Why do plastic materials stretch?

Understanding the Ductility and Failure Mechanisms of Resins through ‘Molecular Motion’

When designing injection-molded parts or selecting materials,

‘This material is prone to cracking’

‘This resin is sticky’

‘It has whitened on impact’

‘It withstood deformation but fractured’

one may encounter phenomena such as these.

Even though they are all ‘plastics’,

why do some materials stretch significantly,

while others are hard and brittle,

some bend easily,

and others break immediately?

The answer lies in the mobility of the ‘polymer chains’ within the resin.

In this article, we will explain in simple terms what determines the ‘extensibility (ductility)’ of resin materials, incorporating perspectives from both the molding shop floor and practical design work.

Plastics are made of ‘molecular strings’

Unlike metals, plastics are formed by extremely long molecules intertwined with one another. This long molecular structure is called a ‘polymer chain’.

To give you an idea,

it is like

threads,

strings,

or

spaghetti

all tangled up in a complex web.

When the resin is pulled, these molecular chains

unravel,

align,

slide,

and

stretch,

causing the material to deform.

In short, ‘how freely the molecules can move’ is the most important factor determining a resin’s elasticity.

The difference between elastic resins and brittle resins

For example,

PE (polyethylene)

PP (polypropylene)

and similar materials are extremely tough and highly elastic.

On the other hand,

PS (polystyrene)

PMMA (acrylic)

are hard but, conversely, are characterised by being relatively brittle and prone to cracking.

This is because differences in molecular structure

determine whether molecules can move easily or with difficulty.

The difference between crystalline and amorphous resins

When considering the stretchability of resins, the difference between ‘crystalline resins’ and ‘amorphous resins’ is crucial.

Crystalline resins

Crystalline resins possess regions where molecules are arranged in a regular pattern.

Typical examples: PP, PE, PA (nylon)

The crystalline regions act as the material’s skeleton, whilst the amorphous regions deform flexibly, resulting in high ductility.

Consequently, crystalline resins tend to be relatively ‘tough’ materials.

Amorphous Resins

Amorphous resins have a structure in which molecules are arranged randomly.

Typical examples: PS, PMMA, PC

However, even among amorphous resins, properties vary significantly.

For example, PC (polycarbonate) is highly impact-resistant and tough, whereas PMMA, whilst offering excellent transparency, is a relatively brittle material.

In other words, one cannot simply say that ‘amorphous = brittle’; the characteristics are determined by the overall molecular structure.

Glass Transition Temperature (Tg) and Elongation

Resins have an important temperature known as the ‘glass transition temperature’ (Tg).

At the Tg threshold, the mobility of the molecules changes significantly.

T > Tg

Temperature higher than Tg → Molecules move easily

Temperature lower than Tg → Molecules become fixed

In other words, this results in the difference between:

Becoming soft and ductile

Becoming hard and brittle

This is a major factor in why resin products are more prone to cracking in winter.

Higher molecular weight leads to greater toughness

The longer the molecular chains of a resin, the more intricately the molecules intertwine.

Consequently,

molecules are less likely to break away

the material is less prone to fracture

and ductility increases.

These characteristics become apparent.

On the other hand,

flowability deteriorates

and higher molding pressure is required

resulting in a trade-off with mouldability.

In injection molding, the balance between ‘flowability’ and ‘strength’ is crucial, which is why material grades are categorised in detail according to their intended application.

Why does glass fibre (GF) reduce elongation?

In injection molding, glass fibre (GF) is sometimes added to improve strength.

For example,

PP GF30

PP GF40

are typical examples.

GF materials offer benefits such as:

Increased rigidity

Increased strength

Improved dimensional stability

However, on the other hand,

they become less ductile

and more prone to cracking under impact.

This is because the glass fibres restrict the deformation of the molecular chains.

In other words, the resin molecules, which would normally be able to deform freely, have their movement restricted by the fibres.

Necking and Crazing

In terms of resin failure mechanisms,

necking

and crazing

are also important phenomena.

Necking

Necking refers to a phenomenon in which only a specific section becomes locally thinner under tensile stress.

This tends to occur in ductile materials and

can be described as a state in which

‘the material is deforming whilst exhibiting viscous behaviour’.

Crazing

Crazing, on the other hand, is a whitening phenomenon caused by microscopic cracks.

This is an early stage of internal damage,

residual stress

contact with chemicals

excessive stress concentration

and is more likely to occur under such conditions.

In molding operations, care must be taken as this can lead to cosmetic defects and reduced durability.

Molding conditions also affect ‘ductility’

Even with the same material, ductility varies significantly depending on the molding conditions.

For example,

rapid cooling

low mold temperature

excessive molecular orientation

residual stress

can all cause the resin to become brittle.

In injection molding in particular,

the problem of

‘the part is molded successfully but cracks in subsequent processes’

can occur.

In many cases, this is caused not by the material itself,

but by

molding conditions

stress state

orientation

Summary

The ductility of a resin material

is determined by

‘how freely the molecular chains can move’.

The final material properties are determined by the complex interplay of factors such as

crystallinity

Tg

molecular weight

cross-linking structure

glass fibre (GF) content

molding conditions

and so on.

In injection molding, it is not just simple strength, but also

toughness

ductility

impact resistance

residual stress

lead to improved quality and reduced defects.

If we understand resin not merely as ‘plastic’,

but as a ‘dynamic molecular structure’,

our perspective on molding defects and failure phenomena changes significantly.