COMMON MISTAKES LEADING TO NONCOMPLIANCE (ISO 527 & ASTM D638)
We are finding that more and more lab personnel are not following standards correctly. The majority of the time it is due to either a misinterpretation of the standard or from personnel not being aware of changes that may have occurred to that standard. We have witnessed firsthand some of the resulting implications, including:
- Failed audits
- Lost time
- Production delays
This article will address some of the most common causes of non-compliance, particularly, those associated with ISO 527-2.
Many of the labs that struggle with ISO 527-2 also test to ASTM D638 and this is not a coincidence. These standards appear to be quite similar, but there are subtle yet distinct differences that make them not technically equivalent. Failure to recognize and understand these differences are likely the source of much of the confusion. The image below highlights the 5 most common mistakes made in ISO 527.
The first 4 of the 5 common mistakes also apply to ASTM D638. The fifth common mistake specific to ASTM D638, which is similar to #3 above, relates to the use of an inappropriate specimen measuring device. Unlike ISO 527-2, flat tip anvils are required but yet, many labs use spherically shaped micrometers. This could be due to habit, particularly for those used to testing to ISO 527.
ISO 527-1 / ISO 527-2 2012 Changes
In 2012, several important changes were made to these standards, four of which resulted in further differentiating these standards. In other words, requirements that were once common among both standards now differ.
Change 1: The Definition of Modulus
The previous versions of this standard referred specifically to Young’s modulus, but in 2012, “Young’s” was replaced with “Tensile Modulus of Elasticity (Chord or least square fit)”. It’s important to understand the differences between the various types of modulus calculations and how they are calculated. In the case of Bluehill® Software, the standard “Young’s” calculation is no longer acceptable; the correct choice would be either a Chord or Segment modulus.
Change 2: The Definition of Tensile Strength, Particularly for Materials with Yield Points
Similar to ASTM D638, tensile strength was defined in earlier versions of ISO 527 as the maximum stress during the test. However, in 2012, the description changed to “stress at the first local maximum”. Since the majority of plastics have yield points or inhomogeneous strain distribution, this is a critical change that will significantly impact strength results. This change has no impact on plastics that do not exhibit yield points.
Change 3: A New Alternate, and Preferred, Method for Determining Nominal Strain
In this case, the previous method or definition of nominal strain was not replaced; a new method was introduced and identified as the preferred method. The original method, Method A, is synonymous with ASTM D638, whereas the new (preferred) method, Method B, is quite different. It is important to understand the differences since each method is likely to yield different results.
Method A (~ ASTM D638): Nominal strain is calculated using crosshead displacement.
Method B: Nominal strain is calculated using an extensometer up until the yield point and then it is based off crosshead displacement. The two strains are added together.
Similar to “Change 2”, this change applies only to materials that exhibit yield points, which is the case for many plastics.
Change 4: Speed of Test
In the latest version, the restriction of only allowing one speed in a given test was lifted, making it possible to measure both modulus and strain at break in one test. It is important to note that ASTM D638 is still limited to only one speed in a given test.
Change 5: Gauge Length of Multipurpose Specimens
The gauge length of the multipurpose specimens changed from 50 mm to 75 mm. Prior to this, both ASTM D638 and ISO 527-2 utilized 50 mm gauge lengths for the preferred specimen types. Again, this is another example of once similar standards becoming much different. It is important to note that although the requirement has changed, due to historical references, 50 mm is still acceptable for QC labs only.
In regards to compliance, one of the most challenging requirements of ISO 527-2 is the accuracy requirement of the extensometer when measuring modulus. This difficulty is one of the reasons why the gauge length changed in the first place because with the larger gauge length, the requirement lessens, making it easier to achieve.
In the next issue of Plastics News, we will address the changes and difficulties associated with ISO 178.
Improving Efficiency: Get Results in Less Time
In the testing world, higher efficiency and throughput are often the two areas where lab managers look for improvements. Faster or more efficient testing often translates to lower testing expenses, but can also prevent or minimize significant costs associated with producing out of spec or “bad” product. When testing machines are used to support production lines, the results obtained are often fed back into the process, equipping line managers with the information needed to make important decisions regarding the production line. These may include adjustments of various parameters or immediate shutdown of the line in the event that bad product is being produced. The sooner they are able to respond, the less “bad product” is produced, saving both time and money. There are multiple ways to improve efficiency, including procedural changes, minor product changes, as well as automating processes or the entire process.
From many lab visits, surveys, and conversations, I feel confident in saying that one of the most inefficient components of testing relates to specimen measurement. It is tedious, time consuming, and can be very error prone, particularly when companies are not utilizing or realizing capabilities they already have. The majority of labs today rely on a typical micrometer or Vernier caliper, a calculator, and a notebook. Measurements are taken, written down (or entered in a spreadsheet), averaged, and then manually inputted into the software. One specimen involves 6 individual measurements, which is equivalent to 42 key strokes (420 if you test 10 specimens).
What many may not realize is that some micrometers can be integrated into the testing system, allowing measurements to automatically be averaged and inputted into the software. The capability is standard in all Bluehill packages.
When using an integrated micrometer, which is really just a micrometer connected to your PC, the 42 key strokes mentioned above become 7, and the 420 key strokes become 70. That is 6 times less key strokes or opportunities to incorrectly enter a value. Time wise, you can expect to save about one minute/test, which could reduce your overall cycle time by 50%, depending on the length of the test.
