Plastic material types and how to choose the right polymer for processing

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Understanding plastic material types starts with behavior under heat

Plastic material types are usually grouped by how they behave during processing, not only by their chemical names. The first split is between thermoplastics, thermosets, and elastomers. Thermoplastics soften when heated and harden again when cooled, which makes them suitable for injection molding, extrusion, blow molding, thermoforming, and many recycling streams. Thermosets cure into a crosslinked structure and cannot be remelted in the same way. Elastomers are selected for elastic recovery and may be either thermoset rubbers or thermoplastic elastomers.

For buyers, designers, and processors, the practical question is not simply “What plastic is this?” It is which polymer family gives the right balance of strength, heat resistance, chemical resistance, appearance, cost, and processability for the part and production route. Standards such as ISO 1043-1 help normalize polymer abbreviations, while ASTM D7611/D7611M defines resin identification coding used on many plastic articles. Even so, a code or abbreviation is only a starting point. The final grade, additives, fillers, and processing method can change performance significantly.

The three broad families of plastics

Thermoplastics

Thermoplastics are the largest practical category in everyday and industrial plastic processing. They soften when heated, flow under pressure, and solidify when cooled. This reversible behavior is why materials such as PE, PP, PET, PVC, PS, ABS, PC, PA, and POM are widely used in molding, extrusion, film, sheet, pipe, packaging, housings, and machined parts.

Thermoplastics may be amorphous, semi-crystalline, or modified with rubber, glass fiber, mineral filler, flame retardants, UV stabilizers, pigments, plasticizers, or impact modifiers. As a result, one PP or ABS grade can perform very differently from another, even when the base polymer abbreviation is the same.

Thermosets

Thermosets include materials such as epoxy, phenolic, melamine, unsaturated polyester, and many polyurethane systems. During curing, they form a crosslinked structure. Once cured, they generally cannot be melted and reshaped like a thermoplastic. They are selected when dimensional stability, heat resistance, chemical resistance, adhesion, electrical insulation, or structural rigidity is more important than remeltability.

Thermosets are common in coatings, adhesives, composites, electrical components, high-performance laminates, encapsulation, and reinforced parts. Their main limitation is that reprocessing and mechanical recycling are more difficult than with common thermoplastics.

Elastomers and thermoplastic elastomers

Elastomers are polymers designed to stretch and recover. Traditional rubber materials are often crosslinked thermoset elastomers. Thermoplastic elastomers, often called TPE or TPR in commercial settings, combine rubber-like flexibility with thermoplastic processing. TPEs are used for grips, seals, overmolding, soft-touch surfaces, flexible components, footwear parts, and vibration-damping applications.

The key selection question is whether the part needs permanent elasticity, a soft-touch feel, repeated flexing, chemical resistance, outdoor stability, or compatibility with overmolding onto a rigid substrate such as PP, ABS, or PC.

Common plastic material types compared

The following table summarizes widely used plastic material types. Use it for early screening, not as a replacement for supplier data sheets, regulatory review, or prototype testing.

