Hardness Rockwell M

What is Rockwell M hardness of plastics?
Which factors influence the Rockwell M hardness of materials?
What are the advantages of Rockwell M hardness?
Which test methods measure the Rockwell M hardness of plastics?
What are the Rockwell M hardness values for various plastics?

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:

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:
Spheroconical Diamond Indenter (Source Springer)

Spheroconical Diamond Indenter (Source Springer)

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)

What are the Rockwell M hardness values for various plastics?

Polymer Name	Min Value	Max Value
ABS Flame Retardant	20.00	70.00
ABS High Heat	20.00	50.00
ABS High Impact	10.00	30.00
ABS/PC Blend - Acrylonitrile Butadiene Styrene/Polycarbonate Blend	50.00	70.00
ABS/PC Blend 20% Glass Fiber	50.00	80.00
ABS/PC Flame Retardant	50.00	80.00
Amorphous TPI Blend, Ultra-high heat, Chemical Resistant (Standard Flow)	112.00	112.00
ASA - Acrylonitrile Styrene Acrylate	10.00	20.0
ASA/PC Blend - Acrylonitrile Styrene Acrylate/Polycarbonate Blend	30.00	60.00
ASA/PC Flame Retardant	30.00	70.00
ASA/PVC Blend - Acrylonitrile Styrene Acrylate/Polyvinyl Chloride Blend	30.00	70.00
CA - Cellulose Acetate	1.00	80.00
CAB - Cellulose Acetate Butyrate	1.00	70.00
CP - Cellulose Proprionate	1.00	75.00
CPVC - Chlorinated Polyvinyl Chloride	40.00	65.00
ETFE - Ethylene Tetrafluoroethylene	1.00	10.00
EVA - Ethylene Vinyl Acetate	1.00	1.00
EVOH - Ethylene Vinyl Alcohol	85.00	104.00
FEP - Fluorinated Ethylene Propylene	1.00	1.00
HDPE - High Density Polyethylene	1.00	1.00
HIPS - High Impact Polystyrene	1.00	1.00
HIPS Flame Retardant V0	1.00	1.00
Ionomer (Ethylene-Methyl Acrylate Copolymer)	1.00	1.00
LCP - Liquid Crystal Polymer	40.00	76.0
LCP Carbon Fiber-reinforced	99.00	99.0
LCP Glass Fiber-reinforced	77.00	87.0
LCP Mineral-filled	63.00	79.0
LDPE - Low Density Polyethylene	1.00	1.00
LLDPE - Linear Low Density Polyethylene	1.00	1.00
PA 11 - (Polyamide 11) 30% Glass fiber reinforced	116.00	116.00
PA 11, Rigid	1.00	25.00
PA 12 (Polyamide 12), Glass Filled	107.00	107.00
PA 46 - Polyamide 46	92.00	92.00
PA 6 - Polyamide 6	30.00	80.00
PA 6-10 - Polyamide 6-10	1.00	50.00
PA 66 - Polyamide 6-6	30.00	80.00
PA 66, 30% Glass Fiber	30.00	80.00
PA 66, 30% Mineral filled	30.00	80.00
PA 66, Impact Modified, 15-30% Glass Fiber	66.00	96.00
PA 66, Impact Modified	50.00	94.00
PAI - Polyamide-Imide	105.00	120.00
PAI, 30% Glass Fiber	95.00	110.00
PAI, Low Friction	95.00	110.00
PAN - Polyacrylonitrile	45.00	78.00
PARA (Polyarylamide), 30-60% glass fiber	110.00	110.00
PBT - Polybutylene Terephthalate	70.00	90.00
PBT, 30% Glass Fiber	90.00	95.00
PC (Polycarbonate) 20-40% Glass Fiber	70.00	95.00
PC (Polycarbonate) 20-40% Glass Fiber Flame Retardant	70.00	95.00
