Today, more and more plastics processing companies are buying so-called “universal” testing machines that use tensile, bending, compression and shear tests to evaluate materials, apply development studies and quality control. Developed electronics have improved the performance of these devices, making them easier to operate and less expensive.

The universal testing machine UTM stretches, bends, compresses or pulls plastic material splines through adjustments at different speed levels. This is the most common equipment in plastic compounding laboratories. UTM test materials can be used to determine whether a material is suitable for certain processing applications or end applications during the preparation of the compound. UTM can also be used for product quality control to ensure consistency between batches of product quality.

UTM is now increasingly appearing in plastic molding and extrusion laboratories. On the one hand, they are increasingly involved in the development of new products and processes. On the other hand, they are accurate in the quality control of raw materials and finished products. In some areas with strong social responsibility, such as medical equipment or the automotive industry, plastic processing equipment is required to test their products, and UTM needs to play a greater role. At the same time, internal testing can improve the quality of process control, reduce the scrap rate, and thus achieve real benefits.

Different trials

The UTM tester consists of one or more vertically supported columns with a fixed horizontal base and a movable horizontal crosshead (cross beam) at the top. In the current UTM test machine, there is usually a ball screw on the column to fix the movable crosshead. The size of the UTM is characterized by the maximum load level of the frame and the dynamometer that measures the load/pull. The dynamometer is attached to a movable crosshead that is driven by an electric motor or hydraulic device. A series of dynamometers with clamps measure the force and can display the results on a digital display or PC. Many UTMs have interchangeable dynamometers and can therefore be matched to the different materials tested. Static testing is performed using a standard electronic universal testing machine with a typical loading speed range of 0.001 to 20 in./min1 in. = 2.54 cm. Dynamic tests or cycle tests such as crack growth and fatigue tests are usually carried out using a hydraulic servo system UTM tester for longer periods of time and lower loads.

Early UTM testers had similar electronic components and recorders. It has now been replaced by CNC equipment and PC software. New automatic control devices can run tests, display data, and sometimes even record while running. In previous digital recorders, including the digital display era before PC software, the test information obtained by the user was the load/deformation curve, the Y-axis represents stress, and the X-axis represents deformation. These curves also need to be calculated and interpreted. The latest systems still provide these curves, but at the same time they can calculate data such as yield strength, damage strength and modulus.

Currently, the most common items tested with universal testing machines are tensile strength and tensile modulus, flexural strength and modulus. When tensile tests are carried out in accordance with ASTM D 638 and ISO 527, clamps are clamped at both ends of the spline, one clamp is stationary and the other is fixed on the crosshead, moving away from the fixture and pulling the spline until the spline appears Break, the crosshead will stop automatically when it breaks. In the bending test, ASTM D790, D6272 and ISO178, the splines were placed on the two supports of the test machine fixed machine. In this test, the direction of the crosshead movement was opposite to the direction of movement in the tensile test, pushing against a center without support, rather than pulling the spline until the spline was bent or even broken. Since most thermoplastic materials do not break in this test, it is impossible to calculate the fracture bending strength. Thus, the standard test method requires the calculation of the bending stress at a strain of 5%.

UTM testing machines are rarely used in compression testing, although this is a major test for rigid plastic foams in ASTM D1621 and ISO 844 standards. The UTM tester can also be used for crush testing of injection molded products of any shape, such as bottles, to compress or deform the sample at a certain height, ie the compressive strength value.

According to UTM suppliers, shear tests are rare for plastic materials. The shear strength value is measured by placing the sample in a punch-type shearing device. The impact velocity was 0.005 in./min until the movable portion of the sample completely exposed the fixed portion. The shear strength value is the value of the stress/shear area. This is important in film materials and sheet products because this type of damage is likely to occur in such products, but this is not the most considered factor in the design of other extrusion and injection products. When tested in accordance with ASTM D732 (without the equivalent ISO standard) standards, the commonly used specimens are plastic sheets or injection molded discs of 0.005 to 0.500 in. thick.

Electromechanical UTM testing machine

The commonly used electronic UTM tester has a capacity of 100 to 135,000 lb. The larger the size, the higher the cost. In general, vertical equipment is smaller and easier to handle, and thanks to the gravity of the sample itself, tricky samples such as films can be handled as easily as heavier injection molded parts. The single-column UTM tester has a lower force range and a lower price, and the structural capacity is generally 1000 lb. The door type UTM test machine has a structural capacity of 1000 to 135,000 lb. The dynamometer is also categorized by a maximum of force values ​​applicable to the structure and sample of the UTM test machine. For example, a 100 lb dynamometer installed in a 1000 lb mold can provide 100 lb of test load. The capacity of the dynamometer should not exceed the predicted fracture load of the sample, otherwise it will affect the accuracy of the test results.