Material	Full name	Typical strengths	Common uses	Selection cautions
PET	Polyethylene terephthalate	Clarity, toughness, gas and moisture barrier in packaging grades	Bottles, trays, fibers, films, thermoformed packaging	Moisture control and drying are important for many processing routes
HDPE	High-density polyethylene	Chemical resistance, toughness, low moisture absorption	Bottles, containers, pipe, tanks, sheet, liners	Lower stiffness and heat resistance than many engineering plastics
LDPE / LLDPE	Low-density and linear low-density polyethylene	Flexibility, sealability, impact resistance	Film, bags, wraps, squeeze bottles, liners	Limited rigidity and temperature capability
PP	Polypropylene	Low density, chemical resistance, fatigue resistance, living hinge performance	Caps, containers, automotive parts, fibers, medical and lab items	UV and low-temperature impact performance often need grade-specific review
PVC	Polyvinyl chloride	Versatility, chemical resistance, rigid or flexible formulations	Pipe, profiles, cable insulation, flooring, medical tubing	Additives, plasticizers, stabilizers, and regulatory requirements matter
PS / HIPS	Polystyrene and high-impact polystyrene	Stiffness, ease of processing, clarity in general-purpose PS	Foodservice items, packaging, trays, foam, housings	General-purpose PS can be brittle; heat and solvent resistance are limited
ABS	Acrylonitrile butadiene styrene	Impact resistance, surface quality, dimensional stability	Housings, appliance parts, automotive trim, consumer products	Outdoor UV resistance and chemical resistance vary by grade
PC	Polycarbonate	Impact resistance, transparency, higher heat performance than commodity plastics	Guards, lenses, electrical housings, glazing, durable components	Can be notch-sensitive and may require drying before processing
PA	Polyamide or nylon	Strength, wear resistance, toughness, good performance in moving parts	Gears, bearings, fasteners, cable ties, under-hood components	Moisture absorption can affect dimensions and mechanical properties
POM	Polyoxymethylene or acetal	Low friction, stiffness, dimensional stability, wear resistance	Gears, clips, bearings, precision parts, mechanisms	Bonding and some chemical environments require care
PMMA	Polymethyl methacrylate or acrylic	Optical clarity, weathering resistance, surface hardness	Displays, light guides, signage, lenses, transparent panels	Less impact resistant than PC unless modified
PTFE	Polytetrafluoroethylene	Low friction, chemical resistance, high temperature capability	Seals, gaskets, bearings, chemical handling, non-stick applications	Processing differs from standard melt-processable thermoplastics

Commodity, engineering, and high-performance plastics

Another useful way to organize plastic material types is by performance tier. This classification is not a strict standard, but it helps match material cost with application requirements.

• Commodity plastics include PE, PP, PVC, PET, and PS. They are used in high-volume applications such as packaging, film, pipe, containers, consumer goods, and disposable or semi-durable products.
• Engineering plastics include ABS, PC, PA, POM, PMMA, PBT, and certain PET engineering grades. They offer better mechanical, thermal, dimensional, or appearance performance than many commodity plastics.
• High-performance plastics include materials such as PTFE, PPS, PEEK, PEI, and high-temperature nylons. They are chosen for demanding heat, wear, chemical, electrical, or structural conditions, usually at a higher material cost.

This tiered view is useful in procurement and design reviews. Over-specifying a polymer can raise cost without improving the part, while under-specifying can lead to failures in heat, chemicals, load, UV exposure, flame requirements, or dimensional control.

Resin identification codes are not a full material specification

Many plastic articles carry resin identification codes numbered 1 through 7. Under the ASTM resin identification system, the common codes are 1 for PET, 2 for HDPE, 3 for PVC, 4 for LDPE, 5 for PP, 6 for PS, and 7 for other plastics or multi-material categories. These markings help identify the general resin family, especially in packaging and recycling contexts.

However, a resin identification code does not prove that an item is recyclable in every local system. It also does not describe additives, fillers, colorants, food-contact status, flame rating, mechanical grade, or processing history. A part marked “5 PP” may be a thin packaging item, a reinforced automotive component, or a medical-grade part. Those applications may share a polymer family but have very different performance requirements.

For processing and procurement, resin codes should be treated as broad labels. Material selection still requires a technical data sheet, application requirements, regulatory review where needed, and trial processing. For more context on forming and conversion methods, see the Plastic Processing section.

How processing method affects material choice

Injection molding

Injection molding favors materials that can flow into a mold, fill thin or complex geometry, cool predictably, and release without excessive warpage or shrinkage. PP, ABS, PC/ABS, PA, POM, PC, and many TPEs are common choices. Important factors include melt flow, mold temperature, shrinkage, moisture sensitivity, wall thickness, gate design, and whether the part needs a cosmetic finish or structural performance. See also: Buying Guides.

