Plastic injection molding material selection guide

Because the type of resin determines the mechanical, thermal, and chemical properties of the final product, engineers must carefully evaluate a number of factors to determine the appropriate material for their specific application. Let’s review some of the most important ones:  

Mechanical properties play a significant role in determining how a material will perform under various loading conditions. Key things to consider include: 

Yield and tensile strength indicate a material's ability to withstand applied forces without undergoing permanent deformation or failure. For structural components or parts subject to mechanical stress, selecting materials with high yield and tensile strength is imperative to ensure longevity and reliability.  

Modulus, or stiffness, refers to a material's resistance to deformation under applied stress. Higher modulus materials exhibit greater rigidity and are suitable for applications where dimensional stability and structural integrity are paramount.  

Impact resistance measures a material's ability to absorb energy without fracturing or breaking. Materials with high impact resistance are ideal for applications subjected to sudden shocks or impacts, such as automotive components or consumer electronics housings.  

Thermal properties are critical considerations, especially for applications exposed to elevated temperatures or fluctuating thermal environments. Important thermal properties include:  

Maximum short-use temperature is the highest temperature a material can withstand for short durations without undergoing significant degradation. Choosing materials with appropriate short-use temperature ratings ensures that the final product can withstand the intended operating conditions without compromising performance or safety.  

Temperature and aging considerations are important to take into account since some materials exhibit changes in mechanical properties or dimensional stability when exposed to elevated temperatures over time. Understanding how a material's properties evolve with temperature and aging is essential for predicting long-term performance and durability.  

Chemical resistance is crucial for applications exposed to harsh chemical environments or corrosive substances. Key considerations include: 

Stress cracking refers to the susceptibility of a material to cracking or crazing when exposed to certain chemicals or environmental conditions. Selecting materials with high resistance to stress cracking ensures longevity and reliability in chemically aggressive environments.  

Compatibility with specific chemicals or solvents is essential for ensuring the integrity and functionality of the final product. Materials must be carefully chosen based on their resistance to chemical attack and compatibility with the intended operating environment. 

Acrylonitrile butadiene styrene (ABS) is a thermoplastic polymer that is a popular plastic for injection molding due to its versatility and good dimensional stability. Composed of three monomers—acrylonitrile, butadiene, and styrene—ABS combines the strength and rigidity of acrylonitrile and styrene with the elasticity of butadiene. Its versatility allows for a wide range of applications, including automotive components, electronic housings, toys, and consumer goods.  

Pros 

• High impact resistance. ABS is known for its toughness, making it suitable for applications where durability is crucial. 
• Good surface finish. It can be easily polished and painted, offering versatility in design aesthetics. 
• Excellent moldability. ABS can fill intricate molds with ease, making it a preferred choice for complex geometries. 

Cons 

• Limited heat resistance. While suitable for many applications, ABS may deform at high temperatures, restricting its use in certain environments. 
• Not suitable for outdoor applications. UV exposure can cause ABS to degrade over time, making it less ideal for outdoor use. 

Applications 

• Consumer electronics casings 
• Automotive interior components 

Polystyrene, a versatile thermoplastic polymer derived from styrene monomers, offers a balance of affordability and versatility. With a linear molecular structure and relatively low density, PS exhibits excellent transparency, surface finish, and dimensional stability.  

Pros 

• Low cost. PS is one of the most economical plastics available, making it suitable for mass-produced consumer goods. 
• Easily processable. It can be easily molded and extruded into various shapes and forms, offering versatility in manufacturing. 
• Good electrical insulation. PS exhibits excellent electrical insulation properties, making it suitable for electronic applications. 

Cons 

• Brittle. PS is relatively brittle compared to many other plastics, limiting its use in applications requiring impact resistance. 
• Low impact resistance. It may crack or fracture under moderate impact, requiring reinforcement in certain applications. 

Applications 

• Disposable utensils 
• Packaging materials 
• Consumer electronics 
• Medical devices 

Polycarbonate, a transparent thermoplastic polymer, has exceptional transparency and a high resistance to UV radiation. Derived from bisphenol A and phosgene, PC molecules are arranged in a linear chain structure, providing superior toughness and dimensional stability.  

