The beginning of plastic injection molding was quite humble.
Back in 1868, John W. Hyatt invented the process of creating celluloid (the first synthetic industrial plastic) and used it in billiard balls via the injection molding technique.
Other items, such as buttons, collar stays, combs, etc., were also produced using this process.
In World War II, rubber production was disrupted, and plastic stepped in to fill the gap in military equipment. Thermoplastics, such as cellulose acetate (CA), polystyrene (PS), polyvinyl chloride (PVC), nylon, and polyethylene (PE), were being used in injection molding
What about now?
Putting it simply, things are very, very different.
For your information, about 85,000 commercial plastic materials from 45 polymer families are available that can be used in injection molding machines.
And all these plastic types can be split into two groups: thermoplastics and thermosets.
Thermoplastics vs Thermosets
Thermoplastics and thermosets are considered different polymers due to their different reactions upon heating.
Thermoplastics become soft when heated and hard when cooled. Therefore, they can be reused again and again. They are ideal for creating molded parts.
Thermosets, on the other hand, undergo an irreversible chemical curing process. They become permanently hardened and stay fixed once formed.
Here is a comparison table for both:
| Thermoplastics | Thermosets | |
| Response to Heat | Melts into a liquid | Decompose/burn |
| Recyclable/Reprocessed | Yes | No |
| Structure | Linear polymer chains | Cross-linked molecular network |
| Properties | More flexible Lightweight Chemical-resistant | More stronger More rigid Have higher thermal |
Let us give you an example.
Think about a plastic water bottle vs an electrical switchboard.
A water bottle is made from a thermoplastic like PET. It can be melted and reused to make new bottles of other products.
Electrical switches are made from thermosetting plastics like Bakelite. They cannot be melted, softened, or reused.
Just so you know, this guide only discusses thermoplastic injection molding materials.
Most Commonly Used Materials In Injection Molding
Let’s keep it simple.
Injection molding is more than just shaping plastic. It is about deciding your final product and its properties, and then choosing the right material to make this happen.
Think about it.
A food container, a car part, and a medical device cannot use the same material.
Each product has different needs. While some should be flexible, others need strength or must be safe for human contact.
This is where material selection becomes critical.
That said, here are some of the most commonly used thermoplastics in injection molding.
1. PP (Polypropylene)
Polypropylene is a thermoplastic polymer, produced via polymerization of propylene.
Due to its semi-crystalline structure, it has high-impact resistance, chemical resistance, greater strength, and stiffness than polyethylene.
It is a perfect balance of mechanical strength and lightweight structure.
Processing Temperature: 390°F to 510°F / 200°C to 265°C
Mold Temperature: 70°F to 140°F / 20°C to 60°C
Tensile Strength: 4,500 psi to 18,500 psi
Applications: PP is used to manufacture automotive parts, snap-over lids (shampoo bottle tops, etc.), toys, medical pipette tubing, containers, plastic kitchenware, power tools bodies, toys, packaging, crates, washing machine drums, and much more.
| Pros | Cons |
| Inexpensive to produce Lightweight, easy/affordable to transport Remains stable at processing temperatures (160°C and 170°C) Resists acids, bases, and wear Great electrical insulator | Can shrink and warp UV exposure for a long time causes brittleness and discoloration Highly flammable Prone to oxidation |
2. PET (Polyethylene Terephthalate)
Polyethylene Terephthalate (PET) is a type of polyester (a specific type of synthetic polymer).
It is formed when ethylene glycol and terephthalic acid are polymerized together.
It also has a semi-crystalline structure, just like the PP. It offers great long-lasting resistance against moisture, chemicals, and regular wear.
PET is mainly used in single-use bottles and packaging.
Industries chose PET since it balances performance and affordability.
Processing Temperature: 480°F to 580°F / 250°C to 305°C
Mold Temperature: 50°F to 200°F / 10°C to 90°C
Tensile Strength: 7,000 psi to 23,000 psi
PLEASE NOTE that PET has a low softening temperature (about 160°F/70°C). Therefore, it is not a suitable fit to carry hot foods.
