Dimensional stability in plastics

Dimensional stability describes the ability of a plastic to reliably maintain its shape and dimensions even under external influences such as temperature, mechanical forces, moisture or chemical media. In industrial practice, this material behaviour is of decisive importance, since even the smallest dimensional deviations can lead to malfunctions, higher wear rates or even the failure of complete systems.

Unlike metals, which usually exhibit very constant thermal and mechanical properties, plastics are significantly more strongly affected by external influences. As a result, they require a comprehensive understanding of their properties in order to be used precisely and reliably in sensitive areas such as medical technology, aerospace or electronics. Precisely where the highest accuracy is required, the dimensional accuracy of a plastic determines the functionality and service life of the component.

Fundamentals of dimensional stability

The dimensional stability of a plastic is closely linked to its molecular structure and its physical properties. While metals possess a homogeneous crystal lattice at the atomic level, plastics consist of long polymer chains whose mobility varies. This mobility is influenced by temperature, moisture or the addition of additives.

Thermoplastics are reversibly deformable when heated and therefore react particularly sensitively to thermal influences. Thermosets, by contrast, possess a rigid, cross-linked structure that gives them high dimensional stability even at elevated temperatures. Elastomers are characterised by their flexibility, but are only conditionally suitable for dimension-critical applications, since they are intended more for applications in which elastic properties are the priority.

High-performance plastics such as PEEK or PPS ultimately combine excellent thermal and mechanical properties with high dimensional stability and are therefore established in numerous key industries. The glass transition temperature (Tg) is also particularly relevant, as it indicates the point at which a plastic transitions from a brittle to a rubber-like state – a decisive point for classifying its stability.

Influence of heat: thermal expansion and linear expansion in plastics

Heat directly influences the dimensional accuracy of plastic components. When the temperature rises, most plastics undergo thermal expansion. On cooling, they contract accordingly. How pronounced this change in length is is described by the coefficient of linear thermal expansion, often given as α or CTE.

For design purposes, the linear expansion can be determined approximately using the following relationship:

ΔL = α × L₀ × ΔT

Here ΔL stands for the change in length, α for the coefficient of linear expansion of the plastic, L₀ for the initial length and ΔT for the temperature change.

The thermal expansion of plastics is in many cases significantly greater than that of metallic materials. It must therefore be taken into particular account with tight tolerances, fits, larger component lengths and in designs in which plastic and metal are joined together. Different expansion coefficients can cause stresses, distortion or malfunctions there.

The actual expansion coefficient depends on the plastic, its composition, temperature range and any reinforcing materials present. Glass or carbon fibres can considerably reduce thermal expansion. With fibre-reinforced materials, it must additionally be taken into account that the linear expansion can be direction-dependent depending on the fibre orientation.

If components are regularly exposed to changing temperatures, then in addition to pure thermal expansion, the resistance of engineering plastics to temperature changes must also be taken into account. For applications with elevated temperature under mechanical load, the heat deflection resistance of plastics is also relevant.

When does the linear expansion of a plastic become critical?

Linear expansion is particularly relevant with:

• tight dimensional and positional tolerances,
• long or large-area plastic components,
• strongly fluctuating service temperatures,
• precise guides and bearing points,
• plastic-metal joints,
• components with limited freedom of movement.

In such applications, the coefficient of linear expansion of the plastic should be taken into account already during material selection and design. In this way, necessary clearances for movement, tolerances and fastening concepts can be determined at an early stage.

Then continue with the existing section "Influence of mechanical load".

Further influencing factors

In addition to temperature and load, other factors also influence dimensional accuracy:

• Moisture: Hygroscopic plastics absorb water from their surroundings depending on the material, temperature, air humidity and duration of exposure. This can change the volume and cause the component to swell. Particularly in plastics production, defined storage, drying and ambient conditions are therefore important when tight tolerances must be maintained.
• Chemical media: Solvents or oils can cause plastics to swell or alter their molecular structure.
• UV radiation: Prolonged exposure to sunlight can promote ageing processes and embrittlement and thereby impair long-term dimensional accuracy.
• Ageing and time: Even without immediately visible load, the dimensions of a plastic component can change over the long term. Causes include relaxation processes or post-crystallisation.

