How is the Hardness of Plastic Determined?

When selecting a plastic for a particular application, there are many material properties to consider — weight, chemical resistance, fire rating, and hardness among them. Yet hardness is one of the most misunderstood entries on a plastic datasheet, and is frequently confused with a related but very different property: toughness.

This post is the first in our Datasheets Demystified series, in which we explain what the numbers on your material datasheet actually mean — and why they matter when choosing the right plastic for your project.

Hardness vs Toughness — What's the Difference?

These two terms are often used interchangeably, but they refer to entirely different material behaviours.

Hardness is a material's ability to resist permanent surface deformation — including indentation, scratching, or cutting — when a force is applied.

Toughness is a material's ability to absorb energy and resist breaking when subjected to sudden forces, such as impacts, drops, or bending.

A practical example helps illustrate the distinction. Cast Acrylic is a harder material than Polycarbonate — it resists scratching and surface wear very well, making it ideal for signage and glazing where maintaining a clear, polished appearance matters. Polycarbonate, on the other hand, is far tougher — it can absorb significant impact without cracking or shattering, which is why it is the material of choice for applications such as riot shields, machine guards, and safety glazing.

Neither material is inherently better. The right material depends entirely on the application's requirements.

What Affects the Hardness of a Plastic?

Plastic hardness — its resistance to scratches, dents, and surface wear — is influenced by several factors:

Molecular Structure: Crystalline, tightly packed polymers (such as HDPE and rigid PVC) tend to be harder. Amorphous polymers (such as acrylic and polycarbonate) are comparatively softer, though their other properties often make up for this.

Additives: Fillers, such as glass fibres, significantly increase hardness. Plasticisers, by contrast, are deliberately added to make plastics softer and more flexible — which is why flexible PVC feels so different to its rigid counterpart.

Processing Methods: The way a plastic is manufactured affects its final hardness. Slow cooling and controlled molecular orientation during production create harder, more structured materials. Rapid cooling results in a softer, less ordered structure.

Temperature: All plastics soften as the temperature rises and become harder (but more brittle) as it falls. This is worth bearing in mind if a material will be used in environments with significant temperature variation.

Environment and Ageing: UV light, moisture, and general wear can alter hardness over time — either softening a material or making it more brittle. This is one reason UV-stabilised grades exist for outdoor applications.

How is Hardness Measured?

This is where datasheets can seem intimidating — there are several different hardness scales, each designed for different material types and applications. Here are the three you are most likely to encounter for plastics.

Rockwell Hardness (R, M, L scales)

The Rockwell test works by pressing a steel ball into the surface of a material under a defined load, then measuring the depth of the indentation. The result is given as a Rockwell number on one of several scales — for plastics, the R scale (for softer materials like polyethylene) and the M scale (for harder plastics like acrylic and nylon) are most common.

A higher Rockwell number means greater hardness. Cast Acrylic typically registers around M97–M100, whereas Polycarbonate comes in at around M70–M75, which neatly explains why acrylic scratches less easily despite being the more brittle of the two.

Rockwell hardness is the scale you will most commonly see on engineering plastic datasheets.

Shore Hardness (Shore D)

The Shore D scale uses a spring-loaded indenter pressed into a material's surface for a defined period of time. It is most commonly used for softer or more flexible plastics and elastomers. Shore D values run from 0 to 100, with higher numbers indicating a harder material.

For reference, a standard PVC foam board (Foamex) will have a much lower Shore D value than a rigid engineering plastic. If you are working with flexible or semi-rigid materials, Shore D is likely the scale on your datasheet.

Vickers Hardness (HV)

The Vickers test uses a diamond pyramid indenter to create a very small indentation and measures the indentation's surface area under a microscope. It is more commonly associated with metals, but does appear on datasheets for some harder engineering plastics and composites.

Hardness of Common Plastics

The table below gives typical hardness values for the plastics we supply, to help you compare materials at a glance. Note that values can vary between grades and manufacturers — always refer to the specific datasheet for the material you are ordering.

Cast vs Extruded Acrylic — A Hardness Perspective

There is ongoing debate in the plastics industry about whether cast or extruded acrylic is the better material, and that discussion usually centres on chemical resistance, machinability, or ease of forming. Hardness is often overlooked — but it is an important factor.

Cast acrylic is produced by polymerising a liquid monomer between glass moulds, a process that allows for higher-molecular-weight chains and a more uniform, tightly packed structure. This results in a measurably harder surface than extruded acrylic, which is produced by pushing molten material through a die at high speed—a process that is efficient and cost-effective but yields a slightly less ordered molecular structure.

In practice, this means cast acrylic resists scratching more effectively than extruded acrylic. For applications where the surface will be handled, cleaned regularly, or exposed to abrasion — such as display cases, signage, or picture frame glazing — cast acrylic will maintain its optical clarity for longer.

It is worth noting that neither variant is universally superior. Extruded acrylic offers tighter thickness tolerances, is generally more economical, and is better suited to certain forming operations. But if scratch resistance is a priority, cast acrylic is the stronger choice—and the hardness figures on the datasheet back this up.

We supply both cast acrylic and extruded acrylic cut to size, with real-time pricing available on our website.

Reading the Hardness Entry on Your Datasheet

When you download a datasheet from our website, the hardness entry will typically look something like this:

Hardness: Rockwell M97 / Shore D 87

Here is how to read it:

• Rockwell M97 — on the M scale (mid-range plastics), this material scores 97 out of a possible 130. This is a high value, indicating very good scratch and indentation resistance.
• Shore D 87 — on the Shore D scale (0–100), this is a high score, confirming the material is hard and rigid rather than flexible.

If you see only one scale listed, that is simply because it is the most appropriate test for that material type. Softer or flexible materials are more likely to be tested on the Shore D scale; rigid engineering plastics more commonly use Rockwell.

Summary

Hardness is a measure of a plastic's resistance to surface deformation, and it is a critical factor whenever scratch resistance, wear, or surface appearance matters in your application. The three scales you are most likely to encounter on a datasheet — Rockwell, Shore D, and Vickers — each measure hardness in a slightly different way, but all tell the same story: the higher the number, the harder the material.

If you are unsure which material is right for your project, our team is always happy to help. You can reach us on 01206 638056, email [email protected], or browse our full range and datasheets online.

Next in the Datasheets Demystified series: Impact Strength — what IK ratings and Charpy values really mean, and why toughness matters more than you might think.