Types of plastic explained for material selection and recycling

Understanding the main types of plastic

The phrase types of plastic can mean different things depending on the context. In consumer recycling, it usually refers to resin identification codes 1 through 7: PET, HDPE, PVC, LDPE, PP, PS, and Other. In plastic processing, the discussion is broader. It also covers engineering plastics, thermosets, elastomers, blends, and filled compounds.

That distinction matters. A resin code identifies the base polymer family. It does not confirm that a product is recyclable in a local system, approved for food contact, heat-resistant, or suitable for injection molding, extrusion, thermoforming, or blow molding.

For manufacturers, buyers, and recyclers, material selection is more reliable when resin codes are considered alongside real processing behavior. A plastic that works well for a bottle may not be suitable for a hot-fill package. A polymer that molds easily may still lack the impact strength required for an industrial housing. This guide reviews the major plastic types from a processing and application perspective.

How plastics are commonly classified

Plastic materials are usually grouped by polymer chemistry, processing behavior, and application requirements. The most familiar consumer-facing system is the resin identification code system described in ASTM D7611/D7611M, which uses numbers 1 to 7 to identify the primary resin in a plastic article. The number inside the triangle is useful for sorting, but it should not be treated as a universal recycling guarantee.

From a processing viewpoint, plastics are often divided into three broad categories:

• Thermoplastics: Materials that soften when heated and harden when cooled. PET, PE, PP, PVC, PS, ABS, PC, PA, and PMMA are common examples. Most high-volume packaging and molded products use thermoplastics.
• Thermosets: Materials that cure into a crosslinked structure and do not remelt in the same way as thermoplastics. Epoxy, phenolic, melamine, and some polyurethane systems are examples.
• Elastomers: Flexible materials with rubber-like behavior. Some are thermoset rubbers, while thermoplastic elastomers can be processed more like conventional thermoplastics.

Because this article focuses on plastic processing and material selection, the resin-code plastics are covered first, followed by engineering and specialty plastics that often appear under code 7 or outside consumer recycling labels.

The seven resin identification codes at a glance

The table below summarizes the seven resin identification categories most often seen in packaging, consumer goods, and recycling systems. Actual use varies by country, grade, additive package, and local recycling infrastructure.

Code	Plastic type	Common name	Typical applications	Processing and selection notes
1	PET or PETE	Polyethylene terephthalate	Beverage bottles, food containers, thermoformed trays, polyester fiber	Good clarity and strength; commonly used in stretch blow molding and thermoforming; moisture control is important before processing.
2	HDPE	High-density polyethylene	Milk jugs, detergent bottles, caps, pipes, crates, tanks	Tough, chemically resistant, and relatively stiff compared with LDPE; widely used in blow molding, extrusion, and injection molding.
3	PVC	Polyvinyl chloride	Pipes, profiles, flooring, cable insulation, medical tubing, films	Can be rigid or flexible; stabilizers and additives are central to performance; processing requires careful temperature control.
4	LDPE	Low-density polyethylene	Films, bags, squeeze bottles, liners, flexible packaging	Flexible and easy to seal; common in film extrusion; lower stiffness than HDPE.
5	PP	Polypropylene	Food containers, caps, hinges, fibers, automotive parts, appliances	Lightweight, fatigue-resistant, and suitable for many injection-molded parts; heat resistance is generally better than PE.
6	PS	Polystyrene	Rigid packaging, disposable foodware, insulation foam, appliance parts	Available as general-purpose, impact-modified, and expanded foam grades; brittle behavior can be a limitation.
7	Other	Other plastics and multi-material structures	PC, ABS, PA, PLA, acrylic, multilayer packaging, blends	A mixed category; properties and recyclability depend entirely on the specific resin, blend, or structure.

A key limitation is that the code identifies the resin family, not the exact grade. A PP film grade, a PP random copolymer, and a glass-filled PP compound can behave very differently in processing and end use, even though all are polypropylene.

