Injection molding material choices are hard to reverse without significant financial losses. The first thing that comes to mind for most manufacturers is choosing injection molding plastics that are easier to process to save manufacturing costs. But far-sighted manufacturers also consider materials that will work reliably and reduce the total cost of ownership for parts.
In this precarious balancing act of future needs vs. present goals you’ll also discover that no two polymers are alike. Even materials of the same chemical composition may not be swappable. Your budget, application environment and part geometry matter a great deal in the final decision. This guide explores the best injection molding plastics available and provides the selection criteria that can get you closer to your objectives.
What Are Injection Molding Plastics?
Injection molding plastics are polymer resins of special characteristics used in the mass production of goods and industrial parts.
The plastics can be melted at high temperatures and forced into a metallic mold at high pressure. They can be quickly cooled and solidified into complex parts.
Before the manufacturing process, injection molding plastics are packaged, shipped and stored as small pellets or granules. They are melted and molded into parts of diverse shapes during processing,
Thermoplastics are the plastics that can be reheated and reshaped after they solidify. You get full recyclability.
Thermosets are the plastics that form permanent bonds during injection molding. Scraps cannot be re-melted for recycling.
Common Plastics Used in Injection Molding
Selecting injection molding resins for your project is just as critical as design optimization because the wrong material can ruin everything. The best place to start is understanding the unique mechanical and chemical profiles of your resins.
- Acrylonitrile Butadiene Styrene (ABS)
Use this material for parts that need to be impact resistant and robust without sacrificing aesthetics.
ABS is easy to process because of its low shrinkage rate. It retains its dimensional tolerances, which is great for automotive and consumer goods applications like dashboards and LEGO bricks.
Watch out discoloration whenever you use ABS parts. The material doesn’t survive long hours of exposure to UV light. It turns yellow and brittle.
Engineers often work around this limitation by using additives like antioxidants and UV absorbers.
- Polypropylene (PP)
For parts that need to survive environments with regular chemical spillage or water leaks, Polypropylene is the material of choice. It is a low-cost crystalline thermoplastic made from propene monomers.
It can withstand alkalis and solvents and resist moisture absorption. It can bend countless times without snapping.
Top applications for PP include plastic bottle caps, microwave-safe containers, medical syringes and car bumpers.
The molecular arrangement of PP is symmetrical, giving the material a low melt viscosity. Molten PP flows with little resistance which is good for mold longevity. However PP has a high mold shrinkage rate and is more susceptible to warping.
- Polycarbonate (PC)
When your parts demand extreme impact resistance and high optical transparency, molders will recommend PC. The engineering plastic can yield parts that are virtually shatterproof even in high vibration conditions.
PC has long molecular chains and rigid benzene rings which offer it more dimensional stability and resistance to warping during. However this same molecular arrangement makes PC more viscous and difficult to inject except under extremely high pressures using novel design playbooks.
Because of its high optical transparency, the demand for PC in the manufacture of glass windows, safety goggles and headlight lenses is on the rise.
- Polyamide / Nylon (PA6 / PA66)
Nylon is an engineering plastic with special characteristics including:
- High mechanical strength
- Superior thermal stability
- Exception wear resistance
Nylon’s molecular structure comprises hydrocarbon and polyamide chains. These form chains rigid bonds when the material is melted, making it stiff and impact resistant.
This same internal configuration gives nylon a low coefficient of friction and utmost dependability in high mechanical wear applications.
The uses of nylon parts include gear teeth, bearings and engine manifolds and zip ties. The major limitation is that nylon is hygroscopic.
- High-Density Polyethylene (HDPE)
If you need an affordable commodity plastic with high mechanical strength and moisture resistance, HDPE fits the profile.
The semi-crystalline resin has a linear molecular architecture with carbon atom chains that pack tightly together as they cool after melting.
This packing efficiency gives HDPE its high density properties including high impact strength at low temperatures and superior chemical resistance. Its common applications include cutting boards, pipes and recycling bins.
However HDPE’s internal structure makes it highly susceptible to warping and mold shrinkage. Molders must ensure that designs feature uniform wall thickness and rounded corners.
- Low-Density Polyethylene (LDPE)
For applications that demand semi-crystalline thermoplastics, LDPE is highly recommended. It has low structural rigidity and high impact resistance at low temperatures.
It is reliable when your parts need to be flexible and chemically inactive. Top applications include:
- Cling films
- Bubble wraps
- Cosmetic bottles
- Water hoses
Beware that LDPE has very weak inter molecular chains compared to HDPE, which gives it a low tensile strength and low heat resistance. It cannot withstand continuous high loads.
