Molding properties of thermoplastics
The molding properties of thermoplastics refer to their physical characteristics, such as flow, cooling, and shrinkage, exhibited during processes like injection molding and extrusion. These properties directly determine the choice of molding process and the stability of product quality. Unlike thermosets, thermoplastics are reversible, softening upon heating and hardening upon cooling. Their molding properties are primarily determined by their molecular structure, molecular weight distribution, and the type of additives. Understanding the molding properties of thermoplastics can help engineers optimize process parameters and design appropriate molds, thereby improving production efficiency and product quality.
Melt flow is the most critical molding property of thermoplastics, typically measured by melt flow rate (MFR) or melt viscosity. A higher MFR indicates better melt flow, making it easier to fill complex cavities; a lower MFR indicates poor flow, requiring higher injection pressures and temperatures. For example, the MFR of polyethylene (PE) typically ranges from 0.1 to 30 g/10 min. PE with a high MFR is suitable for molding thin-walled parts, while PE with a low MFR is suitable for molding thick-walled structures. Melt viscosity is significantly affected by temperature and shear rate. Most thermoplastics are non-Newtonian fluids, and viscosity decreases with increasing shear rate (shear thinning). For example, polypropylene (PP) exhibits significantly improved flow during high-speed injection. Increasing temperature can also reduce viscosity, but temperature sensitivity varies significantly among different plastics. Polycarbonate (PC) is more sensitive to temperature, while polyethylene (PE) is more sensitive to shear rate. Therefore, during molding, temperature and injection speed must be adjusted according to the plastic’s flow characteristics to balance flow and part quality.
Thermal stability refers to the ability of thermoplastics to resist degradation at high temperatures, directly impacting the stability of the molding process and the mechanical properties of the finished product. If a plastic remains at the molding temperature for too long or at too high a temperature, degradation reactions such as molecular chain breakage and oxidation can occur, leading to melt discoloration, decreased viscosity, and even defects such as bubbles and scorch marks. The thermal stability temperature ranges of different plastics vary significantly: Polyethylene (PE) has a thermal stability range of 180-230°C and is susceptible to degradation above 250°C. Polyoxymethylene (POM) has poor thermal stability, decomposing and producing formaldehyde gas if held above 240°C for more than 10 minutes. Polyetheretherketone (PEEK), on the other hand, has a thermal stability range of over 380°C, making it suitable for high-temperature molding. To ensure thermal stability, the barrel temperature and melt residence time must be strictly controlled during molding. For plastics with poor thermal stability (such as PVC and POM), heat stabilizers should be added and the molding cycle shortened to prevent prolonged melt retention in the barrel.
Shrinkage is the characteristic of thermoplastics, whereby they lose volume during cooling. It is a major cause of dimensional deviation in finished products and requires careful consideration during mold design and process adjustments. Plastic shrinkage typically ranges from 0.5% to 3%, with specific values depending on the plastic type, molding process, and product structure. Crystalline plastics (such as PE and PP) experience higher and more uneven shrinkage due to the volumetric change associated with the crystallization process. Amorphous plastics (such as PC and ABS) experience lower and more stable shrinkage. Shrinkage is also affected by molding parameters. Increasing injection pressure and holding time reduces shrinkage because more melt compensates for cooling shrinkage. Increasing mold temperature increases the crystallinity of crystalline plastics, leading to higher shrinkage. For example, the shrinkage of PP products is typically 1.5% to 2.5%. Increasing the holding pressure (from 80 bar to 120 bar ) can reduce this to 1.2% to 1.8%. During mold design, cavity dimensions should be calculated based on this shrinkage. During molding, shrinkage should be controlled by optimizing holding parameters to ensure that part dimensions meet specified requirements.
Hygroscopicity is a key property of some thermoplastics (such as polyamide and polycarbonate). It refers to the plastic’s ability to absorb water during storage and processing. Excessive moisture content can seriously affect molding performance and product quality. Moisture absorbed by hygroscopic plastics vaporizes at high temperatures, leading to bubbles in the melt and defects such as silver streaks and voids in the molded product. Furthermore, moisture can cause hydrolysis in some plastics (such as PA), reducing molecular weight and mechanical properties. Hygroscopicity varies significantly among different plastics: Nylon 6 (PA6) can absorb up to 3%-4% water, requiring rigorous drying. PC absorbs approximately 0.3%, but even trace amounts of moisture can cause silver streaks in the finished product. Non-polar plastics such as PE and PP have very low hygroscopicity and generally do not require drying. Hygroscopic plastics require drying before molding. For example, PA6 should be dried at 80-100°C for 4-6 hours, and PC at 120-140°C for 6-8 hours, to reduce the moisture content to below 0.05%. The dried plastic needs to be stored in a sealed container to avoid re-absorption of moisture and ensure a stable molding process.
Compatibility refers to the mutual solubility of different thermoplastics during mixing and processing, and is crucial for blending, modification, and the use of recycled materials. Plastics with good compatibility (such as PP and PE) can be mixed and processed to form a homogeneous blend, combining the advantages of both materials. Plastics with poor compatibility (such as PE and PVC) will delaminate and crack when mixed, resulting in reduced product performance. Compatibility can be assessed by observing the appearance and mechanical properties of the blend. Homogeneous, transparent blends generally indicate good compatibility, while those that exhibit delamination or turbidity indicate poor compatibility. To improve compatibility, compatibilizers (such as maleic anhydride-grafted polymers) can be added to promote bonding between the different plastic molecules through chemical reactions. For example, adding maleic anhydride-grafted PE to a blend of PE and PA significantly improves their compatibility, increasing the impact strength of the blend by over 30%. Understanding plastic compatibility can help rationally utilize recycled materials, reduce costs, and develop new materials with specific properties through blending and modification.