Applications: Widely used in producing bottles, food containers, beverage bottles, textile fibers (upholstery, clothing, industrial textiles), etc.
| Pros | Cons |
| Inexpensive to produce Lightweight, low transport cost High optical clarity, which improves product visibility and shelf appeal Resists acids, bases, wear, moisture, and gases Perfect for manufacturing thin-walled items, such as bottles, disposable containers, etc. Low impact resistance and mobility | Not suitable for carrying hot food Non-biodigradable Heat and UV rays weaken PET structure Leach chemical beyond softening temperature Contaminated PET is difficult to recycle |
3. ABS (Acrylonitrile Butadiene Styrene)
Acrylonitrile Butadiene Styrene (ABS) is a type of thermoplastic copolymer. It is a combination of three monomers: acrylonitrile, butadiene, and styrene.
It carries the properties of all three components.
Acrylonitrile makes it resistant to chemicals and heat. Butadiene brings in elasticity and retention properties. Styrene gives it a glossy finish, good moldability, strength, and rigidity.
It is a suitable material for complex molding and lightweight designs due to a combination of strength and flexibility.
Processing Temperature: 425°F to 500°F / 220°C to 260°C
Mold Temperature: 120°F to 190°F / 50°C to 90°C
Tensile Strength: 5,000 psi to 7,500 psi
Applications: Used to manufacture automotive parts (trim components, bumper bars), cosmetic parts, golf club heads, electronic housings, luggage and protective carrying cases, toys, helmets, LEGO bricks, and keyboard frames.
| Pros | Cons |
| Inexpensive Melts uniformly and flows well inside the molds to give a consistent output Absorbs shocks without cracking Mix of performance and affordability Resist electrical conductivity | Relatively low heat resistance Can be brittle in a cold environment Degrades under constant exposure to sunlight Not suitable for outdoor use |
4. HDPE (High-Density Polyethylene)
High-Density Polyethylene (HDPE) is a thermoplastic polymer that comes from ethylene.
HDPE has a very dense, linear molecular structure, resulting in a high strength-to-density ratio. It is widely used in injection molding due to its durability, moisture resistance, and low cost.
When the final product has to be rigid and heat-resistant, HDPE is the best material to use.
HDPE is also safe to use in the food sector since polyethylene (PE) is a food-grade plastic.
Processing Temperature: 356°F to 536°F / 180°C to 280°C
Mold Temperature: 68°F to 194°F / 20°C to 95°C
Tensile Strength: 1,900 psi to 4,500 psi
Applications: Used to manufacture bottles with short-life screw caps (milk, soft drinks, cleaning agents, shampoo, bleach), drums, pallets, containers, refrigerator containers, sacks, and bags.
| Pros | Cons |
| Low-cost Durable Resists heat, moisture, impact, and environmental stress cracking Consistent mold filling Low melting point (≈130°C) means shorter cycle time | Lower stiffness Can deform or crack under load Limited temperature resistance |
5. LDPE (Low-Density Polyethylene)
Low-Density Polyethylene (LDPE) is a less dense version of HDPE, produced via high-pressure, free-radical polymerization of ethylene gas.
Compared to HDPE, LDPE is soft and more flexible. It is more ductile but has lower hardness, stiffness, and strength.
It has a strong but stretchy nature. It is primarily used to create items that can hold heavier weights without snapping or disintegrating.
It is also lightweight, chemically stable, and resistant to moisture.
Processing Temperature: 355°F to 535°F (180°C to 280°C)
Mold Temperature: 68°F to 158°F (20°C to 70°C)
Tensile Strength: 1,400 psi
Applications: Used to make shrink wraps, foils, plastic bags, trays, squeeze bottles, food packaging, plastic bags, six-pack rings, ice cream lids, juice containers, cling wrap, etc.
| Pros | Cons |
| Flexible Can bend and stretch Very cheap to produce | Flammable Difficult to bond |
6. PS (Polystyrene)
Polystyrene (PS) is an amorphous thermoplastic made by polymerizing styrene.
It is widely used in injection molding since it has a low cost but excellent processability.
It melts at a relatively low temperature (about 180–240°C) and flows very easily, which makes it great for making thin and complex parts with high accuracy.
PC is in two forms: high-impact polystyrene (HIPS) and general-purpose polystyrene (GPPS).
HIPS is rubber-modified for rigidity, high impact resistance, and is opaque/translucent. GPPS, on the other hand, is brittle, clear, and rigid.
Processing Temperature: 350°F to 525°F (175°C to 275°C)
Mold Temperature: 50°F to 150°F (10°C to 65°C)
Tensile Strength: 7,000 psi to 23,000 psi
Applications: Food boxes, egg cartons, disposable spoons, forks, cups, computer housing, television frames, refrigerator trays, petri dishes, model kit toys, and much more.
| Pros | Cons |
| Lightweight Cost-effective Easy to mold Good rigidity and clarity Good dimensional stability Low shrinkage | Inherently brittle Low impact resistance Susceptible to certain solvents (like acetone) |
7. PVC (Polyvinyl Chloride)
Polyvinyl Chloride, also known as PVC or vinyl, is produced via vinyl chloride polymerization.
