Plastic materials that are frequently tested for hardness include plastic sheeting and molded parts. Products such as rigid thermo-plastics, Plexiglas, thermo-polystyrene, vinyl sheeting, cellulose acetate, and thermosetting laminates such as Formica are commonly tested for hardness too.

Hardness testing measures a material's resistance to deformation by applying an external force or load to the material. Hardness testing for plastic involves pressing an indenter, which protrudes out of the center of a tester's presser foot, into the plastic material. The indenter is pushed into the plastic perpendicularly with test force until the presser foot makes complete contact with the plastic. How far the indenter penetrates into the plastic under a known force determines the plastic's ability to resist the indentation. A sensor measures the indenter's displacement. The resistance to this penetration can be an indication of the plastic's cure or mixture. Basically, the deeper the indenter travels, the softer the plastic.

Scale testing on a candle pin bowling ball can accurately verify that "force of depth" is meeting N.I.S.T. traceability requirements. Source: The L.S. Starrett Co.

"There are three main hardness tests that are used for plastics: ASTM D785, D2240 and E384/E92," says Ed Tobolski, hardness product manager at Wilson/Shore Instruments (Norwood, MA). "The D785 standard produces Rockwell hardness, D2240 produces Shore-durometer-hardness and E384/E92 can produce either Knoop or Vickers results."

Different countries have different standards for hardness testing of plastic. But, "overall, ASTM is a higher standard which coordinates with higher quality," says Alice Chen, general manager at Timetrade USA Inc. (Upper Saddle River, NJ). "Products from other countries only can be accepted by a lot of U.S. industries or government purchasing organizations if they meet the ASTM standard. It definitely helps to bring the whole industry to a higher standard."

Digital testers typically are more accurate and easier to use than analog units. Source: Wilson/Shore Instruments

The hardness tests

D2240 is a Shore, commonly known as a durometer, scale test standard that is used extensively for testing rubber. The Shore D and C scales are used for testing some grades of plastics. This test measures the penetration of the indenter into the surface of the sample as the result of a spring force. The maximum force for the D and C scales is approximately 4.5 kg.

Rockwell and durometer testers are the most commonly used hardness testers for plastic. "Durometers include inexpensive handheld versions for less than $500, to electronic motorized versions that cost several thousand dollars," says Bob Ellis, quality manager at David L. Ellis Company Inc., (Acton, MA). "Rockwell testers can cost between $1,500 to $18,000 depending on whether it is manual or computerized. The choice between a durometer or Rockwell tester will depend on the hardness scale that is specified. A durometer cannot perform an HRR hardness (ASTM D785) test and a Rockwell cannot perform a Type A scale (ASTM D2240) test."

E384/E92 are the standards for Knoop and Vickers testing. These tests use a diamond pyramid indenter to make an indent using test forces from 1g to 1,000 g for E384, and up to 100 kg for E92 (Vickers only). The indent is then measured optically and the size of the indent determines the Knoop or Vickers hardness. "Some plastics do not lend themselves to these type of tests because the indents can be difficult to see," Tobolski says. "Other types of plastics such as polycarbonate will work very well."

The durometer test method is based on a full scale of 100 points-some Rockwell scales may reflect values of more than 100. "The hardness value is actually a inverse indication of how deep the indenter penetrates the material, where the higher the number the less the amount of penetration in," says Richard Miller, technical salesman at Newage Testing Instruments Inc., (Southampton, PA). "The durometer test method takes a relatively simple approach where the indenter is pressed into the material and a hardness value is achieved while the indenter is under load. The Rockwell test is a bit more involved where the indenter is first pressed into the material at a lighter preload, then the load is increased to full amount, after which the load it reduced back to the preload amount and the a value is produced. Each of the steps in the Rockwell test have time requirements to control the consistency of the test."

Hardness testing for plastic involves pressing an indenter sticking out of the center of a tester's foot plane into plastic. Source: TimeTrade USA Inc.

What makes plastic testing unique?

"If you make a loading-unloading cycle with an indenter into a polymer over a 10-second period and over a 100-second period, the measured hardness may be different," says Dr. Nicholas Randall, vice president of business development at CSM Instruments (Needham, MA). "This is because the hardness is calculated from the applied load divided by the contact area. After 100 seconds the contact area will recover more than after 10 seconds and so the measured hardness will be greater. It is therefore important to develop adequate test standards for testing of polymers."