PC - Polycarbonate, high heat	70.00	90.00
PCTFE - Polymonochlorotrifluoroethylene	10.00	40
PE - Polyethylene 30% Glass Fiber	1.00	20.00
PEEK - Polyetheretherketone	55.00	100.00
PEEK 30% Carbon Fiber-reinforced	70.00	107.00
PEEK 30% Glass Fiber-reinforced	70.00	103.00
PEI - Polyetherimide	100.00	110.00
PEI, 30% Glass Fiber-reinforced	90.00	125.00
PEI, Mineral Filled	90.00	120.00
PESU - Polyethersulfone	85.00	88.00
PESU 10-30% glass fiber	90.00	99.00
PET - Polyethylene Terephthalate	50.00	100.00
PET, 30% Glass Fiber-reinforced	70.00	95.00
PET, 30/35% Glass Fiber-reinforced, Impact Modified	62.00	62.00
PE-UHMW	1.00	1.00
PFA - Perfluoroalkoxy	1.00	1.00
PI - Polyimide	110.00	110.00
PMMA - Polymethylmethacrylate/Acrylic	70.00	105.00
PMMA (Acrylic) High Heat	90.00	100.00
PMMA (Acrylic) Impact Modified	35.00	80.00
PMP - Polymethylpentene	100.00	100.00
PMP 30% Glass Fiber-reinforced	1.00	20.00
PMP Mineral Filled	1.00	20.00
POM - Polyoxymethylene (Acetal)	75.00	94.00
POM (Acetal) Impact Modified	35.00	79.00
POM (Acetal) Low Friction	58.00	94.00
POM (Acetal) Mineral Filled	83.00	90.00
PP - Polypropylene 10-20% Glass Fiber	20.00	50.00
PP, 10-40% Mineral Filled	1.00	20.00
PP, 10-40% Talc Filled	10.00	45.00
PP, 30-40% Glass Fiber-reinforced	40.00	50.00
PP (Polypropylene) Copolymer	1.00	20.00
PP (Polypropylene) Homopolymer	1.00	30.00
PP, Impact Modified	1.00	1.00
PPA - Polyphthalamide, 33% Glass Fiber-reinforced	124.00	126.00
PPA, 45% Glass Fiber-reinforced	124.00	126.00
PPE - Polyphenylene Ether	50.00	70.00
PPE, 30% Glass Fiber-reinforced	50.00	70.00
PPE, Flame Retardant	50.00	70.00
PPE, Impact Modified	50.00	70.00
PPE, Mineral Filled	60.00	70.00
PPS - Polyphenylene Sulfide	70.00	85.00
PPS, 20-30% Glass fiber	80.00	103.00
PPS, 40% Glass fiber	100.00	104.00
PPS, conductive	70.00	100.00
PPS, Glass fiber & mineral	66.00	102.00
PPSU - Polyphenylene Sulfone	80.00	80.00
PS (Polystyrene) 30% glass fiber	80.00	91.00
PS (Polystyrene) Crystal	60.00	75.00
PS, High Heat	75.00	85.00
PSU - Polysulfone	69.00	91.00
PSU, 30% Glass fiber-reinforced	87.00	100.00
PSU Mineral Filled	70.00	74.00
PTFE - Polytetrafluoroethylene	1.00	1.00
PTFE, 25% Glass Fiber-reinforced	1.00	1.00
PVC (Polyvinyl Chloride), 20% Glass Fiber-reinforced	50.00	70.00
PVC, Plasticized	1.00	1.00
PVC, Plasticized Filled	1.00	1.00
PVC Rigid	1.00	70.00
PVDC - Polyvinylidene Chloride	60.00	65.00
PVDF - Polyvinylidene Fluoride	75.00	75.00
SAN - Styrene Acrylonitrile	10.00	80.00
SAN, 20% Glass Fiber-reinforced	80.00	100.00
SMA - Styrene Maleic Anhydride	30.00	40.00
SMA, 20% Glass Fiber-reinforced	30.00	40.00
SMA, Flame Retardant V0	30.00	40.00
SMMA - Styrene Methyl Methacrylate	65.00	75.00
XLPE - Crosslinked Polyethylene	1.00	20.00

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