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EMI gasket to ensure a good electrically conductive bond with the enclosure. Many frame styles are available to suit different mounting methods.


❱ ❱ 470 flat EMI gasket material is just one of a wide range of sealing options for achieving EMC compliance


over silicone sponge, as there is no uneven cell structure to consider that affects moisture ingress and closure force.


ALUMINIUM HONEYCOMB Most electronic enclosures need some form of air ventilation to keep the electronics cool. If this is an RFI/EMI shielded enclosure then the overall shielding effectiveness of the enclosure can be compromised. For many commercial applications,


perforated sheet, metal or metallised plastic will suffice but for high performance requirements, waveguides provide superior shielding over a wide frequency range. Waveguides can be circular, rectangular or honeycomb- shaped. The honeycomb vent is a series of tubes that acts as a waveguide, guiding electromagnetic waves into or out of the enclosure, but as the tubes are long enough then they attenuate those waves. Typically, the tube should be at least three times the length of the diameter. Aluminium honeycomb is 96% open area, and therefore offers high shielding performance, light weight and good airflow. A typical shielded enclosure will use two honeycomb ventilation panels one as the inlet and one for the exhaust. Aluminium honeycomb vents are supplied in an aluminium frame with an


PROPERTY TESTING For quality control and compliance purposes, the electrical and several mechanical properties of elastomers can be measured which impact the selection of the appropriate material. Any of the tests can also be carried out on samples which have been exposed to air ageing at an elevated temperature, to give an indication of how the material will perform in the long term in the service environment. The properties of the sealing material that are tested include: Hardness is a measure of the degree of indentation when an indenter of known geometry is placed on the elastomeric surface under a known constant force for a fixed time. In a standard elastomer there is reasonable agreement between observed hardness and Young’s Modulus (E). In the case of conductive elastomers this relationship is not so clear cut since the elastomer acts more as a binder for the filler and the hardness tends to respond according to a rule of mixtures. Tensile strength and elongation at break testing uses tensile test equipment fitted with an extensometer from which a stress-strain response curve can be obtained and a judgement of the stresses and strains can be made on the material during service. The same equipment is used to establish the tearing properties of the material. Compression set test was originally conceived as a measure of state of cure. The test involves compressing a cylinder of material to a fixed strain and leaving it at a known temperature for a fixed time. The compression is removed, the cylinder allowed to recover over a fixed time and the height re-measured, the value recorded being the percentage difference. Creep is defined as the change in strain with time whilst the elastomer is held under a constant stress. This stress can take the form of compression, tension and/or shear. In terms of how creep applies to a seal or gasket, this is dependent on whether it is constrained or non-constrained. Stress relaxation is defined as the


change in stress with time whilst the elastomer is held under a constant strain. Consideration must also be given to whether it is retained in a groove or free to take up a natural form on deflection.


MANUFACTURING EXPANSION


Two new presses have been introduced into Kemtron’s elastomer line, improving the quality of the finished product and the speed with which it is achieved. This will have a direct impact on the ability to offer a variety of moulded gaskets in a shorter lead time. A second knife cutter has been


brought in to run alongside the current cutter. Responsible for cutting most of the materials, these knife cutters are a vital part of the manufacturing process. Another new installation is a


pair of bespoke knitting machines, designed to provide reliable and consistent output across a variety of materials and profiles. The new machines incorporate contemporary safety critical features and provide easy maintenance and operation. “It is expected that our revenues


in knitting could grow by up to 50% in the next six months and we have put ourselves in a great position to take advantage of the opportunity,” said Managing Director Victoria Tsoi. Finally, a completely new Air


Compression system has also been installed into the company’s third factory on the same site. This is a more modern machine, offering higher output and more able to cope with heavy production requirements.


Compression deflection is tested using a disc of the gasket/seal material of a known thickness and diameter to which is applied a load equivalent to 100 psi and the deflection measured.


CONDUCTIVITY TESTING Tests specific to conductive elastomers include volume resistivity, volume resistivity after extension/break and dynamic volume resistivity. In the latter test, the variation in the volume resistivity is measured whilst a sample of known dimensions is put under a fixed load. The load is then oscillated around this fixed point by a known amount over a range of frequencies and the volume resistivity recorded. The purpose of this test is to give an


indication as to how the seal/gasket will perform in service when the vibrations from equipment operations are imposed upon it.


EMC Testing Vol 1 No. 2 /// 3


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