d. Aligning other parameters with universal screw designs maintains a balance between industry standards and material-specific requirements, as substantiated by historical processing data.

3. Nylon (PA)

Features:

Crystalline plastics with diverse types and a narrow melting point range, exemplified by PA66 with a melting point between 260°C and 265°C.

Low viscosity, excellent fluidity, a distinct melting point, and moderate water absorption.

Screw Parameter Selection:

a. The choice of mutation type screws with an L/D ratio of 18-20 is underpinned by historical data illustrating the correlation between screw type and the characteristics of crystalline plastics.

b. A compression ratio between 3 and 3.5, coupled with a specific h3 value, is recommended to prevent overheating and decomposition, as evidenced by thermal stability data for PA.

c. Fine-tuning the gap between the check ring and barrel, as well as the screw and barrel, is informed by data suggesting that a smaller gap is preferable due to PA's low viscosity. The consideration of a self-locking nozzle is supported by data indicating enhanced processing efficiency in specific scenarios.

d. Adhering to universal screw design principles for other parameters ensures a balanced approach, integrating industry standards with material-specific requirements based on extensive historical processing data.

4. PET (Polyethylene Terephthalate)

Features:

Boasting a melting point spanning 250°C to 260°C, blow molded PET exhibits a broader molding temperature range, approximately 255°C to 290°C.

Blow molded PET presents high viscosity, with temperature exerting a significant impact on viscosity, resulting in less-than-ideal thermal stability.

Screw Parameter Selection:

a. For L/D, the optimal ratio is generally considered as 20, featuring a three-segment distribution with L1 at 50%-55% and L2 at 20%.

b. Employing screws characterized by low shear and a low compression ratio, typically around 1.8-2, helps mitigate issues such as discoloration or opacity caused by shear overheating. To further address these concerns, set h3 at 0.09D.

c. The absence of a mixing ring at the screw's front end serves a dual purpose: preventing overheating and minimizing material storage concerns.

5. PVC (Polyvinyl Chloride)

Features:

Lacking a distinct melting point, PVC softens at 60°C, enters a viscoelastic state at 100°C to 150°C, and fully melts at 140°C. Simultaneously, it undergoes decomposition, releasing HCl gas. Rapid decomposition occurs at 170°C, with the softening point closely aligned with the decomposition point.

PVC exhibits poor thermal stability, where elevated temperature and prolonged exposure lead to decomposition and hindered fluidity.

Screw Parameter Selection:

a. Stringent temperature control measures are imperative, necessitating a low screw design to prevent overheating.

b. Corrosion-resistant materials for the screw and barrel are essential due to PVC's corrosive nature.

c. In the injection molding process, maintaining strict control is paramount to address the material's peculiar characteristics.

d. Ideal screw parameters include L/D within the 16-20 range, h3 at 0.07D, ε ranging from 1.6-2, and a segmented distribution of L1 at 40% and L2 at 40%.

e. To thwart material accumulation, omitting a check ring and incorporating a 20°-30° head taper is recommended, particularly suitable for soft adhesives. Alternatively, for heightened product requirements, a separate screw without a measuring section proves advantageous, especially for hard PVC. In such cases, integrating cooling water or oil holes in the feeding section screw, along with cold water or oil grooves in the machine barrel, ensures precise temperature control within ±2°C. This nuanced approach optimizes processing efficiency for PVC in diverse applications.