This article provides engineers with a systematic framework for understanding plastics, helping readers quickly establish connection between "essence of plastic performance - classification logic - selection methods," and providing clear guidance for material selection in product design.

Plastics are polymers formed by polymerization and cross-linking of molecular monomers and functional additives. Taking typical engineering plastic PA66 GF30 as an example, its composition is roughly: 70% PA66 monomer (determining basic performance) + 30% glass fiber reinforcement (modifying mechanical strength) + a small amount of small molecule additives (such as antioxidants and lubricants, to optimize processing or weather resistance).
The key conclusions drawn from this article are:
Basic properties of plastics are determined by monomers, while specific physical properties (such as strength, heat resistance, and color) can be achieved through additive modification (such as glass fiber to enhance mechanical strength, masterbatch to adjust appearance, and flame retardants to meet fire protection requirements).

When selecting from a wide variety of plastics, it's important to first narrow scope through classification. Core classification logic is as follows:
1. Primary Classification: Thermosets vs. Thermoplastics (Mainstream in Engineering)
Thermoplastics: These can be repeatedly heated, melted, and reshaped (such as PA, PBT, and PP) and are mainstream in engineering applications.
Thermosets: These are permanently fixed after curing and cannot be reprocessed (such as epoxy resins). They are only used in specific applications (such as high-temperature insulation).
Motto in engineering practice is: "Thermoplastics can be reshaped, while heat-set plastics are permanently fixed." Prioritize focusing on thermoplastics based on processing requirements.
2. Secondary Classification of Thermoplastics: Based on "Structure + Temperature Resistance"
Thermoplastics can be further divided into semi-crystalline plastics (such as PA, PBT, and PP) and amorphous plastics (such as PVC and PC). Core difference between these two types of materials lies in arrangement of their molecular chains during cooling and solidification (whether they form ordered crystals):
Semi-crystalline plastics: have a clear melting point and possess superior mechanical strength and chemical resistance;
Amorphous plastics: have no fixed melting point and possess superior transparency and impact resistance (such as PVC wiring harness insulation, which relies on its excellent insulation properties).
Thermoplastics can also be divided into three tiers based on their "long-term operating temperature" ("thermoplastic performance pyramid"). Performance, processing difficulty, and price increase from top to bottom:

 

By combining product's actual operating temperature, you can quickly define material range. Taking automotive connectors as an example:
Common required temperature is T3 (125℃), making PA66 and PBT mainstream choices (matching temperature resistance range of engineering plastics).
If higher flowability is required (such as for precision thin-wall connectors), LCP can be considered. If even higher temperature resistance is required (such as for engine peripheral components), specialty engineering plastics such as high-temperature nylon should be evaluated, and further screening should be conducted based on physical properties.

After initial screening, focus on evaluating core properties of plastic to ensure it matches product design requirements:
1. Mechanical Properties: Determine mechanical reliability of part
Key indicators of interest: Elastic modulus, tensile strength, and notched impact strength (all derived from stress-strain curves; FEA analysis can be used to verify whether part meets mechanical requirements).
Elastic modulus: Affects a component's rigidity. For example, in automotive buckle designs, PA66, with its higher elastic modulus compared to PP, maintains a more stable engagement force during repeated insertion and removal, preventing deformation and failure.
Tensile strength: Determines a component's resistance to damage. When a buckle is subjected to extreme tension, materials with higher tensile strength can withstand greater loads without breaking.
Notched impact strength: Reflects a material's toughness. Plastics with high notched impact strength (such as toughened PA) are less likely to break when dropped or subjected to vibration.
2. Thermal performance: Ensures stability in high-temperature environments.
Key indicators of interest: Relative Thermal Index (RTI) and Heat Deflection Temperature (HDT).
RTI (Relative Thermal Index): Key for assessing long-term heat resistance. For example, "RTI Str: 120℃" on UL Yellow Card indicates that plastic retains at least 50% of its initial mechanical properties after 100,000 hours of continuous use at 120℃ (meeting requirements for long-term automotive use).
HDT (Heat Deflection Temperature): Reflects short-term resistance to deformation at high temperatures. For example, if a part needs to withstand a certain pressure at 100℃, select a material with an HDT > 100℃.
3. Electrical Performance: Impacts insulation design and product dimensions.
Key Indicators of Interest (all related to insulation):
Surface/Volume Resistivity: The higher resistivity, the better insulation.
Dielectric Strength: Limiting voltage required to resist breakdown (dielectric strength of air is 3 kV/mm).
CTI (Comparative Tracking Index): Resistance to surface conductivity (for example, a CTI ≥ 600V is required for parts exposed to humid environments to prevent leakage).
Key Benefit: Better insulation reduces distance between two insulated metal parts, facilitating product miniaturization.
4. Manufacturing Performance: Determines Molding Feasibility
Key Indicators (Part Moldability can be verified through CAE Mold Flow Analysis):
Low shear rate applications (e.g., thick-walled part injection molding): Focus on melt volume flow rate (MVR). A higher MVR indicates better melt fluidity, making it easier for thick-walled parts to fill mold cavity.
High shear rate applications (e.g., thin-walled precision part injection molding): Focus on high-shear melt viscosity (measured using a capillary rheometer). Lower viscosity facilitates molding thin-walled structures while minimizing defects such as sink marks and bubbles.

Following table provides a visual understanding of impact of monomer differences and glass fiber modification on performance (specific grade supplier data should be used for verification when selecting a suitable material):

 

Conclusion: Adding glass fiber significantly improves mechanical strength and temperature resistance, but reduces melt flowability (a balance must be struck between mechanical requirements and molding difficulty).