PA plastic
Polyamide clearly explained: properties, differences between PA 6 and PA 66, moisture absorption, processing and industrial applications.
Polyamide, or PA for short, is one of the most important
. The
combines high mechanical strength with good toughness, wear resistance and favourable sliding properties. As a result, polyamide is suitable for numerous mechanically and dynamically stressed components. However, polyamide is not a single material grade but an extensive material group. The most common variants include PA 6 and PA 66. In addition, cast, extruded, glass-fibre-reinforced, impact-modified, sliding-optimised and other application-specific polyamides are available. Which PA type is suitable depends on, among other things, the mechanical load, the service temperature, the ambient humidity, the required tolerances and the media acting on it.
What is polyamide?
Polyamides are semi-crystalline thermoplastic plastics whose molecular chains are characterised by recurring amide groups. This chemical structure contributes to the good mechanical properties of the material, but at the same time is responsible for its comparatively high moisture absorption. PA is processed both into moulded parts and into semi-finished products. Available are, for example, sheets, rods, tubes, profiles and custom-manufactured components. Due to its versatile properties, polyamide is used among other things in
mechanical and plant engineering
, in
conveyor technology and automation
, in
as well as in the
.
The most important properties of polyamide
High strength and toughness
Polyamide offers a balanced combination of strength, stiffness and toughness. The material can absorb mechanical loads, impacts and alternating stresses and is therefore suitable for functional machine elements and structural parts. However, the specific characteristic values differ significantly between the various polyamide types. Unreinforced materials are often tougher and more elastic, while glass-fibre-reinforced PA types can achieve higher stiffness and dimensional stability.
Good wear resistance and sliding properties
Many polyamides possess good abrasion resistance and favourable friction properties. They are therefore frequently used for moving components such as plain bearings, bushes, rollers, gears, chain guides and guide rails. Special additives can further adjust the tribological properties. Depending on the application, sliding-optimised or solid-lubricant-modified PA materials are available, among others. Further fundamentals on friction, wear and material pairings can be found in the article on the
tribological properties of engineering plastics
.
Temperature resistance
Depending on the material variant, polyamide can also be used at elevated temperatures. PA 66 generally possesses a higher heat deflection temperature and a higher melting range than PA 6. Glass fibres, mineral fillers or heat stabilisers can further improve the thermal load capacity. Nevertheless, a blanket maximum service temperature is not sensible. The permissible continuous service temperature depends on the specific PA type, the mechanical load, the duration of use, the component geometry and the ambient conditions. For the design, the technical data of the intended material should therefore always be consulted.
Chemical resistance
PA is resistant to many oils, greases, fuels, lubricants and weak alkalis. However, the actual resistance is influenced by the polyamide variant, the concentration of the medium, the temperature and the duration of exposure. Polyamide can be sensitive to strong acids, certain solvents and some oxidising media. For chemically stressed components, an application-specific resistance test should therefore be carried out.
Moisture absorption
Moisture absorption is one of the most important criteria when selecting polyamide.
PA is hygroscopic and absorbs moisture from the environment. As a result, the dimensions and mechanical properties of a component can change. As moisture content increases, stiffness and hardness generally decrease, while toughness and impact strength can increase. At the same time, the material can swell. For precision components, tight fits or changing climatic conditions, this behaviour must be taken into account already during design, material selection and tolerancing. Detailed information on this topic can be found in the technical article
How does moisture affect the properties of PA?
as well as in the overview of the
differences in moisture absorption among thermoplastics
.
Electrical properties
Unreinforced and dry polyamide possesses good electrical insulation properties. PA is therefore used, for example, for connectors, insulating parts, switch components and housings. Since absorbed moisture can influence the electrical characteristic values, the conditioning state, the environment and the specific material modification must be taken into account in demanding electrical applications. Further suitable materials and selection criteria are covered in the article on
plastics for electrical engineering and electronics
.