Key characteristics of major plastic types

PET for clarity, strength, and packaging performance

PET is valued for clarity, strength, and good barrier performance in many packaging formats. It is widely used for beverage bottles because it can be stretch blow molded into lightweight containers with strong mechanical performance. PET is also used in thermoformed trays and polyester fibers.

In processing, moisture control is critical. Wet PET can degrade during melting, which can reduce both performance and appearance. PET is often one of the more familiar recyclable plastics in bottle form, but product form still matters. A clear PET beverage bottle presents a different recycling challenge than a multilayer tray, heavily colored container, or label-contaminated package. For processors, design for recycling starts with compatible labels, adhesives, colors, and closures, not just the resin code.

HDPE and LDPE for toughness, flexibility, and chemical resistance

Polyethylene is one of the largest plastic families and includes HDPE, LDPE, LLDPE, and other density grades. HDPE has a more linear molecular structure, giving it higher stiffness and strength than LDPE. It is common in blow-molded bottles, drums, crates, pipes, and molded industrial parts. LDPE is softer and more flexible, making it useful for film, bags, liners, and squeeze applications.

In production, polyethylene grades are selected by melt flow, density, molecular weight distribution, impact resistance, stress-crack resistance, and sealing behavior. A grade optimized for blown film, for example, will not necessarily perform well in injection molding. Resin specification sheets are therefore essential when moving from concept to production.

PVC for profiles, pipes, flexible products, and controlled formulations

PVC is unusual because it can be formulated into either rigid or flexible products. Rigid PVC is common in pipes, window profiles, siding, and sheet. Flexible PVC is used in cable insulation, tubing, flooring, and some coated fabrics.

This versatility depends heavily on compounding. Stabilizers, plasticizers, fillers, lubricants, pigments, and processing aids all influence performance. For processors, PVC also requires careful thermal management because excessive heat can cause degradation. Regulatory, environmental, and application-specific requirements should be reviewed closely, particularly for food-contact, medical, construction, and electrical uses.

PP for lightweight molded parts and heat-resistant applications

Polypropylene is widely used in packaging, household goods, automotive components, fibers, and living hinges. Compared with many polyethylene grades, PP generally offers better heat resistance and stiffness at low density. It is popular in injection molding because it flows well, is available in many copolymer grades, and can be reinforced with fillers such as talc or glass fiber.

PP can be a strong choice for containers, caps, closures, appliance parts, battery cases, and interior automotive parts. However, unmodified PP can become brittle at low temperatures, so impact-modified grades may be needed for cold-chain packaging or outdoor applications.

PS for rigidity, insulation, and low-cost forming

Polystyrene is used in several forms. General-purpose polystyrene is clear and rigid but brittle. High-impact polystyrene improves toughness through rubber modification. Expanded polystyrene and extruded polystyrene foams are used for insulation, cushioning, and lightweight packaging.

PS is easy to process and form, but its environmental and recycling profile is challenging in many local systems, especially for lightweight foam products. Low density makes collection and transport less efficient unless the material is compacted or handled through a dedicated recovery stream. See also: Buying Guides.

Code 7 and engineering plastics are not one material

Code 7 is a catch-all category. It may include polycarbonate, ABS, nylon, acrylic, PLA, multilayer structures, blends, or newer materials that do not fit codes 1 through 6. That makes code 7 useful as a label category but limited as a technical description.

Engineering plastics are selected when commodity plastics cannot meet performance requirements. ABS is common in housings and consumer products because it balances toughness, appearance, and processability. Polycarbonate offers high impact strength and transparency. Nylon provides strength, wear resistance, and temperature capability but absorbs moisture. Acrylic is valued for optical clarity and weatherability. For more processing-focused material discussions, readers can explore the Plastic Processing section.

How processing method affects material choice

The best plastic type depends not only on performance requirements but also on the intended process. A resin that looks suitable on a property chart may still fail in production if its melt flow, cooling behavior, shrinkage, or moisture sensitivity does not match the equipment and tooling.

