What is the difference between amorphous and semi-crystalline plastics?

Amorphous plastics and semi-crystalline plastics differ fundamentally in their molecular structure. This structural difference influences mechanical properties, temperature resistance, chemical resistance, wear behaviour, dimensional stability and long-term behaviour under load. For design, mechanical engineering and industrial applications, the classification into amorphous or semi-crystalline forms the first and central material decision.

This guide explores the physical background, typical property ranges, design implications, processing characteristics, typical defect patterns and practical application examples.

Difference between amorphous and semi-crystalline – structural principles

Molecular structure in detail

Amorphous plastics have a statistically disordered molecular structure. The polymer chains are entangled and do not form a regular lattice. The molecular distances between the chains are comparatively larger.

Semi-crystalline plastics consist of amorphous regions and crystalline lamellar structures. In these crystalline zones, molecular chains are densely packed and regularly arranged. This results in a higher local density and stronger intermolecular bonding.

This structural partial order leads to:

• higher stiffness
• higher strength
• better media resistance
• improved temperature stability

Isotropy and anisotropy

Amorphous plastics generally exhibit isotropic properties. Mechanical values are similar in all directions.

Semi-crystalline plastics can develop anisotropic properties during processing (e.g. extrusion, injection moulding). The orientation of the crystalline regions influences stiffness and shrinkage in different directions.

Thermal properties and temperature behaviour

Property	Amorphous plastics	Semi-crystalline plastics
Glass transition temperature (Tg)	clearly pronounced	present
Melting point (Tm)	–	clearly pronounced
Continuous service temperature	limited by Tg	higher due to crystalline phase
Coefficient of thermal expansion	higher	lower

Amorphous plastics lose stiffness abruptly above their glass transition temperature.

Semi-crystalline plastics retain their mechanical stability up to close to their melting point. This property is particularly relevant for machine elements subjected to temperature loads.

Typical materials with property ranges

Below are typical representatives of amorphous and semi-crystalline plastics with practice-relevant property ranges. The values serve as orientation ranges and may vary depending on type, manufacturer and modification.

Material	Structure	Density (g/cm³)	E-modulus (MPa)	Tensile strength (MPa)	Typical applications
PMMA	amorphous	1.17–1.20	2,400–3,300	60–75	Viewing windows, covers
PC	amorphous	1.18–1.22	2,000–2,600	55–70	Safety glazing, machine guards
ABS	amorphous	1.03–1.07	1,700–2,400	40–50	Housings, technical cladding
POM	semi-crystalline	1.39–1.42	2,800–3,200	60–75	Gears, plain bearings
PA 6 / PA 66	semi-crystalline	1.12–1.15	2,500–3,500	70–85	Bearing bushes, machine elements
PE‑HD	semi-crystalline	0.94–0.97	800–1,500	20–35	Tanks, containers, linings
PP	semi-crystalline	0.89–0.91	1,200–1,800	25–40	Apparatus engineering, chemical components

The property values illustrate the design differences: semi-crystalline plastics often achieve higher mechanical load capacity and better media resistance, while amorphous materials offer advantages in transparency and dimensional stability.

Mechanical properties in extended comparison

Fatigue strength and fatigue behaviour

Semi-crystalline plastics show better fatigue strength under cyclic loading. The crystalline regions act as stabilising structural segments.

Amorphous plastics are suitable for static applications with moderate continuous load.

Creep behaviour

Under long-term load, the deformation of amorphous materials increases significantly above Tg.

Semi-crystalline plastics show a lower tendency to creep, particularly at moderate temperature.

Wear and friction

POM and PA offer favourable friction coefficients and high abrasion resistance. They are suitable for:

• gears
• plain bearings
• guides
• conveyor elements

Amorphous materials are preferably used for structural, non-sliding components.

Dimensional stability, shrinkage and machining

Shrinkage behaviour

Amorphous plastics show low, uniform shrinkage. This facilitates compliance with tight tolerances.

Semi-crystalline plastics exhibit higher shrinkage. Crystallisation during cooling leads to volume reduction.

Machining and milling

Amorphous materials can be milled and sawn very cleanly.

Semi-crystalline materials require adapted cutting parameters to reduce internal stresses.

Moisture absorption

PA absorbs moisture. Water absorption leads to dimensional changes and influences stiffness. Designs account for this effect through tolerance planning.

Chemical resistance in an industrial environment

Semi-crystalline plastics offer high resistance to:

• acids
• alkalis
• fuels
• oils
• many solvents

PE-HD and PP are therefore used in plant engineering, container construction and chemical plant engineering.

Amorphous plastics are suitable for applications with lower chemical exposure.

Economic aspects and material selection

The structure influences not only technical properties but also cost-effectiveness:

• longer service life reduces maintenance effort
• lower weight reduces system load
• precise dimensional stability reduces rework
• chemical resistance increases process reliability

The correct classification between amorphous and semi-crystalline has a direct effect on total costs over the life cycle.

Practical examples from industry and mechanical engineering

• Machine cladding with viewing window (PMMA, amorphous): High light transmission and good dimensional stability enable precise, visually clean solutions in mechanical engineering.
• Conveyor technology – gears (POM, semi-crystalline): The high wear resistance and low friction ensure a long service life under continuous load.
• Chemical drip tray (PE-HD, semi-crystalline): The pronounced media resistance ensures reliable use in chemically exposed environments.
• Bearing bush in mechanical engineering (PA 66, semi-crystalline): High toughness and load capacity enable stable function under mechanical stress.
• Electronics housing (ABS, amorphous): Good machinability and dimensional stability support precise housing designs.

Summary structural comparison

| Criterion | Amorphous plastics | Semi-crystalline plastics | ||--|-| | Molecular order | disordered | partially ordered | | Transparency | often clear | opaque | | Shrinkage | low | increased | | Wear resistance | moderate | high | | Chemical resistance | medium | high | | Temperature range | up to Tg | up to near Tm | | Continuous load capacity | limited | high |

Systematic material selection

Amorphous plastics are suitable for transparent, dimensionally stable and visually demanding applications.

Semi-crystalline plastics offer high mechanical load capacity, excellent wear values and high chemical stability.

The structure-related differences – amorphous or semi-crystalline – determine the performance, service life and cost-effectiveness of a component in industrial use.

FAQ – amorphous and semi-crystalline plastics

What does semi-crystalline mean in plastics? Semi-crystalline describes a material structure with crystalline and amorphous regions. The crystalline zones increase strength, chemical resistance and temperature stability.

What are the advantages of amorphous plastics? Amorphous plastics offer transparency, uniform shrinkage and good dimensional stability in cut pieces.

When should a semi-crystalline plastic be chosen? For high mechanical load, wear stress or chemical exposure.

Why do semi-crystalline plastics shrink more? The formation of crystalline regions during cooling leads to volume reduction.

Which plastics are transparent? PMMA and PC are typical amorphous, transparent materials.