Post-crystallisation in plastics

Post-crystallisation can occur in semi-crystalline plastics when further crystalline regions form within the material after processing. This is particularly possible if an equilibrium state had not yet been reached during the original cooling.

Since polymer chains are more densely arranged in the crystalline state, later post-crystallisation can be associated with post-shrinkage and thus a subsequent change in dimensions. In precision components, this can affect tolerances, fits and long-term dimensional stability.

Processing conditions such as mould temperature, cooling rate and suitable thermal post-treatment therefore influence not only the dimensions achieved immediately after production, but also their long-term stability.

Influence of heat

Heat leads to changes in length in plastics, which are described by the coefficient of thermal expansion (CTE). While metals exhibit comparatively low values, plastics can have CTE values up to ten times higher. In practice, this means that even small temperature changes can lead to malfunctions in precision bearings or fine-mechanical components.

In vehicles or aircraft, it is essential to use plastics that retain their shape even under changing climatic conditions. A further aspect is the possibility of reducing thermal expansion through targeted additions of fillers such as glass fibres or talc. In addition, thermal relaxation occurs, in which internal stresses in the material are reduced by temperature cycles. This effect can cause permanent changes in shape if the material has not been correctly pre-treated.

Influence of mechanical load

Mechanical forces act on plastics in the form of tension, compression, bending or vibration and lead to different effects. Under sustained load, creep occurs, a slow, time-dependent deformation that does not stop even under constant load. In parallel, internal stresses can be reduced under sustained load, which is referred to as relaxation.

Under repeated cyclic loading, stress cracks also form, which weaken the component over the long term. Reinforcement with glass or carbon fibres can considerably improve dimensional stability. Component geometry also contributes to this: large-area contact surfaces and uniform wall thicknesses have a stabilising effect and prevent localised overloading.

Further influencing factors

In addition to temperature and load, other factors also influence dimensional accuracy:

• Moisture: Hygroscopic plastics such as PA6 can absorb up to 10 % water, which leads to swelling and changes in length.
• Chemical media: Solvents or oils can cause plastics to swell or attack their molecular chains.
• UV radiation: Prolonged exposure to sunlight leads to ageing, embrittlement and thus also to distortion.
• Ageing and time: Even without external load, plastics can change over years (relaxation, post-crystallisation).

Test methods for determining dimensional stability

Various methods are available for a reliable assessment of dimensional stability. Thermomechanical analysis (TMA) measures the change in length under defined temperature cycles and provides precise data on expansion behaviour. Dynamic mechanical analysis (DMA) records the viscoelastic behaviour across different temperature and frequency ranges.

In addition, creep and relaxation tests provide information about the time-dependent deformation behaviour under constant load. Standard tests such as ISO 75 for heat deflection resistance or ISO 306 for the Vicat softening temperature provide practical comparative values that are indispensable for material selection in development.

Comparison of selected plastics

Plastic	Coefficient of thermal expansion (CTE)	Creep behaviour	Moisture absorption
Polyethylene (PE)	very high	strongly pronounced	very low
Polyamide (PA6)	medium	moderate	high (up to 10 %)
Polycarbonate (PC)	high	moderate	low to medium
PEEK	very low	very low	very low
PTFE	high	low	practically none

This overview illustrates how greatly plastics differ in their properties. While PE is less suitable for precision components due to high expansion and creep tendency, high-performance plastics such as PEEK impress with their combination of low thermal expansion, low creep behaviour and minimal water absorption.

Practical examples of applications

• Electronics: Connectors and housings must retain their shape even when heated by current flows in order to avoid contact problems.
• Automotive engineering: Intake manifolds or housings in the engine compartment must remain permanently stable despite vibrations and temperature peaks.
• Aerospace: Materials must withstand extreme temperature fluctuations without showing dimensional changes.
• Medical technology: Devices and instruments must remain precise after sterilisation processes at 120 °C.
• Mechanical engineering: Plastic gears or bearing bushes must not change their geometry even under sustained load.

Strategies for improving dimensional stability

The optimisation of dimensional stability begins already with the choice of material and design. Plastics with low water absorption, low thermal expansion and suitable long-term behaviour offer advantages in precise applications.

