How to choose a universal plastic testing machine

Universal testing machines stretch, bend, compress or pull plastic specimens at controlled speeds until they break. They are the most common instruments in the labs of plastic compounders. These companies use universal testing machines during the compound development process to determine the suitability of materials for a process and end use. Universal testing machines are also handheld for production quality control to ensure consistency between batches.

Universal testing machines are also increasingly common in the labs of plastic injection molders and extruders today. One reason is that they are increasingly used in cutting-edge product and process development. Another reason is that they provide more stringent monitoring of incoming material and finished product quality control.

Many OEMs, especially those in the medical device or automotive fields, require plastic processors to perform their own testing at the end of a production run. Another reason for in-house testing is improved process control, which can reduce scrap rates and achieve real returns.

Various types of testing

Universal testing machines consist of one or two vertical load-bearing columns mounted on a fixed horizontal baseplate with a movable horizontal crossbar on top. In today's universal testing machines, the columns are usually driven by thread rollers to determine the position of the movable crossbar.

The specifications of a universal testing machine are expressed by the maximum load of the frame combined with the maximum load of the load cell. The load cell is mounted on a moving crosshead that is either motor-driven or hydraulically driven. The load cell, which is connected to the grips, measures the force, which can be read from a digital display or computer. Many universal testing machines have interchangeable load cells so that the sensor can be matched to the material being tested.

To induce strain in the plastic, the universal testing machine applies force to the specimen. Specific tests in tension, bending, compression, or shear are classified according to the direction of the induced strain in the specimen and the speed at which the force is applied. Electrostatic testing is done by standard electromechanical universal testing machines. They are usually loaded at speeds ranging from 0.001 in./min to 20 in./min. Dynamic or cyclic tests such as crack growth and fatigue are generally done on servo-hydraulic universal testing machines for longer periods of time and with lower loads.

Early universal testing machines had electronic components and chart recorders. They have now been replaced by digital controllers and PC software. New controllers automatically run the test and display the data, sometimes even while the test is ongoing. In the old days of chart recorders, and even in the early digital age before computers, all the user got from a test was a load/deformation curve with force on the Y axis and deformation on the X axis. These curves required calculation and annotation. The latest systems still provide these curves, but they also calculate the required data such as yield and failure strength and modulus.

The most common tests for plastics performed by general purpose testing machines to date are tensile strength and modulus, and flexural strength and modulus. For the tensile test specified in ASTM D638 and ISO 527, the specimen is clamped at both ends. One clamp is fixed and the other is in a crosshead that moves away from the fixed clamp, pulling the specimen until it breaks, whereupon the crosshead automatically stops.

The flexure test (ASTM D790, D6272, and ISO178) is performed by placing the specimen on two supports on the fixed base of the testing machine. For this test, the crosshead moves in the opposite direction to that of the tensile test, pushing rather than pulling the unsupported center of the specimen until it bends and possibly breaks. Because many thermoplastics do not break in this test, it is possible to not calculate the ultimate flexural strength. Instead, the standard test method requires calculation of the bending stress at a strain of 5%f.

Although universal testing machines play an important role in testing rigid plastic foams according to ASTM D1621 and ISO844, they are not commonly used for compression testing. Universal testing machines can also be used for crushing tests of any type of molded product, such as bottles.

According to universal testing machine suppliers, plastics are rarely tested in shear. Shear strength is obtained by placing the specimen in a shear device like a punch. At a speed of 0.005in./min, the punch is pushed down until the moving part of the specimen clears the fixed part. Shear strength is important for film and sheet products, which will break due to this type of load, but it is rarely a consideration in the design of other extruded and molded products. According to ASTM D732, plastic sheets or injection molded discs 0.005-0.500in. thick are used in this test.

It is important that the specimen is held in the bottom of the machine because different types of tests require different fixtures. There are hundreds of fixtures with different mechanical styles. They can range in price from a few hundred dollars to $4000-5000 for hydraulically operated grips.

For tensile testing of plastics, the most common grip is a wedge-type, self-tightening type. It self-tightens as the load is increased. For flexure testing, the most common is a three-point bend fixture. Disc-shaped platens are used for compression testing. For shear testing, the grips are usually made by the user or custom-made by a general testing machine supplier.

Electromechanical general testing machines

Common electromechanical units have load capacities from 100 to 135,000 pounds. The larger the size, the higher the cost. Whether single-post or dual-post, the most common machine for testing plastics is a vertical bench-top unit. The same principles apply to horizontal machines, which are mainly transferred to automated processes that use robots to continuously handle specimens. Vertical machines take up less space and are easier to operate.