According to marketing personnel at Tinius Olsen, most plastics users are accommodating a 5000 lb single-column or gantry device with three dynamometers. Tinius Olsen's most popular UTM test machine is a 2000 lb test machine. Instron's MacManuis says non-reinforced plastic testing rarely uses equipment with a frame capacity specification of more than 2000 lbs. The frame capacity of equipment used for filling and reinforced plastics is often 5000 to 7000 lb. However, for continuous composites of glass, carbon black or other fibers, the frame capacity may require 60,000 lbs. If the purchased equipment exceeds the required capacity, not only does the input cost increase, but it also requires more test time because the device runs more slowly. For example, a 250 lb device typically runs at 40 in./min, while a 5000 lb frame runs at 20 in./min.

Software development

The development of software technology has improved the testing speed of UTM devices and made the operation of the devices easier. The reading of the test data, the whole process of the test, whether the sample is stretched before the break, the deformation and the external force are proportional. These answers can help the material engineer or product designer to evaluate the performance of different materials, determine the safety margin and more. Goodly simulate the application of end users.

The new software automates experimentation, collects data, analyzes data, records output, stores data, and repairs it. The user can enter a certain running load speed and the system will automatically adjust the operation of the crosshead. The new software also allows the user to obtain real strain values ​​during the test using a displacement sensor that accurately measures the displacement of the crosshead. The new software also allows the dynamometer to automatically recognize and calibrate when the sensor is replaced.

The software of the new PC UTM tester now performs the entire operation, so the cost is reduced by eliminating the digital display and some electronic components. Last year, Tinius Olsen introduced the PC T-Series single-column and portal UTM testers with a capacity of 22,250 lbs. It is estimated that the T-Series tester includes test navigation software for PC and data acquisition, which is 10% lower than the S-Series on the workbench with digital control panel. When using the S-Series tester, the user needs to reprogram to change the test procedure. The PC-driven T-Series tester can easily complete this process by simply selecting a program option.

MTS System's gantry-based PC with a frame capacity of 2000 to 5000 lbs, the Alliance RT workbench, is priced at $25,000 to $50,000, including machines, PCs, TestWorks 4 software, a dynamometer and a set of fixtures. Installation and training costs. The RT Series digital control system has a single-loop card in the load frame that is said to improve confidence and reduce downtime. The advanced six-point navigation system is said to provide excellent guidance for crosshead positioning through x, y and z axes, thus improving measurement reliability and accuracy.

Testing Machine recently introduced the LabMaster single-column UTM system for low stress testing with a capacity of 250 lb. This R&D device includes a tensile tester and a dynamometer. The PC window that can be connected to the network is placed on a flat touch screen, reducing the operating space required for the workbench. The improved LabMaster software, called the Experimental Method Package, simplifies setup and automatically sets the test sequence. In addition, the most noteworthy feature of the device is its high resolution, which can detect subtle changes in load or sample position. The price is about $16,000~16,500.

Ametek's Test and Calibration Equipment Division has introduced two new single-column UTM testers from Lloyd Equipment, UK. Cross-beams are operating at a wider range than previous models, and electronic components have improved. The unique PC window Nexygen material analysis and testing software runs faster. One of them is the LFPlus series of digital testers, and the low stress equipment has a capacity of 250 lbs. The speed range is 0.002~50 in./min, the performance of electronic components is improved, and the data scanning speed is increased by 8 times. The other is the LRXPlus series with a capacity of 1100 lb. The cross beam runs at 0.004 ~ 49 in/min. It is possible to perform 10 program test setups and store 600 test results.

Dynamic Testing

Unlike the electronic UTM tester used in static testing, the hydraulic servo UTM tester can be used for dynamic testing as well as fatigue testing. In this way, stress must be applied repeatedly to perform a load-release cycle. For example, in the fatigue crack growth test, the user expects to know how many cycles of the material to be tested will break.

The stress required for dynamic testing is less than the stress required for static testing of an electronic universal testing machine. The hydraulic servo system test machine has a frame capacity ranging from 100 lb to several tons, and the price is usually 2 to 3 times that of the electromechanical testing machine. It is basically used for fatigue testing of metallic materials, but it is also becoming more and more widely used in automotive plastics, aerospace plastics, biomedical plastics, and plastics for electronic components, because structural components with good fatigue resistance are required in these fields.

In the past two years, MTS has developed two PC-driven electric servo single-column UTM testers, which are said to perform well and provide low-stress dynamic and static testing of materials and small components. Despite the high price, these devices are suitable for dynamic testing than any previous test machine. Compared with hydraulic servo testing machines, electric servo testing machines have the advantage of avoiding the use of hydraulic oil, pumps and cooling water. The first electric servo models on the market are special equipment for low stress. One is the MTS Tytron 250 MicroForce, a 50 lb horizontal frame unit that has been used for thin film packaging testing, biomedical tube and suture testing, and microelectronic component testing. The other is MTS NanoBionix, which is mainly used in biomedical materials research and product development. The maximum load is only 51g and the cycle speed is 2500 times per second. It is suitable for quasi-static and dynamic performance testing of viscoelastic materials and performance testing of small microelectronic components.