Plastics have a very high flow rate compared to most metals when they are hardness tested. This material flow, caused by the indenter's force, must be considered when testing plastics. Fortunately, "plastic testers allow for the flow of material," says Bob Ellis, quality manager at David L. Ellis Company Inc., (Acton, MA). "A drawback for a manually operated plastic tester is that test dwell times will vary. This leads to result variation."

For a Rockwell test on metals, the normal dwell time for the total force is from 1 to 3 seconds. Because plastics flow, the dwell times may be extended to 15 seconds to allow most of the flow to occur before the result is determined. The same is true for the Knoop and Vickers tests. Additional dwell times must be considered to get repeatable results.

"The Shore tests are not affected as much since the forces are low," Tobolski says. "The Knoop and Vickers tests also have to consider the problem of operators seeing the indent. Sometimes the indent will recover to the point that it is not visible and sometimes it is just difficult to see the indent due to the color or translucent nature of the material. There may not be a discernible indent to measure."

Even the terminology regarding plastic hardness testing is unique. "It seems kind of odd to me that in the metals testing area we uses terms like elastic, plastic and permanent deformation," says Walt Wardzala, major instruments product manager at Mitutoyo America Corp., (Aurora, IL). "In plastics hardness testing, these terms are not generally used. Because of flow and creep, the timing cycle of the plastic test is much more critical and so is temperature."

Hardness testing measures a material's resistance to deformation by applying an external force or load to it. Source: Mitutoyo America Corp.

Analyzing the testers

There are both analog and digital hardness testers. Digital testers have been on the market for several years and are generally more accurate and easier to use than analog units. Analog testers leave a certain degree of interpretation to the operator when arriving at a test result. Analog test results are gone after the indenter is taken away from the material. Digital testers can display and store results and statistics, providing a greater degree of accuracy over analog units that have to be observed by the operator.

There are both dead weights and closed-loop testers. "Rockwell, Vickers and Knoop testers are now available with closed-loop test force control," Tobolski says. "These types of control systems are better than dead weight systems especially when trying to test materials like plastics that have high flow characteristics."

There are different hardness testers for rigid and nonrigid plastics. "Softer, nonrigid plastics have to have lighter loads and larger indenters to resist penetrating right through the material," says Scott Robinson, tech support at The L.S. Starrett Co., (Athol, MA). "The harder, rigid plastics like acrylics need more force and a smaller penetrator because of the possibility of fracturing the material."

High-end testers can measure more than just hardness. "They are also capable of measuring elastic modulus, scratch resistance, viscoelasticity, stress, creep, fracture toughness, impact resistance and wear," Randall says.

This plastic test block was calibrated using the Rockwell "R" scale-60-kg major load/1/2-inch Carbide ball indenter-with the major and minor loads being applied for 15 seconds each in accordance with ASTM D-785 specifications. Source: United Testing Systems

Plastic hardness testing's importance

However, hardness testing of nonmetallics is an area that is still overlooked and undervaluated by many. This may be because the inherent performance characteristics of most plastic materials exceed the performance expectations of their respective applications. "Many companies may ask themselves, ‘Why bother spending the extra time and money to hardness test a product that I believe will work just fine?'" Miller says. "I can only say that times are changing and we as a manufacturing society need to be more forward thinking to avoid learning critical and costly lessons.

"For example, how many people cared about doing hardness tests on bolts and fasteners until it was learned that their failures played critical roles in several airline mishaps. In the arena of nonmetallics, more companies started considering the value it of and actually began testing parts such as O-rings after the Challenger disaster. Should it take an extreme wake-up call of this type to direct our attentions toward the importance of hardness testing? Hopefully not." Q

• CSM Instruments,
  www.csm-instruments.com;
• David L. Ellis Company Inc.,
  www.hardness-testblocks.com;
• The L.S. Starrett Co.,
  www.starrett.com;
• Mitutoyo America Corp.,
  www.mitutoyo.com;
• Newage Testing Instruments Inc.,
  www.hardnesstesters.com;
• Timetrade USA Inc.,
  www.timetradeusa.com;
• United Testing Systems,
  www.tensiletest.com; and
• Wilson/Shore Instruments,
  www.wilsoninstruments.com and
  www.shoreinstruments.com.

Tech tips

1. Make sure the indenter is perpendicular to the sample surface.

2. Press the indenter gentle and steady into the sample surface.

3. Make sure the tester's presser foot makes thorough contact with sample surface.

4. Take the result after a specific dwell time that takes into consideration the flow of the material.