Tips & Tricks for Packaging Testing

Explore best practices to better provide quantitative information about tear resistance, puncture resistance, peel strength, heat seal strength, and durability of materials used in flexible and rigid packaging, and finished packaging products.

Tips & Tricks for Packaging Testing from Instron

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Helping to Standardize High-Rate Testing of Composites

Instron has joined a new international group that is seeking to develop a best practice guide and test standards specifically for testing composites at high-strain rates.

As the automotive industry seeks ever-more-urgently to embrace composites, there is an increasing demand for testing composite material behavior at high-strain rates. The need for detailed data to inform crash simulation models first drove a renewed demand for equipment over the last 3 years, and now there is a need for international standardization in methodologies and data handling. The group’s aim is to facilitate generation and exchange of reliable and comparable test data in this highly challenging area.

The working group has been coordinated by the University of Dayton Research Institute, and currently composes about 20 organizations including major automotive manufacturers, composite materials producers, test houses, and research institutes. As a world leader in high-rate servohydraulic testing systems, the dynamic systems team at Instron are very pleased to share their expertise with this initiative that will make a tangible difference to the industry. Similarly, Instron CEAST will be contributing to work on drop-weight based techniques for high rate testing.

The working group is looking for more European contributors especially, but we would strongly encourage all our customers with expertise in this area to join us in supporting the project. Please feel free to contact Instron applications specialist, Dr. Peter Bailey, if you would like to know more.

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A Case for Extensometry

A universal testing system very simply measures 2 things during a basic mechanical test: force (via the load cell) and displacement (via the crosshead encoder). To obtain a basic stress-strain curve, you might think that’s all you need. With the force measurement from the load cell, the cross-sectional area of the material can be used to calculate stress; and with the crosshead extension, the original distance between the grips or fixtures can be used to calculate strain throughout the test. How simple!

It may be simple, but it’s not the best option for all material tests. Even when using the proper equipment for your test – machine, load cell, grips, fixtures, etc. – the system is compliant, i.e., it bends and stretches a little bit when you’re running a test! This means that what the crosshead encoder is reading and sending to the software may not truly represent the distance your specimen has travelled. But don’t panic just yet! This is the fundamental reason why we use extensometry in materials tests. An extensometer measures strain directly at the specimen – only taking into account the strain directly at the material, and not anywhere else in the system (the crosshead, the grips, the load cell, the couplings, etc.).

International testing standards will specify if extensometery is required for your testing – so have a look through the standards you follow and make sure you’re using the proper device for your tests. Aren't following a standard? Here are some recommended cases where you should use extensometry:

• Stiff materials (composites, metals, plastics)
• Quality control environments 
• Comparing different materials
• Comparing the same material on different machines
• When you want truly accurate strain data!

Note that some extensometers work best for certain materials and situations. If you’re not sure which you should use for your test, we’ll be happy to help you figure it out.

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Challenges of Rigorous Demands

The world of materials testing is changing …

• materials are getting stronger, stiffer, and lighter 
• test standards are becoming stricter 
• testing labs are asked to perform more complex analytical tests

With all of these changes affecting the way labs test, it’s important to think about the following questions: How does your lab environment challenge test results? Is your lab equipped to handle the new strength of specimens? Are you testing under load or position control parameters, or do you require the use of strain control?

These questions, when coupled with the changes above, are all factors we have discussed with our customers and are sharing with you.

Challenge #1

The automotive and aerospace industries are growing and demanding that materials become stiffer, stronger, and lighter. This new breed of materials is helping the industry produce lighter and stronger products, but lab operators are finding that the large energy release may damage contacting extensometers. To reduce the cost of maintenance for labs testing these challenging materials, video extensometers are an ideal  solution – they don’t contact the material, and therefore, they are unaffected by the high-energy breaks of materials, such as carbon fiber composites or rebar.

