The commonly used electromechanical units range in capacity from 100 to 135,000 lb. The larger the size, the higher the cost. Whether single- or dual-column, the most commonly used machines in testing plastics are vertical benchtop units. The same principles apply to horizontal machines, which are primarily relegated to use in automated processes that utilize robots to handle samples continuously. Vertical machines have a smaller footprint and are easier to operate. They have gravity in their favor, which makes them suitable for delicate samples such as films as well as heavier molded parts.
Single-column universal testing machine machines have lower force capacity and lower cost. They come in frame capacities up to 1000 lb. Dual-column universal testing machines are available in load-frame capacities from 1000 to 135,000 lb. Load cells are also rated for a certain maximum force that should be appropriate for the universal testing machine frame and the sample. For example, a 100-lb load cell placed in a 1000-lb frame allows testing with up to 100 lb of force. Capacity of the load cell should not exceed by too large a margin the estimated breaking load on the sample or measurement accuracy will suffer.
Many plastics users do well with single- or dual-column units with up to 5000-lb capacity and a set of three load cells, according to Wayne Hayward, Tinius Olsens marketing manager. His firms most popular universal testing machines have 1000- and 2000-lb capacity.
Craig Stanton, marketing manager at MTS Systems, agrees that machines most commonly used in plastics testing are within the 2000- to 5000-lb frame-capacity range.
MacManuis from Instron says testing of unreinforced plastics rarely uses more than 2000-lb frame capacity. For filled and reinforced plastics, machines with frame capacities of 5000 to 7000 lb are often needed. But for composites with continuous glass, carbon, or other fibers, load frames of up to 60,000-lb capacity or more may be required.
If you buy a machine with much more capacity than you need, you will not only pay more in cash, but you will also pay in testing time. Larger units run more slowly. For example, a machine with a 250-lb frame commonly runs at up to 40 in./min, and a 5000-lb frame runs at up to 20 in./min.
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