There are many types of engineering plastics for injection molding. In order to facilitate understanding and selection, they can be classified according to following main methods:

1. Classification by chemical structure/polymer type (the most basic and commonly used classification)

Polyamide (PA - Nylon):
Features: excellent mechanical strength, toughness, wear resistance, oil resistance, and self-lubrication. Hygroscopicity is its significant feature, which will affect dimensional stability and performance.
Common grades: PA6, PA66, PA46, PA11, PA12, PA610, PA612, semi-aromatic nylon (PA6T, PA9T, PA10T, etc.).
Applications: gears, bearings, automotive parts (intake manifolds, door handles, radiator tanks), power tool housings, sports equipment, connectors, etc.
Polycarbonate (PC):
Features: extremely high impact strength (especially at low temperatures), excellent transparency, good dimensional stability, heat resistance, and electrical insulation. Chemical resistance (especially solvents and alkalis) and scratch resistance are relatively poor.
Applications: automotive lampshades, instrument panels, safety helmet masks, electronic and electrical housings (mobile phones, computers), medical devices, protective glasses, water bottles, etc.
Polyoxymethylene (POM - Delrin):
Features: extremely high rigidity, strength, hardness, excellent fatigue resistance, wear resistance and low friction coefficient, good dimensional stability, solvent resistance. Toughness is not as good as PA and PC, and it is easy to decompose at high temperatures.
Applications: precision gears, bearings, cams, automotive fuel system components (pump cores), zippers, buckles, sprayer components, etc.
Polybutylene terephthalate (PBT):
Features: good mechanical strength, rigidity, heat resistance, electrical insulation, chemical resistance (especially solvents and fuels), low moisture absorption, easy molding and processing. Toughness and impact strength are not as good as PA.
Applications: automotive parts (lamp holders, connectors, sensor housings), electronic appliances (switches, sockets, coil skeletons), optical fiber sheaths, etc.
Polyphenylene ether (PPO/PPE):
Features: very high heat resistance, dimensional stability, excellent electrical insulation (especially at high frequencies), low moisture absorption, good hydrolysis resistance. Pure resin is difficult to process and is usually blended with PS or PA (such as Noryl, Prevex).
Applications: automotive heat-resistant parts (wheel covers, dashboard frames), electronic appliances (connectors, coil frames, water treatment equipment parts), medical equipment, hot water parts, etc.
Polyphenylene sulfide (PPS):
Features: extremely excellent high temperature resistance, flame retardancy (itself V-0), chemical corrosion resistance (almost insoluble in any solvent), dimensional stability, high rigidity. Poor toughness and high price.
Applications: automotive high temperature resistant parts (ignition system, sensor, pump housing), electronic appliances (SMT connectors, coil frames), chemical equipment parts, pump valve impellers, etc.
Polyetheretherketone (PEEK):
Features: engineering plastic pyramid tip material. Excellent long-term high temperature resistance, excellent mechanical strength (especially at high temperatures), outstanding chemical resistance, excellent wear resistance, self-lubricating, high purity, flame retardant. Very expensive.
Applications: aerospace, high-end automobiles, oil and gas, semiconductors, medical implants and other high-demand fields.
Polyimide (PI):
Features: top high temperature resistance (long-term > 260℃), excellent mechanical properties, electrical insulation, radiation resistance, low friction coefficient. Difficult processing and extremely high cost.
Applications: parts in extreme high temperature environments (aerospace, semiconductor equipment), high-performance friction parts, insulating films, etc.
Liquid crystal polymer (LCP):
Features: extremely high strength, rigidity, heat resistance, dimensional stability (extremely low thermal expansion coefficient and shrinkage), excellent flame retardancy, chemical resistance, excellent fluidity (suitable for thin-walled complex products). Obvious anisotropy (large performance differences in different directions), high price.
Applications: precision electronic connectors (SMT), coil skeletons, sensor housings, pump and valve components, high-temperature resistant tableware, optical communication devices, etc.
Polysulfones (PSU, PES, PPSU):
Features: excellent heat resistance, dimensional stability, rigidity, good electrical insulation, transparency (PSU, PES), hydrolysis resistance (PPSU is especially good). Solvent resistance is slightly worse than PPS/PEEK.
Applications: medical devices (sterilizable), food contact parts, automotive electrical parts, water treatment parts, aircraft interior parts, etc.
Special nylons (such as polyaramide PPA - semi-aromatic nylons are usually classified as PA, but sometimes listed separately):
Features: greatly improve heat resistance (such as PA6T, PA9T, PA10T), dimensional stability and chemical resistance on the basis of PA. Hygroscopicity is usually lower than PA6/PA66.
Applications: high-temperature parts around automotive engines, high-reliability electronic connectors, etc.