Challenge #2

With the growing demands come newer standards that place stricter requirements on the type of extensometer you are allowed to use. For example, ISO 527-2012 now requires that devices have an accuracy of just 1 micron in order to measure the modulus of the material. 1 micron is exceptionally small (about 100 times smaller than the width of the average human hair) and is exceedingly difficult to measure. Although this accuracy can be attained with some traditional clip-on extensometers, clip-ons have limited travel and typically can’t measure strain through failure for ductile specimens. In addition, they may cause premature failure due to stress risers from knife edge contact.

Until recently, there hasn't been a video device that could meet the new standard requirement due to problems with lighting and air flow that naturally occurs within a lab, which ultimately affects the device’s ability to provide accurate results.

To prevent the lab lighting from making a difference, we have incorporated a patented lighting system into our new video extensometer that floods the specimen with polarized light and use a polarized filter on the lens. This enables the video extensometer to produce consistent results no matter the lighting conditions, including flicker from fluorescent lights or the difference in lighting from a lab window.

Secondly, all video systems are affected by air flows in the room from sources like air ducts and heat sources. These air flows are similar to what you may see on asphalt on a hot day. The air currents and heat sources in your lab are probably much smaller, but since 1 micron is so small even minor perturbations make the measurement impossible. Our engineers incorporated a patented system of fans into the video extensometer to prevent these air flows between the specimen and the camera, enabling the test to yield accurate results whether your air system is off or running at full speed.

Challenge #3

New standards are also placing requirements on the way that tests are being controlled. In the past, most – if not all – tests were run under load or position control parameters, but newer standards may prefer or require the use of strain control. This can be done with contacting extensometers, while pre-existing video extensometers struggle to run strain control tests because the images taken have to be processed by the computer and then sent to the software to adjust the frame movement accordingly. There is a large delay between the images being taken and the processing by the computer, which means that by the time the frame reacts to the strain measurement the value has changed.


To resolve this, the new video extensometer now measures strain at the camera in real time and then sends the data directly to your test frame. This drastically reduces the response time of the camera and allows for clean strain control.

Challenge #4

Extensometers are excellent for materials testing, but only give you a point-to-point measurement. Other measurement techniques, such as digital image correlation, can give you more information about material behavior during a mechanical test, but they typically require expensive and complicated equipment and software. Because of the high barrier to entry, only a few labs take advantage of this technology and are able to make key insights about their material behavior that other labs can’t.

Instron has lowered the barrier to entry for digital image correlation by offering our video extensometers along with dedicated digital image correlation software. The extensometer images are automatically synchronized in time to the data from the load frame, which means that you can now run a test and begin producing full field strain maps in less than one minute. Read more

Question From A Customer: Air Bubbles In Extrudate

Q: We have an MF30 Melt Flow Indexer and started running tests on various polymers in our lab. Some of the samples have a lot of air bubbles in them. I believe this is contributing to inconsistencies in melt flow values. How do we minimize this?

A: There are a lot of reasons you could be seeing air bubbles in the filament sample. Ultimately, it comes down to keeping the testing and cleaning processes as consistent as possible. To start, keep the following points in mind:

1. The form of test sample can affect how the material melts within the barrel. Pellets or granules are the most consistent. Flakes, bits of component, and powders are not as easily compounded and may leave small air pockets, resulting in air bubbles in the melted sample.


2. It is necessary that the sample be compacted with a suitable and repeatable pressure. For some materials. hand-pressure is fine, while a higher force may be necessary for others. Too little or too much pressure can allow the sample to swell or trap air bubbles. Keep in mind that some materials are more sensitive to operator-to-operator inconsistencies than others.


3. Preconditioning is an important part of preparing the sample for melt flow testing, especially for hygroscopic materials, such as PET or PA. Excess water gets trapped inside the material and generates gases when heated, which often triggers degradation processes within the sample. Degradation can also be promoted by residuals of sample from previous tests, when the barrel is not cleaned properly.

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