Compared with other materials, plastic materials have their own suitability. Selection of plastic materials aims to maximize their advantages and minimize their disadvantages.
1. Various Situations Where Plastics Are Suitable
① Situations requiring lightweight materials, where wood and other materials cannot meet requirements;
② Environments requiring both weight reduction and ability to withstand medium and low loads;
③ Structural products under various medium and low loads;
④ Products with complex shapes; for these products, plastics offer efficient, accurate, and rapid molding characteristics;
⑤ Corrosion-resistant materials for various applications;
⑥ Self-lubricating moving parts;
⑦ Products requiring shock absorption, sound insulation, and heat insulation properties;
⑧ Insulating materials;
⑨ Products requiring good overall performance, such as components combining lightweight, rigidity, hardness, toughness, heat resistance, corrosion resistance, and insulation.

 

2. Situations Where Plastic Materials Are Not Suitable
(1) Products requiring very high mechanical strength, especially structural components requiring high rigidity and materials with tensile strength exceeding 300 MPa;
(2) Environments with long-term operating temperatures reaching 300℃ or 350℃ or higher;
(3) Ultra-high voltage insulating materials above 550 kV;
(4) Except for special occasions, it is not suitable to make highly conductive materials and products from plastics.

Fundamental purpose of selecting plastic materials is to meet performance requirements of product at the lowest cost. This is primary factor to consider in plastic material selection. For a specific plastic product, performance requirements are multifaceted. In selection process, it is necessary to distinguish between primary performance, related performance, and secondary performance.
What are primary, related, and secondary performance requirements for plastic greenhouse films, electrical wires and cables, and plastic food bags?

Service environment refers to temperature, humidity, medium, and other conditions that material or product is subjected to during use, as well as influence of factors such as wind, rain, snow, fog, sunlight, and harmful gases. Therefore, when selecting plastic materials, adaptability of selected material to environment should be considered. (1) Ambient temperature, (2) Ambient humidity, (3) Contact medium  

Processability of plastic materials refers to the ease with which they can be transformed into finished products. Intrinsic properties of material affect its processability; different types of plastics have different processing characteristics, some are easy to process, while others are difficult. Choice of processing technology also has a significant impact on material's properties.
(1) Thermal stability of resin
(2) Suitable molding and processing methods for resin;
(3) Viscosity of resin melt, etc.

Economic feasibility is an important factor in plastic material selection. If selected plastic material meets requirements in terms of performance and processing, but cost is too high, it will be difficult to put into production, let alone enter market. Only high-quality and inexpensive products can be competitive in the market. Cost of plastic products mainly includes:
Raw material price, processing costs, service life, and maintenance costs.
(1) Raw material price: Raw material price accounts for the largest proportion of product cost, approximately 60% to 70% for injection molded products and 70% to 80% for extruded products.
(2) Processing costs: This is an important component of product cost, and product should be completed with the lowest possible processing cost. Processing costs mainly include: Equipment costs, processing energy consumption, and waste materials, etc.
(3) Service life

 

1. Mechanical Properties of Product
Requirements for mechanical properties of product should be considered from three aspects: first, magnitude of force, which can be divided into medium-low load and high load; second, type of force, including tension, compression, bending, impact, and shear; third, nature of force, including fixed load and intermittent load. Different products have different requirements for mechanical properties of plastic materials due to different stress conditions.
(1) General Structural Parts: These mainly include nuts, bolts, washers, brackets, pipe fittings, handles, steering wheels, etc. They are subjected to relatively low stress, generally under fixed loads. UPVC, HDPE, PP, HIPS, and thermosetting resins can be used. For special applications requiring higher mechanical properties, PA, POM, PC, glass fiber reinforced plastics can be selected.
(2) Products subjected to intermittent loads: Such as gears, racks, sprockets, chains, piston rings, cams, etc. These products require high bending strength, impact resistance, fatigue resistance, excellent wear resistance, and certain heat resistance. In some cases, self-lubrication is also required to ensure stable performance during long-term use. Commonly used materials include: PA, GFPA, POM, PPO, PC, GFPC, GFPET, GFPBT, UHMWPE, PTFE, PEEK, PI, and fabric-reinforced phenolic resins.
2. Thermal Properties of Products
Compared with metals and ceramics, plastic materials have a larger coefficient of linear expansion, while their specific heat capacity and thermal conductivity are lower, exhibiting excellent thermal insulation properties. Foam plastics, in particular, are widely used as thermal insulation materials.
(1) Indicators for measuring heat resistance of products
Heat distortion temperature, Vicat softening point, Martin heat resistance temperature, Tg, Tm.  Heat distortion temperature is the most commonly used. Values of above heat resistance indicators are not same for same type of plastic, and general relationship is as follows:
Vicat softening point > Heat distortion temperature > Martin heat resistance temperature.  Based on heat distortion temperature, heat resistance of plastics can be divided into four types.
(2) Other issues related to heat resistance
① Heat resistance of plastics can be greatly improved through filling, reinforcement, blending, and cross-linking modification;
② Heating environment should be fully considered, such as duration of heating and heating medium. For example, for wet heat resistance, low water absorption plastics should be selected, and PA types should be avoided to prevent high-temperature degradation; when in contact with chemical substances, corrosion resistance of plastic should be considered;
③ Magnitude of load on product when heated: heat resistance is good under no load or low load, and low under high load.

