They can be also reinforced with fibers, rubber particles, minerals or flame retardant agents to modify their physical properties. For example fiberglass can be mixed with the pellets at ratios of 10%, 15% or 30% resulting in parts with higher stiffness.

Polypropylene (PP)
The most common Injection molding plastic. Excellent chemical resistance. Food-safe grades available. Not suitable for mechanical applications.

ABS
Common thermoplastic with high impact resistance, low-cost & low density. Vulnerable to solvents.

Polyethylene (PE)
Lightweight thermoplastic with good impact strength & weather resistance. Suitable for outdoor applications.

Polystyrene (PS)
The Injection molding plastic with the lowest cost. Food-safe grades available. Not suitable for mechanical applications.

Polyurethane (TPU)
Thermoplastic with high impact strength and good mechanical properties & hardness. Suitable for molding parts with thick walls.

Nylon (PA 6)
Engineering thermoplastic with excellent mechanical properties and high chemical & abrasion resistance. Susceptible to moisture.

Polycarbonate (PC)
The plastic with the highest impact strength. High thermal resistance, weather resistance & toughness. Can be colored or transparent.

PC/ABS
Blend of two thermoplastics resulting in high impact strength, excellent thermal stability, and high stiffness. Vulnerable to solvents.

POM (Acetal/Delrin)
Engineering thermoplastic with high strength, stiffness & moisture resistance and self-lubricating properties. Relatively prone to warping.

PEEK
High-performance engineering thermoplastic with excellent strength and thermal & chemical resistance. Used to replace metal parts.

Silicone rubber
Thermoset with excellent heat & chemical resistance and customizable shore hardness. Food-safe and medical grade available.

An additive that is commonly used to improve the stiffness of the injection molded parts is fiberglass. The glass fibers can be mixed with the pellets at ratios of 10%, 15% or 30%, resulting in different mechanical properties.

Colorant can be added to the mixture (at a ratio of about 3%) to create a great variety of colored parts. Standard colors include red, green, yellow, blue, black and white and they can be mixed to create different shades.

Surface finishes and SPI standards
Surface finishes can be used to give an injection molded part a certain look or feel. Besides cosmetic purposes surface finishes can also serve technical needs. For example, the average surface roughness (Ra) can dramatically influence the lifetime of sliding parts such as plain bearings.

Injection molded parts are not usually post-processed, but the mold itself can be finished to various degrees.

Keep in mind that rough surfaces increase the friction between the part and the mold during ejection, therefore a larger draft angle is required.

The Society of Plastics Industry (SPI) explains several standard finishing procedures that result in different part surface finishes.