- Polystyrene (PS / HIPS)
Choose general purpose polystyrene if you want a commodity thermoplastic with high optical transparency. Choose high impact polystyrene, the rubber modified version, if you prefer strength and toughness.
The molecular structure of polystyrene comprises of hydrocarbon chains with benzene rings between every carbon atom. This formation blocks crystallization when the resin is melted, forcing it to cool as an amorphous solid. This is good for the optical clarity and dimensional stability of parts.
However, this internal architecture makes polystyrene very brittle. It cracks and shatters easily. High Impact Polystyrene (HIPS) solves this hiccup by mixing rubber pellets into the material before melting. The additives act as shock absorbers and strengtheners but they also make PS more opaque.
Top applications for PS includes disposable cutlery, diagnostic petri dishes and internal refrigerator liners. Both GPPs and HIPs are highly flammable.
- Polyoxymethylene / Acetal (POM / Delrin)
Fewer materials deliver exceptional mechanical strength and low coefficient of friction like Polyoxymethylene (POM) does. It is also good for dimensional stability.
Acetal’s molecular chains are tightly packed (consisting of carbon, oxygen and hydrogen atoms), which makes the resin highly crystalline. This arrangement gives the material many of its desirable properties including creep resistance and fatigue resistance.
POM’s main advantage is its ability to withstand continuous mechanical loads and bending without snapping.
Common applications include:
- Zipper teeth
- Gears
- Electrical switches
- Fuel system parts
- Polyethylene Terephthalate (PET)
This polyester thermoplastic is valued for its strength, transparency and lightweight properties. PET parts are extremely resistant to moisture and gaseous ingress.
The molecular arrangement in PET parts is determined by the speed of cooling. If the material is cooled fast, its benzene and ester chains freeze in place leading to amorphous highly transparent solids.
If it’s cooled slowly the chains form a crystalline and opaque matrix.
Common use cases for PET in injection molding include the manufacturing of beverage bottles, textile fibers and structural automotive housings.
10. Polyetherimide (PEI / Ultem)
Aerospace and automotive injection molding project managers frequently choose Polyetherimide for its high mechanical strength and flame resistance.
PEI can remain operationally reliable when temperatures exceed 180 degrees Celsius. It is flame resistant and maintains its shape even when under heavy mechanical loads.
However, polyetherimide is a high cost material. It is also not easy to injection mold as it is highly viscous.
Popular use cases for PEI include:
- Medical autoclaving trays
- Aircraft galley parts
- Automotive engine bay sensors
11. Liquid Silicone Rubber
Liquid silicone rubber is a thermoset elastomer prized for its high thermal stability and exceptional biocompatibility.
LSR is made of two liquid components that are mixed and cured to produce solid flexible parts that do not melt or degrade when exposed to heat.
The biggest advantage of LSR is its biocompatibility. It’s non-toxic and hypoallergenic and thus useful in medical and food grade applications.
Common use cases of liquid silicone rubber include:
- Prosthetics
- Catheters
- Blood filter sealers
- Pacifiers
- Bottle nipples
12. Polyether Ether Ketone (PEEK)
This is an ultra-performance injection molding plastic used in extreme environments where metals are impractical.
PEEK is the ultimate engineering plastic, delivering incomparable chemical resistance, heat resistance and structural rigidity.
Its top applications include
- Structural brackets for the aerospace industry
- Medical implants
- Oil exploration drilling equipment
PEK is very costly and requires extremely high molding temperatures to achieve desirable outcomes.
13. Thermoplastic Elastomers (TPE / TPU)
Thermoplastic elastomers give you the elasticity of rubber and the recyclability of plastics in one material.
Their molecular architecture includes alternating copolymer chains of soft blocks (amorphous polyol chains) and hard blocks (crystalline polyamide segments).
Because of this, TPE and TPU can sustain extensive elastic deformation and return to their original shapes.
TPE and TPU can be molded onto other plastics or metallic parts, cutting the costs and complexity of assembly. This is called over molding.
Common applications of thermoplastic elastomers include
- Phone cases
- Gaskets
- Wearable fitness bands
14. Acrylic / Polymethyl Methacrylate (PMMA)
Acrylic is the premier injection molding plastic for the production of shatterproof and transparent parts used instead of glass. Its best properties include high optical clarity (92% light transmission), scratch resistance and resistance to weathering.
PMMA is inherently incapable of crystallizing so it solidifies as an amorphous, disordered solid matrix. This explains its transparency and rigidity. Its internal bonds are incredibly resistant to electromagnetic radiation and UV degradation.