• Injection molding: Common for PP, ABS, HDPE, PS, PC, PA, and many filled compounds. Key factors include melt flow, shrinkage, warpage, cycle time, and part thickness.
• Extrusion: Used for film, sheet, pipe, profiles, tubing, and coating. PE, PP, PVC, PS, and PET all have extrusion applications, but grades are process-specific.
• Blow molding: Common for PET bottles and HDPE containers. Melt strength, parison control, and impact performance are important.
• Thermoforming: Often used with PET, PP, PS, PVC, and ABS sheet. Sheet quality, heat sag, draw ratio, and trimming behavior affect results.
• Rotational molding: Frequently associated with polyethylene tanks, containers, and large hollow products. Powder quality and sintering behavior matter.

Material selection should therefore start with the product requirements and production route together. Changing the process later can require a different grade or even a different polymer family.

Recyclability depends on more than the resin code

Global plastic recycling remains limited. The OECD’s Global Plastics Outlook reported that 353 million tonnes of plastic waste were generated in 2019 and that about 9% was ultimately recycled after accounting for losses. Those figures show why resin identification alone cannot solve collection, sorting, contamination, economics, or end-market demand.

Several factors affect whether a plastic item is practically recyclable:

• Local collection rules: A material accepted in one city may be rejected in another.
• Product form: Bottles, rigid containers, flexible films, foams, and multilayer pouches follow different recovery paths.
• Color and additives: Dark pigments, fillers, flame retardants, and incompatible additives can reduce recycled material value.
• Contamination: Food residue, labels, adhesives, metal parts, and mixed materials complicate sorting and washing.
• Material combinations: Multilayer packaging may improve shelf life but can be difficult to separate mechanically.

For packaging and product designers, the practical goal is not only to choose a recyclable resin. It is also to reduce unnecessary material complexity. Mono-material structures, compatible closures, removable labels, and clear specifications can improve the chance that material will retain value after use.

A practical checklist for selecting a plastic type

When comparing plastic materials, avoid choosing by resin name alone. Use a checklist that connects application needs with processing reality:

• Mechanical performance: Does the part need stiffness, impact strength, fatigue resistance, wear resistance, or flexibility?
• Temperature exposure: Will the product face hot filling, sterilization, freezing, outdoor heat, or repeated thermal cycling?
• Chemical contact: Will it contact oils, detergents, solvents, acids, fuels, food, or medical fluids?
• Regulatory requirements: Does the application involve food contact, medical use, toys, construction codes, electrical safety, or flame retardancy?
• Processing route: Is the part molded, extruded, blown, thermoformed, welded, printed, or assembled?
• Appearance: Is transparency, gloss, color consistency, texture, or scratch resistance important?
• End-of-life plan: Can the material be collected, sorted, recycled, reused, or reduced without compromising function?
• Total cost: Consider resin price, tooling, cycle time, scrap rate, secondary operations, logistics, and quality control.

In many cases, the best material is not the one with the highest single property. It is the one that meets the required performance with stable processing, manageable cost, and a realistic end-of-life path.

Frequently asked questions

What are the most common types of plastic?

The most common resin-code categories are PET, HDPE, PVC, LDPE, PP, PS, and Other. In industrial material selection, common plastics also include ABS, polycarbonate, nylon, acrylic, thermoplastic elastomers, and various filled or reinforced compounds.

Does the recycling number mean a plastic is recyclable?

No. The number identifies the resin family. Recyclability depends on local collection systems, product form, contamination, additives, color, sorting technology, and market demand for the recovered material.

Which plastic is best for injection molding?

There is no single best plastic for injection molding. PP, ABS, HDPE, PS, PC, PA, and many engineered compounds are widely injection molded. The right choice depends on strength, heat resistance, appearance, cost, shrinkage, cycle time, and application requirements.

What is the difference between HDPE and LDPE?

HDPE is generally stiffer, stronger, and more suitable for rigid containers, pipes, crates, and tanks. LDPE is softer and more flexible, making it common in films, bags, liners, and squeeze applications.

Why are some plastics grouped as Other?

Code 7 is used for plastics that do not fit codes 1 through 6 or for multi-material structures. It may include engineering plastics, bioplastics, blends, and multilayer packaging, so the exact material must be confirmed before processing or recycling decisions are made.