To demonstrate this point, we conducted a side-by-side comparison of two systems testing the same material to the same standard but with different setups. The video demonstrates how a system equipped with an integrated micrometer, as well as other higher efficient components, compares to a basic set up. From this video, you will be able to get a sense for potential areas for improvement, as well as the expected time savings.
How many times do you enter the same information into your test reports, whether it’s an operator name, a date and time, sample conditioning information, or material type?
Every step or keystroke that you take requires time and creates opportunities for typos and errors. For this reason, eliminating steps wherever possible is advantageous. If you use Bluehill software, many of these inputs can be automatically included in your report and the others can be included as “choice inputs”, which effectively creates pull-down menus, to replace fields where you would normally and repetitively type in the same type of information. Choice inputs and fields that automatically populate saves time, reduces errors, and makes data easier to sort post-test.
Where else can we save time? Think about:
- How much time it takes to open and close your grips (if using manual grips)
- The amount of time it takes to align your specimen in the grips
- The care and time required to attach a manual extensometer onto your specimen
- How long you wait by the machine before you can remove your extensometer and continue your test
Different grip and accessory options (pneumatic vs. manual), as well as the extensometer type (manual vs. automatic), can reduce your overall cycle time by 50%. The case study and video referenced above, demonstrates all of these points.
Bridging the Gap Between Industry Process and Material Rheology
Have you ever thought about the implications of a mold not being properly filled?
Plastics are used in many different ways and how they perform and appear are important to both consumers and producers. To ensure that the performance and the aesthetics are optimum, producers must be able to select the right materials, build the appropriate tooling, and understand how to process them. To do this effectively, it is necessary to understand the true rheological behavior of the material.
Thermoplastic materials pass through a forming process and are, therefore, processed as fluids by means of molding, extrusion, and blowing techniques. Their flow properties during these processes are complex and affected by many parameters. Rheological measurements are essential when investigating or monitoring the conversion of thermoplastics from raw material to finished parts.
Setting correct dimensions and tolerances are critical when designing products or tooling. How confident are you that the material you select will perform as expected?
Rheological relationships help us to understand the properties of plastics so that we can either know how they are behaving or how we can force them to behave. The interrelation between rheology and the product often makes the measurement of a material’s rheology the most sensitive or convenient way of detecting changes in color, density, stability, effect of fillers, and molecular weight.
Do you struggle in getting rheological measurements?
Melt Flow Rate (MFR) and Melt Volume Rate (MVR) are important properties that allow us to verify the incoming material and also help us select the appropriate material suppliers. However, sometimes this type of test does not provide sufficient information. When companies encounter processing issues, they realize the need for a different testing technique: rheological testing.
Rheological tests can reproduce processing conditions with various combinations of deformation speeds and temperatures. In an extruder, a constant temperature is maintained, but the speed changes due to the different rotational screw speed along the barrel. In injection molding, both the temperature and speed change along the barrel length, since the screw acts as a piston by quickly injecting the material into the mold. By performing a rheological test it is possible to set a temperature and measure the viscosity at different speed of deformation.
In R&D, capillary rheometers have the capability to provide additional information about molten polymer behavior. This is possible due to the availability of optional accessories and software features that enhance the testing capabilities of the rheometer.Read the Full Article in Quality Magazine
Q&A: Melt Flow Testing
Q: My general manager is discussing potential changes to our plastic materials to increase our productivity. However, after some test runs with my melt flow tester, in accordance to ISO 1133-1, I am facing some issues because my new material shows a high rheological sensitivity to the time-temperature. What do you recommend?
A: In 2011, ISO 1133-2 was released to provide the appropriate test method for high thermo-sensitive materials; the main difference is the temperature tolerance requirements. ISO 1133-2 specifies tighter tolerances on temperature than ISO 1133-1. It reports a tolerance of temperature of ± 0.3°C along the whole barrel in the complete range of temperature, resulting in more reproducible and accurate measurements.
Q: Why am I getting inconsistent results on melt flow tests performed on the same sample, at different times?
A: One of the greatest perils of melt flow testing using ASTM D1238 - Method “A” is the human/operator variability. The operator has to watch the test being performed and precisely time every second of it, which is not always possible. This variability can be eliminated by switching to ASTM D1238 Method “B”, where a digital encoder is used to accurately measure the movement of the piston and report repeatable Melt Volume Rate (MVR) and Melt Flow rate (MFR) for your sample. However, apart from operator error – variation in material preparation, system cleanliness, and preheating times could also be factors causing inconsistent test results.
Q: I have been using my melt flow system for several years now. My equipment is sturdy, fool-proof, and performs its function satisfactorily. How often is calibration and service required on these machines?
A: Generally, melt flows are most commonly used for QA/QC testing, where it is extremely important to have the equipment function accurately. While a melt flow system is fairly basic, and all Instron equipment is designed for rigorous use in a QC environment, it is important to remember that recent revisions to ASTM D1238 and ISO 1133 standards prescribe a very specific temperature tolerance of ±0.3° C. We recommend annual temperature calibrations on your melt flow equipment, as well as annual service to verify the piston, barrel and die dimensions. If you visually notice any chipping and deformation on your die or piston – please replace these parts immediately, as this could greatly affect your test results.
- Plastics Testing: Increasing Laboratory Efficiency & Throughput
- Fundamentals of Melt Flow Testing
- Understanding Changes to Key Plastics Standards
- Reducing Variability in Test Results