Hardness Rockwell M

What is Rockwell M hardness of plastics?

Rockwell hardness test is one possible method to measure the hardness of plastics. This test method is named after Stanley P. Rockwell, who invented it in 1914. It is generally used for hard materials, like:

• polyamide (PA),
• polycarbonate (PC),
• polystyrene (PS), or
• polyoxyethylene (POM).

Here, the resilience or creep of the polymer is less likely to affect results. For rubbers, elastomers, and softer plastics, shore hardness is often preferred.

Principle of Rockwell hardness scale

The Rockwell scale determines the hardness by measuring the depth of penetration of an indenter. This happens under a large load compared to the indentation made by a pre-load. Different scales use varied loads or indenters such as:

• diamond tips,
• steel, or
• tungsten carbide balls.

The result is a dimensionless number. There are different scales that are denoted by a single letter. The three most common scales used for testing the hardness of plastics are Rockwell E, Rockwell M, and Rockwell R.

Which factors influence the Rockwell M hardness of materials?

As the method is extended from metals to plastics, one must be careful to consider:

• • Type of plastics
    Different plastics have varied hardness levels. For example, PA is generally harder than polyethylene (PE). Thus, a PA sample will have a higher Rockwell M hardness than a PE sample of the same thickness.

• • Thickness of the material
    The Rockwell M hardness test is not valid for plastics that are less than 0.05 inch (1.27 mm) thick. This is because the indenter can penetrate too deeply into thin materials, giving a falsely high hardness reading.

• • Temperature
    Small changes in temperature can result in first and second-order phase transitions. This can alter the hardness of the material.

• • Viscoelasticity
    Unlike metals, plastics are viscoelastic. Stress-strain relationships are time-dependent. The results obtained by dragging the weighted indenter will be rate-dependent.
• Surface finish
  The surface finish of the plastics can affect the Rockwell M hardness reading. A rough surface can cause the indenter to dig in more deeply, giving a falsely high hardness reading.

 

What are the advantages of Rockwell M hardness?

 

• The Rockwell M hardness test is used to measure the hardness of a wide range of materials. For example, metals, plastics, ceramics, and composite materials (such as fiberglass and carbon fiber).
• It ensures the meeting of the required quality standards of the materials.
• It helps to monitor the progress of a manufacturing process and ensure that the desired hardness is being achieved.
• It can be used to identify the cause of a failure in a material.
• It enables to study the effects of different factors on the hardness of a material. For example, a researcher may use the Rockwell M hardness test to study the effects of heat treatment on the hardness of acrylonitrile butadiene styrene (ABS) plastics.

 

Which test methods measure the Rockwell M hardness of plastics?

• • ASTM D785 — It determines the Rockwell hardness of plastics and electrical insulating materials.
• ISO2039-2 — It determines the hardness of plastics.
  • Part 2: Determines the Rockwell hardness.

 

Rockwell Hardness Test Method (Source: MaterialsScience2000)