Extrusion

Extrusion is widely used for pipe, tube, film, sheet, profiles, and wire coating. PE, PP, PVC, PS, PET, and certain engineering plastics are commonly extruded. The material must deliver stable melt strength, consistent output, and suitable cooling behavior. PVC extrusion also requires careful formulation and processing control because stabilizers, plasticizers, and additives play a central role.

Blow molding

Blow molding is used for bottles, tanks, containers, ducts, and other hollow parts. HDPE is a major material for extrusion blow molded containers because of its toughness and chemical resistance. PET is widely associated with stretch blow molded beverage and packaging containers. PP and PVC may also be used in suitable grades and applications.

Thermoforming

Thermoforming starts with sheet that is heated and shaped over or into a mold. PET, HIPS, PVC, ABS, PP, PMMA, and PC can be used depending on the temperature window, draw ratio, clarity, impact resistance, and end-use requirements. Sheet uniformity and forming behavior are as important as the polymer name.

Machining and fabrication

Plastic sheet, rod, and tube are often machined, cut, welded, bent, or bonded for industrial parts. HDPE, PP, PVC, PC, PMMA, POM, PA, and PTFE are common fabrication materials. In these applications, machinability, dimensional stability, bonding method, welding behavior, and chemical exposure can matter more than melt flow.

A practical selection checklist

Choosing among plastic material types is easier when the application is translated into measurable requirements. Start with the operating environment and likely failure risks, then narrow the polymer family and grade.

• Mechanical load: Consider impact, tensile load, flexing, creep, fatigue, wear, and whether reinforcement is needed.
• Temperature: Review continuous and short-term heat exposure, cold impact, hot filling, sterilization, or outdoor thermal cycling.
• Chemical exposure: Check oils, fuels, solvents, acids, alkalis, cleaners, disinfectants, and environmental stress cracking risks.
• Dimensional control: Compare shrinkage, moisture absorption, creep, tolerance requirements, and post-mold conditioning.
• Appearance: Define color, gloss, transparency, texture, scratch resistance, and weathering needs.
• Regulatory requirements: Confirm food contact, medical, electrical, flame, drinking water, automotive, or regional compliance needs where applicable.
• Processing route: Match the material to injection molding, extrusion, blow molding, thermoforming, compression molding, casting, or machining.
• End-of-life target: Consider mono-material design, disassembly, recycled content, local recycling infrastructure, and labeling requirements.

For production parts, the safest approach is to compare candidate grades using supplier data sheets, then validate them with processing trials and application testing. The polymer abbreviation is the beginning of the selection process, not the end.

Frequently asked questions

What are the main plastic material types?

The broadest material families are thermoplastics, thermosets, and elastomers. In everyday processing, the most common thermoplastics include PET, HDPE, LDPE, PP, PVC, PS, ABS, PC, PA, POM, PMMA, and PTFE.

Which plastic material is best for injection molding?

There is no single best plastic for injection molding. PP is often used for lightweight and chemical-resistant parts, ABS for impact and surface quality, PC for impact and transparency, PA for mechanical strength and wear, and POM for precision moving parts. The right choice depends on geometry, performance, cost, and processing conditions.

Are resin identification codes the same as recycling symbols?

Resin identification codes identify the general polymer family, such as PET, HDPE, PVC, LDPE, PP, PS, or Other. They do not automatically mean the item is accepted in a local recycling program or that it is made from recycled content.

What is the difference between commodity and engineering plastics?

Commodity plastics are generally high-volume materials used where cost and processability are major drivers. Engineering plastics provide improved strength, heat resistance, dimensional stability, wear resistance, or impact performance for more demanding applications.

Why can two parts made from the same plastic perform differently?

The same polymer family can include many grades. Fillers, glass fiber, rubber modifiers, plasticizers, flame retardants, UV stabilizers, pigments, molecular weight, moisture level, and processing conditions can all change the final performance of the part.