The application a rubber product can be used for is determined based upon a number of factors, durometer being one of them. Measuring the durometer of any elastomer, using silicone as an example, can be a tricky task to a non-professional. It is imperative for the end-user to have a crash course of what silicone durometer is and what numbers like silicone 60A mean. A better educated client is one that will make better choices and be a more satisfied client because they have a better probability of returning. Learning how to measure a rubber sheet’s or sheet silicone durometers can help in any application where specific requirements need to be met.
When it comes to off the shelf silicone sheet rubber products, the following silicone hardness are available: silicone 40A, silicone 50A, silicone 60A, and silicone 70A. These are listed from a soft silicone sheet, silicone 40A, to a hard silicone rubber, the silicone 70a. Super soft silicone sheet varieties like 30a or 20a are available but most likely are made to order products. As one measures up the gradient from 40A up, the rubber sheet becomes more dense, heavier, and less easily compressed.
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Measuring Silicone Rubber’s Durometer
“The principle used to measure hardness is based on measuring the resistance force of the penetration of a pin into the test material under a known spring load; the amount of penetration (max. 2.5 mm) is converted into a hardness reading on a scale with up to 100 units” (http://albright1.com/durometer-liquid-silicone/). A super soft silicone is more easily penetrated with the measuring device called the durometer, which will offer a lower reading. Soft silicone is generally used in applications that requires the rubber to absorb some force, which protects the materials it rests on as well as the surface or equipment it comes into contact with. Hard silicone rubber is used when wear is a concern. Hard silicone products are denser, which make them more abrasion resistant, consequently showing less wear over time.
When measuring silicone durometer there are a few notes that should be kept in mind.
- Know your Scale: The scale in which durometer is measured is referred to as a shore scale. Make sure to select the appropriate shore scale. Many different types of materials can be measured for durometer and the types of products being tested will determine which shore scale to use. Most elastomers are measured on a shore “A” scale, so a product that says “silicone 70” means it is a silicone product with a shore “A” scale durometer of 70. A super soft silicone product on the shore “A” scale may have a number as low as 20 or 30. A general rule for measuring durometer is that on any chosen scale readings below 10 or over 90 should be disregarded as they are most likely inaccurate. In these cases it is important to go back and check the material’s recommended scale to ensure that you are using the proper type of scale.
- Consider Tolerances: Keep in mind there are many factors when measuring a silicone durometer. The silicone rubber sheet will have different hardness across the sheet much less across rolls. A silicone 60 shore “A” sheet of silicone may measure 55 to 65 across the roll. There are tolerances that are a function of the production process and cannot be avoided. In addition, the measure device used will have a degree of error which needs to be accounted for. Wider tolerances seem to be apparent in super soft silicone products since the soft silicone’s measurement, like a silicone 40 shore “A”, are more likely to be affected by the subsurface. The subsurface does play a large role in determining the silicone durometer rating. The harder the subsurface the more likely of getting a higher measurement. Most accurate measurements are usually derived from sheet rubber products between 1/8” and 1/4" in thickness.
- Where to Measure: Measure away from the edge of the sample. The durometer measurement should always be taken from a minimum of 0.5’’ away from the edge of a sample. Regardless of the durometer of an elastomer, the measurement tends to change as you move out to the edges. Super soft silicone rubber may get a reading even less than what it actually is if measured in the incorrect location. If the durometer reading is taken in the wrong location this can also mislead the shore scale selected for the product. Always play it safe and measure a sheet silicone durometer away from the edge of a sample.
- Use a thick sample. The thickness of a sample can also throw off a durometer reading. Silicone durometer, as well as with any other elastomer, should be measured on a sample that is at least 0.25’’ thick to ensure a proper and accurate reading. If a thicker sample is not available, then adding an additional layer of the same material beneath the sample being used to measure, will be an acceptable alternative. Ensuring that there is a proper thickness before measuring the durometer will guarantee that the silicone durometer reading is accurate.
- Test the Correct Silicone Sheet: Do not use test products or samples for calibration verification. As with most other types of products, rubber products will experience characteristic change over time. These changes can occur due to many outside factors including temperature, light exposure, and extended use. Regardless of the accuracy of the shore durometer measured when the product is new, this number can and will change as the product ages. An elastomer, such as silicone 40, may not necessarily have that same durometer reading months or years down the road.
At Rubber-Cal, we inventory silicone 40A, silicone 50A, silicone 60A, and silicone 70A. These are all available in a red/orange color and in an assortment of thickness. In silicone 50 and silicone 60 shore “A”, we offer colors such as, translucent, white, gray, black, brown, and even blue. Giving more choices in colors, thickness, and durometer is one way to ensure whether looking for a soft silicone sheet or a silicone 60 shore “A” that we have the proper product for your use.
A silicone durometer can be a difficult process that requires many specifications to be met to ensure an accurate reading. Keeping in mind all of these suggestions when measuring any type of material will help eliminate any discrepancies that would otherwise present themselves. Choosing the proper scale, measuring away from the edge, using an appropriate size sample, and performing calibration verification can greatly reduce the risk of a misleading silicone durometer reading.