Ten commonly used 10 plastic resin materials for injection molding

injection molding

All thermoplastics can be injection molded. Some thermosets and liquid silicones are also compatible with the injection molding process.

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.

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.

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.

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.

Finish Description SPI standards* Applications
Glossy finish The mold is first smoothed and then polished with a diamond buff, resulting in a mirror-like finish. A-1
A-3 Suitable for parts that require the smoothest surface finish for cosmetic or functional purposes (Ra less than 0.10 μm). The A-1 finish is suitable for parts with mirror-like finish and lenses.
Semi-gloss finish The mold is smoothed with fine grit sandpaper, resulting in a fine surface finish. B-1
B-3 Suitable for parts that require a good visual appearance, but not a high glossy look.
Matte finish The mold is smoothed using fine stone powder, removing all machining marks. C-1
C-3 Suitable for parts with low visual appearance requirements, but machining marks are not acceptable.
Textured finish The mold is first smoothed with fine stone powder and then sandblasted, resulting in a textured surface. D-1
D-3 Suitable for parts that require a satin or dull textured surface finish.
As-machined finish The mold is finished to the machinist’s discretion. Tool marks will be visible. – Suitable for non-cosmetic parts, such industrial or hidden components.

When selecting a glossy surface finish, remember these useful tips:

A high glossy mold finish is not equivalent to a high glossy finished product. It is significantly subject to other factors such as plastic resin used, molding condition and mold design. For example, ABS will produce parts with a higher glossy surface finish than PP. To find the recommended material and surface finish combination visit the appendix.
Finer surface finishes require a higher grade material for the mold. To achieve a very fine polish, tool steels with the highest hardness are required. This has an impact on the overall cost (material cost, machining time and post-processing time)

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