2. Classification by performance level/application temperature

General engineering plastics: Refers to several types with large output, wide application and relatively moderate price. Usually refers to: PA, PC, POM, PBT, PPO/PPE. Long-term use temperature is roughly in the range of 100℃ - 150℃.
Special engineering plastics: Refers to plastics with better performance (especially high temperature resistance), but relatively small output and higher price. Usually refers to: PPS, PSF/PES/PPSU, LCP, PEEK, PI, PPA, etc. Long-term use temperature is usually >150℃, and can even reach above 250℃ (PEEK, PI).

3. Classification by crystallinity (affecting material properties and processing technology)

Crystalline plastics:
Features: It has obvious melting point and solidification point. Crystallinity has a great influence on performance (strength, rigidity, heat resistance, dimensional stability) during processing. Shrinkage rate is large and anisotropic. Usually opaque or translucent (unless very thin). Good wear resistance and chemical resistance.
Typical materials: PA, POM, PBT, PPS, PEEK, PP, PE, PPA.
Non-crystalline plastics:
Features: No obvious melting point, only softening temperature range. Small shrinkage and isotropic. Usually transparent or translucent (such as PC, PS). Good dimensional stability and not easy to warp. Chemical resistance (especially solvents) is usually not as good as crystalline plastics.
Typical materials: PC, PPO/PPE, PSF/PES/PPSU, ABS, PS, PMMA.

4. Classification by whether it is enhanced/modified (one of the most practical classifications, most engineering plastics in actual applications are modified)

Pure resin: unmodified base polymer. Performance usually does not meet many application requirements (such as insufficient strength, rigidity, heat resistance, large shrinkage, easy to warp).
Reinforced plastics: Add reinforcing materials (mainly fibers) to resin matrix to greatly improve mechanical strength, rigidity, dimensional stability and heat resistance.
Glass fiber reinforcement (GF): most commonly used, cost-effective. Such as PA6-GF30, PBT-GF20, PC-GF10, etc.
Carbon fiber reinforcement (CF): better than glass fiber reinforcement (strength, rigidity, dimensional stability, wear resistance, conductivity), high cost. Such as PEEK-CF30, PA-CF20.
Mineral filling: adding talcum powder, mica, calcium carbonate, etc., mainly improves dimensional stability, reduces warping, reduces cost, and increases hardness/rigidity (but improvement is not as good as fiber). Such as PP-Talc20.
Modified plastics: improve specific properties by adding various additives or blending with other polymers.
Flame retardant modification: add flame retardants to meet fire protection requirements (UL94 V-0, V-1, V-2, 5VA, 5VB). Such as FR-PA, FR-PC, FR-PBT.
Toughening modification: Add elastomers (such as TPU, POE) or toughening agents to improve impact strength, especially low-temperature toughness. Such as Toughened-PA, Toughened-PC.
Wear-resistant/lubricating modification: Add PTFE (Teflon), silicone oil, graphite, molybdenum disulfide, etc. to reduce friction coefficient and improve wear resistance. Such as POM+PTFE, PA+MoS2.
Conductive/antistatic modification: Add carbon black, metal fiber, carbon fiber, permanent antistatic agent, etc. to give conductivity or eliminate static electricity. Such as Conductive-PA, Anti-static-PC.
Alloy/blend: Physically blend two or more polymers to complement each other. Such as PC/ABS (combining PC's heat resistance and toughness & ABS's easy processing), PC/PBT (chemical resistance & toughness), PPO/PS (improving PPO processing), etc.
In actual material selection and injection molding applications, the most commonly used and practical classification combination is: basic chemical structure + enhancement/modification type. For example:
"30% glass fiber reinforced polyamide 6" (PA6-GF30); "Flame retardant polycarbonate / acrylonitrile-butadiene-styrene alloy" (FR-PC/ABS), "Carbon fiber reinforced polyetheretherketone" (PEEK-CF30), "Toughened polyoxymethylene" (Toughened-POM).
Choice of engineering plastic depends on specific requirements of product: temperature to be withstood, load (strength, toughness, rigidity), chemical environment, dimensional accuracy, appearance (color, transparency), friction and wear requirements, electrical properties, cost, etc. Understanding these classification methods will help to systematically screen and compare materials.