Production
Material form
finished product
PA 6 and PA 66 in comparison
PA 6 and PA 66 are among the most frequently used polyamide types. Both materials possess high strength and wear resistance, but differ in important properties.
| Property | PA 6 | PA 66 |
|---|---|---|
| Toughness | Generally higher | Good, often somewhat lower than PA 6 |
| Stiffness | Good | Generally higher |
| Impact strength | Often higher | Material-dependent |
| Heat deflection temperature | Good | Generally higher |
| Moisture absorption | Comparatively high | Also relevant, often somewhat lower |
| Processing | Comparatively easy to process | Higher processing temperatures required |
| Typical applications | Rollers, bushes, guides and general structural parts | Moulded parts subject to higher thermal and mechanical stress |
The table serves as a basic guide. Reinforcements, additives, manufacturing processes and conditioning state can significantly change the properties of both materials. The material selection should therefore not be made solely on the basis of the designation PA 6 or PA 66.
Further polyamide variants
Extruded polyamide
During extrusion, molten polyamide is continuously passed through a shaping tool. In this way, for example, sheets, rods, hollow bars and profiles are produced. Extruded semi-finished products are then often further processed by machining into individual components.
Further information on properties, processing and possible applications can be found on the material page for extruded polyamide.
Cast polyamide
Cast polyamide is produced by the polymerisation of the starting material directly in a mould. The process enables in particular larger semi-finished product dimensions and near-net-shape moulded parts. Cast polyamide is frequently used for large-format rollers, cable sheaves, gears, sliding plates and heavily stressed structural parts.
Further information can be found on the material page cast polyamide.
Glass-fibre-reinforced polyamide
Reinforcement with glass fibres can increase stiffness, strength, creep resistance and heat deflection temperature. At the same time, impact strength and elongation at break can decrease. In addition, the direction-dependent shrinkage of fibre-reinforced moulded parts must be taken into account during design.
Modified PA types
Polyamide can be specifically adapted to different requirements. Available are, among others:
- impact-modified polyamides,
- sliding-optimised and wear-resistant types,
- heat-stabilised materials,
- UV-stabilised variants,
- flame-retardant polyamides,
- electrically conductive or antistatic types,
- glass-fibre- or mineral-reinforced materials as well as
- versions suitable for food contact.
Production and processing of polyamide
Extrusion
Extrusion is predominantly used for the production of continuous semi-finished products and profiles. These include plastic sheets, rods, tubes and custom profile cross-sections.
Extruded PA semi-finished products are particularly suitable for small to medium batch sizes, spare parts, prototypes and components manufactured by milling, turning, drilling or sawing.
Injection moulding
In injection moulding, complex moulded parts can be manufactured economically in high volumes. The process enables the integration of ribs, fastening elements, functional surfaces and different wall thicknesses into a single component.
Since polyamide absorbs moisture, the granulate must generally be dried in a controlled manner before processing. Insufficient drying can impair the surface quality, the mechanical properties and the process reliability. Further information is provided on the page about plastic injection moulding.
Machining
PA semi-finished products can be machined by turning, milling, drilling, sawing and other machining processes. For good dimensional accuracy and surface quality, sharp tools, suitable chip removal and the lowest possible heat input are important.
Excessively high temperatures can lead to smearing, distortion or internal stresses. For larger material removal volumes, step-by-step machining is often recommended. The moisture state of the material must also be taken into account when tight tolerances are required.
Welding and bonding
Depending on component geometry and material variant, thermoplastic polyamides can be joined by hot-plate, friction, vibration or ultrasonic welding, among others.
Bonding PA is more demanding, since the surface is difficult to wet for many adhesive systems. Suitable cleaning, pre-treatment or surface activation can improve adhesion. More on this can be found in the article on the influence of surface treatment on plastics.
Typical applications of polyamide
Mechanical and plant engineering
In mechanical engineering, polyamide is used for components that must combine strength, wear resistance and low weight. Typical examples are:
- plain bearings and bearing bushes,
- gears and sprockets,
- guide strips and sliding plates,
- rollers and idler rollers,
- scrapers and stops,
- cable sheaves as well as
- housings and structural elements.
A comparison with other suitable materials is available in the article POM, PA or PEEK – which material for mechanical engineering?.
Conveyor technology and automation
In conveyor systems, PA is used among other things for guide elements, rollers, chain guides and wear-stressed machine parts. Compared with metallic components, the material can reduce component weight, noise development and lubrication requirements.
Vehicle construction
In vehicle construction, polyamide is used, for example, for housings, brackets, connectors, line components, covers and components in the engine compartment. For thermally stressed applications, reinforced and heat-ageing-stabilised PA types are frequently used.
