What Types of Plastics Can Be Processed by Injection Molding Machines?

As the core equipment of plastic processing, Injection molding machines achieves high precision and efficiency by heating thermoplastic or thermoset plastics to melt state and injecting mold cavities. It processes a wide range of products, from everyday products to industrial components, covering more than 20 types of plastic. In this paper, the types of plastics used in injection molding machines are analyzed systematically from four aspects: material classification, performance characteristics, processing adaptability and typical application scenarios.

Commodity plastics account for over 70% of global plastic consumption and are characterized by high production, low cost and stable processing performance. They are the main materials used in injection molding machines.
1.1 Polyethylene
Polyethylene (HDPE) and LDPE are the two kinds of plastics with the highest yield. High density polyethylene (HDPE) molecular chain is closely arranged, has good rigidity and chemical resistance, and is often used in the manufacture of large containers such as turnover boxes and trash bins. Low density polyethylene (LDPE) has good flexibility due to its branched structure and is widely applied in flexible products such as films and packaging bags. During injection molding, HDPE needs to keep the temperature in the barrel between 220 ° C and260°C to prevent degradation, while LDPE can be processed at temperatures between 180 ° C and220°C.
1.2 Polypropylene (PP)
Polypropylene has the lowest density of plastics (0.9-0.91 g/cm3), excellent fatigue resistance and heat resistance (melting point 160 -170°C) and is the preferred material for bumpers and appliance housings. During injection molding, polypropylene warps easily due to the different crystallization rates, so it is necessary to optimize mold temperature (recommended 80-100°C) and maintain pressure (recommended 60-80% injection pressure) to control shrinkage.
1.3 Polystyrene
PS is known for its excellent optical properties (88-92% light transmittance), but its brittleness and heat resistance (heat deflection temperature 70-90°C) limit its application. High-impact polystyrene (HIPS) is a butadiene polymer that is three to5 times stronger than PS and is widely used in toy housings and computer keyboards. Exhaust injection molding technology effectively solves the problem of volatile residue in PS processing and improves product surface quality.
1.4 Acrylonitrile-Butadiene-Styrene Copolymer (ABS)
ABS combines the chemical resistance of acrylonitrile, the toughness of butadiene and the processability of styrene to become the most widely used engineering plastics. The Vicat has a softening temperature of 100-105°C and an impact strength 3 to4 times that of PS, and is particularly suitable for manufacturing electronic housing and car interior parts that require external force resistance. Two-color injection molding makes it possible to combine ABS with other materials,such as PC, to meet diverse product design requirements.

Engineering plastics have excellent mechanical properties, thermal stability and chemical resistance. Expensive as it is, it plays an irreplaceable role in high-end manufacturing.
2.1 Polyamide (PA, nylon)
Of the PA family, PA6 and PA66 are the most widely used due to their balanced mechanical and processing properties. The melting point range is 220-265 ℃, the water absorption rate as high as 2-3%, and the size stability is obviously affected by humidity. Fiberglass Enhanced PA (GFPA) increases tensile strength to over 200 MPa and heat deflection temperature to more than 200°C, making it ideal for precision mechanical components such as gears and bearings. During injection molding, strict control of barrel temperatures (PA6: 240-260°C; PA66: 270-290°C) and dry conditions (120°C + 4 hours of drying) is essential.
2.2 Polycarbonate (PC)
Known as "bulletproof glue," PC showed 300 times the impact strength and 89 89% light transmittance of 89 glass, but poor abrasion resistance (surface was just 2H). PC/ABS alloys blended with ABS have greatly improved the overall performance of the product and have become standard material for laptop housings and car instrument panels. During injection molding, high injection pressure (80-120 MPa) and rapid injection (injection speed ≥100 mm/s) are required to prevent melt freezing, and mold temperatures at 80-100 ℃ is recommended.
2.3 Polyoxymethylene (POM)
POM is known as "super steel" with hardness (Rockwell hardness M85-105), abrasion resistance close to metal, and excellent dimensional stability (linear expansion coefficient only 6×10−5/°C). By adding polytetrafluoroethylene (PTFE) lubricant, friction coefficient can be reduced to less than 0.1, is an ideal material for gear, slider and other transmission components. During injection molding, the temperature in the barrel should be controlled between 190 and210 degrees Celsius to avoid decomposition of formaldehyde gas.