It was first synthesized back in 1872, B.F. Goodrich Company made it available commercially in the 1920s. It is affordable and the world’s third-most-produced synthetic thermoplastic.
It can be either rigid or flexible, based on the type and amount of plasticizers being used.
It resists salts, alkalis, and polar solvents. It is flame-retardant due to the presence of chlorine in the structure. It also has a high gloss and is easy to print and weld.
Processing Temperature: 330°F to 400°F (165°C to 205°C)
Mold Temperature: 70°F to 125°F (20°C to 50°C)
Tensile Strength: 10,000 psi to 12,000 psi
Applications: It is used to manufacture pipe fittings, conduit boxes, cable insulation, valves, automobile dashboards, interior door panels, toys, garden hoses, electrical connectors, etc.
| Pros | Cons |
| Durable and lightweight Easy to process, cheap Easy to procure Has flame-retardant properties | Relatively brittle (especially at lower temperatures) Soften at a higher temperature |
8. PA (Polyamides / Nylon)
Polyamide (PA) is a synthetic polymer commonly known as nylon.
It has a semi-crystalline structure, which gives it excellent mechanical strength, thermal stability, and wear resistance. These properties make it widely used in injection molding.
PA is also chemically resistant and can handle moderately high temperatures.
In cases where higher stiffness and strength are required, reinforced grades, such as glass-filled nylon, are used.
Processing Temperature: 446°F to 572°F (230°C to 300°C)
Mold Temperature: 158°F to 212°F (70°C to 100°C)
Tensile Strength: 10,000 psi to 15,000 psi
Applications: Automotive parts (engine covers, door handles), industrial components (gears, bearings, pump parts), consumer goods (power tool housings, fasteners, electrical insulation)
| Pros | Cons |
| Resists chemical Flexible and has high tensile strength High impact resistant Easy to process in injection molding | Can absorb moisture Can degrade when exposed to UV rays or light |
Properties of Injection Molding Materials
Here is a comparison table for the most used thermoplastics based on their properties:
| Material | Tensile Strength (psi) | Flexibility | Impact Resistance | Heat Resistance | Chemical Resistance | Cost |
| PP | 4,500–18,500 | Medium | High | Medium | Excellent | Low |
| PET | 7,000–23,000 | Low–Medium | Low | Low | Excellent | Low |
| ABS | 5,000–7,500 | High | High | Low | Medium | Low–Medium |
| HDPE | 1,900–4,500 | Low | Medium–High | Low–Medium | Excellent | Low |
| LDPE | ~1,400 | High | Medium | Low | Excellent | Very Low |
| PS | 7,000–23,000 | Low | Low (GPPS) / Medium (HIPS) | Low | Moderate | Low |
| PVC | 10,000–12,000 | Low–Medium | Medium | Medium–High | Excellent | Low |
| PA | 10,000–15,000 | Medium–High | High | High | Excellent | High |
Categorizing Different Thermoplastics Based on Properties
Each thermoplastic is best known for some attributes.
This table categorizes different thermoplastics based on their attributes:
| Transparency | PS (Polystyrene) PC (Polycarbonate) PMMA (Acrylic) |
| Rigid Plastic Materials | Polyamides / Nylon (PA) PBT (Polybutylene Terephthalate) PC (Polycarbonate) PE (Polyethylene) POM (Acetal / Polyoxymethylene) PVC (Polyvinyl Chloride) |
| Affordable | HDPE PS PE |
| Elastomer/Rubber molded materials | Thermoplastic elastomer (TPE) |
| Chemical Resistant | PMMA ABS HDPE PA PC PP |
| Flame Resistant | PEI |
| Wear Resistant | PA TPE POM |
Please NOTE that some attributes of certain thermoplastics can overlap. So, don’t get confused with that. One material can be put into multiple categories based on its attributes.
Which Materials are Suitable for Thin-Wall Molding?
Some materials that can be suitable for thin-wall molding are:
- PP (Polypropylene)
- PET (Polyethylene Terephthalate)
- ABS (Acrylonitrile Butadiene Styrene)
- PS (Polystyrene)
This is because all these materials are of low-viscosity grades.
