Glossary: What is Dynamic Mechanical Analysis (DMA)?

A clear explanation of evaluation methods that visualise the ‘hardness’ and ‘toughness’ of resins

In the field of plastics, differences in properties—such as ‘this resin softens easily at high temperatures’, ‘this material is impact-resistant’, or ‘it is hard at room temperature but becomes prone to deformation as soon as the temperature rises slightly’—have a significant impact on mouldability and product quality. One of the evaluation methods used to accurately understand these differences is Dynamic Mechanical Analysis (DMA).

Although the name may sound complicated, DMA is, simply put, a test that applies small vibrations to a material to examine its susceptibility to deformation and the delay in its response. This allows for the evaluation of both the elasticity (the property of a material to return to its original shape) and the viscosity (the property of deformation occurring with a delay) of the resin.

In this article, we will explain the basic principles of dynamic mechanical analysis, what it reveals, and how it is useful for injection molding and material evaluation, in as clear a manner as possible.

Resins possess the properties of both ‘elastic bodies’ and ‘viscous bodies’

Firstly, a key point to understand regarding DMA is that resins are neither pure solids nor pure liquids.

For example, an ideal solid, such as a spring, deforms immediately when a force is applied and returns to its original shape as soon as the force is removed. This is the concept of an elastic body.

On the other hand, substances such as syrup or oil flow slowly when force is applied and do not return to their original shape. These are viscous materials.

However, actual plastic materials lie somewhere in between. In other words,

・part of the material behaves like a spring, attempting to return to its original shape,

・whilst another part exhibits viscosity, deforming with a time delay.

In this way, they exhibit the behaviour of viscoelastic materials.

A major feature of DMA is its ability to evaluate these ‘elastic’ and ‘viscous’ properties separately.

Basic Principles of Dynamic Mechanical Analysis (DMA)

In DMA, a very small cyclic deformation is applied to the test specimen. For example, by applying vibrations at a constant frequency through methods such as tension, bending, shearing or compression, the system measures how the material responds.

The key point here is that a time lag occurs between the applied deformation and the force response returned by the material.

In a perfectly elastic material, the force is returned at exactly the same moment the deformation is applied. However, in viscoelastic materials such as resins, the response is slightly delayed due to the movement of molecular chains and internal friction. This delay is referred to as the phase difference.

By analysing this phase difference, DMA primarily determines the following three values:

1. Storage modulus (E’)

This represents the portion of the energy absorbed by the material that is stored elastically. The higher the value, the harder the material is considered to be and the less likely it is to deform.

2. Damping modulus (E”)

This represents the portion of the applied energy that is lost as heat or in other forms. It is related to internal friction and molecular motion, and indicates the viscous properties of the material.

3. tanδ (tangent delta)

This is the value obtained by dividing the damping modulus by the storage modulus, and represents the balance between viscosity and elasticity. The higher the tanδ value, the more viscous the material’s behaviour is considered to be.

What can DMA tell us?

One of the major advantages of DMA is that it allows us to examine not only whether a material is ‘hard’ or ‘soft’, but also to observe in detail how its properties change with temperature.

Of particular importance is the determination of the **glass transition temperature (Tg)**.

At low temperatures, resins are hard and glass-like, but as the temperature rises, the molecular chains become more mobile, causing the material to approach a rubber-like, soft state. The temperature range in which this change occurs is known as the glass transition temperature. Using DMA, this Tg can be clearly identified as a decrease in E’ or a peak in tanδ.

Furthermore, DMA can provide the following information:

Changes in stiffness across the operating temperature range

An indication of the material’s heat resistance

Differences in behaviour due to variations in molecular structure

The influence of fillers and glass fibre additives

Changes in the material before and after degradation

Performance comparisons between different grades

In short, DMA is an evaluation method that visualises changes in the internal properties of resin materials—changes that cannot be discerned by their ‘appearance’—along the temperature axis.

Why DMA is useful in injection molding

In injection molding, the resin is heated and melted, flows into the mold, and then cools and solidifies. The viscoelastic properties of the material play a significant role in this sequence of processes.

For example, understanding to what extent a particular resin softens near the molding temperature, or at what temperature range it suddenly hardens during cooling, is useful for the following considerations:

▸ Optimisation of molding conditions

By understanding the temperature dependence of the material via DMA, it becomes easier to establish a sound basis for determining resin temperature, mold temperature and cooling conditions. This is particularly effective when considering the impact of temperature settings on warpage, dimensional stability and demouldability.

▸ Comparison of material selection

Even for the same application, the temperature-dependent changes in Tg and stiffness vary depending on the resin grade. By comparing DMA data, it becomes easier to determine whether a material ‘has sufficient stiffness at room temperature’ or ‘does not degrade too significantly in high-temperature environments’.

▸ Investigation of Defect Causes

Problems with molded parts, such as deformation, deflection, cracking and abnormal noises, may be caused not only by a simple lack of strength but also by viscoelastic behaviour. DMA provides clues to uncover the underlying causes.

Specific Example: Materials with a Sharp Drop in Stiffness upon Temperature Rise

For instance, even if a particular resin is considered sufficiently rigid at room temperature, DMA analysis may reveal a sharp decline in storage modulus from around 60°C. In such cases, whilst the material may function without issue at room temperature, there is a risk that it will deform more readily than anticipated in heat-generating environments, such as inside a car during summer or within equipment.

Whilst tensile and flexural tests are effective for assessing strength under specific temperature conditions, the strength of DMA lies in its ability to observe continuous changes in properties as temperature varies. It can therefore be considered a highly practical method for conducting evaluations that take the product’s operating environment into account.

Test modes commonly used in DMA measurements

DMA offers several measurement modes. The main ones are as follows.

Tensile mode

Used for test specimens that are easy to evaluate under tension, such as films and thin sheets.

Three-point bending mode

Commonly used for resin sheets and similar materials; this method makes it easy to observe changes in stiffness.

Cantilever mode

A method in which one end of the test specimen is fixed and the other is vibrated; this is relatively common.

Shear mode

Suitable for evaluating rubber and soft materials.

It is important to select the appropriate mode depending on the material’s shape, hardness and the properties you wish to determine.

Points to note when interpreting DMA results

Although DMA is a useful technique, simply comparing numerical values can lead to misunderstandings. This is because measurement results are influenced by conditions such as specimen shape, frequency, heating rate and measurement mode.

Therefore, when evaluating data,

・Are the comparisons being made under the same conditions?

・Are there any differences in the dimensions of the test specimens?

・Are the frequency conditions close to those in actual use?

・At what temperature should the tanδ peak be read?

It is important to pay attention to these points.

In particular, the Tg may vary slightly depending on the measurement method. It is important to clarify whether it is determined at the inflection point of E’ or at the tanδ peak.

Summary

Dynamic Mechanical Analysis (DMA) is a technique that evaluates the elasticity and viscosity of resins separately by applying minute cyclic deformation to the material and measuring the resulting response.

This measurement allows us to determine:

The temperature dependence of the material’s stiffness

The glass transition temperature (Tg)

The magnitude of internal friction

The susceptibility to deformation under service conditions

Differences between material grades and states of degradation

and other factors.

In injection molding, DMA is not merely a material test; it is a vital evaluation method that contributes to material selection, the examination of molding conditions, defect analysis and product reliability assessment. When seeking a deeper understanding of a resin’s properties, DMA provides extremely valuable insights.

DMA should provide clear answers to questions such as: ‘Why does this material suddenly weaken at this temperature?’ and ‘Why does it deform in a high-temperature environment when there are no issues at room temperature?’