Thermoset plastics solidify through chemical reactions during molding, forming irreversible crosslinking structure, suitable for special conditions requiring high temperature and chemical resistance.
3.1 Phenolic Plastics (PF)
PF is the earliest industrialized thermosetting plastic with excellent electrical insulation, heat resistance (continuous temperature of 150°C) and flame retardancy (oxygen index ≥30%), but brittle and poor processing flowability. By adding toughening agents and flow improvers, it can manufacture electrical switches and sockets. During injection molding, barrel temperatures should be controlled between 70 and90°C, mold temperatures between 150 and180°C, and injection pressures between 120-150 MPa to overcome high viscosity resistance.
3.2 Epoxy Resins (EP)
EP is known for its excellent adhesion (shear strength ≥20 MPa) and chemical resistance and is widely used in electronic packaging and composite matrix. By adding inorganic fillers (such as silica powder), shrinkage can be significantly reduced (from 2% to below 0.1%) and dimension accuracy can be improved. Injection molding requires specialized screws (aspect ratio ≥20:1) and high-pressure injection molding systems (pressure ≥200 MPa), with mold temperatures controlled between 120 and150°C.

Specialty plastics provide special properties through molecular structure design to meet high-end demands in aerospace, medical and other fields.
4.1 Polytetrafluoroethylene
PTFE (PTFE) has the widest range of operating temperatures (-200 to 260°C), the highest chemical stability (resistance to king water corrosion) and the lowest friction coefficient (0.04). However, its extremely high melt viscosity (1010-1012 Pa. S) makes traditional injection molding difficult. Cold pressing, sintering or modification of polytetrafluoroethylene (such as filled glass fibers) is required to reduce melt viscosity. Modified PTFE products are widely used in seals and bearing holders.
4.2 Polyetheretherketone
PEEK has a long-term temperature of 240°C, a short-term temperature resistance of 315°C, and a a tensile strength 90-100 MPa. Carbon-fiber-reinforced PEEK improves modulus of elasticity to 140 GPa, close to the level of aluminum alloy, and is widely used in aerospace structures and medical implants. Injection molding requires specialized high-temperature screws (barrel temperature 380-420°C) and high-pressure systems (pressure ≥250 MPa), with mold temperatures controlled between 180 ° C and200°C.
V. Trends in processing technology
With the development of materials science and equipment technology, the processing capabilities of injection molding machines is expanding:
Multi-material composite molding: The laminated bonding of PC/ABS, PA/GF and other composites can be achieved by co-injection technology to improve the comprehensive performance of the product.
Microinjection molding: Development of ultra-precision screws (diameter ≤10 mm) and high-response control systems to produce microcomponents such as gears and connectors weighing less than 0.1 g.
Gas auxiliary injection molding: high pressure nitrogen gas auxiliary filling is used to reduce sink marks caused by product wall thickness differences. It is suitable for car door panels, large appliance housings and other thick-walled components.
Bioenergy source plastics processing: adapt to the characteristics of polylactic acid, polyhydroxyapatite and other materials, develop special screw structures and temperature control system, promote the industrialization of environmental protection materials.
Conclusion:
injection molding machines have expanded from traditional commodity plastics to engineering plastics, thermosetting plastics and even specialty plastics, covering production needs from micrometer-scale precision parts to micron-scale large structural components. With the development of material modification technology and equipment intelligence, injection molding processes will continue to play a central role in high-end manufacturing, pushing the plastics industry toward high performance, high-performance, multifunctional, and sustainable development.