Tips on Using Environmental Chambers & Furnaces

Does your system incorporate "specimen protect" or "load protect"? If you are not sure, ask us.

If it does you can use this feature to compensate for load string and specimen expansion/contraction while your furnace or chamber reaches set-point temperature.

Having inserted the specimen into the grips, set the desired load threshold value and enable the feature before switching on the furnace or chamber. As the load string expands or contacts, the crosshead or actuator will move to keep the force on the specimen below the threshold value.

• To prevent loss of heat around the pull rods and push rods, always use any convection shields provided or lightly pack the space between the pull rod and the furnace or chamber port with a suitable refractory fiber.
• Make sure you use the recommended anti-seize compounds and lubricants on your grips to help aid specimen removal.
• Use the smallest grips possible to reduce heat-up times and to maximize the available travel in an environmental chamber.
• After you have finished testing at below ambient temperatures, you can remove any condensation from the grips and pull rods by warming them in the chamber and allowing them to cool naturally.

Interested in more tips on tensile testing in high temperatures? Download our complimentary High-Temperature Guide. Read more

What is the Bagley Correction and Why do I Need to Perform One?

When testing using a capillary rheometer, it is desirable to know the pressure drop along the capillary. Realistically, it is not possible to test for this inside the die itself, and pressure transducers are typically located just above the die. While this makes the machine design much simpler and more budget-friendly, it does introduce a very particular kind of error that affects readings of pressure drop, shear stress, and viscosity.

When the melt is pushed through the die, there are two drops in pressure that it experiences. First of all, the pressure drop due to the ‘entrance’ effects experienced at and around the die-mouth. This is related to the shape and size of the die and the compaction of the melt from a large to a small geometry. Additionally, there is a second pressure drop experienced between the start and end of the capillary. It is this second quantity that we are really interested in, and the separation of the two quantities is the product of the Bagley correction. The Bagley correction is unique for each configuration and set of parameters; it is only changed in die length that does not require a new correction to be performed.

Performing a Bagley correction might seem daunting, but it’s actually rather simple involving two basic rheology tests. Each test should be set up with identical parameters (a simulation of the anticipated testing conditions) and dies of the same geometry, but differing lengths. The analysis then consists of a mere pinch of high school algebra. Essentially, the two tests (ideally performed simultaneously on a dual bore machine) produce data that can be expressed as simultaneous equations:

E + A = x   (1)

E + B = y   (2)

Where E is the pressure drop due to entrance effects; A and B the drop in pressure in each barrel due to the lengths of the dies; x and y the pressure drop recorded by the transducers in each barrel. 

Then:

B = cA   (3)

Because:
1. The ratio of the length of one die to the other is known (c).
2.  The drop in pressure across the die is proportional to the shape, size and length of the die.
3.  The dies have the same shape and size.
4.  Thus the ratio of the die lengths is also the ratio of the pressure drops between the two dies.

Then taking equation 2 from equation 1:

A – B = x – y   (4)

A – cA = x – y   (5)

As c, x and y are known, you can now work back through the equations and calculate A, B and E. Software can be used to automatically perform the necessary calculations and execute a Bagley correction on all data collected following the procedure.

Having said this, if the data is purely for Quality Control purposes, then you won't need to perform the Bagley correction. As long as each test is executed using strictly the same equipment in the same configuration, then the results are comparable. 

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Plastics Testing Challenge 5: Testing in Non-Ambient Conditions

Most plastic products are expected to be used in various environments and conditions, some of which include cold and hot temperatures. Mechanical properties of plastics are influenced by temperature which makes it very important for designers to consider the plastic’s performance over the temperature range a given product is expected to perform in. While environmental chambers are available to simulate both cold and hot temperatures, it is challenging to identify the appropriate extensometers and fixtures to use at these extreme temperatures.

Solution: The majority of our basic standard clip-on style extensometers are designed to work within a temperature range of -100 to 200° C (-148 to 392° F), which covers a vast range of the testing needs. However, there are some applications that have higher temperature requirements, and for these applications, we recommend our high-temperature clip-on extensometers, which can be used in temperatures as high as 540° C (1004° F).

Tip: Video extensometers, such as the AVE, can also satisfy requirements for a variety of hot and cold applications. The AVE can mount to the outside door of most Instron® environmental chambers, allowing it to measure strain through the viewing window while not subjecting it to the various temperature extremes. Read more

4 Things to Consider When Testing at High Temperatures

Whether you’re testing the ceramics used in wind turbine blades for power generation or the metal used in a jet engine, high-temperature tests present some unique challenges and require specialized testing equipment and accessories. 

Do I need a Chamber or a Furnace?

• Temperature chambers are used up to 600o C (1112o F) – this is when the wavelength of the   electromagnetic radiation heat energy reaches the visible spectrum.
• Split-furnaces are used for testing between 400o C and 1200o C – this is when radiative heat transfer becomes greater then convective. Models are also available that apply heat (up to 1600°C) only to the specimen, and not the grips.

Do I need an Extensometer and What Kind?

• Various types of extensometers can be attached to the specimen before or after heat soaking.
• Water cooled extensometers are available if your test requires accurate strain measurement at very high temperatures.

How can I Improve the Alignment of my Load String?

• Alignment of the load string is very important and easily achieved with self-aligning quick-change adapters. These adapters allow quick and efficient removal of the tested specimen and pullrods so a new specimen can be promptly installed and tested.

Does Specimen Type Matter?

• Round specimens with threaded ends or flat, pin-loaded specimens are ideal for high temperatures.
• High-temperature lubricant is necessary on all threaded connections.

To learn more, read A Guide to High-Temperature Tensile Testing
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Common Sources of Error When Testing at High Temperatures

Testing at high temperatures can prove to be erroneous if you begin testing before things have stabilized. When you first turn on your chamber, the load string and temperature are not stabile. A transient temperature will cause your signal to drift and show a load when there isn’t one. Just like preheating your oven before cooking, you need to allow your load string to warm up and stabilize, as well as reaching your desired temperature before placing your specimen within the grips – this will give you accurate data.

Side tip: We have found that HVAC systems or other types of vents can impact your temperature testing. Make sure to move your system away from vents before testing at high temperatures. Read more