Acrylic is commonly used in the manufacture of:
- Optical lenses
- Display screens
- Automotive tail light housings
How to Choose the Right Plastic for Injection Molding
There is no single best injection molding plastic because a material that is excellent for one industry can be disastrously unreliable in another industry. To make the best decision, carefully consider the needs of your application including the mechanical functions needed and the realities of the usage environment. Then of course, choose a material that best aligns with your budget.
- Mechanical Strength
A material’s mechanical strength defines its ability to remain intact amid external forces. The 3 most important mechanical strength factors to consider for your injection molding plastic include:
- Tensile strength – resistance to stretching
- Flexural strength -resistance to bending
- Impact resistance – ability to absorb sudden shocks without breaking
- Environmental Exposure
After parts leave the molder’s facilities, their lifespan and performance may be majorly dictated by weather, chemicals and temperature in your application environment. When buying the best injection molding plastics, it’s often wise to consider what solvents, oils, radiation, UV or moisture your parts will contact.
- Thermal Management
Consider both the processing and operational thermal limits of your preferred injection molding resin. A plastic with extremely high melting temperatures can be difficult to process. For operational temperature, look at how quickly the material begins to warp when temperature rises. Choose a plastic with the highest maximum continuous service temperature for parts longevity and operational efficiency.
- Cosmetic and Optical Requirements
Choose transparent polymers for parts that need maximum optical clarity. Amorphous resins have the highest rate of light transmission. For high-gloss surfaces and finishes, choose materials that offer the best surface quality and processing stability.
- Budget
High performance engineering resins cost more than commodity injection molding plastics. Also consider production volume. You might be able to comfortably foot the bill of a costly high performance plastic during prototyping but when you switch to mass production, the high costs start eating into your margins.
Best Injection Molding Plastics by Property
These are the overall best performing plastics for injection molding:
Polyether Ether Ketone (PEEK) – strongest plastic for injection molding
This material can withstand high tensile stress of over 100 Mpa. It doesn’t deform or creep under constant mechanical load. It can be reinforced with carbon fibers to deliver the same level of performance as metal in its various use cases.
Polypropylene (PP) – cheapest plastic for injection molding
Polypropylene costs between $1.01 and $1.57 per kg in its standard virgin resin form. It has an ultra-low density of 0.90 g/cm^3 and it’s very effortless to process because it’s hydrophobic with a low melting point.
Polyoxymethylene (Acetal / POM) – most durable injection molding plastic
Earlier we discovered that POM’s internal molecules are tightly packed which makes it super robust and dimensionally stable. Its high tensile strength makes it immune to deformation when exposed to persistent loads. It is self-lubricating and that means more longevity even in applications where friction is unavoidable.
Polycarbonate (PC) – best injection molding plastic for transparent parts
The amorphous molecular structure of polycarbonate allows it to transmit the maximum amount of light (95% light transmission) with very minimal scattering. Its refractive index of 1.584 means that it can be molded into clear transparent parts without the risk of distortion.
Thermoplastic Polyurethane (TPU) – best plastic for flexible injection molded parts
As mentioned earlier, TPU has a block copolymer molecular structure consisting of soft and hard sections. Partially this means it can stretch significantly more than other resins and still come back to its original shape without tearing or losing strength.
Injection Molding Plastics Comparison Table
| Material | Cost | Tensile Strength | Flexibility | Max Operating Temp | Unique Feature |
| PP | Ultra-Low | Low | High | 100 °C | Integrated living hinges; excellent fatigue life. |
| HDPE | Low | Low | Medium | 80°C | Moisture barrier; food and chemical safe. |
| ABS | Medium | Medium | Low | 85°C | Excellent impact resistance and cosmetic surface. |
| Nylon | Medium-High | High | Medium | 120°C | Self-lubricating wear resistance; high friction life. |
| PC | Medium-High | High | Low | 135°C | Bulletproof impact resistance with optical clarity. |
| POM | Medium-High | High | Low | 105°C | High dimensional stability; zero structural creep. |
| PMMA | Medium | Medium | Rigid/Brittle | 90°C | Exceptional UV resistance and optical transmission. |
| PEEK | Ultra-High | Extreme | Low | 250°C | Metal replacement polymer for extreme settings. |
| LSR | High | Low-Medium | Extreme | 200°C | Thermoset rubber; extreme thermal and chemical stability. |
| TPE | High | Low | Ultra-High | 70°C | Elastomeric flexibility; ideal for soft overmolding. |
Conclusion
Choosing the best injection molding plastic is a decision that pays off well if done in alignment with the part design, desired processing windows and manufacturing margins. Every polymer has unique chemical and physical compositions and that makes mold interchangeability practicality impossible. You cannot swap a low cost commodity plastic into a mold designed for a premier engineering resin. Because of that, always choose your plastics before the mold is cut. Follow this guide to make the best material decisions.