Reinforcements with glass or carbon fibres increase stiffness and can at the same time reduce the coefficient of thermal expansion. Further information on structure, properties and selection can be found under fibre-reinforced plastics.

Mineral fillers can also influence shrinkage and expansion behaviour. In design terms, uniform wall thicknesses, suitable transitions and sufficient clearances for movement help to reduce stresses and distortion.

Reproducible processing is equally important. Cooling rate, mould temperature, holding pressure and, where applicable, thermal post-treatment influence the internal structure and thus the subsequent dimensional accuracy of the component.

What role does thermal stability play in longer production runs?

In longer production runs, the most constant possible thermal conditions are important so that the material and process do not gradually change. Fluctuating mould, material or ambient temperatures can affect shrinkage, crystallisation and internal stresses and thereby favour dimensional deviations between the manufactured components.

For dimension-critical series components, relevant process parameters should therefore be kept stable and monitored regularly. The tighter the required tolerances, the more important reproducible thermal process control becomes.

Challenges and limits

Despite diverse possibilities for improvement, plastics reach certain limits with regard to their dimensional stability. Extreme temperatures above 300 °C cannot be managed by most polymers, since their molecular chains are thermally destroyed. Even under sustained high load or intensive chemical attack, even high-performance plastics lose their dimensional stability.

Ageing represents a further problem: UV radiation and exposure to oxygen lead to embrittlement over long periods, which can favour deformation. Processing also plays a central role – faulty parameters in injection moulding or extrusion lead to internal stresses that later manifest as distortion or crack formation.

In such cases, hybrid solutions are often suitable, in which plastics are combined with metals in order to utilise the advantages of both material worlds. Nevertheless, the precise maintenance of tolerances under extreme load conditions remains a challenge that designers must always keep in view.

In conclusion

The dimensional stability of plastics is a complex interplay of material properties, design configuration and application conditions. It is decisively determined by heat, mechanical load, moisture and chemical influences. For developers and users, this means that careful material selection must already take place in the early planning phase, supplemented by suitable tests and simulation-supported predictions.

Only in this way can risks such as distortion, dimensional changes or malfunctions be minimised. Modern high-performance plastics and reinforced variants offer the possibility of being used reliably even in high-precision areas, but still reach their limits under extreme conditions. A deep understanding of materials, combined with design know-how and controlled processing, is therefore the key to ensuring the dimensionally stable performance of plastic components over the long term.

FAQ

What is meant by dimensional stability in plastics?

Dimensional stability describes the ability of a plastic to keep its shape and dimensions as constant as possible under influences such as temperature, mechanical load, moisture and media.

Why do plastics expand when heated?

As temperature rises, the mobility of the polymer chains increases. This changes the dimensions of the material. The magnitude of this thermal linear expansion is described by the coefficient of thermal expansion or coefficient of linear expansion.

What is the expansion coefficient of a plastic?

The coefficient of linear expansion α indicates how greatly the length of a material changes with a given temperature change. The value is material-dependent and can additionally be influenced by fillers, reinforcing fibres and the temperature range under consideration.

How can the linear expansion of a plastic be calculated?

For many design estimates, the relationship ΔL = α × L₀ × ΔT can be used. What is required is the coefficient of linear expansion of the material, the initial length of the component and the temperature difference that occurs.

What does post-crystallisation mean in plastics?

Post-crystallisation refers to the subsequent formation of further crystalline structures in a semi-crystalline plastic. It can be associated with post-shrinkage and thereby change the dimensions of an already manufactured component.

Which plastics exhibit high dimensional stability?

Materials such as PEEK, PPS, PEI or other suitable high-performance plastics can offer high dimensional stability depending on the application. The actual suitability additionally depends on temperature, mechanical load, moisture, media contact and the required tolerances.

How can the thermal expansion of plastics be reduced?

One possibility is reinforcing the plastic with glass or carbon fibres. In addition, the choice of base polymer, fillers, fibre orientation and a design suited to plastics play an important role.

What role does air humidity play in plastics production?

In hygroscopic plastics, air humidity and the moisture content of the material influence both processing and the subsequent component dimensions. For precise components, material-appropriate drying, storage and defined ambient conditions are therefore important.