Pros 

• Exceptional impact resistance. PC is virtually unbreakable, making it suitable for applications where impact resistance is crucial, such as safety equipment. 

• Transparency. It offers excellent optical clarity, making it ideal for applications requiring visibility, such as lenses and windows. 
• High heat resistance. PC can withstand high temperatures without deformation, ensuring stability in demanding environments. 

Cons 

• Prone to scratching. While durable, PC surfaces can be prone to scratching, requiring additional protective measures in certain applications. 
• Relatively high cost. PC tends to be more expensive than some other materials, impacting overall project budgets. 

Applications 

• Safety goggles 
• Automotive headlight lenses 
• Electronic displays 

Cons 

• Higher material cost compared to standard engineering plastics. This may impact project budgets. 
• Limited availability of PPSU grades suitable for injection molding. This may require specialized sourcing. 
• Higher processing temperatures and pressures may be required. This increases energy consumption and costs. 
•  Requires proper ventilation during processing. Due to emissions, special attention to ventilation is necessary.  

Applications 

• Medical devices such as surgical instruments, dental instruments, and implantable components 
• Aircraft interiors, including seating components, cabinetry, and trays 
• Automotive components requiring extreme temperature and chemical resistance 

Polypropylene (PP), a thermoplastic polymer derived from propylene monomers, offers a unique combination of properties. With a high strength-to-weight ratio and low density, PP is widely used for applications requiring lightweight yet durable components, such as automotive parts, packaging, and consumer products. PP exhibits excellent fatigue resistance, processability, and thermal stability, making it a versatile material choice for various engineering applications. 

Pros 

• Lightweight. PP is one of the lightest thermoplastics available, making it suitable for applications where weight reduction is critical. 
• Chemical resistance. It's resistant to a wide range of chemicals, including acids and bases, ensuring compatibility with various environments. 
• Low cost. PP is relatively inexpensive compared to many other engineering plastics, offering cost-effective solutions. 

Cons 

• Low stiffness. PP exhibits lower stiffness compared to some other materials, limiting its use in high-load applications. 
• Poor heat resistance. It softens at relatively low temperatures, restricting its suitability for high-temperature environments.  

Applications 

• Bottle caps 
• Containers 
• Automotive parts 

Polyethylene (PE), a thermoplastic polymer composed of ethylene monomers, is popular due to its simplicity, versatility, and low cost. With a linear molecular structure and relatively low density, PE offers excellent electrical insulation and ease of processing. 

Pros 

• Excellent chemical resistance. PE is highly resistant to chemicals, acids, and bases, making it suitable for applications in aggressive environments. 
• Low cost. It is one of the most cost-effective plastics available, offering economical solutions for various applications. 
• Food-safe. PE is approved for food contact, making it suitable for packaging materials in the food industry. 

Polyethylene terephthalate (PET) is a thermoplastic polymer commonly used in plastic injection molding. It is derived from terephthalic acid and ethylene glycol through polymerization. PET exhibits excellent transparency and mechanical properties, making it suitable for a wide range of applications. 

Pros 

• Excellent clarity. PET exhibits a high level of transparency, making it ideal for packaging applications. 
• High mechanical strength and stiffness. It provides durability and structural integrity. 
• Good chemical resistance. Resistant to oils, greases, and most solvents, ensuring compatibility with various substances. 
• Recyclable. Contributes to sustainability efforts in the packaging industry. 
• Versatile processing capabilities. PET allows for intricate designs and complex geometries. 

Cons 

• Susceptible to stress cracking. Prone to cracking when exposed to certain chemicals, requiring careful consideration in material selection. 
• Limited thermal resistance. Compared to some engineering plastics, its thermal resistance is limited, restricting its use in high-temperature applications. 
• Relatively high moisture absorption rate. In humid environments, its dimensional stability is impacted. 

Applications 

• Beverage bottles and food packaging. 
• Polyester fibers for textiles and clothing. 
• Engineering components such as gears, bearings, and electrical insulators.