 

3. Other Properties of Product
(1) Chemical Properties of Product
This includes chemical resistance, solvent stress cracking resistance, and environmental stress cracking resistance. Factors affecting chemical stability of plastic materials mainly include chemical composition, aggregation state, and additives contained; environmental factors mainly include temperature, humidity, and stress conditions.
(2) Optical Properties of Product
Generally, pure amorphous plastics are mostly colorless and transparent, while the greater crystallinity of crystalline polymers, the worse light transmittance and transparency. However, transparent crystalline plastic products can be obtained by changing crystal structure through methods such as adding nucleating agents and stretching, such as biaxially oriented PET and PP films.
(3) Barrier Properties of Product
This refers to product's ability to provide a certain degree of shielding against gases, liquids, aromas, and odors. In practical applications, various composite materials can be used, such as LDPE/PP/LDPE, LDPE/PET/LDPE, PE/PA/PE, PP/PA/PE, BOPP/PP, etc. Barrier properties of plastics can also be improved through stretching, blending, surface plating, surface coating, and surface chemical treatment to meet different packaging requirements.
(4) Electrical Properties of Product
These can be characterized by parameters such as dielectric constant, volume resistivity, dielectric strength, and arc resistance. Overall, plastic materials have excellent electrical properties and come in many varieties, suitable for various applications with different electrical performance requirements. ① For general-purpose wires and cables, PVC can be used. For high-frequency and high-voltage applications, PE and PE-CL with high resistivity, low dielectric constant, and high dielectric strength should be selected. For rigid parts, HDPE, PP, PTFE, PI, etc., can be used; ② As electrical insulation materials and products (such as switch boxes, relays, contactors, packaging materials, etc.), high dielectric strength, arc resistance, and heat resistance are required. Thermosetting plastics are generally used, such as amino plastics, phenolic plastics, and epoxy plastics; ③ In electronics industry, in addition to electrical performance requirements for various components, there are also high requirements for mechanical properties, heat resistance, and molding and processing performance. Selection can be made according to specific situation. Commonly used plastics include PP, PA, PC, POM, PET, PBT, PSU, PPS, PI, and PTFE.
(5) Combustion and Flame Retardancy of Products
① Combustion and flame retardancy of general plastics can be characterized by oxygen index (OI): OI < 22 indicates flammable plastics, OI between 22 and 27 indicates self-extinguishing plastics, and OI > 27 indicates flame-retardant plastics.
② Factors determining combustion performance of plastics: If macromolecular structure contains halogen, nitrogen, phosphorus, sulfur, or aromatic rings in main chain structure, material will have a certain flame retardancy or self-extinguishing property.
③ OI values and flame retardancy of commonly used plastics.
④ For applications requiring flame-retardant products, in addition to selecting flame-retardant plastic varieties based on product's performance, flame retardants can also be added to achieve purpose of flame retardancy.

Material selection based on product use is essentially still material selection based on product performance. In addition to considering product's application field, factors such as product's operating environment, stress conditions, and target users should also be considered. Considering product's usage environment and stress conditions is equivalent to considering product's performance.
Material selection based on product application can be categorized according to different requirements of plastic products in various application fields, such as plastic products for construction industry, packaging industry, chemical industry, household appliances, agriculture, automotive industry, and mechanical parts. Alternatively, materials can be classified according to their function to meet requirements of different application fields, such as structural materials, corrosion-resistant materials, optical materials, and low-friction materials.

Plastic material selection methods require considering numerous factors, summarizing and comprehensively evaluating them to ultimately select required material, a process that is quite complex. In recent years, systematic material selection methods have been developed, including star-shaped profile model method, statistical quantitative comprehensive evaluation method, value analysis method, and computer-aided selection method, which can be used as references.