Plastic materials used for injection molding include commodity polymers, speciality plastics and polymer blends. There are hundreds of different plastic resins, each with different end-use properties and processing requirements. In addition, there are different grades of the same plastic material, including resin types with fillers such as glass fibre or carbon fibre, and specific brand names. With so many choices, selecting the right material for your project can be daunting. Whether your project involves medical plastic injection moulding, automotive plastic injection moulding or something else, PROTO MFG’s experts can help you quickly complete the entire process from design to production.
These are the most common plastic materials for injection molding:
Acrylic is a strong, transparent thermoplastic that is lightweight and unbreakable, making it an alternative to glass. This injection moulded material also has excellent optical clarity, allowing high light transmission. Resistant to ultraviolet (UV) light and weathering, acrylic is also known as PMMA or its full chemical name: polymethyl methacrylate.
Unlike some plastics, acrylics can withstand water attack and maintain tight tolerances. PMMA does not absorb odours but is not resistant to solvents and is easily stained by grease. Injection moulded acrylics have high tensile strength but are susceptible to stress cracking under high loads.
Applications for acrylic injection moulding include windows, greenhouses, solar panels, bathroom vanities and other transparent parts for architectural, lighting and outdoor applications.
ABS has a relatively low melting point and is easy to mould. This opaque polymer can be used with colourants and a variety of textures and surface treatments. Its butadiene component provides excellent toughness, even at low temperatures, while the styrene component gives injection moulded parts a glossy, attractive surface finish.
ABS is known for its strength and impact resistance, but lacks strong resistance to sunlight (UV), water and weathering. Compared to acrylic resin, injection moulded ABS is not suitable for outdoor applications. In addition, ABS produces a lot of smoke when burned and, unlike nylon, has poor resistance to high friction.
ABS injection moulding is used to produce many types of plastic parts, including
Nylon is a synthetic polyamide (PA) that combines toughness, high heat resistance, high abrasion resistance, good fatigue resistance and noise reduction properties. It is not flame retardant, but flame retardant nylon is available. Similarly, although nylon is degraded by exposure to sunlight, UV stabilisers can be added to improve outdoor performance.
Nylon is less resistant to strong acids and bases than other plastics. Nylon is not as strong as polypropylene and not as impact resistant as polycarbonate. Injection moulding nylon is challenging because it tends to shrink and does not fill the mould well.
Applications for nylon injection moulding include:
Polycarbonate is strong, lightweight and naturally transparent. This injection moulded plastic has excellent optical properties, allowing light to pass through but retaining colour and strength when pigmented. Although not scratch resistant, polycarbonate is significantly stronger than glass, making it very durable.
Injection moulded polycarbonate sometimes replaces acrylic because it retains its physical properties over a wider temperature range. However, polycarbonate requires higher processing temperatures, which can make it more expensive to mould. However, polycarbonate has predictable, uniform shrinkage, which allows precise dimensional control and tighter tolerances.
Applications for polycarbonate injection moulding include:
Polyethylene is the most widely used plastic in the world and is a commercial polymer selected on the basis of density. Both high density polyethylene (HDPE) and low density polyethylene (LDPE) are chemically resistant, but differ in hardness, flexibility, melting point and optical clarity.
Polyethylene plastics include polyethylene terephthalate (PET, PETE), a material that is not defined by density. Like low-density polyethylene, PET can be as clear as glass, but designers can also choose PET or PETE grades with different optical clarity. High-density polyethylene (HDPE), low-density polyethylene (LDPE) and polyethylene terephthalate (PET) are all resistant to moisture and chemicals, but low-density polyethylene (LDPE) is softer and more flexible than high-density polyethylene (HDPE), which is an opaque material.
Polyethylene injection molding applications may be limited to indoor applications due to their poor UV resistance. These plastics may not be suitable for certain processing applications due to their inability to withstand higher operating temperatures. Injection moulded polyethylene is commonly used in products such as housewares, toys, food containers and automotive parts.
Polyoxymethylene (POM) is an engineering plastic also known as acetal. It combines excellent rigidity with thermal stability and has a low coefficient of friction. It has low water absorption and good chemical resistance. In terms of appearance, POM plastics are naturally opaque and white in colour.
In its pure form, acetal has low impact strength and very high thermal expansion. However, POM can be reinforced with glass fibres or minerals to improve strength and stiffness. Compounds reinforced with both provide an excellent balance of mechanical properties. The lack of sunlight resistance of injection moulded POM limits its use in outdoor applications.
Due to its low coefficient of friction, injection moulded POM is used in bearings, gears, conveyor belts and pulleys. Other applications include fasteners, spectacle frames, knife and gun parts, locking systems and high performance engineering components
Polypropylene is the second most widely used plastic in the world. It offers good chemical resistance, retains its shape after twisting or bending, has a high melting point and doesn’t degrade when exposed to moisture or water. Injection moulded Polypro, as PP is sometimes called, is also recyclable.
Despite its comparative advantages, polypropylene degrades under UV light and is highly flammable. At temperatures above 100°C (212°F), this injection-moulded plastic breaks down into aromatic hydrocarbons such as benzene and toluene, which are harmful to humans. Polypropylene is also difficult to bond and paint.
Applications for polypropylene injection moulding include:
Polystyrene plastics are lightweight, relatively inexpensive and resistant to moisture and bacterial growth. These commodity plastics also have good chemical resistance to dilute acids and bases, and excellent resistance to gamma radiation used to sterilise medical devices.
There are two main types of polystyrene: general purpose polystyrene (GPPS) and high impact polystyrene (HIPS). GPPS is brittle and has less dimensional stability than HIPS, which is compounded with butadiene rubber to improve its material properties. GPPS also has a glass-like clarity, whereas HIPS is opaque.
Polystyrene injection molding is used in medical, optical, electrical and electronic applications. With its higher impact strength, HIPS is often used in appliances and equipment, while injection moulded GPPS is used in plastic toys, cases, containers and trays.
