How to measure plastic hardness - One of the key reasons to measure plastic hardness is to classify finished products by quality, especially those that go directly into consumers' hands. The hardness of a plastic often signals whether it’s fit for everyday use or likely to fail under pressure.
Manufacturers, therefore, rely on hardness data not just for compliance, but for making clear decisions in material selection and product validation.
In this article, Qualitest walks you through the full scope of plastic hardness testing. We’ll explain what plastic hardness really represents, explore the most widely used test methods, and guide you step-by-step through the measurement process.
You’ll also find a helpful comparison of hardness scales, an overview of trusted testing instruments, and a breakdown of common testing errors. Let’s jump into the explanation below.
What Is Plastic Hardness?
Different classes of plastics respond differently under pressure. Thermoplastics, acrylics, polystyrene, and composites like Formica all exhibit unique hardness behaviors.
In practical terms, plastic hardness refers to the material’s resistance to surface indentation or deformation when force is applied. It’s one of the most straightforward ways to assess how well a plastic will hold up.
For manufacturers, understanding how to measure plastic hardness is often used to control the quality of molded parts, sheets, or components designed for real-world use. Will the surface scratch too easily? Will it deform under pressure?
As a result, hardness becomes a key checkpoint in both material selection and production quality control, especially for products expected to endure repeated contact, handling, or environmental exposure.
Common Methods to Measure Plastic Hardness
When evaluating plastic components, the choice of hardness testing method should be considered by the material’s structure and how it will be used. Here are some of common methods to measure plastic hardness in the industry:
1. Rockwell Hardness Test
In this method, the process begins with a preliminary minor load to seat the indenter, followed by a heavier load. After removing the major force but keeping the preload in place, the depth of the indentation is recorded.
In order to answer how to measure plastic hardness, scales such as Rockwell M, R, and E are commonly applied, depending on the material stiffness.
2. Shore Hardness Test
This method is widely used for softer and mid-range plastics to offer quick and reliable results. It involves a spring-loaded pin or needle that presses into the surface of the plastic.
The depth of penetration correlates directly with the material’s hardness. Shore A is typically used for softer and more elastic materials, while Shore D applies to harder plastics. This one often becomes the go-to choice for quality checks in production lines.
3. Vickers Hardness Test
Vickers testing is less common for plastics but highly precise when fine detail is required, especially in lab environments. It uses a diamond-shaped indenter and measures the diagonal length of the indentation under a microscope.
However, since it works across a wide range of loads, Vickers is more suitable for thin sections or small samples where other tests might fall short. The results of this method are expressed in HV (Hardness Vickers) or DPH (Diamond Pyramid Hardness).
Step-by-Step: How to Measure Plastic Hardness
The right way to measure plastic hardness depends on two things: the type of plastic and how it’s expected to perform. Flexible consumer-grade polymers and high-performance engineering plastics don’t belong in the same test setup.
Below is a practical breakdown of process typically works on the production floor:
1. Start with the Right Test for the Material
How to measure plastic hardness - For softer plastics, like thermoplastic elastomers, flexible PVC, or silicone, Shore A is the standard choice. It gives a quick read on how much the surface yields to pressure, which is useful for consumer goods, seals, or footwear materials.
If you’re working with something stiffer, say ABS, nylon, or filled polypropylene, then Shore D or Rockwell might be more appropriate. Rockwell is particularly useful for semi-rigid or rigid plastics where surface resistance matters, like in gears or appliance housings.
Then there’s Vickers testing, which takes a more detailed look. It's often used in research or high-precision applications, especially for engineering-grade plastics like PEEK or polysulfone. If you’re dealing with thin cross-sections or composite blends that need microscopic inspection, this is the go-to.
2. Prep the Sample Properly
Once you’ve chosen your method, make sure the sample is ready. It needs to be clean, flat, and thick enough to give an accurate reading.
- For Shore testing, aim for a thickness of at least 4 mm.
- Rockwell usually needs about 6 mm, since the deeper indent requires more material to absorb the load.
- Vickers, on the other hand, can handle thinner pieces, but the surface needs to be well-polished—especially if you’re measuring under a microscope.
Also, be mindful of where you test. Stay away from edges, weld lines, or any area that may have residual stress or inconsistent density.
3. Make Sure Your Equipment Is Calibrated
Prior to how to measure plastic hardness, verify that your durometer or hardness tester is properly calibrated. Use reference blocks that match the hardness range you’re expecting.
For instance, if you're measuring a Shore D part, don’t rely on a Shore A block to calibrate your device, it’ll throw off your baseline. Similarly, Rockwell and Vickers testers require specific indenters, loads, and dwell times depending on the material and the scale being used.
Make sure everything is aligned with the right standard (as ASTM D785 for Rockwell, ISO 868 for Shore, or ASTM E384 for Vickers).
4. Perform the Test
Now, carry out the test depending on which method you're using:
- Shore Durometer (Type A or D): Press the device gently but firmly onto the surface, making sure the indenter is fully in contact. Hold it steady and read the value after one second. It’s fast and commonly used in QC checks during production.
- Rockwell (Scales M, R, or E): Secure the sample, apply the preload, then increase to the full load. After holding for a few seconds, release the main load and read the penetration depth while the preload is still applied. This gives you a clear sense of how rigid or dent-resistant the plastic is.
- Vickers (HV or DPH): Place the diamond indenter, apply a precise load, and hold it for a set time. After removing the load, measure the diagonal length of the indentation under a microscope. It’s not as quick as the others, but it’s highly accurate—especially for detailed failure analysis or thin molded parts.
5. Record and Repeat for Reliability
Don’t rely on a single reading. Test multiple points across the part, especially if it was molded or extruded, since hardness can vary with cooling rates and wall thickness.