Electrical engineering and electronics
Polyamide is used for electrical and electronic components such as connectors, coil formers, switch parts, cable fastenings and housings. Depending on the safety requirement, flame-retardant, reinforced or electrically modified variants are available.
Advantages of polyamide
- High mechanical strength and good toughness
- Good wear and abrasion resistance
- Favourable sliding and friction properties
- Good resistance to many oils, greases and fuels
- Lower weight than many metallic materials
- Good electrical insulation in the dry state
- Versatile modifiability through fibres, fillers and additives
- Processing by extrusion, injection moulding, casting and machining
- Broad range of semi-finished products and moulding compounds
Disadvantages and limits of polyamide
Polyamide is not automatically the best solution for every application. The most important limitations include:
- Moisture absorption with possible dimensional changes
- Change in mechanical and electrical properties due to moisture
- Limited resistance to strong acids and certain chemicals
- Possible UV ageing in non-stabilised types
- Dependence of properties on the conditioning state
- Sometimes lower dimensional accuracy than materials such as POM or PET
- Demanding bonding without suitable pre-treatment
Polyamide or POM?
PA and POM are frequently used for similar mechanical components. In many versions, polyamide offers high toughness, good wear resistance and high mechanical load capacity. POM, on the other hand, absorbs significantly less moisture and therefore often achieves better dimensional accuracy.
For precision parts with tight tolerances or components in changing climatic conditions, POM can be advantageous. For highly stressed rollers, guides, large-format components or applications with high impact loads, PA can represent the more suitable solution. The decisive factor is always the specific requirement profile.
Which polyamide is suitable for your application?
When selecting the suitable polyamide, the following questions should in particular be answered:
- What forces and mechanical loads act on the component?
- What service and peak temperatures occur?
- Is the component exposed to moisture or water?
- What dimensional and shape tolerances must be maintained?
- Do oils, fuels, cleaning agents or other chemicals act on it?
- Are special sliding or wear properties required?
- Must the material be electrically insulating, conductive or flame-retardant?
- Is a semi-finished product, a machined part or an injection-moulded part required?
- What batch size and component size are intended?
Only from the combination of these requirements can it be determined whether, for example, PA 6, PA 66, cast polyamide, extruded polyamide or a reinforced or modified PA type should be used.
Polyamide for industrial components
Polyamide is a versatile engineering plastic for components subject to mechanical, thermal and tribological stress. Its high strength, toughness and wear resistance make PA a proven material in mechanical engineering, in conveyor technology, in vehicle construction and in electrical engineering.
For reliable material selection, however, the respective polyamide variant and the specific operating conditions must be taken into account. Moisture absorption, dimensional accuracy, temperature load and the chemical environment are particularly decisive. By selecting a suitable PA type and a professional design, durable and economical plastic components can be realised.
Frequently asked questions about polyamide
What does the abbreviation PA stand for?
PA stands for polyamide. This is a group of thermoplastic plastics, which includes PA 6 and PA 66, among others.
Is polyamide the same as nylon?
Nylon is a well-known designation for certain synthetic polyamides. In the technical field, however, the material designation PA together with the respective type number is predominantly used.
What is the difference between PA 6 and PA 66?
PA 6 is generally tougher and comparatively easy to process. PA 66 often possesses higher stiffness and heat deflection temperature. However, the actual properties depend heavily on reinforcements, additives and the moisture state.
Is polyamide waterproof?
Polyamide does not dissolve in water, but it does absorb moisture. As a result, dimensions, stiffness, toughness and electrical properties can change. For permanently humid environments, it must therefore be checked whether the selected PA type is suitable.
Is polyamide suitable for gears?
Yes, polyamide is frequently used for gears. The material offers good wear resistance, dampens noise and can be operated without corrosion. At high precision requirements, however, the moisture-related dimensional change must be taken into account.
Can polyamide be milled and turned?
Polyamide can be machined well. Sharp tools, controlled heat input and suitable chip removal help to avoid dimensional deviations, smearing and distortion.
What alternative is there to polyamide?
Possible alternatives include POM, PET, PE-UHMW or PEEK, among others. Which material is suitable depends on the requirements for dimensional accuracy, wear, temperature, chemical resistance and cost.
