Nowadays, more and more plastic processing companies are purchasing so-called universal testing machines, which are used for tensile, bending, compression, and shear tests to evaluate material properties, conduct application development research, and quality control. The advanced electronics have improved the performance of these devices, making them easier to operate and increasingly affordable.

The universal testing machine can stretch, bend, compress, or pull plastic material samples by adjusting different speed levels. This is the most common equipment in plastic mixing laboratories. In the preparation process of mixed ingredients, the use of testing materials can determine whether the material is suitable for certain specific processing or end use applications. It can also be used for product quality control to ensure consistency between different batches of product quality.

The universal testing machine includes one or more vertical load-bearing columns, with a fixed horizontal base installed on the columns and a movable horizontal crosshead (crossbeam) at the top. Nowadays, universal testing machines usually have ball screws on the columns to fix the movable crosshead. The size of the universal testing machine is characterized by the maximum load-bearing level of the frame and the force gauge used to measure the load/tension. The dynamometer is attached to a movable crosshead driven by an electric motor or hydraulic device. The series of force gauges with fixtures can measure the magnitude of force, and the results can be displayed on a digital monitor or PC. Many universal testing machines have interchangeable force gauges, so they can be matched with different materials being tested. Static testing is conducted using a standard electronic universal testing machine, typically with a loading speed range of 0.001 to 20 in./min (1 in.=2.54cm). Dynamic or cyclic tests such as crack growth and fatigue tests are usually conducted using a hydraulic servo system universal testing machine, which takes a long time and has a low load. At present, the most common items tested using universal testing machines are tensile strength and modulus, bending strength and modulus. When conducting tensile tests according to ASTM D 638 and ISO 527, both ends of the spline are clamped with fixtures, one fixture is stationary and the other is fixed on the crosshead, moving away from the fixed fixture and pulling the spline until it breaks. When the spline breaks, the crosshead will automatically stop. During the bending test (ASTM D790, D6272, and ISO178), the spline is placed on two supports fixed to the testing machine. In this experiment, the direction of movement of the crosshead is opposite to that of the tensile test, pushing instead of pulling the spline towards an unsupported center until the spline bends or even breaks. Because most thermoplastic materials do not break in this test, it is impossible to calculate the flexural strength at break. Therefore, the standard test method requires calculating the bending stress at a strain of 5%. The capacity of commonly used electronic universal testing machines is 100-135000 lb. The larger the size, the higher the cost. Generally speaking, vertical equipment has a smaller volume and is easier to operate, and can also rely on the gravity of the sample itself to make difficult samples, such as thin films, as easy to handle as heavier injection molded parts. The force range of a single column universal testing machine is relatively low, and the price is also low. The structural capacity is generally 1000 lb. The structural capacity of a gantry universal testing machine can reach 1000-135000 lb. The force gauge is also classified based on the maximum force value applicable to the structure and sample of the universal testing machine. For example, a 100 lb load cell installed in a 1000 lb mold frame can provide a test load of 100 lb. The capacity of the dynamometer should not exceed the predicted fracture load of the sample too much, otherwise it will affect the accuracy of the test results.

Unlike the universal testing machine used for static testing, the hydraulic servo universal testing machine can perform dynamic testing and fatigue testing. This requires repeated application of stress and a loading release cycle. For example, in fatigue crack growth tests, users expect to know how many cycles the tested material will break. The stress required for dynamic testing of universal testing machines is smaller than that required for static testing of electronic universal testing machines. The frame capacity of hydraulic servo system testing machines ranges from 100 lb to several tons, and the price is usually 2-3 times that of electronic mechanical testing machines. It is mainly used for fatigue testing of metal materials, but its applications in automotive plastics, aerospace plastics, biopharmaceutical plastics, and plastics for electronic components are becoming increasingly widespread because these fields require structural components with good fatigue resistance.