Then, average the values and make sure to document all test conditions, such as load, time, temperature, and the hardness scale used. These details are crucial when you’re comparing results across batches or troubleshooting production issues.
Plastic Hardness Scales Comparison Table
Shore hardness testing (also known as durometer testing) is particularly practical for assessing plastics and elastomers due to its simplicity and versatility.
Below is a closer look at the different Shore scale types used in how to measure plastic hardness, and how each one aligns with specific material characteristics.
1. Shore A
Though traditionally used for softer rubbers, Shore A is also suitable for flexible plastics like polyacrylics and soft thermosets. You’ll often see this scale used to evaluate soft plastic seals, flexible connectors, or polymer coatings.
Plastics falling in the 20–90 A range will usually feel pliable but still hold their shape.
2. Shore B
This scale handles moderately hard thermoplastic elastomers (TPEs) and fibrous plastic composites. Materials like stiffer grades of TPE used in consumer electronics, or semi-rigid packaging materials, often fall into this range when they exceed the limitations of Shore A but aren’t rigid enough for D scale.
3. Shore C
Shore C is used for medium-hard thermoplastics and thermoplastic elastomers—the kind often found in industrial components or automotive trims. Think of materials like mid-grade polyurethanes or polyethylene blends that are too stiff for Scale B but not quite in the Shore D territory.
4. Shore D
If you’re testing harder plastics and rigid thermoplastics, Shore D is where you’ll land. This includes materials like polycarbonate (PC), glass-filled nylon (PA GF), acrylonitrile butadiene styrene (ABS), and high-impact polystyrene (HIPS).
These are the kinds of plastics used in everything from structural housings and enclosures to gears and mechanical parts. Anything harder than 90 A usually qualifies for Shore D testing.
5. Shore DO
This scale is occasionally used for dense thermoplastic elastomers that are too rigid for Shore C but still not rigid enough for full D-scale application. While not common in everyday plastic testing, you might encounter DO in specialized applications like tightly packed thermoplastic cable jackets.
6. Shore M
Shore M is especially useful when you're working with delicate thermoplastic elastomer parts like micro-molded connectors or flexible circuit components. These materials may deform under heavier durometer pressures, making M a more precise fit.
7. Shore O
Some very soft thermoplastics and low-durometer TPEs—used in cushioning or flexible packaging—fall under Shore O. This scale ensures accurate readings for materials that are too soft to be tested on Shore A or B.
8. Shore OO
While most commonly used for sponges and foams, Shore OO also applies to extremely soft thermoplastics found in specialty medical devices, soft-touch consumer packaging, or prototype skins.
Recommended Equipment for Measuring Plastic Hardness
How to measure plastic hardness - Choosing the right equipment for measuring plastic hardness depends on the material’s characteristics and the level of precision needed. Below are three commonly used instruments, each with its own strengths depending on the application:
1. Rockwell Hardness Testing Machine
If you're working with hard, rigid plastics, especially those reinforced with fillers or glass fibers, the Rockwell Hardness Tester is a solid pick. It uses a steel ball or cone-shaped indenter and applies a specific load to determine resistance to deformation. This machine is especially helpful if you're comparing the hardness of various thermoplastics under standardized conditions.
Best used for: High-hardness plastic parts, automotive components, or structural polymer materials where indentation resistance matters.
Discover Rockwell Hardness Testers match your requirement here.
2. Vickers Hardness Testing Machine
Need ultra-precise readings on small plastic samples or thin films? The Vickers tester might be your go-to. This equipment uses a diamond-shaped indenter and is typically applied under lighter loads, making it great for micro-hardness testing.
Best used for: Lab-based research on polymer composites, coatings, or multi-layered plastic materials.
Explore our range of Vickers Hardness Testers here.
3. Shore Durometer (for Shore A/D Scales)
For softer plastics like rubbery thermoplastics or flexible polyurethane, Shore hardness testing is the most user-friendly method. The handheld durometer presses a calibrated spring-loaded indenter into the surface, giving a quick reading from 0 to 100.
Best used for: Soft-touch plastic parts, gaskets, elastomers, or flexible packaging materials.
Find the right Shore Durometer for your application here.
Common Mistakes to Avoid
- Using the Wrong Hardness Scale: Always match the scale with the material’s expected hardness range. For soft, flexible plastics, go with Shore A; for harder types, consider Shore D or Rockwell.
- Pro tip: If you're unsure on how to measure plastic hardness, run a quick trial on a scrap piece first before committing to a full test.
- Skipping Sample Conditioning: Always allow your samples to stabilize at standard room conditions (typically 23°C and 50% RH) for at least 24 hours before testing.
- Inconsistent Pressure or Dwell Time: In both Shore and Rockwell methods, applying inconsistent force or releasing the load too early is a common mistake. Shore durometers, for instance, require steady, vertical pressure for about 1–3 seconds to get a stable reading.
- Improper Sample Thickness or Surface: Hardness testers rely on good surface contact. As a general rule, the material should be at least 4 or 6 mm thick (or stacked, if needed) and flat.
- Neglecting Calibration: Using a durometer or tester that’s out of calibration can create a domino effect of bad data. Make it a habit to verify calibration with test blocks or certified reference materials before starting your measurements.
Final Thoughts
Whether you're developing flexible packaging or high-impact components, understanding how to measure plastic hardness helps you know how it behaves under pressure.
If you're ready to bring more precision into your materials testing, World of Test offers an impressive range of plastic hardness testers, including Universal Testing Machines, HDT/VICAT Heat Deflection Testers, Melt Flow Indexers, Flexible Packaging Equipment, Color Testing, Sample Prep tools, Impact Testers, and PVC Pipe Testing Equipment.
Explore the full selection on our Hardness Testers, and if you’re not sure where to start, don’t hesitate to contact us for a personalized consultation.