Single-post general testing machines have lower force levels and lower costs. They have a load capacity of 1000 pounds. Dual-post general testing machines have frame load capacities of 1000 to 135,000 pounds. Load cells are also rated for a certain maximum force, which should be appropriate for the general testing machine frame and specimen. For example, a 100-pound load cell placed in a 1,000-pound frame can test stresses up to 100 pounds. The capacity of the load cell should not exceed the estimated breaking load of the specimen by too much, otherwise the measurement accuracy will be destroyed. [page]

Many plastics users can do well with a single- or double-legged unit with a load capacity of 5,000 pounds and a set of three load cells. The most commonly used machines for plastic testing are those with frame capacities of 2,000 to 5,000 pounds. Testing of unreinforced plastics rarely requires frame capacities exceeding 2,000 pounds. For filled and reinforced plastics, machines with frame capacities of 5,000 to 7,000 pounds are often required. But for compounds containing glass, carbon black, or other fibers, a frame capacity of 60,000 pounds may be required.

If you purchase a machine with a load capacity far greater than you need, you will not only pay more money, but you will also pay for testing time. Larger units run slower. For example, a machine with a 250-pound frame load capacity will typically run at 40 in./min, while a 5,000-pound machine will run at 20 in./min.

Single or dual-pillar?

How much of a maximum load you will typically need depends on the type of material you are testing, and is one of the important things to consider when deciding between a single-pillar machine and a dual-pillar machine. Also consider whether you will need an environmental compartment for testing at controlled temperatures. A dual-pillar unit is taller, allowing larger specimens and larger heating cabinets to be inserted between the pillars. If you will be doing any compression testing on foam, you will generally want a dual-pillar unit because the specimens tend to be large.

Dual-pillar machines are also inherently stiffer. So there is less deflection during testing. Finally, there is the difference in cost. A single-pillar general purpose testing machine may only cost $7,000 to $10,000, while dual-pillar models typically range from $13,500 to $30,000. These prices are just for the machine. The load cell may range from $1,500 to $5,000. Data acquisition and analysis software might cost $2,500, not including the computer. Fixtures are another separate cost, as is installation and training. If you need a heated chamber, it typically costs between $8,000 and $20,000.

Advances in software

The electromechanical design of general purpose testing machines is relatively mature. The new advances are in the control software. Advanced computer software offers greater productivity and accuracy, and is easier to use. These software add a level of repeatability that was not available before.

Rather than relying solely on readings at the breaking point, designers and quality managers can now test to find out what happened. Did the material stretch or deform before breaking? Was its deformation proportional to the stress? The answers can help them evaluate materials, determine safety margins, and better simulate end-use applications.

New software automates testing, data collection, analysis, report output, data storage, and retrieval. Users can tell the machine to run at a certain load rate, and the system will automatically adjust the crosshead speed. The new software also allows users to get the true value of the tension during the test through position sensors, which accurately measure the distance the crosshead has moved. The change in specimen length divided by its original length gives an automatic strain result. The

new software also allows automatic verification and calibration of load cells when sensors are replaced. It "reads" the electronics on the load cell and establishes the parameters, eliminating the need for mechanical corrections that were previously performed by the operator.

Newer software with faster data acquisition can more accurately capture load peaks and examine strain/strain curves in more detail at higher speeds or when loads fluctuate. Typical data acquisition rates are about 50Hz, (50 readings per second), although sampling rates can reach 5kHz.

The latest computer-driven universal test machines are less expensive than the machines with digital control panels that have been used for more than 10 years. Newer models of computers have simpler controls and often do not have digital displays with graphic readouts. Computer-based universal test machines now drive the entire operation, which keeps costs down by eliminating digital displays and some electronic components.

Affordable Dynamic Testing

Unlike electromechanical universal test machines that perform static tests, servo-hydraulic universal test machines perform dynamic or fatigue tests. These tests necessarily require the application of force continuously in a cyclic action of loading and releasing. For example, in a fatigue crack growth test, the user wishes to determine how many cycles it takes for the material to break. Dynamic tests performed on electromechanical machines require less force than static tests.

Servo-hydraulic machines have load capacities ranging from 100 pounds to several tons and generally cost two to three times as much as electromechanical general-purpose testing machines. They are primarily used for fatigue testing of metals, but are increasingly being used to test plastics for the automotive, aerospace, biochemical, and electronics industries, where plastics have replaced metals in structural parts subject to fatigue.

Computer-driven servo-electric general-purpose testing machines are said to have the unique ability to perform a range of low-stress dynamic and static tests on materials and small parts. These units, while expensive, are more affordable than they used to be. The advantage of servo-electric drives over servo-hydraulic drives is that they avoid the use of hydraulic oil, pumps, and water cooling.